78 924 alternator not charging

[helowrench]

I have a 78 924 that will not sustain itself running. The alternator has been removed and checked good (under load). the battery also was removed and checked ok. however if you start the car you can watch the voltmeter drop as the car sucks the life out of the battery and dies. Is there a hidden fusible link/fuse/relay I should look for? Is there a wire that commonly goes bad and causes this?
thanks in advance.
Rob

[Sabyre]

Voltage reulator could be bad.

[toolboy62]

If the alternator is similar to the VWs I've worked on (and that's a big assumption so check everything I say) there ought to be a blue wire on the back of the alt. that actually passes through the alt. light on the instrument panel; if that blue wire is disconnected your alt. will not charge *and* the light on the IP will never come on, not a good situation. The alt. needs that feed to energize and start charging...

FWIW I believe the regulator is integral to the alt. so I don't think that is the problem - if the alt. checks good on the bench the reg. should be OK.

nate

[bearone2]

if the alt lite bulb in the ip is bad, the alt won't charge the batt.

[Smile]

I've would check for grounding problems or x-wireing somewhere.

[nickd]

Your problem is in the wiring, it's somewhat common. The alt/starter wires run directly beneath the exhaust manifold. The manifold heat leads to increased wire resistance which leads to heat build up in the wires which leads to the wires melting to each other and shorting out/etc.

You'll need to 1st pull the starter wires off and unplug the wiring harness near the battery box. Then pull the alternator out with the wiring attached (keep an eye on how it snakes from the battery box down to the alt/starter).

Then you can either rebuild the harness (not too bad) or buy a new one at the dealer. New harnesses used to be reasonable, I don't know what they're charging for them lately. After replacing, you'll probably find that the starter will spin quicker too.

FYI- the 924 charging system doesn't route the wiring through the dash light like 928's. And if the alt was tested then the regulator is fine (it is internal).

-nick

[bearone2]

before you disect the electrical, does the alt lite come on when the key is turned on?

[Mike B]

Originally posted by Smile
I've would check for grounding problems or x-wireing somewhere.

Good advice Smile...Bad grounds can cause a host of problems and that's where I would look first.

[bearone2]

the grounds/wiring can/could be ok, but if the alt lite doesn't come on when the key is turned on THE BATTERY WON'T CHARGE.

[bearone2]

are you a real helo wrench/where?

[Flormat]

The discharge light in the dash excites the field of the alternator. Unless the 924 has an alternate 12 volt source other than the dash light to excite the field.