RSuave08

I own a 1988 Nissan 300zx and the car starts and drives great, but when I brake or put electrical load on the car the volt gauge drops to around 9.5 volts. When I press the gas the volts will go back up to 12.5 volts. I heard it could be that the alternator belt needs to be tightened?

86tcoupe

Is it loose ?

zxguy1986

12.5 is weak output from a Z alternator. Why not get battery load-tested at your battery place. They will tell you the alt output. May be time for a new one.

RSuave08

Sorry for the wrong information, it reads 13.5 volts when charging. I do have a multimeter and the battery seems to be fine. I even bought a new car battery and tested it but still the problem was occurring, so I returned the battery. I have a feeling my alternator is malfunctioning and will just pull the alternator out and go get it tested. Is the alternator a pain to remove?

zxguy1986

Sort of a pain from above - stuff in the way. But I've don it a couple of times. If you can use a lift, easier from below.

RSuave08

Okay, yes I will be doing it from underneath. Is there a way I can test the alternator before I pull it out? I heard of pulling a cable off of the battery while the car is running to see if the car dies or not, but that can be bad for the car.

zxguy1986

Sure. Those nice folks who want to sell you a new battery will do a hand-held computer-ized 'load test' of your current battery for free - and part of their test is to gauge your alternator output within a fraction of a volt. They may even tell you that your battery is fine but your alternator is heading to the junkyard if they're honest. Those guys usually don't do alternators or any other repairs themselves - so that IS honest.

Try the big chain auto parts stores in your area. Look for the "free battery check" sign in the window. Takes 5 mins with you in the driver's seat, turning stuff off and on, hitting the gas, idling, stopping, restarting, etc, etc. It's free. So get the guy a cup of coffee on your way there. Then say, thanks, you'll be back later for that part they want to sell you. Or buy it right then.

RSuave08

I went to O'Reilly Auto Parts and he checked it with his machine and said the battery and alternator are good and said it could be a possible short in the system.

zxguy1986

OK as long as he didn't recommend his own repair shop to solve your 'system short' problem. Did he give you any voltage numbers for the alt output? It still sounds to me like your alt/battery combo are not always cooperating like they should. A reading of 9.5v with the brakes lights on is a 'not-charging-the-battery' reading. That should not happen, even at idle. If the voltage regulators are built-in to these alts (that's what they tell us), you need a new alt. May be a bad voltage reg. Assuming your battery is good and charging when it should be. Your call.

RSuave08

Well he has no shop and just simply hooked up some machine to the battery and said my alternator was putting out roughly 14 volts and my battery was good at idle, but could be the voltage regulator, he was saying it could be a diode. I guess the best thing to do would be to take out the alternator and have it tested somewhere or just bring it to a garage.

zxguy1986

Agree. Those alt diodes are likely controlling the charge to your batt. Since replacement alts aren't that expensive and you are ready to do the switch anyway, just change out the alt with a good rebuilt - with warranty. The Bosch if you have the bucks. In general, when a Z31 has problems, the first thing to check and get right is the voltage supply coming from your alt/batt combo. Just about everything else expects it to be right all the time. No down time or part-time performance allowed.

RSuave08

I was looking through the current history of papers on repairs of my car and the last owner put a new alternator in it, this leads me to think maybe it is a weak connection or faulty alternator.

RSuave08

It was a corroded ground wire, finally found it out.

PredatorZ

The ground on these cars can be improved. You can connect the neg battery, ecu and intake / plenum with a ground wire, eliminates ground loops and open grounds in your EFI system, a problem these cars display after 30 odd years of corrosion. This mod is refereed to as a "star ground", will help your car remain reliable, and its really an easy fix.