tj90

I have been trying to track down the reason why my alternator (battery) bulb is not illuminating on my early 84. Part of my problem is that I don't have a good wiring schematic. I found the one at http://www.pelicanparts.com/944/elec...l_diagrams.htm but its seems like its missing pages. If you look at part 4 page 1 and 2 do not line up.
I am not able to get the bulb to illuminate under any condition. I jumpered the exciter line on the back of the alternator to the positive lead of the battery and still could not get alternator to operate. The belt is in good condition and no perceived slippage. I will get the alternator bench tested tomorrow.
In the meantime I tried ohming out the wires at the gauge. From what I can see on part 4 page 2, the wires are #11, #7 and #12. Not sure where #7 and #12 go? From the schematic, it looks like they are tied together which I confirmed. Ohming out between #11 and the others gives me about 3-4 Ohms which is the resistance of the bulb. Therefore, I know my bulb and connection to gauge is good. The volt meter is giving me 13V between each of the wires and body ground when the key is turned on or when car is running. That would explain why my light doesn't come on. Can anyone help?

968workaholic

#7 & #12 are switched positive power. You need to apply ground to #11 to check the bulb. Its possible that the exciter wire is somehow getting 12V when the car is off. Did you check the starter connections? If they are loose you won't get full power to the battery, even though the alternator is working

Jfrahm

The voltage regulator could be blown, they are cheap. Huco brand has been good for me although they are not made in Germany anymore so who knows from here on out.

tj90

Thanks for the ideas guys. The bench test shows that the alternator and regulator is good. The knowledgeable parts guy told me that touching the exciter to the positive terminal of the battery wouldn't do anything. He said to ground it for maximum output. Now I'm really confused - it sounds like #11 should be grounded when the ignition is turned on. Mine is at 13V. Why? Ill clean the starter terminals but they were tight.

marc abrams

You need to edit your post from knowledgeable to unknowledgeable parts guy. Find a good auto electric shop to test that alternator. If you were to ground the excite circuit when the alternator is running, you will be letting the smoke out of it.

MAGK944

Most batt/alt failure lights work with a voltage difference rather than a positive and ground wire. One side of the light has battery +ve voltage and the other side has alternator +ve voltage. When you turn the ignition on without starting the engine the light sees a +ve only on one side from the battery and will therefore turn on. When you start the engine it sees a +ve on both sides so will turn off. With the engine running if either the battery or alternator fail the light will drop the +ve on one side and the light will turn on.

marc abrams

You mean -ve on the alternator side

Basically what goes on here current flows from the ignition switch, to the light, and to the voltage regulator. Once the alternator is producing it's own power it can feed the voltage regulator itself and current stop's flowing through the light.

MAGK944

No I mean +ve. Both sides should be +ve if the alternator and battery are good. With ~12v on both sides of that light it will be off.

marc abrams

Sorry, but I interpreted that line in you post as the alternator is not changing.

MAGK944

Maybe I didn't explain that too well. You don't need a +ve on one side of a light and a -ve on the other side to turn a light on. You can have a +ve on each side and that is how those alternator/battery lights generally work. The light will come on if there is a difference in the +ve voltage on either side.

Say the battery was pushing out 12v but the alternator was pushing out less, like it was failing, that light will come on even though there is no -ve or ground in the circuit.

That is also why you sometimes see that light dim when the alternator belt is slipping or about to give, it's because the alternator is pushing out less than the battery.

marc abrams

No problem there bro.

Personally I try (if thats possible with electrical problems) to keep it as simple as possible and give just enough information so that the individual can quickly find the problem.

tj90

Problem solved....drum roll please.....


It was corrosion on the connectors sitting above the brake booster. I thought to check this from this thread: https://rennlist.com/forums/924-931-...ht-solved.html

It was exactly the problem that I was having. What is crazy is that I ohmed out the excite line from the alternator to the gauge and it showed only a few ohms of resistance. I guess the take away is that you cant always trust the DMM to tell you that you have a bad connector.

I really appreciate all the help you guys have given. However, I still don't have a good electrical diagram. Do you guys have one?

Jfrahm

Good catch. A DMM can show a good connection, very low resistance or full voltage, but that can be true only with no load on the wire. Sometimes you seem to have a good connection, but when you try to draw some current the connection opens up. Testing with a lamp or other load can be a lot more reliable.

PaulD_944S2

Or run it and then put the DMM leads across the wires in each connector and check for voltage drop.

marc abrams

I'm glad you got to the bottom of it. I'm just wondering why your alternator wasn't charging when you jumped the exciter circuit the first time?