Hello,

I'm new to this forum and Z's. I have a friend (mother of a friend really) that asked me to look at her 1988 300ZX to diagnose the battery going dead. She's an older lady and this was her late husband's car. He passed away and she wants the car gone, but wants the battery issue fixed first. From the receipts I've looked through, this has been going on for 6-7 years. Once the car sits for about 3 days without a charger, the battery is dead. I have confirmed this. I brought it home three days ago, and it is now dead. Battery was at 12.2V after a 45 minute drive to get it here. The next day it was at 12.1V. I forgot to check it the following day, but today it's at 12V and the car will no longer start. The battery has been replaced several times, most recently in July 2021. Shops told her previously the alternator is the cause (it's not) and they replaced it. They told her another time that she left a light on and that drained it. Obviously still not the issue.

Are there common issues that I can start with on this car? Any known weak points where a large enough electrical draw will drain the battery? There are many things that can drain a battery, so I'm hoping someone knows a common issue in these cars. I have confirmed that there were no lights on anywhere in this car while it has been sitting in my garage. Nothing obvious is draining power from being left on. I've never messed with these cars before, as I usually have my hands deep in old Fords or Chevys.

Thanks!

 

Check the voltage at the positive terminal of the battery. While the car is running you should be getting at least 13.5+VDC. If you are not getting that at that positive terminal, the alternator has to be bad since you can't charge a 12 V battery with only a 12 V charge.

 

Please correct me if I am wrong, but a bad alternator won't drain a battery while the car is off. It just won't charge it while it drives, and the car will die. Once jumped the car can run nonstop, and still start for a few days, if the battery charged enough during the drive. That is at least my understanding and experience from working on cars for 20+ years.

I just charged it overnight so I can have a starting point for the battery drain. It's at 13.4V as of this morning. I'll keep checking it with the car/all accessories/lights off for the next couple days.

The alternator has already been replaced as that's what she was told the issue was before. It didn't fix anything.

 

This has been going on for the past 6-7 years? What battery brand is installed in the car? Is the battery the correct group no.? I would avoid AAA batteries because in my experience they didn’t last long. Even so, a car battery shouldn’t be completely drained after three days of non-use of the car.



First, check for easy to spot battery drain causes. It sounds like you’ve already done it. Park the car in a dark garage, turn off the ignition, and wait for the dashboard security lamp to turn off and not blink. Check to see if any of the footwell lights, map reading lamps, dome lights, etc. and exterior lights are lit.



I don’t think the 1988 N.A. model has a fuel injector fan so that can’t be it. For models that do, it can run down an already weak battery because the fan can stay on for 15-20 min. after the ignition is turned off.


If it isn’t one of the above, you’re going to have to use an ammeter and pull fuses to track down where the current draw is coming from. The dashboard clock and security system draw current after the ignition is shut off but shouldn’t drain a battery completely after 3 days. Do a forum search on electrical problems and ammeters for instructions on how to track down electrical problems. Also, You Tube might have one.

Also, lots of short trips don't allow the alternator to charge up the battery fully.

In my case, it was once a map reading lamp that was left on. It’s easy to forget to turn off a map reading lamp. Another time my brake lights wouldn’t turn off because the brake lamp switch was defective.