The Stovebolt Site Forums Welcome! Welcome Centre Do It Yourself (DITY) Gallery 1947 GMC EC152 - "Love at Last"

Hello All
I'm new here. I've always wanted an antique truck - I got lucky and found a beautiful 1946 GMC pickup. She's an EC152, but she's been shortened to a shortbed length. I wasn't sure what I was doing with her - I've had many other projects, but nothing this old. Over time I've fallen in love with this pickup... the beautiful purr of her 228, the starter peddle on the floor, etc.

She came with an old AM radio - a beast of a thing; the size of a toolbox with tubes! But when I got the truck, the previous owner got a Painful wiring harness and stopped after butchering the existing wiring. I've since installed an American Autowire harness, a 12v alternator, new instruments, new windshield, new brakes all around, new brake cylinder, etc.

Maybe one of you can tell me about the various tags around the pickup. Below is a pic of one in the engine bay on the driver's side. This is clearly different than the manufactures identifying tag (often called a cowl tag). There is another tag on the bed.

Enough rambling. Here are a few pics of her current state...

My 1939-54 GMC Master Parts Book says that your "1574-159069" tag is a "cab or body number" for a 100 thru 350 series AC, CC or EC truck. There is also an engine serial number stamped as shown below.

What is the purpose of the two plastic boxes mounted on the firewall above the heater hoses?

The two plastic boxes are really just one - just the shape of the cover. It's a junction for the big HOT cable.

Welcome! Nice looking truck!

I notice the title of your thread says 1947, but the text says '46. I'm assuming the '46 is correct? (My fingers slip too once in a while! )

Bill - Thank you. Good to know. I've never seen these on other trucks - thing of the past probably.

Rich, it's a slight technical thingy... She was built January of 1947 but she's 1946 all around. I hear that most people regard 47's built in the first half of the year to be 46 models.

Post the numbers on the id plate, engine id, etc and know for sure what it was originally titled as.

@Otto: Thanks. I have the original title, so I already know what it was titled as. Thanks.

Interesting. So the vehicle was a '46 design, built in '47, but sold by Chevrolet as a '46.

Otto - Here's some info with a better description of what are known as 1946 2nd Series trucks like his truck.

The "1946 2nd series GMC" refers to the GMC trucks that continued to be sold under the styling of the previous C-Series/CC-Series and CF-Series/CF-Series (for COE models) for the 1946 and 1947 model years. These models were held over into the early part of 1947, with the new Advance-Design trucks being introduced around March to May of 1947. General Motors literature always referred to these trucks as 1946 model year trucks.

Exactly.

Trust me - I ended up with a heap of parts that I purchased over the winter for a 1947 GMC pickup (i.e., 47-49) and nearly none of the fit. So now I just order 46 parts and haven't had an issue yet. So I have come to refer to her as a 46.

Being that she is date-stamped as a January 1947, all the parts were made in 46 - so I makes that much more sense for me.

Well... it's technically a GMC (not a Chevy - but same diff, right?). Pretty much when WWII hit, all manufacturing was for the war effort (well, except farming and a few other essentials). Consumer vehicles didn't change from about 1941 till around 1946 +/- (till they got the manufacturing lines re-jigged; that and some are left-overs from military surplus).

It's a fascinating period of time. Even pennies were made in steel for a period (1943) - the copper was needed for the war effort.

Here - I thought it'd be fun to post a pic of how I got the truck. Actually there isn't much change visibly. Mostly she's dirty, missing lights, windshield cracked and delaminating.

Incidentally, I had a one lens for the front turn-signal (I think that's a turn signal below the headlight). It was cracked but I could read the part number. It cross-referenced to a Dodge truck of the same year. I managed to purchase NOS lenses for the old girl.

Nice truck. Here is mine. I bought it in 1997. If I can help with pictures please let me know. My email address is in my profile. I have been buying and selling parts since 1997 and I have quite a few on hand. I set up at the Hershey car show very October. Good luck!

