Technical What Does This Battery Connector Supply?

[Steve148]

Hi -

2004 JTD. I'm getting recurrent flat batteries, and after endless testing I've isolated it to a 3amp drain via this connector on the battery's positive terminal. Drain is persistent, even with the engine off and keys out.





It's a thick cable, so I'm guessing starter and/or alternator? Can anyone please confirm, and maybe suggest how I can continue testing downstream to identify the faulty component.

I realise a blown alt. diode is likely, but the alt. is only a year old and there's no AC flicker in the lights when the engine is running. When the last alt. failed, my car was like a mobile strobe light - which was worrying, considering the epilepsy angle...

 

[g8rpi]

Hi,
That connection is for the starter motor and alternator. The heavy cable goes to the starter first and then the alternator (this is based on the Croma, but the basic scheme is the same). To tell if it's the alternator or starter motor, disconnect the cable between the starter motor and the alternator. If the current "leak" goes away it's the alternator, if not it's the starter motor. It sounds like a single diode in the alternator has failed.This would not give flickering lights as the frequency produced by the alternator is too high. The fault you had before may have been with the regulator or brushes. If it's the starter at fault, it's probably a build-up of contamination on the terminals or inside around the brushes or contacts. It may just need a good clean.

HTH,
Robert G8RPI.

 

[Steve148]

Thanks Robert, really helpful. Jacking the old crate up now..


Yep, the fault went away when I disconnected the main lead from the alternator. There must be a circuit to earth from the alternator's pos terminal.

 

[g8rpi]

Hi,
Almost certainly a rectifier failure. Most alternators would need two diodes in the rectifier to fail, but the Denso alternators use zener diodes that may (depending on voltage rating) provide a leakage path with only one diode failed.

Robert.

 

[Steve148]

Most alternators would need two diodes in the rectifier to fail, but the Denso alternators use zener diodes that may (depending on voltage rating) provide a leakage path with only one diode failed..

Interesting, especially as I was going to buy a Denso alternator tomorrow. My local Euro Car Parts has Denso, Lucas or RTX brands, all in the £100-130 range. I don't know who RTX are, so that Lucas part is getting more attractive.

Thanks, you've helped me a lot.