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need help please

15 May 2000
I just replaced my battery in my 94 auto. Now when I put it in park it sometimes suddenly cuts off. And sometimes it happens when its in park and I roll up both windows. Has anyone had this problem? I see the battery meter go down a bit then it cuts off. Its a brand new battery. I have no idea whats going on with the car.
Thanks for the help in advance.

It sounds to me like you are getting a voltage drop in the system. First thing I would do is recheck the battery terminals for tightness, and also look carefully for cranks in the terminals. Is the car starting OK, every time?

If it always starts OK, then the other possibility is your alternator is really not working correctly. Could be as simple as a loose belt. Why did you replace the battery? Has the alternator been checked out to see if it is OK?

I ask because what really seems to be happening is you are starving the car for voltage. When you put the car in park, you most likely have your foot on the brake, so the brake lights are on, drawing power. When you raise the windows, same thing. The brake lights and the window motors demand alot of power, so if your alternator is not 100%, you can dip below the proper voltage and all sorts of things may get strange.

To confirm this, you can try a few other accessories to see if you get the same result, headlights, heater blower, etc. I suspect any big electrical demand will produce the same results. If so, get a voltmeter on the battery terminals, read it with the car off, start the car, then read it again, should be about 1.5V HIGHER to properly charge the battery. Then activate the brake lights, voltage drop?? If so, check alternator belt/alternator for proper output.

Any good auto electrical shop can test it fully if you get to the point you think the alternator is the problem. They can bench test it for full output. The alternator, as the name implies outputs alternating current, but you really need direct current to supply the car with. Inside the alternator is a set of diodes, known as the "rectifier", it converts AC to DC. If a diode goes bad in this rectifier, you will not get proper output from the alternator to the car. (this is typically what will go bad in an alternator electrically, or they become mechanically bad, if a bearing and/or slip ring fails.

With all this said, please do not assume the alternator is bad, it could just be a loose belt.

There are 2 surefire ways to check if your alternator belt is loose.

1. With the motor off, try to turn the alternator pulley by hand. If you can turn it, it's loose.


2. With the motor idling, check the voltage at the battery. It should be above 12.6V, ideally up to 14V. If it is on the low side, have someone gun the motor and watch the voltmeter. If it jumps up to 14V, the belt is loose and the alternator needs the extra speed to make up for the slippage.

If this is the case, your alternator would loose efficiency and would not be charging the battery completely. When you apply more load with brake lights, system voltage falls too low and like Larry Says, Strange things can happen.

Another way to see if one of your diodes are bad is to look at the alternator dash light indicator carefully in a dark garage or at night. Sometimes when only one diode has failed in the diode trio that feeds the rotor, instead of being out, the indicator will flicker dimly.

Another thing to consider is your driving habits just prior to this ocurring. If you have spent the last 20 minutes in stop and go traffic with the headlights, blower, rear defogger all on and AC/DC cranking on the CD player all while your foot is on the brake while the auto trans is in drive, the alternator isn't going to keep up. The battery will be the current source as long as it can, but if this kind of deep cycling occurs on a regular basis, the battery life will be shortened.
Thanks for the help guys. I am gonna check the alternator tonight and see what comes up. If that does not cure the problem then I guess its a trip to the dealer. thanks again,
Originally posted by nsxboy7:
Thanks for the help guys. I am gonna check the alternator tonight and see what comes up. If that does not cure the problem then I guess its a trip to the dealer. thanks again,

Make sure you take the car for at least a ten minute high speed (over 43) drive so the computer relearns its base settings from the prom back to the control side of the ecu. When you disconnected the battery it went brain dead and needs to be woken up.

Also, your alternator voltage should be 13.7, not 12.6 and it will always go up a little when you rev it so this is not a good test for an alternator problem. The difference between low and high numbers will be exagerrated if the belt is loose, as Kinnan suggested.

Good Luck,
Mark Basch