Resistor/alternator question

I cooked two alternators this weekend. One because I bridged across two terminals while trying to test output with a multimeter and the second one because I reversed the ground and battery lead on the old Paris Rhone.

So, I'm going to get another alternator as soon as I can, either a Delco 10si or a Delco 3 wire and try again. (I will have the new alternator tested today just in case it still works)

Now to my question: I pulled the resistor wire from the exciter lead to test it with my multimeter. I put the probes on either end of the wire with the resistor in the middle on my workbench and set it to read ohms. The needle moved over to the zero on the right hand side.

Is that what it should read? Speak slowly please, I'm an electrical idiot.

Mike, If what you are referring to is the little short harness looking thingy, it is a "Diode".... even while everything you read in a Volvo schematic will call this a "resistor".

The excite circuit on many of these alternators share the ignition circuit!
When the alternator is up and charging, this circuit may continue to excite the ignition and you cannot turn the engine OFF!

So this diode is intended to prevent a back-flow of current to the ignition.
It is not a resistor! (if you are referring to this little harness)

Recently discussed in this thread.

Ricardo, I beg to differ. For what I have seen, the little rubberized black thinguie that you have connected to the excitor circuit is normally a resistor, not a diode.

A diode conducts power one way, and a resistor opposes resistance either way: the black thinguie (as you see I keep usig a highly technical term for the sake of clarity ) measures resistance either way.

The resistors installed on most of the SEV Marchal/Paris Roneo in the AQ applications are about 72-75 ohm. Don't know if VP has changed their ways lately.

Well, you may be correct!
I trust what you are saying..... I'll have to research this!

I did a correction to a V-8 system whereby this part had failed.
He could not turn his engine off.
We replaced this with a diode, problem solved.

Can anyone answer my original question?

"I pulled the resistor wire from the exciter lead to test it with my multimeter. I put the probes on either end of the wire with the resistor in the middle on my workbench and set it to read ohms. The needle moved over to the zero on the right hand side. Is that OK?"

I'm in the process of installing a rebuilt Paris Rhone alternator and will post back the results.

I completely cooked the original one. I can send you the recipe if you want.

RicardoMarine said:

Well, you may be correct!
I trust what you are saying..... I'll have to research this!

I did a correction to a V-8 system whereby this part had failed.
He could not turn his engine off.
We replaced this with a diode, problem solved.

Click to expand...

Ricardo, I agree 100% that a diode would be a better solution because the whole idea is to prevent backfeeding of the coil when the ignition key is turned off. No idea why VP decided to go for a resistor instead.

el_pescador said:

Ricardo, I agree 100% that a diode would be a better solution because the whole idea is to prevent backfeeding of the coil when the ignition key is turned off. No idea why VP decided to go for a resistor instead.

Click to expand...

Read the write up in the link I posted. The purpose of the resistor is to supply just I = E/R = 12/74 = 160mA of current to alternator's stator winding in order to guarantee that the alternator "bootstraps" itself on start up. The diode would prevent backfeed from the alternator to the ignition system when the key is turned off, but does not sufficiently limit the current from the battery to the alternator stator winding prior to engine start. With a diode instead of the resistor, the current would only be limited by the resistance of the wiring from the battery, through the ignition switch back to the engine and then by the DC resistance of the stator winding itself. I'm guessing that current would be ~10A...

Again I have proven that if you have a mechanical problem that you can't solve, just throw some money at it. The alternator shop confirmed that I had managed to cook both my new alternator and the old Paris Rhone.

So, since I don't understand electric issues worth a damn, I decided that the course of least resistance and highest probability of success was to buy a rebuilt Paris Rhone for a not inconsiderable sum

I installed it and it works so problem solved. And, next time I will be a lot more knowledgeable about alternator and ignition issues. Oh...this learning curve is expensive...

Experience= learning from your own screw-ups.

Yes, I know, I have quite a bit of experience in several areas.....