All
I have been restoring a 69 Camaro with a 307 V8 and Powerglide trans. Engine totally professionally rebuilt. Trans freshened up. Lots of work to get it show worthy.

Here is my saga.

Noticed months ago after installing the fresh engine, the engine would bog down and shake violently under load. Strangely enough it only does it in drive. I have confirmed this by performing "power load test" with foot on brake, and accelerating. In reverse engine RPM increase and the engine power increases. Same test in drive, immediately after attempting to increase power, engine starts shaking violently. I have done all of the usual stuff, plugs, wires, points, rebuilt dist. etc.

I have been suspecting that the issue was with the ignition system – high voltage dropping out. Today I found something rather significant. I measured the voltage going to the positive side of the coil while performing the power load test (foot firmly on brake, and using other foot on the accelerator). In reverse the voltage is a steady 10.5 volts. Repeating the test in drive, the voltage starts at 10.5 volts when there is no load, but under load drops to 7-7.5 volts. This is repeatable and consistent.

Now I believe I that the reason the engine bogs down is because of a voltage problem going to the distributor. However, want to emphasize the problem only happens when the trans is in DRIVE under load. In reverse under load there is no voltage drop and the engine does not bog down.

As a further test, today, I disconnected the combo white resistor wire and yellow wire from the positive side of the coil. In its place I ran a wire directly from the positive battery terminal to the positive side of the coil. I started the vehicle. And low and behold, the in "DRIVE" power load test there is no bogging down and the engine does not shake. I took the vehicle out for a 10 mile test drive and it ran like a champ.

And while in this configuration, doing one more test, I take a volt meter and attach it to the disconnected white resistor wire/yellow wire (that formerly was connected to the coil), put vehicle in drive, power load it with foot on brake and amazingly the voltage goes from about 10.5 volts to less than 6 volts. But in "reverse" the measured voltage does not drop.

Thinking that maybe the steering column mounted ignition switch may be going bad, I swapped it out for a spare that I had. No change.

I also did one more test. I was wondering if the entire Camaro was experiencing a voltage drop across all electrical circuits. So I took a volt gauge and measure the voltage across the terminals of one of the instrument cluster sockets (bulb removed – lights on), while engine was running. Did the DRIVE power load test and the voltage stayed a stead 12 or so volts.

So whatever is causing the loss of voltage while in DRIVE and under load is only affecting the ignition circuit.

I am totally stumped. What in the world is causing my voltage to drop going into the positive side of the coil - only in DRIVE?

One more item. The problem appears ONLY after the engine is fully warmed up. When cold, the DRIVE power load test the engine does not bog down and there is no voltage drop to the positive side of the ignition coil.

Thanks for your ideas. I am at a loss.

Mike V

 

Resistance increases as temperature increases, causing a voltage "drain" on a system. Something else in the ignition circuit is heating up and using more of the 12v than it should. What other components are connection on the ignition circuit?

I should also warn you, I'm a computer/electrical engineer. So while I may be good at understanding how electrical systems are working/interacting, I'm still learning WHAT components are connected to each other in a car. I'll see if I can get my hands on an electrical diagram for the ignition circuit in a Camaro, then I could probably pinpoint what the issue is.

 

Ok here is an update. I am 90% sure now that I have a bad resistor wire going from the bulkhead connector on the firewall to the positive side of the coil. Here is the test that I did to come to this conclusion. I used two volt meters. On first volt meter I probed the resistor terminal in the bulk head connector (that way I can measure the voltage being presented to “input” side of the resistor wire. With the second volt meter I hooked to the positive terminal on the coil, the “output” side of the resistor wire. Now with the engine running and in idle (no load) I noted the voltage on both meters:

Voltage Meter 1 (on bulkhead connector “input” side): 13 volts
Voltage Meter 2 (on positive coil terminal “output” side): 11 volts

With transmission in reverse and engine under a power load test, the voltage remains pretty much the same.

Voltage Meter 1 (on bulkhead connector “input” side): 13 volts
Voltage Meter 2 (on positive coil terminal “output” side): 11 volts

Now with the transmission in drive and engine under a power load test, the voltage at the bulkhead remains steady, while the voltage at the coil drops, as in these readings:

Voltage Meter 1 (on bulkhead connector “input” side): 13 volts
Voltage Meter 2 (on positive coil terminal “output” side): 6.8-7.7 volts

So my conclusion is that somehow only when the transmission is in drive and a load, the resistance in white resistor wire must be increasing, thereby causing the voltage presented to the positive side of the coil to drop. This voltage drop is so low that it is preventing the coil from building up enough high volts for the spark plugs.

What do you all think? Is my conclusion correct? Has anybody else ever seen a similar situation where the white resistor wire broke down under load?

Now the question for the group is where does one purchase replacement GM resistor wire? I would prefer to get new resistor wire instead of replacing the wire with straight copper wire and installing a ballast resistor in line – as I am trying to keep the stock look of the wiring.

Any thoughts on all of this?

Thanks in advance.