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The alternator may not be failed Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Wed, 9 Jun 2004 09:43:43
In Reply to: 86 5 speed turbo keeps burning up alternators?, Walter, Tue, 8 Jun 2004 16:52:55 Members do not see ads below this line. - Help Keep This Site Online - Signup |
Just to be clear - the alternator is just an armature with wire coils, rotating inside a case with more wire coils. There is a diode block, but that's it. It so happens that the voltage regulator is bolted on the back, and it holds the brushes.
I'm getting picky about this, because alternators rarely fail. Yeah, the bearings might die, and I've had a diode blow out once. But the usual reason for a alternator not to charge is either the voltage regulator, or the wiring to it, or just a loose belt.
It would be nice to know if the alternator itself is actually failed (blown diode block), or the voltage regulator, or something entirely different. Since you've replaced a bunch of alternators, I'm guessin that either there is a problem outside the alternator, or an installation problem. Since you've got an '86, my first question is - Are you tensioning the serpentine belt properly? If the belt is loose or in poor condition, there could be slipping of the belt/pulley when the alternator goes under electrical load. Once it starts slipping, the voltage out would drop.
Removing the battery cable isn't a great test. With the engine not running, there is no current flow from battery to alternator. The only current would be to drive the dome light. So I wouldn't expect much of a spark, if any.
First, I'd make sure that you've tensioned the belt properly. It may go on just fine, but loosen up quickly, causing the apparent 'alternator failure'. You drop in a new one, and it works for a while until the tension goes out. Before doing anything else, make sure the belt is still properly tensioned, that the belt is in good shape, and shows no signs of slipping. Look for glazing on the contact side of the belt (bad). When pressing on the center of the belt (engine off!!) between the alternator and tensioning pulley, the belt should deflect about 1/4 inch with 'moderate' force. I don't know what that means exactly (manual words), but the belt should feel pretty darn taut.
Next, make sure that the thin wire going to the back of the alternator (usually green or yellow) is in good shape and not shorting on anything. That's the wire from the BATT light. On older cars, the insulation on that wire usually fails, or at least looks pretty cruddy. If that wire shorts to ground, you'll get both a BATT light, AND the alternator won't work. The voltage you'll see is the battery voltage, which would be in the 11 volt range under load (i.e. powering the car). The purpose of the battery is to start the car - once started, it's dead weight. The alternator should supply all the power for the car, and recharge the battery.
Make sure the alternator is properly grounded. When running, measure the voltage on the alternator case as compared to a good chassis ground. It should be in the few millivolt range. If it's anything over 100 millivolts, fix the ground problem.
It is possible that your electrical system is pulling an unusually high current, but doubtful. Not unless (1) one thing is shorted, and the fuse refuses to blow or (2) a bunch of things start pulling lots of current. However, if that's true, the battery voltage would sag.
Also measure the battery ground post to chassis ground. That should be no more than a few tens of millivolts.
posted by 192.249....
, Wed, 9 Jun 2004 09:43:43 <-- Viewing This Message
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