Where did you find a new generator made by GM for a Toyota? A new one is going to cost between $400.00 and $600.00. That's why we always find rebuilt units. The only time we ever install new generators, steering gears, and transmissions is when the vehicle is under warranty and the manufacturer is paying the bill.
If this is a Nippendenso generator, there is likely something wrong with the internal voltage regulator. It is responsible for turning off the dash warning light, but be aware a lot of newer ones also turn the light on for under-charging and over-charging. Measure the charging voltage at the battery while the engine is running, and right at the generator's output terminal. You must find between 13.75 and 14.75 volts. That's as far as some mechanics go because that test just takes a few seconds to perform. The next part requires a professional load tester to measure the maximum output current the unit can deliver. In this case it should be very close to 130 amps. If you find the maximum is exactly one third of that, the generator has one failed diode of the six. 45 amps is not enough to run the entire electrical system under all conditions. The battery will have to make up the difference until it slowly runs down over days or weeks. You may squeak by with 45 amps, but AC generators are three-phase units. One failed diode kills one phase. That results in a very high "ripple" voltage which professional load testers measure. The voltage regulator may respond to the dips in output voltage by interpreting that as low charging voltage and by turning on the "Battery" light. The clinker is you will almost always find battery charging voltage to be perfect, between 13.75 and 14.75 volts, or possibly even a little high.
If this is indeed a GM generator with Toyota mounting ears, get rid of that and put on what is supposed to be there. GM had, in my opinion, the world's second best generator design up to the '86 model year. For 1987, they redesigned them and now they have by far, the worst design ever. They develop huge voltage spikes that lead to failure of the diodes and internal voltage regulator, and those spikes interfere with computer sensor signals. Repeat failures are very common.
Be aware too that all generator output circuits are fused in some way, and the fuse devices are sized according to the current rating of the generator that came from the factory. If you had a 100 amp generator before, you aren't going to get more current just because the new one has a larger capacity. The only time you'll get the full rated current is during the full-load current test. That test only lasts a few seconds but that is long enough to blow a bolted-in fuse. For vehicles that use a fuse link wire, that wire will usually hold up long enough for that test to be completed.
Tuesday, June 16th, 2015 AT 7:44 PM