Charging System Problem

[FullaBullz]

I have tried everything I can think of but can't fix a charging system problem my car is having. Please help...

PROBLEM: Alternator doesn't charge until around 2000rpm when the car is first started but after the car hits the 2000rpm mark the voltage goes up and alternator charges fine until the ignition is turned off. Also it seems like there isn't enough power when car is just idling after car has hit the 2000rpm mark. The battery and alternator have both been tested are are "good."

CAR: 1993 300zx TT running stock ecu with no electrical mods / alterations to OEM wiring

WHAT I'VE DONE: 1) new / rebuilt TT alternator - no change
2) new optima red battery - no change
3) new battery cables - no change

Is there another voltage regulator other than the one built into the alternator? If so where is it?

Thank you for your help!!!

[marty1mc]

New battery cables really help your starter the most. The Alternator harness may be an issue, though. What voltages are you reading at idle with no accessories, then at 1500, then at 2k? Next, what voltages are you reading with accessories on at those speeds?

ETA: Also, have you checked the engine ground as well? (The one on the top of the transmission passenger side just behind the motor). You should have removed and re-installed it when you put in the new battery cable. For your test with the accessories on, turn on your lights, heater fan and rear window defog.

[FullaBullz]

The voltage is constant with the battery at up to 2000rpm around 12.3v. Once the car hits the 2k mark the voltage goes up to around 14-15 volts depending on rpm and stays at 14ish volts until the ignition is cycled. The ground is tightly held in place bye the 14mm bolt thats long enough to hold on the front bumper! I would understand if it were a constant problem all the time but the charging system works fine after the first time the motor hit the 2k rpm mark. Oh and the local Nissan dealer knows less about the car than I do, when he asked me if it had two turbos or one I took my keys back!

PS - You have a good looking Z!

[marty1mc]

Thanks for the compliment and LOL about the dealer. OK, do you have an aftermarket or an OEM alternator? I will say aftermarket ones from Advance or Autozone are notorious for issues. I would put a used OEM on before a reman'd Adv or AZ one.

To me, it sounds like your alternator output is way low. The alternator usually doesn't put out decent current (and therefore voltage) until after 1k, but 2k is horrible. Have you checked all of the connections from the Alt harness to the battery? Corrosion could also help degrade this. (but I really suspect the alternator). Maybe take it to a rebuild shop, have them do an output map to see if it is really producing anything below 2k.