Former VWDieselParts.com Forum

Hello there VWers, my issue is with the battery light not turning on when the Ignition is engaged in the start and run positions. I have 12.4v @ batt. fully bench tested bosch alt. in working condition. My alt. does not seem to be charging the battery, with vehicle running I am only getting 12.2 to 12.4v when lights turned on the voltage drops to below 12v. Any ideas would be great.
p.s. Battery terminals in good condition, ground cleaned?
Thanks,
Eric

Most likely the blue exciter wire has an internal fracture right by its terminal on the alternator. It's a common issue due to the vibration of the diesel. The fracture is often not visible because the blue insulation can remain intact.

To test the system, run the engine and put a clip-lead or other jumper between the blue wire's terminal and the heavy wire going to the battery + terminal. Give the engine a brief rev to about 1500 rpm and see if the alternator kicks on.

Check the 5 amp fuse marked "radio". It powers the whole instrument cluster. A short anywhere in the cluster will cause it to blow. Without that red light the battery won't charge.