1969 Chevrolet Corvette Starting issue

  • 7.0L
  • V8
  • RWD
  • 114,000 MILES
The Corvette is suddenly difficult to start. I have replaced the ignition switch on the column thinking that was the problem. While it will start, it only starts a you release the key. I have checked all ignition components and they seem to be in order. The clutch does have to be depressed in order for the engine to turn over. Here is my question.

Can the ignition lock cylinder itself cause the problem? Is there a wire that runs from the back of the lock cylinder to the ignition switch to activate the coil and cause the coil to fire?
Do you
have the same problem?
Friday, October 17th, 2014 AT 6:04 PM

1 Reply

You don't have that silly and over-complicated stuff on a '69 model. It sounds like you may have two problems. If I understand correctly, the engine always cranks fine and at the proper speed, but it won't fire up or run until you release the ignition switch from the "crank" position, then it runs normally. If that is correct, there's two things to consider. First, the ignition coil should have voltage on the smaller positive terminal when the ignition switch is in the "run" and "crank" positions. The actual voltage depends on whether the breaker points are open or closed. If they're open, you'll find full battery voltage, 12.6 volts, on both terminals. If the points are closed, you'll find 0.0 volts on the coil's negative terminal and around 6 to 8 volts on the positive terminal.

There's a resistor between the ignition switch and the coil's positive terminal to limit current so the points don't overheat. When the points are closed, current flows through that circuit so some voltage is dropped across the resistor. That's typically 4 to 6 volts, and that's why you'll measure whats left over, 6 to 8 volts, at the coil.

The problem is during cranking, the very high starter current draws battery voltage down to as little as 9.6 volts. Add the drop of that ignition resistor and the ignition coil may be trying to operate on 3 to 6 volts. Obviously you aren't going to get much of a spark, if any. If the coil can only develop 6,000 volts, it won't fire any spark plug that needs more than that. 12,000 volts is typical.

To overcome that resistor's voltage drop during cranking, every manufacturer bypasses it one way or another. Chrysler does that with an additional terminal on the ignition switch. Ford and GM do it with a tap on their starter solenoids. It was common on Chryslers in the '70s for that resistor to burn open. The symptom always was the engine would start right up, but it would stall as soon as you released the ignition switch from the "crank" position.

Chrysler used a "ballast resistor" on the firewall. GM commonly used a resistor wire hidden in the wiring harness. Failure of those was very rare so I can't even tell you what to look for.

First check those voltages at the ignition coil's small positive terminal. With the ignition switch in "run", you must have 9 to 12 volts there, depending on whether the points are open or closed. Next, you must find voltage there when the ignition switch is in "crank". You can unplug the starter relay so it won't crank if you want to. That way you can do all the testing without cranking the engine.

First let me describe how the system normally works. Lets assume battery voltage gets drawn down to 10.0 volts during cranking. 10.0 volts still comes out of the ignition switch and goes to the ignition resistor. The average voltage, (more on this later), drops to we'll say 6.0 volts. That makes for a pretty weak spark. While the starter is cranking and the solenoid is engaged, full battery voltage is placed directly on the coil positive terminal, bypassing the resistor. 10.0 volts is higher than the voltage the coil normally runs on so spark is nice and strong during cranking. That makes for easier starting.

I don't have a wiring diagram to look at, and GM was famous for trying different things on different models, and letting owners find out what did and didn't work, so I can only suggest that you might not have a terminal on the ignition switch to power the ignition coil during cranking. That isn't needed as long as the resistor bypass circuit is working, and it sounds like yours isn't. If you have the starter relay removed so the system is disabled, and you find 12 volts at the ignition coil's positive terminal with the ignition switch in "run" but not in "crank", don't panic. That might be normal. That would be even better because then we have only one problem to diagnose.

That problem is with the resistor bypass circuit. If you look at your starter solenoid, you'll see two large terminals. One is for the battery cable and the other one goes into the starter motor. There will always be a small terminal marked "S" for "solenoid". That one gets the 12 volts from the starter relay to make the solenoid engage. On most GM solenoids you'll have a second small terminal across from the first one marked "R" for "relay". That one gets battery voltage switched onto it by the solenoid when it is engaged and the starter is cranking. That is the wire that goes to the ignition coil's small positive terminal. By applying full battery voltage there, (10.0 volts in my sad story), and regardless if 10.0 volts comes from the ignition switch or not, the ignition resistor is bypassed and is in effect, out of the circuit momentarily. Based on your description of the symptom, this is the circuit that is missing.

