Erratic amp gauge reading

[Gene M.]

Re: Erratic amp gauge reading

When the gauge was "bouncing around" was it favoring the plus side? Mine was. I had this happen almost two years ago and problem was a cell in battery gone bad. It still had enough power left to start the car. But shortly there after it started to fail even after having put on a charger to attempt to keep it up.

Replaced battery and regulator and the problem was gone. Charge is normal and battery is strong.

Make sure the regulator is set up correctly and grounds clean and tight. The battery can be damaged as well as alternator if not.

[Joe R.]

Re: Erratic amp gauge reading

Here's a link to a document that explains how to troubleshoot the gauges:

http://www.lbfun.com/warehouse/tech_...g%20gauges.pdf

The ammeter section starts on page 5.

Good luck.
Joe

[Jerome P.]

Re: Erratic amp gauge reading

For what it is worth.
I had an amp meter that acting erratically on my 72. After I lot of head scratching and testing of the alternator, which pass. I decided to have the alternator rebuilt anyway. Paid just over $50. It was worth it. The gauge stands straight up shortly after starting with very very little movment thereafter.

[Gary C.]

Re: Erratic amp gauge reading

Keith, posted this info earlier today on another thread very similar to yours. In case you've not seen it here's it is.
Get one of the digital voltmeters that plugs into the cigarette lighter. Roy Braatz or another one of the TDB frequent users posted info a while back. Walmart has them for around $15 bucks. That way you can see what the charging circuit is putting out when you're driving around. Gary....
http://www.amazon.com/Vector-VEC008-.../dp/B0002ISEQW

[Jack H.]

Re: Erratic amp gauge reading

That's a decent suggestion, Gary, but it's not a guarantee of diagnostics...

A digital voltmeter works by having a discrete integration period where it examines the voltage waveform present at the probe interface and then reporting either the peak, average or RMS voltage level it saw on the display. The process repeats and you'll see discrete reading differences from integration period to integration period...

If the voltage level is unstable, take the case of an alternator that has one or more of its six rectifying diodes blown or damaged, you may or may not observe voltage fluctuation(s) that the analog ammeter is responding to (depends on that digital VOM's integration period).

So, it IS possible for an alternator to be damaged, yet test 'good' on a simple autoparts store 'go/no-go' tester. Auto electric pros, put an alternator up on an exercise machine and view the DC output on a oscilloscope....

There, they can see 'missing' phase charging pulses from a defective rectifier bridge as well as the degree of 'trash' AC ripple riding the alternator's DC output. Issues like these can explain the cause/effect episode that Jerome Pederson recited (alternator tested good, but he went ahead and had it rebuilt anyway to notice the ammeter pointer needle instability evaporated as a result)...

[Gary C.]

Re: Erratic amp gauge reading

Agree Jack. Prefer analog over digital as well for all the obvious reasons you pointed out. Thought was to try to provide an in car driving diagnostic aid without having to resort to constructing one's own plug in analog volt meter. Regards, Gary....

[Jack H.]

Re: Erratic amp gauge reading

Hey, Gary, there's NOTHING wrong with the suggestion you and Roy Braatz made. That's so long as you understand the limitation(s) of the measuring instrument...

Even an analog voltmeter has needle deflection response time limits and can't be trusted in DC measuring mode to 'really' show what's going on with the underlying waveform. That's why I discussed the pro/con issues associated with displaying the actual waveform on an o-scope or equivalent measuring device.

I've seen more than one alternator with damaged diode(s) that put out 'lumpy/erratic' DC with LOTS of AC trash riding on top of the DC. This kind of characteristic can make an ammeter 'jumpy/twitchy'. So, that's why I chimed in (to clarify and explain the limits of testing with VOM style meters).