Ammeter reads negative only

[Timothy Keith-Lucas]

Here's a strange one:

Ammeter was reading negative, and more so with the lights on. OK, I think, this generator is toast and why not replace it with an alternator? Done. Added a little digital voltmeter while I was at it. Now, cruising around the county, the voltmeter reads 14.4 volts, so that alternator is pumping out, and the ammeter still reads negative, more so with the lights on. No change.

So, perhaps the generator was pumping out all along, but the ammeter only reads the outflow. It doesn't even go to zero when the generator is working; it reads the amps being used independent of the amps being generated.

 

How do you mis-wire an ammeter to do this? It's not stopping me, it's just really odd.

[MV8]

That it reads more negative with a greater load doesn't make sense if it were wired backward. Sounds like it is out of calibration.

[Timothy Keith-Lucas]

Thank you, MV8. I'll try tracing the wiring according to the diagram.. Thanks for posting it.

 

--Tim.

[jbcollier]

Battery and fat feed to the starter on one side and everything else on the other.

 

Disconnect the negative battery terminal (to rule out draws and shorts).  It should read “0”.  Hook the battery back up.  Any change?  There shouldn’t be.  Turn on lights.  It should read negative.  If it reads positive, then the ammeter is hooked up backwards.

 

The generator/alternator feed goes to the opposite terminal on the ammeter from the terminal the battery is hooked to.  All the ammeter does is measure current either flowing into, or out of, the battery.  It does not measure system load or charging current.

[MV8]

The meter reads gen output amps (charging) versus the demand amps (load).

 

Tim, are you checking volts at the gen or across the battery?

 

 

[Timothy Keith-Lucas]

Across the battery. Having replaced the charge used in starting and having a functioning voltage regulator should result in about 14.3V and a balanced ammeter. Disconnect the alternator, or have it fail, and you get a negative reading reflecting amps used by the car. I'm getting 14.3 and a negative reading that is proportionate to the car's uses, including the fan motor and lights. I always run with the lights on, given that drivers around here are predominantly in jacked-up Ford F-150s.

[MV8]

Since you have a volt meter reading what it should, I would ignore the amp meter.

 

Echoing JB, does it read zero with A battery terminal disconnected? What about when the key is off? Even if it does, it could still need calibration. It is a magnetic indicator with a single loop of wire. If the loop is not equidistant to the needle base, it could deflect more one way than the other. Looks like a post rotating in the gauge housing could do that.

It could be safely tested out of the car, wired in series with an accessory (like an cooling or heater fan) and a battery. Do not place directly across a battery as it will melt the copper loop wire inside.

 

Some good info with great pics: https://www.lotuseuropa.org/LotusForum/index.php?PHPSESSID=a62e89f5fe022a4e7051b0998baac507&topic=4840.0

[SENC]

Normal behavior on mine (pos earth and generator): at idle and/or with significant load it will read negative - as rpms rise it will read positive, even with some load.  Obviously, dead zero when ignition is in off position.

[MV8]

I wonder if the gen may have a path parallel to the amp meter.

 

Tim, is anything besides the starter switch and amp meter on the negative side of the battery?