The Stovebolt Site Forums The Shop Area Engine Shop '63 GMC 305 Starts but won't stay on

Hey all,

I'm new here - bought a 63 GMC 1500 305 V6 because classic motorcycles weren't challenging enough : P

When I purchased (from a friend) it ran great! However, as of last week the engine will start but when the key goes back to the ON position it shuts right off. I tried fixing the ignition switch thinking that might be a good place to start but that was a no go. Replaced the plugs because hell, why not? Distributor cap/rotor looked fine but ordered a new one anyways. My mechanic mentioned that maybe the ballast resistor might be shot but I can't seem to find where (if anywhere at all) it's located.

To be honest, the truck has a lot of wiring issues, once I got under the dash and checked it out I noticed a lot of dodgy setups. I'm almost certain there is a ground issue with the entire truck as none of the gauges work. I'm happy to post a picture of the fuses/fuse box as that seems like maybe it's applicable but in the meantime any help would be greatly appreciated!

Sounds like the ballast resistor died. If it's separate from the coil, bypass it to confirm. If it's in the coil, replace the coil to confirm.

Problem is I can't even find the darn thing. I'm slightly convinced it doesn't exist. Is that possible? Assuming it is there, how exactly do I bypass it? Could I also check it with a multi meter?

You might have a resistance wire instead of a ballast resistor:
https://www.stovebolt.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=1087188

I've researched this and found it could be the case. I'll post a picture of the starter tomorrow. I assume that maybe the wires running out of it may be different depending on whether its a ballast resistor vs a resistance wire.

Here are some photos that might be helpful.

Amateur question: is that my ignition coil? Either way, when I've looked up pictures of the original engine - trying to orient myself - I don't see anything mounted there.

Yes, your second and third pics show the coil.

It's been ages since I had a truck with a coil, but wouldn't there be two wires (one resistance or with ballast resistor, and another without) on one of the coil terminals? I seem to remember that. I had an issue with that last truck and went as far as replacing the resistance wire only to find out it wasn't the problem.

The ignition switch may also be at fault. If the "Run" contact is messed up, it would cause a similar issue. The wire going to the coil from the switch should be hot with the key in either "Start" or "Run" position.

Thanks for the insight kevin! I did notice a melted/damaged section on the Red wire on the ignition switch. I believe that is the wire for start though... anyways I taped it over just to be safe. Electrics are really my weakest point. How can I test if it's hot? Do I set my multimeter to 20 ohm and then connect the black to frame (ground) and then put the red into the harness where the "Run" wire is?

I read the link added before and found that a lot of ppl decided to remove ballast resistors but do you think it would make sense to just add one?

To test the switch, temporarily disconnect the output wires and connect your meter (set to voltage) between a ground point and each terminal. You could also just use a test light. You really only need to test the "Run" terminal. It should be hot any time the switch is in the "Run" position.

If you don't have a ballast resistor, you may have reduced coil life, as it will be getting the full 12V across the primary circuit all the time, rather than just when cranking to give you a hotter spark.

The resistance wire on my '61 drops the 12v to about 6v during 'run' operation.

-Kevin

Make sure to use D.C. Voltage on your meter. House voltage is AC.

Ed

Are you suggesting that I try testing the harness on the ignition? I've already replaced this. Or are you saying test the wire that corresponds with "Run"?

Just be sure that you have power on the "Run" terminal on the ignition switch when the switch is in the "Run" position. You could test it either at the terminal or at the other end of the wire that goes to the coil.

A wiring diagram for your truck should help you (and those who are helping you). This one is pretty good (has a few small errors but is still a help)

http://67-72chevytrucks.com/vboard/showthread.php?t=499476

You can see one dark green wire from the coil to the starter R terminal. The resistance wire runs from the starter R terminal to the firewall connector. Inside the cab, the wire is pink.

Interestingly when I look a the fuse panel there isn't a pink wire running to it. In fact there isn't anything plugged into the "ign" plug. Wonder if thats related?

