charging system problem

Hi,

Recently started having problems with my '89 Cabriolet. It had been sitting for a while and the battery had gone dead. I recharged the battery and it ran great. A few days later, the battery starts discharging while running until the car cuts out. I figured the alternator was bad and replaced it. Put the new alternator on and still had the same problem. Took the alternator and the battery down to autoparts place and had both tested. Alternator putting out 14 volts as it should and battery tested good,although with low charge. Put it all back together and seemed to be stable, but only at 11.5 -12 Volts. Went thru the fusebox and disconnected every fuse, except the fuel pump to see if there was something dragging it down. Nothing seemed to change. At this point, I'm charging the battery over night and will try again in the morning. Anyone have any suggestions?

Thanks

spudspuppy

My Mazda has the same problem. I replaced both new battery and alternator but still reading the same voltage (around 12V) at battery posts when engine is off or running. If anyone have the solution please help. Thank you very much.

Does the battery light come on when you turn the key to the on position then go out after starting the engine (it's okay if you have to rev the engine slightly to make the light go out)?

Boschman,

No, the battery light does not come on. When you turn the key to the on position, the temp gage light blinks rapidly and then goes out and the oil light blinks steadily. when you start it, the oil light goes out and everything seems to be normal (however still only putting out 11.5 to 12v).

P.S. Battery is now fully charged and has no effect on the output voltage of the alternator.


Thanks,

spudspuppy

This morning I started the engine then pressed the gas pedal to increase the RPM, the voltage went up to 14V but soon I released the pedal it slowly dropped down to around 12.4V. Please advise for the solution?

My Cabrio always did that when it was working properly. Perhaps the idle is too low for the rated output of the alternator. Alternator will have a spec indicating output voltage and current at a given RPM.

I think that the alternator may rely on the LED feedback to excite the field on the alternator to get it charging. If the LED is burned out it may not charge. They are replaceable once you remove the instrument cluster.

Boschman,
I must say that is an option I have never heard of before. I can't imagine how the alternator would use that for feed back as I've never seen that in an american car, but then again, it is a Volkswagen.

I recently ordered the service manual for the car and should have it next week. Hopefully, with a complete schematic, I should be able to track it down. I'll let you know. Thanks for your input

Spudspuppy

Try this, get a test light & hook the clamp end to battery positive then connect the probe end to the small blue wire coming off the alternator. The test light will act as the light in the dash (except that it will come on with the key off since you're not hooking it through the ignition switch). Start the car & see if the light goes out, if so it is charging & you need to replace the dash LED. Before replacing the LED you might want to be sure that pin #14 (black wire) on the instrument pannel connector is 12V + with the key in the on position.

Boschman,

Well, it looks like you were right. I did as you suggested (the light came on and went off when started). The voltmeter indicated that it was charging at around 13.5 V. I drove it around for awhile and it continued to charge. However, the next day I started it and it was back to its old tricks of not charging. Received the service manual from VW today and it has a specific section for checking the alternator exciter circuit. I will follow their test procedures and see what the result is. They indicate that a failure means you need to replace the printed circuit board. However, I should be able to determine what's going on once I get the dash board open.

As a side note, this all started happening the day my wife got caught in the rain with the top down. I'm wondering if any water got into the dash and possibly shorted something in this circuit. She wasn't in the rain long, but maybe long enough. Then again, maybe it's just a coincidence, as the car is pretty old.

I'll let you know what happens. Thanks again for your assistance.



Spudspuppy

Here's a quick follow-up to the problem so far. Pulled the instrument panel out of the dash and analyzed the diodes. Diodes were good and had good connection between the diode and the connector. However, can't find the connection from pin #12 of the instument panel to the blue wire on the alternator. Traced it to the fuseblock but can't find the other side of it. I figure that's where it is loose or disconnected. Had to order the wiring diagram for the car as it didn't come with the service manual I ordered.

More to come...

The blue wire goes from the alternator to pin C2 of the fuse block. It connects internally to pin A16 which goes to the warning light in the cluster. It is also connected to D4, the elapsed mileage counter for the O2 light (which also turns on the charge light) but that shouldn't be important. If the problem is in the fuse block you can just connect the two wires & it shouldn't cause any problem.

Boschman,

Well, after taking the dash apart and examining the connectors on the fuseblock, I found the problem. Turns out the wire from the alternator was broken in the engine compartment, underneath the fuel filter mount. Wasn't obvious at first, but noticed it when I was doing continuity test between C2 and the alternator. Wire seemed a bit loose and I pulled it and it came right out of the loom.

So, everything works great now. Thanks for all your help.

Regards,

Spudspuppy