Truck just quit running

[1969SurvivorC20CST]

OK guys I've started a different thread on this 69 C20 396 survivor truck. After I installed the factory exhaust system today the truck was running on my lift when suddenly it just died. Just as if you had turned off the key switch. I was baffled for a minute or two. Pulled the coil wire at distributor and had someone crank it. Cranked fast but no fire. Hmm. Checked for 12v at the coil positive with key on, nothing. So I'm standing in front of truck thinking and I turn towards the battery and the plastic junction block with the fuse holder. I had been curious about it and it's function since first working on the truck. So just for chits & giggles I pulled it apart and what was left of a glass fuse fell out. Hmmmm I thought, could this somehow be connected to the truck dying. So I cleaned it out good and jumped both ends with test lead. Fired right up. So can anyone tell me what part this terminal block plays in the starting, running of the truck and what size fuse is supposed to be there??

[1969SurvivorC20CST]

Photo

[jumpsoffrock]

I think it's a 3 amp fuse and that circuit specifically is called a "shunt" and is used for the ammeter gauge in the cluster.

[pjmoreland]

That fuseholder is for your battery ammeter gauge on the dash. It takes a 4A fuse. There is another fuseholder just like it on the driver side. The wires from these two fuses run directly to the gauge and shouldn't be at all related to your coil. The coil should be getting its power from a red wire that is also connected to that plastic distribution block by the battery. This red wire feeds nearly the entire electrical system, including the starter solenoid and coil via the ignition switch. Your starter was working, so maybe there is something flaky going on with your ignition switch or the wiring between the ignition switch and your coil.

Schematics can be downloaded here.

http://67-72chevytrucks.com/vboard/s...d.php?t=185856

[1969SurvivorC20CST]

In that diagram, I lose the fused wire where it goes into the bulkhead connector. Thanks for posting.

[pjmoreland]

Unfortunately that schematic doesn't show where the ammeter wires go inside the cab. You can download a year-specific service manual for your truck here, which contains schematics that show the ammeter wiring:

http://67-72chevytrucks.com/vboard/s....php?p=5781194

You will find that the ammeter wires simply go to the ammeter gauge and nowhere else.

[HotWheelsFan]

Fuses or fusable links are not found in the starting or running of a standard ignition engine, that I know of. Ignition switch and wiring are the main sources of intermittent loss of power to the coil.

If the ignition switch was loosing contact internally, usually wiggling the key a bit you might be able to cause the loss of power to the coil condition. Nice, tight working switches are usually fine... Common losses of power to coil intermittently include bulkhead connectors at firewall/fusebox area, ignition switch connector (as wiring gets older, connectors and connections can have more resistance causing added heat and can sometime start to melt the connector), and of course you have connections at the coil, starter, and battery.

Just a couple things to look at, no fun having an intermittent problem that can leave you stranded somewhere.

Bob

[body bolt]

There's another one of those fuses on the other side. These affect only the ammeter and nothing else.

[Sheepdip]

Try hot wiring to see if it runs, if so more than likely the ignition switch.

[PbFut]

Looking over and cleaning my 72 GMC sierra recently, I traced the ignition wire for the coil. I beleive the harness is original and not hacked. The power to the coil come from 2 sources. In the run poition the power comes from the power block via a yellow stainless steel wire. It is SS because it has a natural high resistance and feeds the coil about 10v on my truck. There is also a splice in the wire pretty close to the coil. That splice runs over to the R terminal on the solenoid. When you turn the key to start the S terminal is energized 12v to trigger the solenoid and Crack the engine. The R terminal is also energized with 12v while cracking and the 12v runs up the splice to the SS wire going to the coil. Since the SS wire splice is close to the coil. You get a full 12v to the coil for start up. When the key goes to the run position the positive lead drops back to 10v at the coil. I assume to protect the points or maybe coil for longer life. So if you fire then die its in the yellow lead to the power block on the fire wall. No fire at Crank look to coil or points or possibly condenser. I typed this on a phone so I am sure it is full of text error. Good hunting

[1969SurvivorC20CST]

Thanks for the input fellas. Like I said once I jumped out that fuse connection it started right up. Coincidence??

[RustyPile]

Originally Posted by 1969SurvivorC20CST

Thanks for the input fellas. Like I said once I jumped out that fuse connection it started right up. Coincidence??

Believe what you want, call it a coincidence if you will.. But that fuse and its "mate" on the driver's side has absolutely no relationship to your issue... It's a shunt for the ammeter ONLY.. Both fuses can be removed and the holders left open and the vehicle will still start and run.. You have a bad or lose connection, failing ignition switch, or other issue going on with your electrical system, and by disturbing the harness, the bad connection possibly restored itself.. It will rear its ugly head again...

[Accelo]

Wiggling the harness likely had more to do with it than anything.

