ammeter jumping around or at +45....

[kenmataya]

found some much older threads about this, but thought i would check for some fresh advice. My 70 240Z ammeter is either jumping around between 0 and 45 or its pretty much pegged out. when its pegged out my interior lighting is nice and bright, but the headlights don't seem any different. and it always runs the same. I've just replaced the voltage regulator with a unit from the z-store a few months ago, because i was getting 20V when i was checking the blinker wiring. Now the battery isn't holding a charge ( i'll take it down to be tested tomorrow ) - could that be causing my problems? i checked the voltage at the alternator and it was about 12.8V at idle.

thanks,
Ken

[SteveJ]

Let's go over some of the details.
1. How old is your battery?
2. Do you know how to use an ohmmeter? If so, have you checked for continuity between the battery negative and chassis and between the alternator E terminal and the chassis and between the battery negative and alternator E terminal?
3. Use the 72 FSM and look through the EE section. Test output according to the FSM and tell us what you get.
4. How old is the alternator? It could be on its last legs.
5. Do you know how to use an ammeter? If so, is the battery discharging when the car is off? What is the current? (Make sure all lights are off, including the dome light and glove box light.)

[Gary in NJ]

found some much older threads about this, but thought i would check for some fresh advice.

I find this funny...looking for fresh advice on a 40 year old car...

Anyway, it sounds like a dead short somewhere in the electrical system, possibly in the ammeter itself. I'd get multimeter on that electrical system before driving it again because if its really producing 45A @ 20VDC you are soon to experience exploding battery syndrome.

[kenmataya]

OK, i'll try to answer these questions:
1. How old is your battery?
unknown, looks a bit weathered - i've had the car since May. took the battery to Pep boys and they said it was bad. I've got a new bosch battery to put in.

2. Do you know how to use an ohmmeter? If so, have you checked for continuity between the battery negative and chassis and between the alternator E terminal and the chassis and between the battery negative and alternator E terminal?
I'll check that, but i'm assuming its OK, the car was running fine for 10 months and i drove it 1500 miles when i bought it.

3. Use the 72 FSM and look through the EE section. Test output according to the FSM and tell us what you get.
I'll do that

4. How old is the alternator? It could be on its last legs.
also unknown, i'll have voltage info once i get the new battery installed.

5. Do you know how to use an ammeter? If so, is the battery discharging when the car is off? What is the current? (Make sure all lights are off, including the dome light and glove box light.)
actually i just replaced the glovebox light which was burned out. i have a dash cap, and fit of the glove box is tight so i have to make sure the door closes. i'll take the bulb out till i get this straightened out.


so, my question is if the battery is bad would that give me the strange ammeter readings? or if the ammeter is correct, would that fry the battery? (as i said, the voltage reg was replaced just a few months ago)

[SteveJ]

so, my question is if the battery is bad would that give me the strange ammeter readings? or if the ammeter is correct, would that fry the battery? (as i said, the voltage reg was replaced just a few months ago)

Fix/replace known bad components. Then check your results again.

Edit: I forgot to add this. Pull the alternator and have it tested. If it's bad, too, you could ruin your new battery.

[FastWoman]

If your old voltage regulator was trying to charge the battery at 20V, it's entirely reasonable that the old battery is toast. As long as your running system voltage doesn't get above maybe 14.5 V, when revving the engine slightly, I think you should be OK to drop in the new battery.

[kenmataya]

OK,

had to deal with some personal business, but finally got the battery installed (bosch). ammeter now reading dead center, voltage at the alternator and battery around 12.5V - is that too low? ( i believe the cause of the first battery dying was replacing glove box light - the dash cap tends to push out the glove box door, not depressing the switch, leaving the light on )

[SteveJ]

OK,

had to deal with some personal business, but finally got the battery installed (bosch). ammeter now reading dead center,

With the car running, I would expect it to be a little on the positive side

voltage at the alternator and battery around 12.5V - is that too low?

At what RPM? Look a the FSM. It has your answers in a chart.

( i believe the cause of the first battery dying was replacing glove box light - the dash cap tends to push out the glove box door, not depressing the switch, leaving the light on )

Been there. Done that.