The Stovebolt Site Forums The Shop Area The Electrical Bay 1936 GMC gauge

I had my amp gauge converted to a volt gauge a couple of years ago and I have lost the paper work that came with it. As you can see in the picture I have two spade connectors with no wires attached. One is marked IN the other is marked OUT. I don't know what to connect to these terminals. Dose "IN" go to the "hot key on" post of the ign switch? Where would the other connection go?

If it's a volt gauge, it should only need a hot wire and a ground. Can't read the markings on that box hanging on the gauge, but guessing IN would come from the ignition switch so the gauge is only active with the ignition on.

Looks to me like the strap that connects from the middle post on the gauge hooks to ground out of sight on the other end. What does the printing on the strap say?

IF (big if) it does, then you'd only need one wire connected to switched 12 volts. I don't know what that big thing that sort of looks like a relay is.

The big strap say's ground. I think it is a shunt for the higher amps from the alt.

Can I use 6ga wire to go from a 60amp circuit breaker to the positive buss bar and then to the ign switch. Would I need a couple of 30amp inline fuses also? One inline fuse before the buss bar and one out of the buss bar to the ign switch? Everything else is wired and fused out of the fuse box.

Shouldn't need a "shunt" (especially to ground) in a voltmeter system. Maybe it was just leftover shunt from the ammeter that was used to provide a ground for the voltmeter.

A 60 amp breaker near the source of power (either battery or battery cable near the starter) then 6 gauge wire (good to 15 feet - see attached wire size table) to distribution point. I personally wouldn't use more than 15 amps through a stock ignition switch. Anything more, I'd control by relays triggered from the ignition switch.

Wow, hold on there bud. Firing off too many questions and pics too small, and can't expand pic. This area is too important for us to start firing off answers.
One of the ways to "convert" a ammeter to a volt meter is to add a huge resistor in series to the ammeter. That big box could be a resistor.
If you can stand a few questions and comments.............
1. Can you post pics with a camera that yields a big pic. Post several close and wide shots.
2. You say you had the ammeter converted, so did it come with that box on it?
3. Are you saying that you clipped the wires and forgot how to install? What is the story here? You installed it years ago and why is it disconnected
4. Is this a 12V system?
5. You are mentioning and asking about shunts and 60 amp circuit breakers and 30 amp inline fuses and wire sizes. We have no idea what you have or have not done and why. We don't know what all that stuff is for and if it was necessary.

The point is that it seems you've been doing a lot of something and it's best we don't skip over it all when helping with a question like this. We may have mixed categories and questions. So lets back up and slow down and get an overview.

If others know what's going on and want to advise in piece meal, I will watch the thread.

A true voltmeter wouldn't draw anywhere near 60 amps. It could be wired inline with any other load wire tapped off the side.

If that box/strap/shunt (or whatever) is a shunt, then DON'T connect a hot wire to it if connected to ground. You should get out your multimeter and check between various terminals. Across a shunt would show very little resistance, and would basically smoke a hot wire connected to it if the opposite side is grounded. Measure ohms per the attached picture between 1-2, 3-4, 4-5, as well as between 1-4 (fork) with the shunt(?) disconnected. Also measure between 3, 4, and 5 to ground (individually).

Has the ammeter actually been changed out or is it the original with that gizmo connected across it? Post a pic of the front of the gauge as well.

You don't just want to connect hot wires willy-nilly or you'll let the smoke out of something.

[on edit] What Bartamos (he got there first) said. We need more info. Do what I suggested and we can help figure it out.

I don't see a "strap" that you call ground.

I don't know why the old ammeter has three posts.

What thing has the words IN and OUT? Who marked it? You or the conversion person? Who converted it?

If the box is a resistor, it would not have IN and OUT because a resistor is not polarized. If it is a circuit breaker/fuse it could be polarized. A person with electrical knowledge would not use the words IN and OUT. It's pretty much meaningless and open for error/misinterpretatio when wiring the back of a converted ammeter. Probably was cleared up in the lost instructions.

Ken, if you're referring to the markups on the picture I posted, I was just indicating a ground point for measuring.

My suspicion is that the third post (marked 4) may be a ground, but hard to tell from the photo. Note that there's another hole opposite that post.

I'll get some more pictures in the next couple of days. Wife and I have Doctor appts. for the next couple days at different times, makes it a pain because its a 1/2 hour to town. So we have to make multiple trips back and forth or waste the whole day in town. As soon as I get the pictures I will answer the questions and do the ohm checking.

Great,

Kevin: I was referring to Bill and fast55 discussing 'strap" back aways. Not your reference numbers.