Mean't to quote this before to reference back to the images I posted

It looks like there's a bundle of wires all wrapped up leading to the fuse panel. The pink one is probably in that bundle. The ign plug would be used for accessory purposes. Foe example, if you wanted to hook up a tach, you could run the power to the tach off that terminal. You can check with the ign switch on - is there power at the ign plug?

Anyway, forget about the fuse panel for now. You want to figure out why the truck doesn't run. Electrical problems can be overwelming - the secret is to concentrate on one circuit at a time. In this case, you want to concentrate on the ignition circuit.

Turn Ign switch on. Use your multi-meter to check the dark green wire at the coil. If no voltage, go back to the starter R terminal. Is there voltage there? If no voltage, go back to the connector on the firewall and check there.

Thanks for the help.

I tested the green at the coil with the key in the "on" position with the multimeter (DC 20ohm) and it it was bouncing between 11 and 12ish. Should I continue to test the other connections mentioned? I followed the pink wire which you mentioned is the resistance wire. I've attached images of that, the bundle I believe it's inside (which maybe I should cut open and inspect?). I also uploaded a picture of the ignition switch and you will notice the wire that goes to "Ign" which is pink I taped because I noticed the insulation was damaged. This isn't the same wire is it? I also uploaded a screenshot of the wiring diagram so those that are interested in helping don't need to download it.

On your picture #2 of the ignition switch. I see that it uses spade type connectors inside the connector plug. I mentioned earlier about replacing the resistance wire on a truck I had that was cutting out on the road. The fix for that (after replacing the resisitance wire and finding that didn't fix it) was removing the spade type connector from the coil and squeezing it with a pair of pliers to tighten it up. It had somehow got loose enough that it would lose contact and kill the engine. The female spade connectors will pull out of the connector body if you release them with a pick. There's a springy bit that engages the connector body that you have to push back to unclip. Could be the vibration from the running engine makes the wire lose connection, since you mentioned an unstable voltage at the coil.
BTW, I hope you had your meter set to DC volts and NOT ohms. Using a meter set to ohms on a hot connection is a good way to kill the meter.

I meant volts not ohm, my bad!

When you say spade connecter from the coil you're referring to the connection on the ignition or the connection on the coil itself? I uploaded a photo of the filthy coil, it's amazing how much crap is caked up on some stuff on this truck. I think I should probably disconnect the battery and clean that with a wire brush...

I meant the connection on the ignition switch. The connector body pulls off revealing spades sticking out of the switch.
The coil connectors (stud and nut) are pretty dependable, although if loose can cause problems.
My coil that I mentioned had connectors that slipped over studs or buttons on the coil. There was a bakelite connector body that held the connectors and slipped around the coil and made both connections at once.
I would clean up every connection that you can and make sure they're all secured.

Voltage at the coil is a good thing. The wire terminals on the coil may need some work - does the lower one have two wires connected to it or does it just look like that in the pictures?

The wire from the distributor to the coil may have an internal break that only causes a problem while the engine is running. Try removing the distributor cap and check the voltage at the points while wiggling the wire and moving the vacuum advance. (this happened to me - I used a wire that was too stiff and the movement of the vacuum advance fatigued the wire until it broke)

There are two wires coming off it. I'm not sure what that other one is all about.

I like the idea here, but could you explain it a little more in laymans terms? Sorry about that. Same voltage test? 20v DC ground black red on each point and wiggle which wire? The dark green? By moving the vaccum advance you mean opening and closing it?

Thank you for all of your patience. Honestly, I got this truck so I could learn more about larger engines which I'm afraid is a dying art, particularly in my generation (I'm 30). None of my friends in a nearby radius could help with this stuff. Plus the proficient mechanic nearby is $170/hr

Yup. Same voltage test. You are using your multi-meter set to DC Volts. Ignition switch on run. Black probe is held on a good clean ground such as distributor body. Use red probe to for check for voltage where the wire from the coil (black) ends at the points inside the distributor.

