mark kibort

any ideas of how the alternator charging light stays on when the car is shut down and the key is removed. the owner tried to get the alternator looked at... its fine. thoughts are , the ignition at the key area, might be bad.
car runs fine.... even starts without the ignition relay # IV. is that a clue?
when the car is turned off, the light stays on and this kills the battery.

any help would be appreciated by the owner (guess who... )

thanks!!

Dave H.

bad alternator diodes

parasitic draw, maybe in the cluster
start pulling fuses.

Mrmerlin

has the rear window defroster been turned on by mistake?,
try turning the **** CCW ,
if no joy,
then start pulling relays till the light goes out,
do the rear window defroster first

dr bob

Do what Stan suggests, although a stuck defroster relay usually leaves the car running as power feeds backwards through that relay to the 15 bus.

-- What Year Is The Car?

-- Use your DMM to test for voltage at the exciter pin in the 14-pin engine connector. Pull the -cover- carefully from the back of the connector to test. If there's voltage there with key off, gently pull the engine side of the connector off with your meter probe still touching the exciter pin. If voltage goes to zero as you pull the connector, the problem is not the regulator in the alternator itself, as it is now disconnected from the car.

-- Test for voltage at the car (bottom) side of the 14-pin at the exciter pin. With key off there should be none. If there is voltage there, test the circuit back towards the cluster via the CE panel, then to the cluster, to find where it's coming from. It -may- be coming through the ignition switch but that's very rare.

James Bailey

You can unplug the ignition switch plug by reaching up under the pod. There is enough room with the pod all the way up ! Also can pull it down far enough to test with a new electrical ignition switch BEFORE pulling the pod off....disturbing the foil instrument circuit board and tabs should be avoided if possible.

Randy V

Bad ignition switch.

mark kibort

thanks! ill pass it on today. he is going crazy trying to trace it down himself.
interesting that the def being on can cause this kind of issue.
interesting that he could start the car with the ignition relay removed

by the way, this is an 88 S4 928

mark kibort

amen to that! ive done that and caused little issues. those foil connectors are very brittle and delicate. (along with the plastic connectors being weakened by age.. fortunately, its been on the previous cars!

ill try this if the defrost thing doesnt show to be the issue.

mark kibort

thats what we thought, but alternator turned out to test fine.

thanks,

mk

mark kibort

But, the car is not running with the key turned off, the car shuts off, but the charging light stays on.

could that be the same issue as you thought? what you are saying,makes me think that if the defrost relay or switch is broken, then the car wont turn off as power is supplied backward through the relay to the ign.

so, sounds like we are back to ignition switch problem....

will try and pulll as Jim recommended.

dr bob

Mark--

With key in "run" or "Start" positions, bus 15 is energized via the ignition switch. Power -from- the 15 bus passes through the charge indicator light until the alternator charging brings stator voltage (upstream of the rectifier diodes) up to match the 15 bus voltage. With zero potential across the indicator bulb, it goes out.

With engine not running, options for lighting the bulb include something else putting power to the 15 bus. The rear defroster relay is one of those things. But, if power is passing to the 15 bus to light the dash indicator, it would also leave the normal "engine-running" circuits from bus 15 energized, and the engine would continue running. Yours stops when the key is turned off.

Next option, as others suggested, is a shorted diode in the alternator, feeding voltage from the alternator back towards the 15 bus through the charging indicator bulb. This is the test I suggested you make, with the DMM on the exciter pin in the engine side of the 14-pin connector. If a diode is shorted, you'll see voltage on the engine side of the harness even after the connector is pulled.


---

Problem solving is a process by which you look at possibilities one by one, eliminating them in a controlled fashion. The guidance I shared for testing instantly narrows the search by determining whether the source of the voltage is on the car side or the engine side of the 14-pin connector. If the voltage source is on the car side, we'll know and go deeper into diagnostics. On the engine side, there are a few possibilities all inside the alternator.

Be aware that a parts-counter alternator test determines whether the unit can generate power. It doesn't test for a shorted diode backfeeding power through the excitation circuit. You get to test for that in the car.


---

It would be a Good Idea to have the owner participate directly in this thread. Saves a lot of "lost in translation" errors as you relate what you think you heard or read back and forth.

Mrmerlin

also check the 14 pin connector for wires shedding their insulation

mark kibort

will do.. sounds like its not DEF button as its off and the car doesnt run with key removed. (but light still on)

will do the check you recommend

thanks again! (he thanks you as well)

Mark

mark kibort

He pulled the ignition relay and then the light goes out, AND the car still starts..... clue?
defrost didnt do anything, but we kind of knew that might not be it anyway.
looks like it might be ignition switch.. thougths?

dr bob

If the car runs without the ignition relay, I suspect he didn't pull the actual ignition relay. TAKE A PICTURE of his CE panel with that relay removed, and post it here for analysis. The relay numbering scheme is not exactly intuitive. Dates to Roman times.

Post the picture please.