FAST55: I read a few of your posts to see motor and your wiring info somewhat.
"I am wiring up the alt in my 1936 GMC. 4.3 vortec. I am running the power wire from the alt to a power block which then powers up the fuse box. Do I need a fuse in that power wire? if so what size 40amp or 60amp? Or am I not wiring this correctly and I should wire it differently."
After market.

"It's a Ron Francis bare bonz fuse block. I have everything else wired in except the amp/volt gauge ( had it converted to volts) When I get done with the wiring I can start to hang Sheetmetal. Right now my son is doing most of the work as I had heart surgery 5 weeks ago and can't really do anything except supervise. Going to be a long summer waiting to get well enough to start working on it myself ."


I sure hope this new post of yours means you have recovered from the surgery and are doing good buddy. Good to know your son was helping you.

Did you buy a aftermarket harness kit or wiring from scratch? If kit, can you give the vendor and part number. I think we should go over the charging, gauges and fuse panel distribution and wiring diagram..... or create one for you to double check things. I wouldn't wire anything or energize anything yet.

This pic may clear up some of the posts/holes we see in the original two gauge can. I think I would have tried putting an actaul voltmeter on/in there. I've done similar. If I had a 36 amp/oil cluster, I could try it. I have some volt meters with the right sweep.

Ok, I'll try and answer the questions from Bartamos.
2, yes it came back with the black box attached
3, No I have not installed to working condition. I sent the gauge's out to be refurbished and have the amp gauge converted to a volt gauge. I am now putting the truck back together after doing a resto mod conversion, 4.3L, auto trans, R/P, disc brakes, 12 volt electrical system.
4, I have wired the alt (self exciting) but not sure if I did it correctly. I have all the accessories wired in, the computer for the engine and trans. I have been told different ways to wire it in using heavy amp fuses to protect the different circuits not protected by the fuse box.
I opened up the magic black box and it looks like it has a glass fuse in it, it's hard to tell but it looks like it has a ground wire soldered in from the mounting plate to the top brass plate. The following are the ohm tests.
1-2=3.7 ohms, 3-4=203 ohms, 4-5= 107 ohms, 1-4= 207 ohms,3 to grd=203 ohms, 4 to grd= .0 ohms, 5 to grd=107 ohms.
I hope this is the info that was asked for and is useful, it seems strange that if I go from the ground strap and touch any of the other connections they all have continuity.

I haven't had time (or maybe the ability) to figure out the ohm readings, but it looks to me like that add-on is some sort of resistance bridge to make the ammeter react like a voltmeter.
From the configuration of the terminals (I assume it came to you that way) you'd connect power to terminal 1. Is that the one that's marked "IN"? I would probably put a reasonably sized fuse (maybe 10A) in the power feed to it.
I didn't see a picture of what the front of the gauge looks like. Has the face been converted to something indicating volts instead of Charge & Discharge? I'm wondering if the needle would just sweep from center position to some position on the right if 12V were applied to terminal 1.

Been trying to upload pictures, but no luck yet. I did one yesterday, but must have lost those brain cells overnight.

I resemble that remark
If you're on a computer (not a phone), instructions should show up on the left side of the page. Or click here for a refresher.

Yes, I printed that out so I could refer to it as I was trying to post pix. I'll work on it some more later today as I have to take the wife and myself to physical therapy and then a couple other quick stops.

Things to contemplate:
1. A volt meter needle pegs to full CCW when off and hovers to middle when operating. A ammeter is at middle when off and slightly CW when operating. If the face is still original ammeter markings, it's still readable as a volt meter.

2. Be sure the converted ammeter can handle 12V as a volt meter.

3. An ammeter is placed in series and a volt meter is placed in parallel. So two thing happening: Resistor to convert and wired like a voltmeter, which is normally one terminal gets 12V and other terminal goes to ground.

4. I'm not sure about your readings. Showing 1-2 as the lowest reading and others at 100 and 200 ohms (I guess I expected 1-2 to be the highest). You are showing 3 grounded and 5 grounded. Trying to figure that out. Where was the black meter probe place when checking ground? What was the mode setting on the meter and what range?

5. You characterize your build as a restomod. Maybe just leave ammeter unwired and add a volt meter in a discrete location or since you have an alternator, add an indicator/idiot light somewhere and call it good. These are not critical gauges needed with a new harness, 12V conversion and alternator, restomod. This goofy unknown ammeter is holding up the work.

Please advise exactly where the words "IN" and "OUT" are located. Pic or two.

I suggest you do the readings Kevin asked for in buzz mode, no numbers. Continuity buzz mode. Everything connected.

He already said that all the numbered terminals has continuity to ground. No need to re-do that.

