Hello everyone, I searched and found this topic spoken about a lot, my gen light is staying on at all speeds and I am only getting 11.56 volts from the output wire on my alt. Its a reman unit from autozone that is about 2 months old. I took it back to them, but they wouldnt return it due to it putting out 14.5 exactly on their machine. I couldnt quite explain to him what is going wrong inside the alt. but I know that I am getting a full voltage drop between the brown wire and red wire of the two pin connector. Its almost acting like the brown wire is a ground when it is running. After I shut the car off, I put a meter on the battery and it crawls back up to 12.3v. Can someone explain to me if I am understanding this circuit correctly, and possibly what has gone bad in this new alt so I can get it replaced?

Thanks.
Chris

 

Hard to know whats wired where over internet. How about with the engine running when you disconnect that wire and rev up a little to take a voltage measurement. You should see something over 13.5 volts. If not that alt has bad/no output. If it is over 13.5v then you have a big short to ground in the downstream wiring. But you can have so many wierd problems with wiring that you may have to bite the bullet and try a Chevy dealer Chris. :yesnod:

 

Well Im a BMW diagnostic tech myself, so I more than likely will not be going to a chevy dealer, I believe I have a basic understanding of the charging system, but I wanted to check to make sure since it really isnt a car I have a lot of information on. From what I gather, on the two pin connector, when doing a voltage drop across both wires with the engine running, there should be little to no voltage across them. I have full batt. voltage across them and wanted to make sure I was looking at the circuit correct.

 

I think YOU may have already identified your problem.

At low engine speed, current flows from the Battery through the Generator Warning Light to the Field Winding (generator). The Generator Warning Light is lit whenever the Battery supplies the field current.

As the engine speed increases the generator output voltage increases and the field current is supplied through the diode trio. When the voltage on each side of the Generator Warning Light is about the same, the light goes off.

So much for normal operation.

Generators have a built-in feature that avoids overcharge and accessory damage by PREVENTING the generator from turning on if there is an open in the wiring harness connected to the No. 2 generator terminal.

Therefore, I think the reason the alternator not charging, is because an open in the wiring harness connected to the No. 2 wiring (alternator). The warning light is lit (correctly) because the BATTERY is providing electrical power. The alternator is working properly as noted in your alternator tests (control circuits are working (correctly) keeping the alternator turned OFF thus preventing an overcharge condition).

Opens between the No. 2 generator terminal and the battery may be at the terminals, at the wire crimp between the harness wire and the terminal, or may be the wire itself.

There is a section in the GM Shop Manual for your year car that outlines a number of tests - the Section is Engine Electrical "UNDERCHARGED BATTERY" (This section deals with different types of charging problems.)

My $0.02 :)

 

On the back of the alt there is a small hole, it looks like a D. You insert the inside of a ball point pen in there (or something similar) - this will "full field" the alternator. You could also install a jumper wire between connections a & b (lower left section of diagram - gen) Putting a jumper there should turn on the alternator bypassing the diodes - jump the small wires, don't connect anything to the bigger wire which has it's own lug.





[Modified by 71coupe, 10:51 PM 5/2/2004]

 

THANK YOU everyone for your extremely informative and fast replies, I found the problem, it was corrosion/arcing marks on the contacts of the Gen. light bulb socket, I replaced the socket and bulb, and boom, 14.5 volts at 1200 rpm.

Gotta love this forum.
---Chris :steering:

 

I just lerned a lot from this post.(spelling)
I'm trying to get my 68 up and running.