Warpig - The light below the headlight in your pic was most likely a "marker light" (but maybe an add on turn signal) and looks identical to the newer AD series 1947 GMC's marker light design. The below pic from a 1947 GMC Maintenance Manual dated October 1947 shows the detail and looks identical to your fixtures. If you zoom in on your pic, I can also see where the earlier style 1941-46 "marker lights" that where designed/mounted on top of the headlight pods seemed to have been on your truck. The other pic below is from a 1945 GMC Parts Book, detailing the exploded parts view of the 1941-46 headlight assemblies. The fender mounted fixture could be an add on, or maybe another mid year switch over thing as they were gearing up for the new 1947 AD series truck style???

Thanks Greenie. That would be a great help. Do you have a pic of your steering wheel? Is it original? Mine is a Chevy steering wheel - cool, but wrong. Also, the original knobs have cool lettering compared to after-market; any pics of those?

Hard to say. I see other people have the headlight mounted market lights. I also see that later model 1947's have the fender marker light. I can say, mine seem original - but hard to say.

My truck is an EC - whereas your manual says CC. I wonder if that is the difference.

It seems turn signals didn't exist (well... your hand existed, but you know what I'm saying). Mine does have an after-market turn signal assembly on the steering column. But the previous owner hacked it up something terrible.

Thanks for the info.

Rick - You can see a couple more existing pics of Greenie's truck HERE, as well as lots of other 1941-46 GMC and Chevy trucks (and as listed on this page as "1947.1 (First Series) Trucks -- The Last of the "Art Deco" Series Trucks") in this 1941-46 Stovebolt Gallery Page. *

If you dive into those galleries a little deeper, some of the Bolters have included personal web pages to provide more details on their trucks. A prime example with tons of great detailed pics is fellow Bolter "JimF4160" showing off his 1941 GMC CC-101 Pickup.

I think you will find a great deal of resource pictures and info in these galleries to keep you busy, and maybe drooling - hahaha, for days to come.

* Note: I previously mentioned that your truck is considered a 1946.2 (2nd Series) truck. In getting reacquainted with this older Gallery Page, I think your truck may be better described as a 1947.1 (1st Series). A mix up in terminology and a technicality that really has no effect on the registration year of your truck. My apologies for that.

Rick - This Old GMC Trucks Pickup Serial Number decoding web page helps break down your "EC" code. It shows 1941-46 GMC Pickups used "CC" and 1946 also identified with "EC".

The Old GMC Trucks web site hasn't been added to/updated in quite some time, but still has a ton of great info that might also help with your project too.

Another example of a later 46 EC152.

16-Jun-2025 : new fuel pump. The old one was leaking. I purchased the one with the glass (I grew up with these and I like to see the gas - easy to check if you have fuel). It was $20 more than without the glass - money well spent. The rebuild kit for the fuel pump was almost as much as the fuel pump itself.

Next on my list: replace the fuel sensor.

It may be that I have the wires reversed on the instrument panel (since the wiring and instrument panel are upgraded). But I figure a new sending unit ($21) can't hurt. My first step in diagnosing would have been to connect a DMM across the old sending unit - but to properly measure I'd have to remove it anyway. Given how the gaskets on this old girl are... crispy... it seemed wise just to get a new unit.

Oh, it also could be that I don't have enough fuel to register. I've hesitated to fill her up - I'm still chasing leaks and such. I guess at this point I could do that.

That might be interesting. I went to that site - ended up in some circular links without finding the info.

Let me ask - what sort of info would I get from this decoding? My tag simply has "EC152", a serial number, hp rating (85.5 @ 3k!!!), and gross weight. It seems like there would be little to decode. I guess I could attempt to dig more (the site seems a little rough around the edges - LOL). Or maybe they are referring to another tag with some other numbers?

According to this page there was only one year for EC-152, 1946.

My 1939-1947 100-970 MPB list both E and F series as 47. Later books show the E series as a 46. The E series came out after October of 46. October 1, 1946 was the model year brake-off point. All trucks built after 10/01/46 are 1947 models. (A, C and E series)

In July of 1947 the "New Design" F series trucks came out.

