Weird Charging Issue

[spencer087]

OK so i have a new bosh rebuilt altenator and a new battery but the vehicle has a weird charging issue. Basically the radio will go out and o will AC so i then rev to approx 3500+ RPM and they spring back to life and are fine for about 30 min then will go dead again and repeat. Why is it the altenator will charge after i rev for like 30 min but then stop again until i rev??? This is very confusing to me. Thanks

[ttrousdell]

the rotating part of the alternator rotates in a magnetic field. as it rotates it cuts through this mag field and a voltage is induced. this voltage varies based on engine speed but is supposed to be maintained at a fixed value by the voltage regulator. The current also has to travel through slip rings to carbon brushes to get from rotor to output terminals. Change the volt reg/brush pack and see what happens.

[jlevie]

If the battery is fully charged, the primary power and ground connections are good, and the alternator is good the system voltage at idle should be at least 13.5v and it should rise to about 14v at 2500rpm. And those voltages should be nearly the same if the load the alternator by turning on the high beams and set the HVAC blower to max.

Charge the battery overnight and check the system voltage, per the above. Also check all of the primary and ground connections, including the ground strap from the engine to the frame.

[spencer087]

OK ill charge the battery overnight and then check the voltage.... currently it is 12.44 not running, started and it became 12.00, rev'd to 3500 and it jumped to 13.5 and maintained that steady

[spencer087]

maintained that even back down at idle... but give it 30 min or so and the altenator will suddenly stop charging like it forgets what it is doing

[jlevie]

OK ill charge the battery overnight and then check the voltage.... currently it is 12.44 not running, started and it became 12.00, rev'd to 3500 and it jumped to 13.5 and maintained that steady

12.44v with the engine off represents a battery that is only at 75% charge. Seeing 12.0 volts at idle says that the alternator isn't working as even with a battery at 75% the system voltage at idle should be close to 13.5v.

Check all the power and ground connections, but it sounds like you have a bad alternator.

[spencer087]

BUt the altenator kicks inn for about 30min if i rev it to 3500 rpm even just briefly... then after about 30min the aternator stops working again. I removed the battery light has been removed from the dash cluster so is it causing the problem? maybe it wont charge until it revs to 3500 rpm as a fail safe or something?

[jlevie]

What year car is this? Unless it is a very late 325i or 318is the battery light is a critical part of the charging circuit and provides exciter power to the alternator. Put the bulb back in, make sure it lights when the ignition is switched on, fir the engine and check system voltage.

[Ryann]

Verify with an ohmmeter that the alternator case has a solid ground connection to the battery.

[7pilot]

I suspect that the battery is failing.
It seems that it accepts a surface charge only, and this is driving the alternator batty.
I think that the battery would probably fail a load test.

m

[spencer087]

It is a 1987 im going to try the light bulb thing...

[spencer087]

OK so currently the brake lining light stays on even though the brake pads are fine. The ABS light stays on. The battery light does not burn at all even when key is turned on so i took out the light bulb and put 12v on it and verified the bulb is good. I am currently charging the vehicle battery to see if turning the key on at normal voltage maybe will reset something...

[jlevie]

Ignore the brake light and ABS warning for now. The important issue is why the battery warning light doesn't come on when the ignition is switched on. Start by checking the fuses that feed the cluster. If you can't find a fuse problem, there could be a problem within the cluster, in the wiring between the cluster and C101 or in the engine harness.

[spencer087]

OK all fuses check out. All instruments/lights work on the cluster except the battery light.
NEW TEST. Started vehicle and took voltage, 13.5.
Removed battery cable, Instantly died.
Replaced cable and started vehicle, Rev'd to 3500 RPM, Removed battery cable vehicle stayed on and voltage was 13.7.

Why does the alternator work AFTER i rev to 3500 RPM But only for a certain period of time that seems to be computer controlled since it is always around the same amount of time?
And of course why doesn't it just charge all the time like its supposed too????

[spencer087]

I think maybe the problem is with that stupid light bulb.... Light bulb works when i connect it to 12v source but not in the cluster... Is there a way to bypass the bulb? Like take one wire from the harness and splice it to a different one so the exciter wire skips the cluster all together?