I want to check my electrical and ignition systems out to make sure they are functioning properly. I have a stock 1970 Chevelle with a 402. I am looking for information on how to check the coil and alternator voltages and currents and what range I should be looking for on each.

I have measured the voltage at the coil positive terminal (negative terminal grounded) with the engine off and the key on. I got 6 volts. Is this correct?

With the engine running I got 12 volts at the coil + and 7.5 volts at the coil -. Are these readings correct?

I would also like to measure alternator output of my original (61 Amp) alternator.

Thanks in advance.

I give this a shot. There seems to be a lot of smart guys out there but maybe they are off for the day. It's raining here anyway.
The voltage on the positive side is ok. The voltage on the negative side is what you are seeing through the coil. If you were to move the engine to a position where the points are closed you would read 0 volts. The points "chop" the 12 volts from the battery by opening and closing. This "chopped" voltage is then raised by the coil which is really a high voltage transformer. It is more important to have a good output (large center wire) but DON"T measure it. Your meter will be toast.
Doesn't matter how much current your alternator is rated for if all you want to know the voltage. Try 13.5 to 15.5 volts. However the more amps you have, the better you will be at idle cranking a 100 watt stereo.
John

Good way to test alternator is to check voltage at battery. Start engine, run about 1500 rpm, turn on evreything in the car that draws current. Voltage should stay above 13 volts steady, may cycle with turn signals. If voltage starts dropping even with engine revved up a little, need some diode work in alt. Shouldn't coil voltage have a resistor in the 12 feed, say down to 9 volts or so. Point ignition?

Remember also, what you are seeing at the pos side of the coil is resisted voltage. The wire that connects to the positive side of the coil is a resistor type wire. GM did this to extend the life of the points. I think the voltage should be around 9 volts, with the key on, engine off, AND the wire disconnected from the positive terminal of the coil. The reason you are seeing 6 volts is because of the coil windings and the additional voltage drop. Disconnect the wire and check it again.

Thanks, I guess I didnt figure in the extra resistance provided by the coil itself when checking the voltage at the coil +. Ill disconnect the wire and check it again.

Kevin;

Don't bother removing the wire from the coil and checking it. You'll just get the same voltage as what you measure at the battery.

To know how well your ignition system is working, you need to know the voltage going to the plugs and the timing settings. As for measuring the voltage going to the plug, I really can't think of a way. Maybe there is some kind of arc guage that measures the voltage my seeing how far it will jump.

I would have thought that 6 volts on the coil while off was a little low, but if it is working well, don't worry about it.

If you are worried, put on an aftermarket coil and a points replacement system (pertronics??). The coil should come with a recommended operating voltage.

PEter

Careful Peter, when you start discussing resistors in a series circuit, Ohm's law comes into play. You have at least two resistors here, the wire feeding the coil, and the coil itself. Voltage drop through the resistors equals the applied voltage. The total resistance of the circuit is changed when you add more resistors to the circuit.