1972 datsun 240z ammeter not working. Easy fix or should i get a used one?

[jalexquijano]

Today i fried one 15 amp fuse which is plugged in a fuse holder of the parking light harness built by dave irwin and installed in my engine. It came out melted because i removed a 15 amp i had plugged on the instrument fuse holder at the fuse box. I believe this damaged the ammeter of the car. I checked all connections and did not find anything broken. My question is: could the short damage the ammeter? My fuel gauge does work. Can this be fixed and how?

[siteunseen]

Sounds like an alternator problem to me.  Fuses pop before causing damage up stream therefore you should always use the recommended fuse.  Never go higher.

I had a bad alternator that burnt my fuse, none of gauges worked.  The internal voltage regulator went bad.  Have you upgraded the alternator?  I think after these years of using mr irwins upgrade without issue you can rule that out.

[jalexquijano]

My alternator was upgraded to 60 amp with internal voltage regulator. I dont think its damaged. Car cranks fine. Can the ammeter be fried? If so, can it be fixed?

[Zed Head]

In the picture your ammeter shows a big current draw.  Over 45 amps.  This could be because your alternator is not charging, like site said. 

Disconnect your battery.  The ammeter should go back to the center point.  If it does it means that you have a very big current draw when the battery is connected.

[siteunseen]

I'm not positive on this but when you put a volt meter on the batteries post it should read 14.50 volts with the car running. If it's more you have a bad voltage regulator.

The gauge most likely isn't bad it's something else.  Don't get tunnel vision on the gauge.

Thanks Zed Head.  You always come up with a better answer.  We could open a shop... Sanford and Son.

 

[jalexquijano]

Im tired now. Ill try that tomorrow and give you the reading. These gauges are very fragile. 46 years old

Edited December 31, 2018 by jalexquijano

[Zed Head]

34 minutes ago, siteunseen said:

We could open a shop... Sanford and Son.

Just tried to break it down to the basic question.  45 + amps is still a giant draw, over the full capacity of an early 240Z generator.  There should be smoke,or a blown fuse. if you see that on your ammeter, I think.

Could be that the needle got stuck when the fuse shorted.  Give it a tap after disconnecting the battery.  At least you'll know if the gauge is damaged or not.

Edit - on the other hand I might not know how the ammeter works.  Maybe the center point is only set with voltage present.  @SteveJ knows though.

Edited December 31, 2018 by Zed Head

[cgsheen1]

3 hours ago, Zed Head said:

  Maybe the center point is only set with voltage present.  @SteveJ knows though.

 

No, the needle sits at the center point without voltage or current.  I have one in my hand (and I'm not sitting in a Z...).

[Mark Maras]

11 hours ago, siteunseen said:

put a volt meter on the batteries post it should read 14.50 volts with the car running. If it's more you have a bad voltage regulator.

 

1

and if it's less than 14.5, it"s a bad alternator.

[SteveJ]

34 minutes ago, Mark Maras said:

and if it's less than 14.5, it"s a bad alternator.

 

11 hours ago, siteunseen said:

I'm not positive on this but when you put a volt meter on the batteries post it should read 14.50 volts with the car running. If it's more you have a bad voltage regulator.

The gauge most likely isn't bad it's something else.  Don't get tunnel vision on the gauge.

Thanks Zed Head.  You always come up with a better answer.  We could open a shop... Sanford and Son.

 

Those are blanket statements and not really accurate.

1. The ammeter is designed to indicate charge/discharge of the battery. The ammeter is situated between the battery and alternator, and most loads are on the alternator side of the ammeter. If the ammeter is on the positive side, the alternator is charging the battery and/or your brake lights/hazard lights are drawing current.
2. Make sure you have a good battery. The specified alternator and the wiring harness are not designed to charge a dead battery.
3. If the ammeter has failed completely, typically the car will not start since current will not flow through the ammeter. For starting the car, power goes from the battery, through the fusible link and ammeter, and to the ignition switch.
4. If the alternator has failed, the electrical system is relying solely upon the battery, and the battery will discharge.
5. If the voltage regulator has failed, typically the battery will reverse power the alternator when the car is off, and the battery will discharge.
6. The FSM describes alternator and voltage regulator testing. See EE-18 for the alternator test and EE-22 for the start of the regulator testing. Of course, you can purchase a clamp DC ammeter to place over the battery cable to simplify testing. (This means you don't need to use the resistor shown in figure EE-55 if you use the clamp DC ammeter.)
7. The voltage reading of 14.5 would be at 2500 RPM, and that is not a go/no-go voltage. A voltage between 14 and 15 VDC at 2500 RPM should be fine. As the FSM states, the alternator should show 12.5 or higher at 1000 RPM when doing the test as described in the manual.

[Dave WM]

you should take a look at the shunt and make sure its installed properly.

[jalexquijano]

14 hours ago, siteunseen said:

I'm not positive on this but when you put a volt meter on the batteries post it should read 14.50 volts with the car running. If it's more you have a bad voltage regulator.

The gauge most likely isn't bad it's something else.  Don't get tunnel vision on the gauge.

Thanks Zed Head.  You always come up with a better answer.  We could open a shop... Sanford and Son.

 

Readings taken both at positive of alternator and at battery with car at idle. Ammeter still at zero. I have a spare one and already verified that my connectios were tight on the one installed.