Kevin, if I ask for a test, it's to help him and help me. Also "4 to grd= .0 ohms" not all had continuity.
I don't want to say someone is not good with a multimeter or know how to test. They may or may not. We all make mistakes. So I may ask for another test or procedure, whatever, If I think I see odd results.....and maybe something that was disconnected can be connected on next test. The more info the better. I'm trying to figure a few things out, avoid mistakes and misinterpretations, see things that maybe should not be grounded. Double check. You said you haven't had time to analyze the tests. Etc., etc. "No need to do that" is subjective.

Ken, you were asking for continuity and he did that, although his definition of continuity (through resistance) and mine are apparently different than yours (zero or very low resistance.)

I wasn't necessarily "correcting" you, just clarifying. I'm sorry if you took offense. No continuity in my book is open circuit, and continuity is any connection, even through a resistance. A buzz or beep test on a continuity setting on my multimeter is only up to 40 ohms. Above that it won't beep.

Fast 55 did only provide resistance to ground from test point #5, although I asked for resistance to ground from 3, 4 and 5. Resistance of ALL terminals to ground would be good to know. I re-read his test values and he did do 3,4, and 5 to ground, but the .0 is a question, depending on the meter display as Ken said below. Someone with more electrical circuit knowledge than I have may be able to see some value in that those readings.

A letter O is open circuit on some meters, so just using beep to confirm O or zero. Wanting to know where probe was. if a person messes up a test, it is a major problem to troubleshoot. Anyway....I am no longer interested in electrical ohm testing this thing. I am switching to "abandon it"..... or try it with what someone else says is the way to hook it up. If it blows, abandon it.

If buzz has range limits, that's a problem.

It is looking like I'm not trusting Fast55. That is not the case. Multimeters are sometimes tricky if you have setting too low or too high or miss the decimal point. Confuse milli and tenths and whole numbers.

Fast55, I'll get off that and assume all test are accurate.

Kevin, thank you for the clarification and info on buzz range. I edited my post.

I'll try to download a picture again, wish me luck. NO luck, I'll contact the IT guy again. I can try and email the pictures to one of you and maybe you could get them to post.

I went through the tests again and I have continuity on all the points ask. Also I have continuity from the body to all points and they all have continuity. Points 2 and 3 are connected by a jumper wire. If I disconnect the jumper wire then 1 and 2 are open and 3,4,5 have continuity.

I think maybe I'll fine another amp gauge and just hook it up as a amp gauge. does the alt put out to many amps for it?

An amp gauge typically has a shunt built into it that divides the current flow thru it. If you drive more amps thru it than it's rated for, it would just peg the meter, with a large percentage of the current flowing thru the shunt. That shouldn't harm the meter unless you're running 1000 amps thru it.
Basically, an ammeter is a voltmeter set up to measure voltage drop across the shunt. (That's probably TMI, so you can ignore that last item.)

I sent you a PM with my e-mail if you'd like to send me the picture of your current ( ) meter, and I can add it to your post.

Is it possible that your existing ammeter might still work if the external "conversion" parts(s) are removed.

No matter what the rated output of an alternator is, it only puts out enough to keep the battery charged and supplement accessories if necessary. It is a load seen and measured, as any other load, by the ammeter. We can't be positive if a 1936 ammeter is robust enough, but most folks who use the original ammeters after conversion to 12V have no problem as Kevin says. The experience for that on Stovebolt is mostly with AD trucks. A 1936 ammeter is too "primitive" to be sure of it's use. 12V components draw 1/2 the amps as 6V components, so that would mean the ammeter has extra amperage "capacity". The extra voltage of the components is no factor. The needle deflection my be less.

You can play around with it on the bench to get a feel for which terminals are "IN/OUT" and if it even works, by using 1.5V up to 9V household batteries. You can us a bulb or two as a load to detect movement of needle. A full test to see if it shows charge and if it blows at 12V will take testing under the hood, not in dash. Some folks here may have other test ideas.

The pic below may help figure out which terminals are which. But even the diagrams were very primitive and poorly drawn. Not many actual automotive engineers back then, especially electrical. Some old Model-T and Model-A guys hired.

I think I already made the point about wasting time on a non critical gauge and that in your restomod 12V conversion, an indicator light is WAY better. This does not sit well with purist, but the original gauge will still be there as it is now and no one will know it's not connected. All of the wires that went to the ammeter would be basically connected together with some fused. We can help with that when you get us info on a harness part number. The light tells you if alternator is charging or not. A 1936 ammeter tells you nothing but that.

FYI: wiring for a refurbish/restore/restomod is done very near last.

Here's a couple pics that Fast55 e-mailed me. No idea what's in the little black box. Also IN and OUT seem backwards to me. I would assume that the hot wire would go to the terminal that's not connected to the gauge, but really no idea.

If that is a fuse, then it's just a rebuilt/refurbished ammeter, NOT a conversion to a volt meter. The original had a fuse across those terminals. They are both "OUT" to loads. They just fused the wire to the light switch. The lone terminal is voltage "IN".


