Ignition system voltage issue

[Canman38]

After searching for a while, I couldnt find an answer. So heres my issue; I am hooking up a pertronix on my 88 cobra 5.0 gm based engine. I have found lots of info on the install which has been very helpful. However, in trying to find a 12v source for the power to the ignition module I am getting thrown a curve ball. From every one I have seen people are hooking up to the purple wire on the alternator. So just to double check, I put a voltmeter on the purple wire, I am getting 11.1 volts. Thinking my battery might have been low, I checked its voltage..
I got 12.5v across the battery terminals.
I get 12.5v across the main battery cables.
I get 12.4v at the ignition switch.
I get 11.1v at the purple wire on the back of the alternator.
I get 11.1v at the coil positive.(which confused me,I thought there was a resistor there)
I get 11.4v on the purple wire to the esa module.
I get 12.5v at the tilt motor terminals.

All these numbers are with the engine off ignition switch on.

What do you guys get on your purple wire?

Do I have to wait for it to be running to see the voltages go normal?

I dont want to burn up my pertronix as people always say that, that is the reason the burn up, by not getting 12v.

Thanks in advance,
Walter

 

[bruceb58]

Re: Ignition system voltage issue

I get 11.1v at the coil positive.(which confused me,I thought there was a resistor there)

The reason it isn't lower like you expected is that the points are open. Close the points and you will see the voltage drop across the resistor. With no current flowing through your resistor, you aren't dropping any voltage across it.

As far as it being 11V instead of the 12.4, you have a resistive connection somewhere. I would pull apart your engine harness and see if there is corrosion in there.

There are three more more measurements you should make. First make sure your points are closed. Measure the voltage at the input of the ignition switch with it on on and off and measure the output of the ignition switch with it on. Ground the meter at the battery for all tests. The reults of these tests will not only see if the ignition swittch is at fault but it will tell you if the drop is between the battery and the helm or the helm and the engine.

If you are adding a Pertronix to solve an existing issue, you may not solve it until you figure out your voltage drop.

 

[Canman38]

Re: Ignition system voltage issue

Ah, good pointers, thank you. Not trying to solve any existing problem. However I am thinking I might have had a problem now.

Im right in my garage so heres what i got.

I already have the pertronix in the distributor, so I just touched a jumper from the neg coil post to the battery neg.
I also put a jumper across the igntion switch terminals.

3.6v at the coil.
7.8v at the purple wire.

with the jumper off the coil and igntion switch:
12.1 into the ignition switch
11.9 out of the ign switch

I think I am going to start checking to see where power comes into the engine compartment from the igntion switch. Any idea where?

walter

 

[bruceb58]

Re: Ignition system voltage issue

Main engine harness.

Basically, there is a large wire at the starter solenoid that then goes through a fusible link and then makes it to the engine harness. Then it travels to the helm, goes through a fuse and to your ignition switch. From there, it goes back to the engine harness and goes to all your items on your engine. Therefore, it has to travel twice through the connections at the engine harness but passes through a few other points as well. All are suspects at this point.

Your measurements already show 0.5V drop by the time it gets through the ignition switch. I would make all measurements with the negative side of the coil grounded to accentuate your loss.

Like I said before, use the ground at your battery for your reference on all measurements. You can get confusing results if you don't.