My paper 1939-54 100-470 models lists EC-152 as 1946 only, with serial numbers 001 thru 3121. This is probably the exact same page of the same manual I pointed to 2 posts above because the page number and "Revised August 1954" at the bottom of the page are the same. FC is listed as 1947-50.

My .PDF copy 1939-53 100-370 models also lists EC-152 as 1946 only, with serial numbers 001 thru 3121. Revised December 1953. FC is listed 1947-50.

My .PDF copy GM Parts Sales United Motors System Master Parts Catalog for GMC Trucks model series 100 Thru 470 also lists EC-152 as 1946 only, with serial numbers 001 thru 3121. Revised 2-1-56. FC is listed 1947-50.

Summary: All 3 of these documents list EC-152 as 1946 models and FC as 1947-59 models.

I won't enter the debate about whether EC-152's are only 1946 or if they continued into early 1947. What I can say is my tag and title say EC152, and my title and original title indicate 1947. It's hard for me debate the items I see in front of me. I have to side with Jim.

But what is the point of the debate anyway? I mean, is Bill attempting to indicate my truck is a forgery?

Oh... so the new fuel pump... isn't quite working well yet. With the old fuel pump my truck started easily everytime... but with the new pump it's a bear to get running. It looks to have good fuel pressure... but I have nothing to measure it by. Despite that, I wonder if this new fuel pump is crap. I can see the fuel in the glass bowl - but I can't verify the fuel to the carb... yet. I figure this weekend I'll open the carb and check the floats. Keep in mind that the carb was rebuilt a couple months ago and worked fine with the old fuel pump.

Rick - Here's a Tech Tip on testing your fuel pump written by Jerry (Hotrod Lincoln) that may help you out.

Before monkeying with the carburetor, just disconnect the line at the carb inlet and crank the engine to see if fuel is reaching the carburetor.

If it is, perform the necessary tests on the fuel pump.

If fuel isn't reaching the carburetor, you have joined the ranks of many others here, including me, who have purchased new fuel pumps that don't pump a drop straight out of the box.

No, just telling you what 3 books say.

If I only had a pressure gauge. Plus, she currently won't start - so that's out too.

As they say in the Netherlands... wat een spelbreker (what a bummer).

Thanks for all the tech-tips. I agree - I'll discon the carb; see if she flows when she cranks. If she flows, I'll attempt to ascertain the amount of flow vs crank time... Otherwise I'll see if the floats may need adjustment.

A typical fuel pump (for carbureted) is only about 4-6 psi... not a lot. If I get flow, then it's likely the carb. But I also have to keep in mind that there may be simple residual pressure in the lines.


Thanks Otto - nice to know this isn't unheard of. I admit it's been a long time since I've had to replace a fuel pump.

Your pump should only be putting out 2 psi - 4 psi. Any more than that will overpower the float valve and flood your carburetor.

My pump operates at 2.5psi.

I forgot to ask if you have an inline filter installed between the pump and carburetor? If so, it could be clogged.

The average human is only capable of producing about 2 psi if he blows as hard as he can. If you can't blow through your fuel filter, neither can your fuel pump. If it's even moderately difficult for you to blow through the filter, a properly functioning 1947 fuel pump will also have trouble.

You can buy a pressure gauge/vacuum gauge (it does both) for about $20 at any auto parts store.

Good to know. I actually don't have a gauge, but I know for most carbureted cars it's 4 to 6.

Yep - and I am considering that.

Again... good suggestion.

I think I got her going. I adjusted the floats (a few times) and examined a few things. Honestly couldn't tell you what happened. She acted like she was flooded - I'll continue to diagnose (try again tomorrow).

So, she still has a few things out of whack. She has a new American Autowire harness, new 12v instrument cluster, etc. I can't recommend them enough - best harness I've seen; made the job easy.

The horn working would be nice - I can see a cut wire at the bottom of the steering column. The steering wheel is not original - but should work. The horn is original. So I might start digging into that.