I'm going to start over as my input. I have a GMC 305.

1. The coil was originally mounted in a hard to reach place and many have been moved like yours has.
2. You have a resistance wire. You don't have a ballast resistor unless a PO added.
3. DO NOT use that diagram. It is some guys redraw of the original. He has two tries at it after finding mistakes. Use the original wiring diagram.
4. If your truck was starting and running fine, AND NO ONE screwed with anything you haven't told us about, and it now shuts off after spring loaded switch returns from "start" position to "on" position, it is the ignition switch. I would say a broken off wire but you said you have 12V at coil with ignition switch ON. You should have TWO wires to the coil positive. Do you have two wires on the positive and one wire on the negative?? The coil is marked - and + if you clean it off!!

"I tried fixing the ignition switch thinking that might be a good place to start but that was a no go." What does that mean exactly?

See if there is voltage at the coil positive terminal with key OFF and then see if there is voltage with key ON. Report results of this test. Then we can proceed. Get a ice pick lighted voltage tester. Forget the multimeter.

Clean up the wires and the coil!!!!! So you and we can see what's going on and see the colors.

ALSO: to be clear, you are saying that the motor FIRES, starts to run, but when you let go of the key it stops. Right?

FIY: Never use a sketch or a redraw of a schematic or wiring diagram on the internet for our old trucks. Find the original from a manual.
Your problem will be very easy to fix. We need to proceed slowly. Don't do any wiring or disconnecting. Just test the coil pos again, key "On" and Off". "On" does not means "start" position.

You are checking ohms and fuse panels and breaking into harnesses and don't know about electrics. Lets back off a little. Getting too confused possibly.

What did this mean? "Are you suggesting that I try testing the harness on the ignition? I've already replaced this." What did you replace?

When you mention these things - probably one month ago it did a similar thing. I walked away, 15 minutes later I came back and tried again and it started up. Several weeks ago, I brought it to the mechanic to look at the faulty gas gauge. He claimed to fix it but I didn't notice the gauge moving (always full). Since then I drove it once and it ran fine. I suppose he could have caused the problem.

In terms of fixing the ignition switch I replaced it with a new one from NAPA. Oddly the old one only had 4 male spades while the harness had 5 female plugs. The new ignition has 5 male spades so now everyone has a home. After replacing this, still the same problem.

I can't find my test light at the moment, I apologize. I went out there and wiped it off and found that there are two plugged into the positive. I'm going to try again tomorrow when it's brighter outside but test results were as follows:

NO reading when key was in OFF position.
It was hard to get a reading when in ON this time. When It seemed like I got a good ground (to the frame near the motor) it read a solid 12.12 for a bit but mostly it was bouncing around seemingly due to how strong my connections were. Usually reading low numbers between 2-8 volts - sometimes less.

Yes, motor fires and will run endlessly if I keep the ignition on start especially when giving a bit of gas but when letting go of the key it dies.

The ohms vs volts was a mistake not how I actually tested it. I was testing 20v on DC.

I mean't to say "harness or ignition" referring to the fact that I already replaced the ignition switch.

Thanks for your detailed help

"Yes, motor fires and will run endlessly if I keep the ignition on start especially when giving a bit of gas but when letting go of the key it dies"

One each new starter and solenoid please.

Holy Cow, did I just read what I think I read? The start position engages the solenoid and sends the pinion gear into the flywheel. Are you saying you ran the motor with solenoid engaged? Never heard of that actually. Don't do that.

Let's be sure ignition switch is wired correctly now with the mechanic and the 5 blade new switch. THIS IS IMPORTANT.
1. Red wire should be plugged into ignition switch terminal blade marked BATT.
2. Pink into blade marked IGN
3. Resistance wire, which is either green or a funky looking wire, plugged into same circuit as pink.
4. Violet onto blade marked SOL
5. Brown into blade marked ACC
6. Red should show HOT all the time at switch.
7. Be sure you have the above wires to correct marked switch terminals no matter how you used the fifth terminal.

