Alternator: Live-side O/P diode short cct
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URL: http://www.bavarian-board.co.uk/forum_posts.asp?TID=47686
Printed Date: 29-April-2024 at 15:34
Topic: Alternator: Live-side O/P diode short cct
Posted By: jimf671
Subject: Alternator: Live-side O/P diode short cct
Date Posted: 04-April-2011 at 20:52
This occurred on an E36 318i with a 70A Bosch alternator but it applies to the majority of alternators with the typical 9-diode set-up that has been standard for years.
The cheap battery that the previous owner had fitted was replaced a few weeks ago after it died of natural causes in old age. It was replaced fairly promptly so I had hoped that the usual combined occurrences of battery and alternator failure would not happen!
The usual signs of alternator failure through live-side output diode short-circuit (one of several possible failure modes) are low charging output, excessive heat (in alternator), and noisy operation (whining).
In this case, a slight whining noise was heard while driving around town. At this stage I suspected the power steering pump and wondered when the fluid had been changed. No change was seen in operation of the charge warning lamp.
The next morning the battery was flat. And I mean FLAT.
After checking the boot light and stuff like that while the battery was on charge, I took the alternator off and checked it. (Diodes checked using both resistence measurements and the diode test function on my multimeter.) Everything pointed to short-circuit of all three output diodes. All three seemed a bit much.
I got out a circuit diagram and then it became obvious, what had never occurred to me before, that the short-circuited diodes allow the battery to drain through the diodes, the stator coils, the field diodes, the commutator and field coils, and the output transistor.
To check this, I put the alternator across the battery and it drew 3.5A. Not difficult to explain the flat battery when something is drawing 3.5A.
Assymetric commutator wear is usual on Bosch alternators but on this one it was particularly excessive. I tend to suspect that this might be connected to the diode failure so if your brushes and commutator are worn then it would be worth checking the diodes.
The old Lucas fault-finding chart can be found in this document.
http://www.lotus-europa.com/manuals/misc/electrical/17acr.pd f
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JimF
"Nemo me impune lacessit"
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