Happily my Prii are immune from alternator problems, but I thought the alternator on my 1996 Subaru might be reaching end of life when another driver described headlight dimming while driving up a steep incline at night. I pulled out my handy digital multimeter that I barely know how to use and found the following: 12.4 volts across the 12v battery when the car is off; 13.9 volts when the car is idling without electrical load; 13.2 - 14.0 volts with electrical load and ICE idling. The drop to 13.2 volts occurred when the alternator would start to spin, and would go up to 14 volts after some 2 - 4 seconds. The alternator would cycle on and off every 30 seconds or so when the load included headlights, radio, and heater on high. The multimeter negative lead was put on the 12v negative terminal. If I just touched a bolt on the car the meter would read less than 1 volt. After I ran the electrical load for a few minutes the alternator was uncomfortably warm but not Ouch hot. Is the alternator bad ? The load readings seem not quite right. Thanks for the help!
If I am interpreting your post correctly you either have an alternator belt slipping or a bad alternator. The alternator should be spinning any time the engine is spinning and should be a fairly constant 14V. It should not be cycling on and off.
Perhaps take the belt off and see if the alternator bearings are loose. Also check the belt tensioner as that idler can go bad too. These might cause problems when warmed up. Check the other belt accessories too as one of them might be on the verge of failing. Bob Wilson
Thanks for the help thus far ... I checked the car again, and now realize that it is not the alternator that starts and stops, but a belt fed device next to the alternator. A picture is below, showing the part. I put a white dab of paint on it for ID, and for a later video. Then I turned on the car, turned on a large electrical load, and took a video while monitoring the voltage at the battery. The video shows that other part cycling, and the change in battery voltage. So kind people -- bad alternator ?
By this site page it seems the alternator is good. How to Troubleshoot a Subaru Outback Alternator | eHow I know it dropped below the 13.5volt limit, but I suspect you don't want non-essential loads running the test. The thing turning on and off is the AC compressor. Try running the test again with the AC and defroster off to make sure the voltage drop is linked to it going on. Was it just the headlights that dimmed or did the dash lights as well. Could the compressor have also come on at the time? When were the belts changed? One search result stated the recommended change interval was 60k miles. Assuming the voltage drop is linked to the compressor coming on, my guess is that it is the belt and the alternator is fine. When the AC engages it puts a sudden load onto the crank pulley driving all those accessories. Perhaps that is causing the belt to slip. Or it is completely normal behavior. The clutch for the AC is electrical, and it engaging might be the reason for voltage drop. Or the compressor is going. I noted it didn't start smoothly and skipped a little at first. A friend had to turn off the AC or lose power going up hill with the family's '80s Subaru. Was the hill steep enough for the car to struggle without downshifting?