Brake lights don't work - despite a new sensor. I can jumper across the sensor to check if the new sensor is bad. Otherwise I have to pull out the wiring diagram. It wouldn't be the first time I made a bad assumption on the nomenclature of a diagram.

No signals - but I'll use my hand. Us older gentlemen are used to using our hands. I don't think the truck originally had directional signals - my truck has an aftermarket item attached to the steering column... but some maniac cut the wires into stubs.

But... I think I'll replace the inner door release handles. The driver's side is worn down a lot. This old girl has seen some miles I imagine.

Realize that the original horn electrical system did not use a relay and that your new harness probably expects to use one.

The original circuit had battery voltage going to one terminal on the horn. The other terminal had a wire that came back into the cabin and up through the steering column to the upper steering column bearing, which also provides a surface for the contact in the steering wheel to touch. The top end of the contact touches the underside of the horn switch in the steering wheel. When you mash the horn button the horn switch closes and completes the circuit to ground.

Check your new wiring diagram to see if it uses a relay. Making the change involves very little work.

You are correct about the turn signals. Your truck was not equipped with them when new. It also probably had only one brake / tail light.

I drive all over the place using hand signals.

Surprisingly, turn signals weren't required on cars sold in the USA until 1968.

Thanks Bill. It does - it's built into the new fuse box. I'll have to review the wiring diagrams. Thanks for the inspiration.

Has only ever worked on one of these old tube radios. My pickup has one. It's nifty. I doubt I'd wiring it in, but at the same time... wouldn't that be cool. Keep it tuned to some oldies station, click it on and wait for the tubes to warm - as the tubes warm the signal becomes clearer. I'm sure I can get a 12v to 6v converter, but it's also missing at least one tube (mud wasp nest in it's place - LOL), dials are frozen, it's missing the cover, and I'm sure there is a speaker in there (that's shot).

This truck is so cool!

Rick - On the radio questions, check out The Radio Bench sub-forum under the Electrical Bay. Some help is available there if you want to dive into your old "radidio".

If you are planning to use the original 2 terminal horn you will need to change the wiring at the horn. One of the wires (I don't think it will make a difference which one - maybe some other Bolter will have more expertise) will need to be connected to ground, the other to the horn relay.

Before you start wiring changes, use your 12v battery and some spare wire to test how your original 6 volt horn reacts to 12 volts. You may decide that you want to change horns.

Radido? Radiditto? Pequeña Raditta? LOL. Gearhead, you are a riot!

Yeah, I tinkered with it the other day. Perhaps I'll just clean it and put it back in. If lucky I'll find a cover and maybe one day I'll get it working again. Bigger fish to fry.

You are absolutely right. Good suggestion.

I did tinker with this yesterday. The old Chevy steering wheel is really stuck on there. I created a "puller" but it didn't work. There appear to be two threaded holes in the steering wheel - that I imagine are for pulling, but the threads are so rusty that I couldn't find a bolt to thread into it. I think before I force this thing off I should have a backup plan (like a replacement steering wheel).

When I got the truck, all the wires were cut. There was a push button under the dash that I imagined could be used for a horn button. Since the starter still uses the floor peddle, this is what makes sense. So likely I'll wire the horn to this push button for the interim.

OK, so this weekend was fun. I've decided to pick up and move for retirement. My neighbors are upset. One of them came over - we had a couple beers and she convinced me to take her for a drive around the block. I have yet to drive the ol' girl more than back and forth in the driveway (I tend to be.... overly cautious). Anyway, my neighbor convinced me - so I took the ol' girl out for a drive around the block. She was fine. It was fun. The tranny makes a bit of noise - don't the all (eyes rolling in head). Anyway, I'm glad someone made me step out of my comfort zone. I did tell my neighbor that if the truck broke down she had to push. LOL.

First drive recording:

Warpig's "Love at Last" first ride

You can see I have the AC on and the air is on full (i.e., the vent and windshield are fully open - LOL).

Not a great recording, but fun to take her for a spin. She feels great. The ignition time is maybe off a little. Or I might have a slight carb issue. I'll play around with it later.

Outstanding Rick! Thanks for sharing and looking forward to more on your truck.