THE PINK WIRE IS GAS GAUGE HOT FEED. So mechanic was playing around near pink wire which, with a four terminal ignition switch, is ganged together with the resistance wire. WHICH IS THE ON/RUN WIRE!!!!!!! Which if not wired correctly, will not allow motor to run in "run" position. Then you changed ignition switch with a five blade. So this is a critical area involving exactly your problem.


After you have double checked above wiring, back to testing:

I will stay with this same simple test until it gives a definite result. Find your light tester. Clean the positive terminal on the coil. Inspect the two wires on the coil positive terminal for tight and check the crimped ring-tongue terminals to be sure they are not half broken off or wire come loose from crimp.
Then be sure battery is at 12.6 volts, or near, using your volt meter/multimeter. Then use the ice pick tester with black lead on the battery negative post and the red lead on the actual coil terminal stud and see if light lights up. There is no "bouncing around" with a 12V test light test. It either lights or It doesn't. Do this test with ignition switch turned to the "ON" position. Report.

Another test you can do is take off both wires at the coil positive and tape the ends. Install a test wire from the battery positive post to the coil positive terminal. Use a clip for easy removal. Start the motor using the key as normal. If it starts and runs, remove the test wire from battery to stop motor. Report.

Word of warning/encouragement. The problem you are dealing with can and is frustrating. Don’t skip tests. Perform them as outlined by the various helpers. Don’t lose focus and don’t get your feelings hurt when criticized by someone for not following instructions. If you do a Stovebolt search on “Ignition Problem “ you will finds hundreds of posts. The common thread is someone usually gets frustrated and then the ****ing match starts. Keep your cool and proceed slowly and methodically until the desired result is achieved. Good luck. 🛠

I've attached a picture of the back of the assembly.

Pink wire is on it's own and I don't see any other wire that seems like it could be the green one we're looking for or a funky one that appears to be hanging or without a home. Otherwise, everything is in the right place and plugged into the right spade. The only outlier is the white wire which is going into "GRD". If it seems to make a difference I can go back to the original ignition switch that was on it as I'm currently running the new 5 spade from Napa.

I just want to re-iterate that the truck did run fine on and out and back trip once after the mechanic messed with it and I also drove it from the shop back to my house. In theory whatever he potentially messed with could have come loose?

I'm going to get a new test light this weekend at the nearby parts shop since I can't find mine so stand by for the rest of the tests.


Thank you for the encouragement.

I want to encourage you in this RScanz. You'll learn a whole new language and some important troubleshooting skills. GMC V-6s are unusual eninges and aside from your present electrical search you'll learn about how industrial engines were built. In some versions 2 351s (the larger variant of your engine and available as an option in pickups) were married together to create a 702 V-12. The one I saw was used to power a piece of backwoods logging machinery and was on skids at the antique truck show near Tacoma WA. It had a really impressive sound when they fired it up. It had been the modern replacement for a steam donkey-engine on a high-lead logging show in the Cascade Mountains pulling huge logs on cables toward the landing where they loaded them on the trucks.

But back to your present problem. Long ago there was an Italian fella named Ohm who was a scientist working with electricity. He developed the concept of electrical resistance and developed a formula used even today called Ohms Law. Ohm worked with a couple other fellas named Ampere and Volta. Scientific history has named three important electrical properties after these men: The Volt, the Amp and the Ohm. Ohms law describes the relationship between these properties: It says that one volt of electrical pressure (abbreviated as E) will push one Ampere (a measure of electrons flowing past a given point in one second) abbreviated as I, through a resistance of one ohm, abbreviated as R; Ohms law formula is I=E/R.

If you know any two properties you can find the third one using this formula. You can algebraically manipulate the formula to get any one of the three on the left side of the = sign. This formula is really handy in automotive work where you often want to find amps or volts but ohms get in the way. Take your resistance wire for instance: I takes battery voltage and reduces it to a safe level for a 6V rated coil, except when you turn the key to the start position when the coil wants full voltage to compensate for the voltage reduction caused by the high current flowing to the starter. Then the resistance wire is bypassed with a normal wire. And ohms goes almost to zero except for a few tenths of an ohm in the wire and connections allowing volts to rise almost to battery voltage (there might be a couple tenths of a volt lost in the wire and connectors).

