When I start my car the ammeter stays straight up until the engine warms up, then it starts dancing quite often from +10 to -10. Once I start driving the needle will either stay calm or start bouncing irratically when coasting from +45 to -45. When I turn on the lights or the fan, it seems to calm down a little but still bounces. I notice the headlights and interior lights pulse consistently from normal to dim, while the ammeter bounces to the same beat. I had the alternator (with internal VR) taken out and benchtested. Was fine. I checked all of the wire connections and cleaned everything from the alternator to the ignition. I checked for bad wires, replaced one and didn't see any others.

I have checked through other threads and only come up with replacing the alternator/VR, but I had it tested in the car and then taken out and benchtested, with positive results.

Any other thoughts on what else to check?

have you installed either the diode or arne's voltage regulator adapter? The original 240Z alternators were externally regulated, where the VR was on the right side of the engine bay. If you have an external vr on an internally regulated alternator, you'll have issues.

My alternator is an upgraded 60 amp with internal VR. Yes, I have Arne's adaptor.

Remove the fuse panel and look on the back where the white/red wires attach. I believe one of these wires is attached by a screw. One feeds the fuse panel through the ammeter and the other is attached to the alternator 12V output (side note: the white wire is a direct feed from the fusible link at the starter). Over time, this could come loose and quite possibly result in your problem. If you don't find the problem there then I'd remove one fuse at a time in the panel to see if a particular circuit was the culprit.

Will check it out, thanks.

Stephen;

I am looking for the correct diagram for your car. Do you have the rear window defogger? As I have said before our wiring is different then what you have been directed above. I am comparing wire diagrams as they come into me. You will not find a white red wire in your fuse box. It comes directly from the engine bay. I am trying to get the all the tie ins correct in my mind and then will give you some ideas on where to look.

When I start my car the ammeter stays straight up until the engine warms up, ....... When I turn on the lights or the fan, it seems to calm down a little but still bounces. I notice the headlights and interior lights pulse consistently from normal to dim,.......... I checked all of the wire connections and cleaned everything from the alternator to the ignition. I checked for bad wires, replaced one and didn't see any others.

Any other thoughts on what else to check?

FWIW, it sounds like a heat related stray ground issue. I know you have checked all of the connections, but it still sounds to me like a ground wire somewhere is making GOOD contact when the engine is cold and when heated it is losing it's good connection. That would also hold true when the lights are on as the load would increase so would an alternative ground path. Just thinking out loud trying to help..........

Have you checked the ground lead/contact point from the battery to the block?

Just another $.02 worth and good luck to finding the problem.

The white lead is shown in the FSM wiring diagram. It runs from the external voltage regulator connector to the ammeter and the fuse block. The quick and dirty wiring diagram at Autozone does not show this (or a lot of other details). Whether it is there or not I can only tell after I've looked at my 11/70 240Z. I will look when I get home.

Jim - no, I do not have a rear defogger (clear hatch glass)

Rxsleeper - the battery ground has been cleaned and firmly connected.

Thanks, everyone, for your responses and please keep 'em coming!!

Rxsleeper - the battery ground has been cleaned and firmly connected.

Yes, but what about the other end of the ground circuit. How clean and solid is the ground cable at the starter bolt, for example. Also double check th eground at the alternator itself.

Arne, will doublecheck but I have wire brushed and spray cleaned the connections to the grounds. Will see if I got them all.

im not sure how similar the 240z wiring is to the 280z, but mine has the gauge itself grounded. you might want to check the grounds coming out of the dash.