1995 Toyota Tercel Charging Issue

HARRINGTON73

MEMBER
1995 TOYOTA TERCEL
4 CYL
2WD
MANUAL
166,000 MILES

Hi. I have read in your archive of questions that alternators cannot run at full load for a very long time, so I want to describe this issue I'm having to see if its normal or not. I replaced my alternator with a high-quality rebuilt unit about 6000 miles ago for the same problem I'm about to describe. I use my car for work and this involves a lot of idling while using most of the accessories at times. When it's raining, I might have the defoggers, headlights, hazard lights, wipers, blower motor, and inevitably the cooling fan operating at idle for a long period of time. I've noticed that after the car has been idling for an hour or so, the engine really slows down in rhythm with the various loads--you can hear the cooling fan really struggle when it cycles. Then the alternator light starts to gradually come on. I checked the battery and alternator with no accessories turned on. The voltage across the terminals was 12.98 with the engine off, and 14.14 at idle. However, I checked the voltage again with the charging system loaded as I've described and the voltage gradually dropped to around 10 at times. Could I have a defective alternator or am I simply asking it to do something it was not designed to do? Any information would be helpful. Thanks.

1 Reply

CARADIODOC

MECHANIC
33,742 POSTS

Hi harrington73. Welcome to the forum. 14.14 volts is right in the middle of the acceptable range. How long did it take to reach 10 volts? If that was after it had been idling for quite a while, I suspect the battery just ran down. If that happened as soon as you turned on all of the loads, suspect a slipping drive belt or a defective diode inside the generator. If the belt is slipping, you will likely have poor power steering too.

The industry standard is to load test the charging system at 2000 rpm. At idle, the generator will not be able to produce the maximum current it is capable of. You'll be lucky to get half. When one of the six diodes is defective, the most output current you will get will be very near one third of the unit's rated output. 35 amps from a 100 amp generator will not keep up with all of the things you mentioned.

The generator should be tested on the vehicle. Most electronic testers will measure maximum output current under load, maximum output voltage under no load, and ripple. A defective diode will cause excessive ripple and wreak havoc on the numerous computers.

The next time you find only ten volts across the battery, try raising engine speed and see what happens to the voltage. Find out too how much current the generator can supply when load tested.

Caradiodoc