Mechanics

ALTERNATOR

1992 Toyota Corolla • 4 cylinder FWD Automatic • 220,000 miles

Original owner: 92 Corolla LE (4AFE) burns out alternator every 3 months or so. Are there relays associated with the alternator which may have failed? My fuses are OK and wiring is too. The alternator is fine w/o load, but falls out of range when under load, even if it's only the fan (with or w/o A/C). I charge the battery and 2 or 3 days later if falls to 11.9 or so, requiring a light jump.
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Ol Scool Hachi
July 2, 2011.




Get a voltmeter and measure the battery voltage not running and running and comeback with both readings-Drops to 11.9 could be a battery drainage due to a short somehwere or something is still on.

Rasmataz
Jul 2, 2011.
12.5 off, 12.8 @ idle. 11.6 to 12.1 when headlights and A/C are on. My Black&Decker Smart charger suggests alternator is bad. Within 5 seconds of putting the charger on the car, the voltage rises past 12 & the car starts. This will go on for 3 days then falls below 12 volts due to lack of charging by alternator. I've replaced the alt. Three times within the last 14 months. Fortunately, it has a lifetime warranty.
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Tiny
Ol Scool Hachi
Jul 2, 2011.
For years, I used to have two stereo amplifiers in the car with a 1/2 farad capacitor but removed them in an effort to ease the load on the electrical system, but the problem still persists. It began about 1.5 to 2 years ago. Right after changing the alternator all is fine, but problems start about two months post repair.

Tiny
Ol Scool Hachi
Jul 2, 2011.
I want you to load test the battery for 10secs and see if it will hang on with a 9.6 volts or higher within that time frame unable to replace the battery-also have the alternator load tested too. The voltages you have is way too low-pointing to a bad alternator-recheck wires going to and coming from the alternator and ignition switch

Rasmataz
Jul 2, 2011.
The battery load test was fine, the alternator failed. I'm about to replace the alternator, however, my issue is with the ongoing alternator failures. Could there be another factor which would cause the alternator to fail prematurely, such as a bad relay? The wiring appears fine with no visible damage/charring/corrosion. The car runs fine for a few months post repairs, then the alternator fails & the problems arise again. The 100 amp fuse next to the battery terminal & the charge fuse in the main fuse box also good.

Tiny
Ol Scool Hachi
Jul 2, 2011.
Hi guys. Let me offer a suggestion. This is a REAL common problem with GM vehicles and might be what's happening here. Many voltage regulators are a "switch-mode" design similar to what's used in a vcr. The switching transistor is either fully turned on or fully turned off and it switches rapidly between those two states, just like a digital computer signal. If you calculate Ohm' Law, power (heat) equal volts times amps. Depending on whether the transistor is on or off, there is either zero current through it or zero volts across it, so there is always zero watts dissipated, and they can use a tiny, inexpensive transistor to do the job. It should never get hot.

Because that current is abruptly switched off, the field coil in the generator develops a huge voltage spike just like the ignition coil primary winding. The battery absorbs and dampens those spikes. As the battery ages, its "internal resistance" goes up due to the lead flaking off the plates. When that gets bad enough, it can't supply enough current to crank the engine. Before it gets that bad, it loses its ability to absorb those spikes. The voltage spikes can interfere with sensor signals for the various computers, but they can also destroy diodes inside the generator and the voltage regulator. From your description, it sounds like you have a bad diode now. The easiest way to prove that is to have a load test performed with a tester that measures voltage ripple. AC generators put out a three-phase output. You'll lose one of those phases with one bad diode, ( that causes the really high ripple), and the unit will only be able to supply exactly one third of its rated capacity. That means you will only get about 30 amps from a 90 amp generator.

It's real common to go through four to six generators in the life of a GM vehicle. Replacing the perfectly good battery at the same time as the generator takes care of most of the repeat failures. In the case of the GMs, the old battery will work fine in a 1986 or older model that doesn't have the current design generator.

I don't know of a test that identifies a tired battery. If you have the opportunity, you might do a load test on your new generator and see how much ripple it is producing, then check it again after replacing the battery. If it's much lower with the new battery, I suspect the new generator will last a lot longer.

I'm not aware of this being a common problem on any car other than GM products, but your description of the symptoms and voltages fits exactly. Most other manufacturers also use switch-mode voltage regulators and they don't have trouble.

Caradiodoc
Jul 3, 2011.

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