Great looking truck you have there Rick! Welcome to the club! I have a spare steering wheel if you want to chat more about it send me a PM. Here is a pic of my 1945 GMC. Great little trucks and you can't beat that dual AC feature!

I'm liking the sound of an old truck! Looked like a nice day for a test drive.

Rick, what a great video. Isn't it fun to have a crank-out front windshield? The truck sounds good to me, and what a pretty drive. Looks like you are a little at the "Find it; Fix it; DRIVE IT" stage.

Only one word.... "wow".

That is an amazing truck. I think I found pics of it online and downloaded them in my archive. Nice to meet you Phillip. I'll send a DM soon.

Rick! Congratulations on getting that old girl on the road. Your little creampuff purrs like a kitten.

Got the feeling watching that video well done

Thanks Paul. Yeah, I think the timing is off but I do enjoy the sound of her engine.

Thanks TPR! I see your signature says, "265 V8 with service tray" - what is the service tray?

The truck is a riot. I've been taking several neighbors for rides. After all the rides, we were talking in front of the house as it got late/dark. So I got a chance to take the old girl for a short drive at night (after everyone was gone). That was pretty cool. Nothing like seeing all the lights on, driving in the dark - those marker lights are super cool.

I need to start digging into the fuel gauge. I think she's got 3 gallons in her - but who knows. That would not be cool to run out while taking a test spin. I figure I might have the wires on the gauge backwards. I have a new sensor - it's generic though, and apparently you have to adjust of the float arm. That blows in a way - I mean, effectively the accuracy would be dependent on guessing the right length. Maybe I'll get luck and the old sensor will make this obvious.

Also, I noticed the speedometer isn't working - which is odd for a purely mechanical (and simplistic) system. I'll dig into this too. Should be easy to figure out.

My speedo wasn't working when I gotr it. It turned out there was no cable inside the sheath.

I installed a new cable and it worked for 5000 miles before the speedo head itself broke, Rebuilt it, no problems now.

Where is the fuel needle pointing? Always full is usually a ground issue. Always empty is usually an issue with the wiring between the sender and the gauge. This assumes the gauge is not damaged. They usually aren't.

Lookinto your sender. There's a ebay seller who sells the best senders for these Ttrucks under the name industrialstuffs.

OMG - I hadn't even considered that there might not be a cable. The instrument cluster is brand new, so I hope it's not the instrument cluster.

I'll look into the connection and sensor... soon.

A bit more of a drive... and some gear grinding...

Quick drive with the pup (still learning non-sychro)

Yeah, and the ol' girl still needs timing.. and maybe to be at a proper engine speed when shifting.

How is your grill in such great shape? I couldn't find a new/replacement grill. I assume you restored it?

That Grill is 4k worth of restoration and re-chroming completed back in 2004. The chrome restore guys are getting harder to find.

My grill may need to remain "rustic". Or maybe one day I'll see what the process entails. Do you have videos of the truck? I love to see things operate and hear their sound. In my opinion there aren't enough videos of these old beasts. When I search I mostly see models being offered for sale - rarely someone who seems to want to share something personal.

Contact S&H Chrome Plating in Tennessee for a quote. They do an excellent job and I found them to be reasonably priced (all things considered) when they straightened and re-chromed the severely crunched rear bumper of my '66 Toronado.

With bumpers, they have fixed prices regardless of how much work your bumper needs. Not sure how they price multi-piece grills.

615-865-0100

Thanks Otto.

I loved listening to you laugh Rick. First drives in an old truck are pretty laugh inducing. It’s like being a kid again, and graduating from our old bikes we delivered newspapers with, to a (hopefully) unbreakable mountain bike. Same laugh, same excitement.
I need to find an Art Deco truck.

Thanks. It is fun. I've driven it every days since getting her running.

Art Deco Truck? I found this on Facebook. It's been listed for 8 weeks - he might be more flexible. I'm considering it, but I'm also getting ready to move so I probably shouldn't. This truck looks sweet:
https://www.facebook.com/marketplace/item/655029767447295

When I talked to the guy, he said it doesn't smoke and has been updated to 12v wiring. New carb, distributor, and brakes. It's very tempting.