Without a fuse you have an unprotected circuit.
Having been in a vehicle fire caused by such a circuit I recommend leaving it off till a proper fuse can be attached. Before you remove the splice see if splicing the wires repaired your amp gauge function. Just turn on your high beams with out the motor running and see if it deflects. Just so your know the amp gauge is wired to the same 12+ wire in two different places. I know it's weird.
This circuit is problematic and the reason GM went to a volt meter in later years.

[HO455]

Originally Posted by RustyPile

Believe what you want, call it a coincidence if you will.. But that fuse and its "mate" on the driver's side has absolutely no relationship to your issue... It's a shunt for the ammeter ONLY.. Both fuses can be removed and the holders left open and the vehicle will still start and run.. You have a bad or lose connection, failing ignition switch, or other issue going on with your electrical system, and by disturbing the harness, the bad connection possibly restored itself.. It will rear its ugly head again...

I agree! It is difficult to troubleshoot a problem that isn't there but, if it happened once it will happen again, and in my world it's always under less than favorable circumstances, on the freeway, during rush hour, on my way to work.

[RichardJ]

If you wiggled that Ammeter wire and had a problem, then start looking for the problem right there. The stud nut on the junction block could be loose or corroded.

[LNP]

I agree start at the spot you messed with to get the truck to start. Could be you moved something to fix the bad connection momentarily. Pull on the wires to see if any of them stretch away for its connector. Just because they look good doesnt mean the wire isnt coroded off the connector in side the insulation. I ran into the alot through my garage work years.

[HO455]

Just realized I should have mentioned you may have wiggled the fuseable link enough to restore its continuity.

The fuseable link is the wire from the positive battery connection to the insulated post on the fender well. It's next to the small ampmeter fuse.

By design a fuseable link will fail internally but look undamaged externally. Give it vigorous tugs while testing with a continuity tester to determine if it has blown.

[1969SurvivorC20CST]

Thanks for all the input guy's...The truck is back at the owners garage but I will have it back next week. Hope it starts for him!

[1969SurvivorC20CST]

Originally Posted by pjmoreland

That fuseholder is for your battery ammeter gauge on the dash. It takes a 4A fuse. There is another fuseholder just like it on the driver side. The wires from these two fuses run directly to the gauge and shouldn't be at all related to your coil. The coil should be getting its power from a red wire that is also connected to that plastic distribution block by the battery. This red wire feeds nearly the entire electrical system, including the starter solenoid and coil via the ignition switch. Your starter was working, so maybe there is something flaky going on with your ignition switch or the wiring between the ignition switch and your coil.

Schematics can be downloaded here.

http://67-72chevytrucks.com/vboard/s...d.php?t=185856

I have the truck back to my shop today. I looked hard for the other fuse holder you mentioned on the drivers side and it is not there. The truck has been starting and running perfect. I will be separating the bulkhead connector next couple of days to inspect the connection.

[franken]

Originally Posted by HotWheelsFan

Fuses or fusable links are not found in the starting or running of a standard ignition engine, that I know of. Ignition switch and wiring are the main sources of intermittent loss of power to the coil.

The + wire that goes to the junction block on the fender that powers everything but the starter should have a fusible link.

[HO455]

Originally Posted by 1969SurvivorC20CST

I have the truck back to my shop today. I looked hard for the other fuse holder you mentioned on the drivers side and it is not there. The truck has been starting and running perfect. I will be separating the bulkhead connector next couple of days to inspect the connection.

Here is a drawing I robbed from VetteVet in this thread. Your looking for the fuse on the bottom (drawn with gray wiring but it should be black in real life.) that goes to the firewall connector.


http://67-72chevytrucks.com/vboard/s...d.php?t=673925

[RustyPile]

Let's not be going down a rabbit hole again... As stated by myself and others in this thread, any problems with those 2 fuses and their related circuitry IS NOT the cause for the dying issue..

69SuvivorC20CST, Before you go digging into areas that may or may not be the problem, there's always the possibility that you'll create additional problems.. "I think I'll disconnect this" is the worst approach to electrical troubleshooting. Use good troubleshooting techniques.. Use a voltmeter, DVM, or a test light. Probe the large gauge RED wire shown in the diagram posted by HO455.. That's the main power distribution wire that travels throughout the engine bay and under-dash.. It should have a fusible link at the battery end and will be "hot" all the time.. Among other places, that red wire carries current to the ignition switch. Check to be sure the pink wire at the switch is hot with the switch in the ON position.. Before you separate the bulkhead connector, probe that wire on both sides of the connector...

After you've check continuity thoroughly, then separate the bulkhead connector and check for bad contacts.

[Steeveedee]

Hmm...my truck doesn't have a fusible link, as brought to my attention by this thread. Sometime in the past, it popped and was incorrectly replaced, I guess. I think I have some wire left over from when my Skylark had a short at the starter. That's where Buick puts them. Lots of fun to replace...