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Alternator puzzle - comments?
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Posted by peter charak (more from peter charak) on Fri, 15 Apr 2011 22:23:31 Share Post by Email
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This is a new to me 92 900S classic convertible 16-valve 2.1L non-turbo. Haven't had a 900 for some years, and those were 2.0L turbos.

Noticed slow cranking. Charged battery and started monitoring voltage at cig lighter socket while driving.

At morning start-up, DMM display shows about 12.5v for about a half hour, then drops to 11.8v or so, by which time I'm close to work. After work, the 11.5v will get me started, but after that it's a lottery. Occasionally will rise to 12.5v volt again, perhaps even 13.5v for a short time, but this is rare.

If voltage doesn't rise while driving home, 13.5A LEAVES the battery continually, (clamp-on meter), voltage keeps dropping and eventually engine stops around 7.5v. I keep spare battery, cables - even a charger - in the trunk, so I can always get home.

Swapping in a known good regulator produced no change.

I then belatedly noticed that among the cluster of idiot lights "batt" light was missing with ignition on (or while driving).

So, some checks. Battery negative to alternator case, 350Ω! I had expected about zero. Some bushings may be non-conductive, but at least the double belt tightener (I assumed) is electrically part of the block. I can't see the bottom of it, but the top is visibly connected to the alternator case. Bentley shows that the case goes to ground at G32, but neither location nor method is stated. So, why the resistance?

Now, another check. With engine running, I see 6v between alternator case and battery negative. Puzzled, I jumpered the case to ground, and got a spark and lowered rpm. Alternator was being loaded! Voltage immediately rose to 14.2v, and current INTO the battery became 12A. Charging! "Batt" light now comes on with ignition, goes off when engine starts, and everything appears normal.

However, the +6v on the case can't be right, some sort of internal failure - or am I missing something. Nevertheless, sinking it to ground makes all the numbers appear OK - except for the hefty 20A flow through the jumper from alternator case to ground. So, what's going on? Is there a reason to fix this further aside from saving fuel by not heating the jumper?

I do have a couple of spare alternators, but I'd rather not spend the time on a swap at this time.


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