Electrical problem
2001 Volkswagen Jetta 4 cyl Front Wheel Drive Automatic 90000 miles
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have jetta, with 90 amp bosh alt. shop test alt it puts out 100 amps when tested, battery checks out at 235 amps cold crank, cables cleaned tightened, when car first started, volts will display 13.8 to 14 after car runs a few minutes less than 5 , voltage drops to 12.5 then to 12. not sure what steps to take now?

HAve you tetsed the fuse on the battery/fuse bracket? ther should be a110 or 150 amp fuse depending on the engine size.Also test continuity from alt to fuse on big wire (black) Unfortunatly the PCM (computer) controls the alt charging in this car, so the problem may go deeper than just an alt. Also check fuse 11 behind the dash, it is part of the circuit as well

2CarPro Jack wrote:

HAve you tetsed the fuse on the battery/fuse bracket? ther should be a110 or 150 amp fuse depending on the engine size.Also test continuity from alt to fuse on big wire (black) Unfortunatly the PCM (computer) controls the alt charging in this car, so the problem may go deeper than just an alt. Also check fuse 11 behind the dash, it is part of the circuit as well

cables ok, fuse 11 ok, tried a new battery, other battery showing 12.5 volts, system now charging 16.8 volts, dropped after about 10 minutes, noting that if engine raised above idle tact and insturment cluster shut off, i guess high voltage protection? after 20 min road test battery died, any sugestion on changing pcm?

Hi stribman,

Check if belt is slipping on pulley or faulty pulley if equipped with damper rubbers..

KHLow2008 wrote:

Hi stribman, Check if belt is slipping on pulley or faulty pulley if equipped with damper rubbers..

no the belt is fine, no oil,grease dirt water ect, not hard either no belt slipage. ive have concluded it has to be something with the computer controls? and I dont look foward to that.

Hi stribman,

If charging voltage keeps dropping, it could be a faulty alternator, internal rectifier or voltage regulator problem when it heats up.

KHLow2008 wrote:

Hi stribman, If charging voltage keeps dropping, it could be a faulty alternator, internal rectifier or voltage regulator problem when it heats up.

Yes it could be , but the alt tested ok , when checked, Im not sure what voltage they put on field to make it check, it was a shop that all they do is work on starters and alt, you would think they know there stuff, been there 30 years, would anyone know what voltage should be going to alt field from the ecm? or how to preform a check on this? I have read it is anywhere from 1.7 to 3 volts, and more than that could fry the regulator? I have had a friend read codes and got a low voltage code and one for recirculation valve fail also one for wastegate fail , could a shorted senoid cause ecm voltage drag?

Hi stribman,

No, I am not doubting their capabilities, but it happens only after running the engine for a period of time before the alternator starts to provide insufficient charge.

Sometimes when contacts are bad, it shows a good reading during testing and initial running. When the circuit heats up, resistance increases and it is unable to provide the proper connection and that is when it starts to give problem.

I would suggest stripping the alternator to check for bad internal connections.

Normal ECU output voltage is 4.5 to 5 v, so it should be around that.

There are ways to test but you need to remove the ECM socket for this circuit at the same time leaving the engine wires to ECM connected. I am not sure of the wiring circuits for your car so I do not know which socket to disconnect.



You can test it by checking the voltage

KHLow2008 wrote:

Hi stribman, No, I am not doubting their capabilities, but it happens only after running the engine for a period of time before the alternator starts to provide insufficient charge. Sometimes when contacts are bad, it shows a good reading during testing and initial running. When the circuit heats up, resistance increases and it is unable to provide the proper connection and that is when it starts to give problem. I would suggest stripping the alternator to check for bad internal connections. Normal ECU output voltage is 4.5 to 5 v, so it should be around that. There are ways to test but you need to remove the ECM socket for this circuit at the same time leaving the engine wires to ECM connected. I am not sure of the wiring circuits for your car so I do not know which socket to disconnect. You can test it by checking the voltage

thanks for the info ill check the voltage at the alternator regulator input.