Power drain on 87 ZX analog

Voltmeters don't go negative… is it dropping to zero? Or is it dropping below 12-14 volts? Could be a bad gauge-- you can validate with a voltmeter on the battery for the same reading. If not, I'd do the normal charging system checks on the battery and alternator. For the alternator, have autozone/advance/napa/oreilly check the diodes to ensure you haven't had one go out.

Get the alternator checkedor get a digital voltmeter and see if the alternator s putting out 14+ volts to the battery. In order to go below 12V, the battery is being drained, and the alternator is not keeping it charged up.

Probably the Alternator is bad, I had that happen 2x in my 35+ years with my 84'

I've already checked the alternator with Autozone. Its Good. The voltmeter DOES run at 14 volts for a while and then it goes negative. When I put on the brakes, the light icon on the dash lights up and sometimes the motor speeds up. Light bulbs are all OK with no corrosion. Grounds have been checked and some were cleaned.
Ive also pulled the fuses one at a time in an effort to isolate the issue - to no avail. Anybody have an other ideas ? Does the ZX have a voltage regulator and if so. where is it.

Does this happen with the engine being heat soaked ? the alternator maybe on it last … It will work cold , but once hot output drops off … the regulator is in the alt. …
try testing the output when you have the problem , go for a drive and carry tools to test it right then and there,.

The moment it goes negative, the alternator should be putting out 14 volts. I don't care what Autozone says, it is a bad alternator. If you had a short to make a battery go negative, you would have a fire somewhere. either a bad alternator or bad voltmeter

I agree with all the other folks here. In the past, I had my alternator take a crap while driving my 88, and the instrument cluster (analog) and other electrical functions went berserk. Fresh alternator fixed my issue.

Look at this logically.

1 A good battery should be at least 12+ volts with the car not running
2. A good alternator should put out 14+ volts to the battery while running.
3. A good voltmeter should be able to read these voltages.

Either the voltmeter in the car is bad (use a separate voltmeter to rule that either in or out), or the alternator is not putting out the 14V and your car is running off the battery. Autozone checked the alternator while cold and you said it happens while warmed up. Also check the power cables, both ends of both of them to see if they are corroded.

Over the time I have been on these forums, I have heard numerous stories of a checked out good alternators really being bad.

Also, in a previous post from you last march, you said when the fuse for the voltmeter was in, all the idiot lights came on, remove it and they go off. Hmmm… See a possible connection here? If you use an external voltmeter, and you see less than 12 V at the battery while the car is running, THE ALTERNATOR HAS TO BE BAD or bad wiring/connections. That could cause all the idiot lights to do weird things.