OK, so recent adventures... I got my brake lights working, got my fuel sensor working, and I found the issue with my speedo. The speed is bad - which blows since it's new.

How do I know it's the speedo and not something else - like the cable, etc. Well... I'm glad you asked. In my head I've been surprised that I've already put on 3 miles on the truck - in other words, the odometer is working. Before I spend $300+ on a new one I'll open the current new one to see if I can spot the issue. Maybe it's a trivial issue. It's a small bummer - it's not like the truck is going to be breaking any land speed records anytime soon.

Chuck the cable into a drill and set the drill in reverse. Turn on the drill and see what happens?

Sometimes the cable may not be in the speedo head all the way or the drive gear in the tranny might be bad, missing, whatever.

But, if that were the case I'd expect the odometer not to be rolling digits. Am I wrong?

Rick - It sounds like your speedo suffered a needle shaft failure. The odometer portion still works but since that shaft is probably broken beyond that, the needle will do nothing but sit there.

Here's an old thread with details on how to Repair your speedo, should you decide to go that route. The detailed instructions are courtesy of fellow Bolter "Jon G" in response to Bolter "Clem Donahue's" request for help.

Let us know what you figure out/decide to do.

I took Otto's advice and connected the speedo to a drive. It works. The shaft is not broken. It would seem that it cannot induce the field or something. I'll look at the link in a bit.

.. To be candid... I'm ticked as all get out at Jim Carter's parts at the moment. I've made orders of parts that are supposed to be in stock.. days after the parts should have arrived I call to find out that the parts are out of stock. When I cancel the order they send other items in the order - at full shipping cost. I ordered a new speedo cable and grommet. They shipped me the $1.85 grommet at a $20.00 shipping rate. I got this huge box today with a grommet. A silly rubber item that can take any shipping method without damage - and they charged me $20 for shipping. That was the shipping for the order I cancelled.

This isn't the first time either. Something has changed with them. They seemed dependable - months ago. Suddenly they can't seem to figure out which way the wind shines. They continue to waste my time and my money. If this is the way they intend to go forward, I cannot do business with them. I cannot afford to spend $20 to ship a part I don't need (especially a $2 part that is so highly generic I could by at walmart).

It's a good thing they are in Missouri - otherwise they'd hear me screaming profanities.

FYI, my speedo is a remanufactured unit from Jim Carter's...

I hate to hear that about Jim Carter. I've ordered a few things from them a couple years ago. My speedo is also from them in October of last year.

I've never had any problems with them but I haven't done a whole lot of business with them either. I hope they can get their act together.

There are other companies, OPGI for one, that I won't do business with for exactly the reason you stated. Out of stock parts sold as if they are in stock and then stringing you along after you've paid for them.

That seems to be the trend with shipping these days. I've seen several instances where some small part could be put in a padded envelope and mailed for a couple bucks, but instead they put it in a shoe box with some crumpled newspaper and bubble wrap and charge more to ship it than what the part costs.
Things are definitely not the way they used to be!

So I think I see the issue with the speedometer - or at least a problem. Not moving the speedo shows roughly 12 MPH. I estimate I'm getting up to 30 MPH, with no movement. Something seems very wrong with this unit.

I have tested the speedo by connecting a drill to the input shaft - the needle moves as expected. I don't have much time to mess with it these days. I'm attempting to retire and move closer to my brother. Of course, that might be turning into a pipe-dream. I was hit by an impaired drive earlier this year - and believe it or not, he's apparently going to attempt to sue me. And if I'm frank, I don't have faith in the justice system. But that's a story for another venue I suppose.

Today, I hope to tinker around on the horn for the old girl. There was a button under the dash that I believed someone used for the horn previously. The current steering wheel configuration has issues. The old steering wheel is brittle - I'm afraid if I pull it, I'll break it. It's on there pretty tight. It appears there are two machine screw holes that would be used with a puller to remove the steering while, but the threads are rusted and fairly shot. So... the button under the dash it is!

