Alternator or Computer

i have a 1994 pontiac grand se quad 4 engine.

the problem that i am having is this…

i had recently gone out to the store came out and my car would not start. i got a jump start and went to auto part store they tested the battery and determined bad. got a new battery. the next very day i have the same issue… i have no way of getting it to the mechanic b/c after a jump start and let the car run for a while the car just dies? when the car is in park the speeedometer jumps to like 100 but the car is in park…

i had someone come to my home and look at it. they tell me that i dont seem to be the alternator but a computer problem but they cant tell for sure. so i just need some help and though that maybe this site could help me pleaase

I suspect the alternator is bad. Have it checked out or you might try just swapping it out. I suspect also the speedometer issue is due to the alternator.

are they able to test the alternator if its off the car? b/c i was told that they cannot do a real test if its off the car.
b/c like i states the car will not stay running long enough to get it to a auto part store b/c the nearest one is about 15-20 mins away

This is pretty straightforwards to test.
A fully charged 12V battery should show 13.6V across its terminals. You can check this with a digital voltmeter. To charge this, the alternator has to be able to get the voltage up to at least 14.5V across the battery terminals with the engine running. So check this again with the engine running. Lastly, if you really want to know if the alternator is good repeat the measurement with the engine idling and the headlights turned on. The voltage should still show above 13.6V at idle.

That is not going to distinguish between the alternator and the voltage controller that usually sits on the end of the alternator.

The above test is a good quick first step for testing an alternator, but it must be accompanied with a load test to verify the alternator can product it’s rated current output.

are the digital voltmeters expenseve

I don’t see how it can be the computer when you can jump start it. The computer is either going to work ALL the time or NONE of the time.

You can get a decent digital meter for $20.

thanks i really appreciate all the help you guys have given

For the record - that is not entirely true. I recently had to replace the ECU in my Matrix because every couple of days it would crash and the car would stall while driving. I would turn the car off and restart [kind of like rebooting your PC] and it would be fine for a couple of days. ECU’s can be intermittent.

However, this situation does sound more like an alternator problem than a computer problem.

ECU’s can be intermittent.

That is NOT the problem the OP is having. It was able to start AFTER the battery was jumped. That will have NOTHING to do with the ECU.

If you only do the digital voltmeter test (per above), you really won’t be able to draw any conclusions on whether your alternator is good or not.

I suppose you do a poor-man’s load test, which is to turn on all the car’s accessories (a/c, blower motor, high beam headlamps, interior door lights, radio, trunk light, brake light,…). Rev the engine a bit to about 2000 rpm. If the voltage across the battery terminals is still in the 14.5V range, you know that your alternator is at least partially working. This shouldn’t be considered a conclusive test, but it will give you an indication of your alternator’s ability to put out “some” current.

In case not everyone realizes it, there was one small typo in dim160’s post.

A fully-charged lead/acid battery should read 12.6 volts, not 13.6 volts. From our high school chemistry book - the potential for a lead/acid cell is 2.1V. Times 6 cells = 12.6V. Sometimes you will see just a bit more “surface charge” right after charging, but if you load it for a few seconds by turning on an accessory, it will normalize to 12.6 volts under no-load fully-charged condition.

Also, in JoeMario’s post below, he suggests that at 2000 RPM, the voltage at the terminals should be around 14.5 volts. Some voltage regulators are set a bit lower than that. Without knowing the spec for this car, if I saw 13.8V or better at 2000 rpm with loads on the system, I would assume that it is OK.

i just wanted to say thanks to everyone for the help on my question!

i actually got the car fixed today and it was my alternator… now the car runs sooo good…

thanks again everyone