There are other ways, besides Chrysler's ignition switch tap, to bypass the ignition resistor, so the "R" terminal was left off on some GM models. If you get a used starter without that terminal, where do you put that wire? If you stick it on the "S" terminal, you'll still get full battery voltage during cranking, which is good, but then once the engine us running, the ignition resistor circuit will try to power the starter solenoid too along with the ignition coil circuit. The lower voltage after the resistor isn't strong enough to PULL the solenoid into engagement, but it can be strong enough to HOLD it there once it is engaged during starting the engine. The symptoms can include a starter that keeps cranking after the ignition switch is released, and repeated failure of the ignition resistor. You could lose spark too when the ignition switch is released because the extra current going through the resistor to try to run the starter solenoid will cause a much higher than normal voltage drop, leaving too little voltage to run the ignition coil.

The more common problem is when someone replaces the starter and they don't reconnect the "R" wire to the solenoid.

As I mentioned, there are some models that didn't use the "R" terminal. At the manufacturing plant they will build solenoids without that terminal to save money. Later, when any starter is rebuilt, it gets a new solenoid with both small terminals. The additional cost is so little, and they don't have to rebuild some starters just for a few specific car models. One solenoid fits all applications, and if the "R" terminal wasn't used on that car, you just don't attach anything to it.

This is where the problem can come in. Mechanics knew that terminal might not be used and if that wire really WAS there but it fell out of sight, it might get overlooked and not be reconnected during starter replacement, especially when one person removed the starter and another reinstalled it. This commonly occurred when an engine was removed or some other work was done that took multiple days allowing the mechanic time to forget how many wires were on the starter.

This is the place to start looking if there's 0.0 volts or something considerably lower than battery voltage during cranking at the ignition coil's positive terminal. If you have that "R" terminal on the solenoid and the wire is attached, measure the voltage on the terminal during cranking. If you do find full battery voltage there but not at the coil, there is a break in that wire. There's two common causes for that. One is the wire is corroded off the terminal. Tug on the wire and it will pull out of the insulation around that terminal or the insulation will act like a rubber band. The second cause is there could be a fuse link in that wire that is burned open. Fuse link wires are smaller in diameter than the wires they protect so they'll be the weak link in the chain, and the insulation won't burn or melt. The fuse link is only a few inches long and it will be spliced into the rest of that wire. The insulation will almost always be a different color too because they only come in a few colors to denote their current rating. To test a fuse link wire, tug on it. If it's good, it will act like a wire. If it's burned open, it will act like a rubber band.

To make this a little more complicated, you can't use a digital voltmeter to measure the voltage on the ignition coil's positive terminal during cranking or running. Digital voltmeters can't measure voltage in a circuit with a square wave pulse, and that's what you have at the negative terminal / breaker point junction, and the positive terminal / resistor junction. At the negative terminal, when the points are closed, you'll have 0.0 volts. When they're open, you'll have full system voltage, (roughly 14.0 volts with the engine running and generator working). At the coil's positive terminal you'll have roughly 8.0 to 10.0 volts, then full system voltage.

A digital voltmeter takes a voltage reading, analyzes it, then displays it while it's taking the next reading. It typically takes a new reading about twice per second. One time it might see 0.0 volts. The next time it might see 14.0 volts. It doesn't average them. Instead, the display will bounce around and be impossible to interpret. The older pointer-type voltmeters do average the voltage but those are hard to find today. An inexpensive test light will give you an idea if the voltage is 0.0 volts, full battery voltage, or something in between, as long as the difference is significant enough that you can tell the difference in brightness.

The test light might be your best bet when looking at the voltage on the ignition coil's positive terminal. You'll be able to see 0.0 volts, full battery voltage, and something in between very easily, and from inside the car.
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Friday, October 17th, 2014 AT 9:57 PM

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