Ammeter X-file - after alternator upgrade

[Stanley]

After the 280zx alternator upgrade with Dave's adapter, the ammeter never seemed to move. I didn't (and still don't) know if it got burned out or if the mod just makes it useless so it doesn't move. Before the upgrade the ammeter needle would jump around if a turn signal was on for example, no jumping after the mod which I thought meant the new internally regulated alternator was working better than the old one.

A few weeks ago it wouldn't turn over or even click. Dead battery. I figured maybe I'd bumped the overhead light on, it's happened before. It sat while I did some work on the AT. Couple of days ago started it with the jumper battery. I figured I'd see the ammeter showing strong charge but the needle didn't move from the middle. I drove around the parking lot for a few minutes to get it warmed up (to check AT fluid level). Shut it off and then tried to start it. I knew I hadn't driven enough to charge the battery, but it did click and try to turn over after the short drive, so I knew the alternator had been charging.

I read somewhere that the ammeter doesn't work after the upgrade; don't know if that's correct. So, main question is what to do about it. The fuel gauge part is OK. The other question is if the gauge got fried could it be draining the battery? And if so is there a way to unhook the ammeter but leave the fuel gauge part connected?  If I need a new gauge then what kind?

Searched the forum and found some threads but electrical is over my head, except the easiest stuff. It's a '73.

 

 

[german240]

I did the same upgrade and my ammeter works fine ('72 240z).

[SteveJ]

If your ammeter was non-functional, it would affect many systems in the car, including making where you had no power to the ignition. A jump start wouldn't solve the problem, either.

For a 73 with the ZX alternator swap, you have to be careful that you don't have the electric fuel pump modification in your car. That modification added some wiring that includes providing power for the fuel pump relay coil. Leaving this wiring modification in place after swapping in the ZX alternator will cause the relay coil to be energized all of the time. That will run down your battery.

Keep in mind that if your electrical system is good, you won't see the ammeter move hardly ever. 

The ammeter was moving with the turn signals before probably because the voltage at idle was low. The voltage dip from the lights lighting up meant that the battery had to prop up the electrical system momentarily. The ammeter measures the flow between the battery and the alternator. All but a couple of electrical components are wired to the alternator side of the ammeter. (I believe the brake lights and hazard lights are two exceptions. They will light up with a blown ammeter.)

[german240]

2 minutes ago, SteveJ said:

 

The ammeter was moving with the turn signals before probably because the voltage at idle was low. The voltage dip from the lights lighting up meant that the battery had to prop up the electrical system momentarily.

I have the same issue with the needle moving while the turn signal is flashing. Since my voltage system is working perfectly fine I guess this is like it is and nothing to be worried about ? Besides that, the needle is always close to middle or slightly right.

[SteveJ]

What is the voltage at the alternator at idle? I have gone to LEDs and electronic flashers for my turn signals, so I don't have enough of a load to move the ammeter.

[Stanley]

Thanks, all. I did remove the power wire to the fuel pump relay per Dave's instructions. But this might make a difference to the ammeter's lack of response (showing it charging) while running the car to charge a dead battery. The mechanic ran a wire from the alternator to the starter to prevent damage to the electrical system and ammeter per Dave's instructions. Dave explained it to me in an email this way:

"The alternator power wire goes to the ammeter (amp gauge) then to the fuse box, where another power wire taps the ammeter wire and goes out to the starter. And from the starter to the battery via the battery cable.  Making a power/charging loop.

But the length of that loop causes heat and resistance. 

Running that new 8 gauge from the alt to the starter, shortens the charging path to the battery but the ammeter and fuse box are still get power. And the ammeter doesn't have all the charging amperage going through it any more because the new 8 gauge redirects the charging amps directly to the battery, via the starter battery cable."

 

Before the swap, if the battery was low and I was driving to charge it up, I could watch the ammeter. When it went back to the middle I knew it was charged and it was OK to park the car.

So maybe I need a voltmeter or idiot light in addition to the stock gauge. Not a lot of places to put gauges in a 240z though. Already have fuel pressure, vacuum and A/F gauges and don't want to mod the dash.