Small amounts of resistance (Ohms) are difficult to measure accurately without specialized equipment. Some high end Digital Multimeters will measure low ohms but few people have them. So we usually must infer that there is more resistance than desirable by measuring the voltage drop between two points. For example in a starter circuit where there may normally be 100 amps in the starter cable even a few tenths of an ohm will cause a no-start condition. For example: 100A=E/0.3 Ohms; where E is the voltage drop between the battery and the starter. Solving for E we get: 100A*0.3ohms=30V. So clearly 12 V won't push 100 amps through 0.3 ohm and the starter won't rotate the engine fast enough to start, if at all. Measuring volts between the battery and starter we would see a voltage drop of nearly all of battery voltage. Time to clean the battery terminals.

Good tips from all. This has been a good read. Electrical is scary? No need to be. Frame and sheet metal is ground. Wire is how things get connected. So why 2 wires on the + on the coil? one is "run" from the key switch. That can be from a 6 volt source, a resistor wire, or a ballast resistor. The resistor wire cuts 12 volts down to 6 volts. The ballast resistor does the same thing. The ballast resistor can hide. It can be on the firewall, or inside the coil. Inside the coil is just a simple way to get a so called 12 volt coil. So why all this fuss about 6 or 12 volts at the coil? Simple actually. The points are a switch. That switch was designed to work with about 4 amps of power. The coil was made to work with those 4 amps. and...the condenser was too. When cars and trucks changed to 12 volts. The engineers could see no reason to change the coil, points, and condenser. Tha accountants would not pay for it any way. So an inexpensive resistor was the fix. Wire, ballast, or inside the coil. Same cheap and effective result. 4 amps at the points. The second wire on the +? That is from the starter. Foot or solenoid. It is to help with cold starts. Temp push of extra current to make a big spark. Off when the starter is off. simple and effective. So you have two simple tests. Test 1: key on in the run position. Volts at the + . 12 volts? you have a 12 volt coil(the one with a resistor inside) . 6 Volts? You have a ballast resistor or a wire resistor. Just a note: the points need to be closed. If they are open the circuit you are testing will be open. Volts reading will be funky. As in not useful. A ground strap is helpful to get past the rubber in the motor mounts remember ground is...the frame and sheet metal. Test 2: Volt meter at the + and turn on the starter. 12 volts? All is good. Let the key spring back, and 6 volts? If you have a resistance wire or a ballast resistor, all is good. 12 volts? If you have a "12" volt coil. All is good. Did you check to see if you have a ground strap? Most ground straps go from the motor/bellhousing to the frame Some(not many but some) just jump over a motor mount to avoid that sneaky bit of rubber. So what can go wrong? Runs in start(starter on) and quits when in run. Bad resistor wire(covered in previous posts). Bad ballast resistor(also covered in previous posts). Bad coil? Not this time. The coil works with the starter. Bad ignition switch? Not this time. Again covered in previous posts. Bad ground strap? Maybe. bad connection on the "run" wire? maybe. That is a start. look at the easy stuff first. Cheap is good. Let us know what you find.
Steve

Wow, really getting some great information from everyone. Learning more than just how to fix this problem but also some historical things as well.

I'm out of town for the week but right before I left I cleaned up the ignition coil and cleaned the connections going to positive and negative. The resistor wire is the cloth covered wired correct? Attached is an image. The wire didn't look like it was in great condition as you can see. I want to trace the wire as I feel like the work the mechanic did on the gas gauge might have been what created this.

At any rate, stand by for more updates next weekend.

This post is getting too long. Let's wait until you do the work, do the test mentioned and answer all the questions. We are getting nowhere.
The resistance wire will probably have a large diameter cloth covering. White'ish or yellow'ish?