Just to share some pics... here's the heater

And finally... does anyone recognize this rear end? It appears to be a Dana, but I can't place it.

Rick - It sounds like the lube in your speedo is kinda dried out??? This 2024 thread on a 1941 speedo problem (speedo is the same design as used in your early '47 truck) I think will offer some helpful info on your issue and solution. If you want to see other previous threads on the topic, use the Stovebolt Search to help you find them. I did a search using "Speedometer Repair 1941-47 trucks" and found lots of existing threads that are on topic and could help you with that.

You can also use that search to find previous posts on steering wheel removal and horn repairs, among most any other topics you come across. Happy hunting!

Rick - Using the info from this Dana/Spicer axle identification guide, it could be a Dana 44. That's just from comparing the rear cover pics with yours, which are a little rounder on the bump out than the 50/60 series axles. The decoding info at the top of the page should be helpful, assuming you can find/read the stampings on your axle?

nice heater

You are a wealth of information.

Problem solved... the needle seems to have fallen off, so it's no longer stuck at 10 MPH. I'll see about taking the instruments back out to see if I can find the needle and get this working.

Horn: I confirmed power to the horn. I cleaned the horn and can hear it making some movement. But it's unclear why it's not working - but I admit I know little about how the horn is meant to operate.

Stock horn had battery power to one terminal (call it A) at all times. Actually, both terminals (A and B) should read battery power when the horn button isn't pushed. Inside the horn there is a normally closed switch connecting terminals A and B. Terminal B has a wire that goes back into the steering column and attaches to the brass ring at the top of the steering column. The steering wheel has a brass contact that connects one side of the switch under the steering wheel cap to the brass ring.

When the horn button is pushed it provides a ground for terminal B which activates an electromagnet inside the horn, moving a diaphragm in one direction and compressing a spring. Once the diaphragm moves a short distance it opens the switch in the horn, stopping electrical flow, turning off the electromagnet and allowing the spring to push the diaphragm back to its original. The process repeats until you let go of the horn button.

Quick troubleshooting steps:

You've already confirmed power to the horn, so ...

1. Confirm power to the other horn terminal. It should be there if the horn button is not being pushed.
... If power, skip to step 2.
... If no power, the contacts or wiring inside the horn are defective.
...........SOME horns can be opened and the contacts could be cleaned with VERY FINE abrasive.
...........Reassemble and try again.

2. Remove one wire from the horn. Check the wire for power.
... If power, reinstall the wire to the same terminal which we will call X.
... If no power, reinstall the wire to the same terminal which we will call Y.

3. Use a short piece of wire, a screwdriver, or practically anything metal to short terminal Y to ground on the engine nearby. The horn should HONK LOUDLY.
... If the horn honks loudly, the problem is between the terminal B wire and the ground provided by the horn button.
... If the horn makes little or no noise the contacts or wiring inside the horn are defective.
...........SOME horns can be opened and the contacts could be cleaned with VERY FINE abrasive.
...........Reassemble and try again.

Thanks Bill. I had already confirmed power. The truck is re-wired to 12v with a negative ground. This is the American Auto-wire harness - it's pretty awesome. I was able to trace the whole thing out and I have power to the horn (via a relay inside the cab). I can ground the horn wire which closes the relay, providing 12v to the horn. The horn does make a relay click sound - but that's it.

I attempted to clean the contacts, but this didn't seem to do much. I suspect the contacts are shot.

I am trying to make sure I understand the mechanism... Since sound is produced by vibration, the horn must be creating a pulsation or vibration. I'm guessing the contacts inside are meant to energize the coil, pulling the bar which moves the metal diaphragm and opens the contact (disconnecting the coil). Once the contact is broken the contacts close again the coil re-energizes... Is this about right? If this is correct, then my contacts are fine - otherwise the coil couldn't energize and make that relay sound. It also implies that the distance of the gap is very important - it would effectively set the frequency of the horn.

I ask about the inner workings because this would help me diagnose.