The ammeter would have been placed in series with the circuit, If that device is a resistor to convert ammeter to volt meter, the resistor is placed in series with the ammeter. But after it's converted to a volt meter, the new volt meter is place in parallel to the circuit. @#%&*)$#!!...sooooooo...... You find the original power terminal and original load terminal and see if a resister is placed between them, thennnnnnnnnnn.....you wire the power terminal to keyed hot and the other to ground. Don't quote me on this.

There are other red and black markings we are not shown. Makes it rough on us.

Fast55, are you taking pics with a cell phone or a camera?????????

Kevin maybe you know the answer to that if you got the file.


We have been informed F55 is getting a new gauge of some sort. So everything now is moot.

here's some pix

Adding to the mystery is the terminal I circled in the attached photo. Is this the elusive "IN" terminal? No way of knowing.

Ken, He sent me the two photos I posted above in .png format. I just resaved them as .jpeg to reduce the size so they'd post. Wouldn't make a difference if he was using a cell phone or a camera, but either way his were initially saved as .png format (which is much larger.)

I was asking to be able to help him find and post pics, I see he is an expert now.

The spade terminal you circled would be a place to wire my 1. or 2. and using the uncrimped blue terminal as well. In other words, two connections to place this gauge in the charging circuit. Basically what you said. We don't know. thanks Kevin.

1. If that is a fuse, then it's just a rebuilt/refurbished ammeter, NOT a conversion to a volt meter. The original had a fuse across those terminals. They are both "OUT" to loads. They just fused the wire to the light switch. The lone terminal is voltage "IN".

2. The ammeter would have been placed in series with the circuit, If that device is a resistor to convert ammeter to volt meter, the resistor is placed in series with the ammeter. But after it's converted to a volt meter, the new volt meter is place in parallel to the circuit. @#%&*)$#!! ...sooooooo...... You find the original power terminal and original load terminal and see if a resistor is placed between them, thennnnnnnnnnn.....you wire the power terminal to keyed hot and the other to ground. Don't quote me on this.

I think what I said was maybe it would be better/easier to just find another original gauge. Don't know how easy that would be. I don't want to drill another hole in my dash as what I have now centered under the gauges are turn signals, high beam indicator, check eng light for computer, parking brake light. So If I drill another hole they wont be symmetrical, my guess is no one would notice, but might drive me nut's every time I look at it The alt has been updated to be self exciting with a different voltage regulator, so I don't know if a light would work unless I change it back to original.

Well I guess the first post and reply's got closed, I sorry if I made anybody mad. Here's the final installment.
I tested the gauge with a 9v battery and when I power up the red jumper wire the gauge moves almost center of the gauge, I tried it with a light bulb in the circuit it was the same, so I'm going to install it, wire up the alt, start it up and see if any smoke escapes anywhere. I want to thank everyone who answered my questions and wish you well. See you in the funny papers

You didn't make anyone mad. We just had to pause for a bit to get some clarification on a few things. We've opened it up again and added your last post here to your original to keep things together.

Thanks for the followup!

It raises a few more questions we'd like answered if you wouldn't mind.

It would be helpful if you showed us the terminals you connected the 9V battery to. Are you saying that the meter needle was to the left and moved to midrange when you connected 9V? Are you able to test with 12V from a battery and see what happens? No need to install it in place before testing.

Definately, no one was mad at you. You are doing fine. We want to help.


I need to correct my comment:

1. If that is a fuse, then it's just a rebuilt/refurbished ammeter, NOT a conversion to a volt meter. The original had a fuse across those two terminals. They are both "OUT" to loads. They just fused the wire to the light switch. The lone terminal is voltage "IN".

I wrote down what terminals I test with the 9v battery. It's out in the shop and I will go fetch it after I get home from my Wednesday job. I have a guy that has a fleet of old trucks and cars, even a 57 m/b that I work on for him., currently putting a motor in his 95 Dakota. He keeps me flush with cash to buy what I need for my 36. So I will post those findings this evening.

OK, here's what I checked with the 9v battery. grd to#1=9v, grd to #2= 9v, grd to #3=9v.
#2 to grd with red jumper wire disconnected = 9v and needle moves
#3 the same as #2,
#4 to grd= continuity.
Another thing I noticed with the magic black box out of the circuit the two pins on the back of the gauge, grd #3 and the needle moves.
I installed the unit, with the black box installed, key on gauge moves about 1/2 of the way to the middle, start the truck and it jumps to about 2/3rd's to 3/4's of the way across the gauge, reads 14.6v on my volt meter.
I'm going to call it good and I want to thank everybody for their help.
Now I'll clean up the wiring and move on to bigger and better stuff.

Sounds like it's working like a voltmeter should. Thanks for following up.
Glad it worked out for you.