The other wire is green.

Nothing is cleaned yet that I see.

That's a mighty big glove you have there...lol

After a long hiatus because of work and lack of time I got back into it today and tested a few things.

When running your test bartamos with the circuit tester grounded to NEG on the battery and key in "ON" position I touched the + terminal on the ignition coil and got nothing. When having a friend try to "START" the vehicle it lit up.

After this test I left the key in the "ON" position and checked the starter. The light test only lit up when I touched the large 12v wire in the middle.

I also checked the wire that runs into the distributor from the ignition coil. With key in "ON" position I took the distributor cap off and with the circuit tester grounded to the NEG on the battery I connected the light to the center of the distributor and got nothing. Oddly, after removing the wire and when doing a continuity test I found that this wire didn't work. I went across the street to the mechanic and got another wire - tested this for continuity - it worked. Still engine didn't stay running. I attached a picture of the inside of the distributor cap because I think it could be replaced regardless.

I'm thinking I will start tracing the red wire in the ignition harness and make sure all of it's connections are clean and solid. I think I will also do the same for the resistance wire.

As always, any input is appreciated!

. The resistance wire must be energized when you come off of the start position on the ignition switch. Turn the switch to the on position and test voltage of the resistance wire at the coil.
While the ignition switch is in the start position, a separate wire is energized, providing full battery voltage to the coil.
If your resistance wire is bad, the engine will fire while the ignition switch is in the start position. Once the ignition switch is allowed to fall back to the run position, the engine will die.

On my 64 GMC, there is a toggle switch that somebody has put in, in the past. Its purpose was to jump a wire around the ignition switch to keep the ignition alive because the faulty switch would quit from time to time, but also would keep the engine running all the time when the switch was turned off too. I will be replacing the switch one day to fix this. Dont know if this pertains to you or not, but my .02 worth.
I might add also, my latest acquisition, a 1964 GMC 305 long bed, comes to me with a few electrical gremlins. First thing I will do before any trouble shooting is to check to see if it has its engine ground straps as well as body ground straps and will also add a hood to body ground strap for the park and turn signal lights in the hood that seem to be working off and on, more off though.

Steve

I see, that would explain why it lights up when in "Start". I suppose I should just replace the resistance wire and see what happens?

I'm going to inspect all the ground straps also because I know there is other electrical issues with the truck including a bad ground somewhere.

Checking grounds is always a good place to start!

Mike B

When you did the test for 12V by touching the coil positive terminal with key "On", and did not get voltage. Obviously that tells you that you are not getting voltage to the coil. This is your only problem. You have no other problem. You don't have a ground problem. Your problem is very simple. You need voltage when ignition switch is at the "ON" position.
SO............start testing for voltage, with switch "ON", follow the wire at the coil positive and follow the wiring step by step, connector by connector, terminal by terminal, wherever it leads, all the way to the ignition switch until you find 12V. Report. One of the two wires will go to the starter, follow the other wire. The long waits between reports is making this last WAY TOO LONG. You are bouncing all over the place and blogging a lot of non info. DO the test I just gave and find out why the ignition switch is not providing voltage.

A section of the path between the ignition switch and coil positive is a resistance wire. Other sections will be regular wire probably.

A battery hot wire comes to the ignition switch and the ignition switch distributes that voltage to various places. (start, run, gauges, accessories)There may be a fuse panel and fuses involved and there may not. Just follow the wire backwards to the switch and see if the terminal on the switch for "ON" is live or not when switch at "ON". This, as I said long ago, is VERY simple. The testing of the wire from coil to distributor is an example of you going off on your own and proved nothing. STOP taking things apart. STOP disconnecting wires. If you want help, follow instructions. As Hot Rod Jerry says......"we don't guess, we test".
With all due respect, you asked because you don't know what you are doing, so stop doing what you don't know. The Cavalry has arrived.
I still don't see cleaned wires and how they are attached to coil and where they go. I know where they are supposed to go. I have a GMC and have wired it.