If I disconnect the battery while the car is running and the car dies, is it safe to assume the alternator is bad?



(BTW, this is for my roommate's '88 Prelude).

 

I say yes, but Sears would argue with me.
~Tom

 

that test is not allways good but it works most of the time. Have the unit tested at a shop or parts store.

 

I've seen someone fry the computer by disconnecting the battery to test the altenator. I wouldn't do it. If the car is drivable get someone with the proper machine to test it. The test will tell the output and amperage of the altenator. If the car will not run just remove it and have it tested at some local parts store. Around here they test them for free.

 

Jonski:

This was a good test when cars did not have computers on board. If the car has an ecu (probably has if 88), you run the risk of frying the computer.

The easiest thing is to take the alt to a place like autozone and have it tested under load (this assumes the counter person knows how to use the equipment). I think they can test with alt on the car if you can bring it there.

You can easily test the alt yourself if you have a voltmeter. Let car idle. Look at voltage (at battery posts), should be at least 14V. Turn on headlights, ventilation, the voltage should drop a bit. Rev up the engine, the voltage should go back to around 13.5-14. You get the picture.

Note that if alt is not working right, the battery charging light should be on since the alt is not charging the battery due to voltage below ~13.5. This means all electrical stuff is running off the battery, meaning it will drain in no time and will leave you stranded on the side of the road.

Hope this helps,
Hugues -

 

We took the car to autozone and we were told that the alternator was fine, so I guess the real test will be time.