1972 car starts and dies - towed home

[Larry M.]

Problem arose today five miles from home - attempting to leave a store after a 5-minute stop, the car (1972, 350 4-speed, standard ignition, tilt/tele steering column) would not start and stay running.

With the key turned to the start position, the engine would turn over and fire; however, after releasing the key to recoil back to the ignition position, the engine would die.

My immediate suspiscion is the steering column ignition switch has failed.

But before I start work on further diagnosis and repair, a coulple of questions:

*Other than the ignition switch, are there any other likely problem items to look at?

*Is there a diagnostic prodcedure to follow that will confirm or eliminate the switch as the source of the problem? I have the Chassis Sevice Manual, but did not see that info there.

The car is parked in the garage; it will probably be a week or so before I have time to dig into it further.

Thanks!

Larry

[Terry M.]

Re: 1972 car starts and dies - towed home

This is basic Kettering Ignition operation:
When cranking the voltage applied to the ignition points is full battery voltage. With a healthy battery, good electrical connections, a good starter and good engine internals that voltage will be around 11-12 or so volts.
With the ignition switch in the run position voltage to the points is more like 8 volts to prolong point life. This reduced voltage is produced by a resistor mounted on the fire wall in C1 & C2s and by a resistance wire in the wire harness on C3s.

Look for an open in the resistance wire, or a disconnection where this wire hooks to the coil. That resistance wire is not very flexible and breaks easily. IIRC (and Duke, Joe L, Bill Clupper or John Hinkley among many many others will help me) the resistance wire connects to the positive coil terminal. If your wireing harness is unmolested in this area there should be two wires connected to the crimp terminal under that coil nut. Check these connections before the ignition switch. If your issue is a disconnected or lose wire it will be far easier to fix than the ignition switch. You do know the ignition switch is on the steering column above the brake/clutch pedals, right?

[Duke W.]

Re: 1972 car starts and dies - towed home

The dash bulkhead connector that carries the ignition hot wire is a problem on C2s. I assume C3s have similar bulkhead connectors that connect the interior wiring to the engine compartment.

There was a lengthy thread on this whole subject in the last couple of weeks.

Duke

[Larry M.]

Re: 1972 car starts and dies - towed home

Terry,

Thanks; the wiring harness is relatively new - I replaced the engine harness in 1988 while the engine was out for a rebuild. I've not molested it since. I'll check that resistance wire.

Yep, I know the switch is on the column down by the pedals; I know I'll need to drop the column for access to it (had the column - and all the rest of the interior - out in 1987 for restoration/replacement/refreshment).


Here's hoping I find a loose wire on the coil!

Larry

[Larry M.]

Re: 1972 car starts and dies - towed home

Duke,

Thanks - I'll look that up.

Larry

[Wayne W.]

Re: 1972 car starts and dies - towed home

You can test the switch simply by turning the switch to run and checking for voltage at the coil. The reason it initially fires is the bypass on the starter solinoid that applies 12V to start when the solinoid is energized. If there is no voltage there at run, just trace back to the switch.

[Larry M.]

Re: 1972 car starts and dies - towed home

Thanks Wayne; I'll check that out.

[Larry M.]

Re: 1972 car starts and dies - towed home

I had some time yesterday to start checking this out - the coil connections are tight; the wire terminations on the coil look fine; my test light shows power on the positive side of the coil with the key turned to the 'start' position, but no power with the key in the 'run' position.

As Terry mentioned, two wires (one black; one yellow cloth with some sort of clear coating) share a common crimped terminal. Both wires disappear into the wiring harness at a point below the wiper motor. The wiring diagram in the CSM shows the yellow wire continuing to the starter solenoid, and I can see where it makes that connection. The black wire is identified as the resistance wire.

My next step will be to identify the location of the black resistance wire on the bulkhead connector; pull the bulkhead connector off the firewall; and test for power going to the firewall terminal that the black resistance wire connects to.

Does that sound like the next logical step?

Larry

[Stephen L.]

Re: 1972 car starts and dies - towed home

When you find both ends of the resistance wire check it for continuity with an ohm meter. If the wire is bad/open, you'll see a high/open resistance. If the wire is good, the resistance will be less than 5 ohms. Disconnect the wire from the coil to insure you have NO influence on the reading from other items in the circuitry.

[Larry M.]

Re: 1972 car starts and dies - towed home

Steve,

Thanks for that suggestion; I'll try it.

The black wire is identified as a 'resistance' wire. Does anyone know what that means? If it needs to be replaced, can it be done with standard wire, or is a special product required?

Larry

[Edward J.]

Re: 1972 car starts and dies - towed home

Larry, The resistance wire cut the voltage to Eight volts with engine running, you may have to open the engine side of harness to trace the wire to make sure you don't have a open in that wire. It does leave the ing. switch and heads through the firewall. you will want to open a wiring dia. for 72 and follow the wire from the switch, testing for voltage leaving the switch and heading through the firewall. Once you find you have power leaving the switch and have power as it leaves the firewall connector you may have to un-tape harness to check for a open.

[Dick W.]

Re: 1972 car starts and dies - towed home

Steve,

Thanks for that suggestion; I'll try it.

The black wire is identified as a 'resistance' wire. Does anyone know what that means? If it needs to be replaced, can it be done with standard wire, or is a special product required?

Larry

Your local friendly auto parts store that has someone behind the counter that knows something besides what the computer tells him can furnish you the wire. It is wire with a specified amount of resistance

[Larry M.]

Re: 1972 car starts and dies - towed home

Ed,

Thanks; yes, my plan is to confirm power to the engine compartment side of the bulkhead connector; and, assuming that I find power there, to then check for continuity in the black resistance wire that goes to the coil.

I have the wiring diagram; I'd be in deep trouble without it.

Larry

[Larry M.]

Re: 1972 car starts and dies - towed home

Dick,

Thanks; now that you mention it, I recall the small spools of black Packard resistance wire I used to see. No idea why my memory wasn't jogged by the fact that I typed 'black resistance wire' several times in my above posts, but thanks for reminding me! Rather than a FLAPS, I may have have better luck seeing if a GM dealer will sell me several feet.

Larry

[Larry M.]

Re: 1972 car starts and dies - towed home

I've been working on this on-and-off since late October, and have confirmed several things, but I still don't have power to the coil when the key is in the 'Run' position. I'm working backward to see where I lose power.

Here's what I know:



The wire in the engine compartment harness that runs from the bulkhead connector to the coil has continuity.

The wire in the underdash harness that runs from the ignition switch to the above-referenced wire has continuity.

Those circuit in those two wires together has continuity.

Reading electrical diagrams in not my strong suit; so even though I have been able to use them to successfully identify several wires and connectors to test, I haven't been able to identify where the ignition switch gets its 12V power from. On the attached photo are the two harnesses that plug into the ignition switch. Should any of those wires be hot at all times? None of them will light up my test light. Underhood, I have power going into the bulkhead connector from the 12 ga black wire.

If one of the wires in the photo should be hot, which is it, and I'll backtrack from there.

Thanks again for the help.

Larry

connectors.jpg