My 72 350/auto has got me baffled. I put a voltmeter on the ignition side of the coil with the key in the "on" position and got a reading of about 6.5 volts. Turning the ignition to "run" shows about 12v and starts the car, all OK so far it seems. But when the key returns to the "run" position, the voltage stays around 12v. Shouldn't this go back to 6.5v? The ignition wire (switch to coil) seemed very warm, almost hot, so I changed the ignition switch, no change. Any ideas?
Thanks

 

You had posted your question in the Help section, which is used for Help with the website itself (forum issues, account issues, website issues, etc.).

You will receive a better response if you post Corvette technical questions in the appropriate Corvette section. In your case, you would want the C3 Tech section.

I've moved your thread to the appropriate Corvette section.

Good luck!

 

J T, Are you an undercover Moderator?

 

how many wires are on the positive side of the coil?

 

Hi,
Two wires. One, black resistance type and another orange with fabric cover. Both wires connect together with one connector at pos terminal. All looks factory.

 

hmmm.. wonder if the solenoid is messing up, sending 12 volts to the coil after the engine starts. It would be easy to test if those were separate wires at the coil.. sure hate to cut the harness for this. You are still running points, right?

Voltage is going to go up after the engine starts, but I still think the coil should be getting like 9 volts. I'll test my 71 in a little bit.

 

The resistor wire going to the pos side of the coil doesn't reduce the voltage , it reduces amperage going to the coil.

 

I have a little under 12 volts at the coil when not running, a little over 12 when running. I imagine that ignition wire is supposed to be warmer, probably why it's insulated. At this point, I'd be more concerned about why it's only getting 6.5 volts when not running. Could be that ignition wire is going bad.

 

I believe you can test that wire at the firewall, I don't think it's resister wire all the way to the ignition switch.

 

OK, Roger, I am going to agree with ya and then disagree with ya. Yes, the amperage is reduced, BUT, by inserting a resistor to do that, you do reduce the voltage at the coil too. Its all related. V=IR you know.

 

Before joining this forum I would have bet that if you removed the resistor wire from the coil and measured voltage it would read less than battery voltage , it won't.
After 69427 educated me about this subject I went to the garage and did some testing. :o
The resistor wire is under 2 ohms , it takes a whole bunch of ohms to start showing any reduction in voltage.

 

Roger, Roger, Roger. If you apply 12 Volts straight on top of the coil the voltage is 12V.

If you place a 1 ohm resistor in line with the coil, as you say, the amperage draw is reduced. The voltage on top of that coil is reduced also. It is reduced in relationship to the added resistance + the coil resistance. Coils have a very low primary resistance, lets say about 3 ohms or so. Now you have 4 total ohms in the line. The voltage on the top of the coil would now be 3/4 of the original voltage or about 9V. You cant say the voltage is not less. You are defying the laws of physics.

Now if you are saying you just measure the voltage at the resistor with it taken loose from the coil, then I agree, thats what you will measure. But, thats not the way the system operates in service. It is a completed circuit.

 

You're half right (the part where you agreed with Roger).

I have corrected people 'til I'm blue in the face, and yet this reduced system voltage claim keeps surfacing. A resistor limits current. It does not set up a regulated voltage for a coil. The coil sees (system voltage) 12v at the start of every dwell period. Every time. It does not see 6 volts or 8 volts or any other number except 12. An ignition coil is not a resistance. It is an inductance (which means V=IR doesn't apply), with a resistive component. The primary current waveform is predominately controlled by the coil's inductance, and the "voltage" one measures is just an unavoidable side effect of the need to have a series resistance to limit the peak current in crude mechanical switching circuits. And as such, measuring anything on an operating coil with a voltmeter yields nonsense. It's a continuously varying current and voltage waveform, and to get anything resembling a true accurate measurement requires an oscilloscope. Period.

The only time a volt/ohmmeter should be used anywhere near a coil is when the engine is not running. This will allow you to remove the effects of inductance from the equation. This will tell you what the resistances are in the circuit, and if the points are open or closed, but little else.

I'm tired of typing (I've gone through all this multiple times before, including correcting stuff in the ignition sticky), but I would be happy to go into depth on any aspect of this anyone may disagree with.

 

Hey, I agree with you and understand exactly what you are saying. I had many hours EE courses in college. I am talking about a steady state with the engine not running.

 

I think if you bump your engine over until the points close you will get a reading around the 6.5 v that the OP is concerned about.

 

Yes I'm still running points. I was thinking about that solenoid also but I didn't know if it could have that effect on the system. I was getting my info out of "CHILTON'S REPAIR AND TUNE UP GUIDE CORVETTE 1963-1982" page 61.
It reads, "When the ignition switch is in the "run" position, voltage from the ignition switch passes through a resistor, which lowers the battery voltage to the coil from 12V down to about 6-8V.
Note: If this resistor is defective, the full 12V from the battery will quickly burn the breaker point contacts and shorten the life of the ignition coil.
When the ignition switch is in the "start" position, the resistor is bypassed. This provides a full 12V to the ignition system to aid in starting the engine."
It will be interesting to see what you get for a voltage reading.

 

See post 8

I get 12 volts basically. I'm not running points, but I didn't change out the resister wire.

 

Thanks Roger.. hadn't thought of that

 

Is it possible there's something not right with the wire connections at the starter solenoid?

 

If the resistor is defective it will most likely be burned in 2 and the circuit will be open. No voltage to the coil in the ON position.