Hey guys - please help me.

Originally posted this over on the C3 board, but limited responses:

Hey guys, so I'm really more of a lurker than a poster. Alway thought you could learn more by listening than talkin'. So here it is - fully restored '72 BB coupe - 4-spd/AIR/AC/3.70 rear/fully numbers match/MM red/49K miles, she's a beauty and has taken about 9 years out of my life to do right. So I want the following problem gone, naturally.
Big problem - was working on freshening the behind-the-dash A/C seals last year and acidentally shorted something across the ammeter. Big spark at the back of the ammeter and nice white smoke in the engine compartment coming from the harness right under the wiper door vacuum pot. So I opened the harness wrap at that location and found 2 burned fusible links, which I repaired. Re-wrapped the harness and fired her up, all was fine - EXCEPT no response + or - from the ammeter. Thinking I melted another link somewhere in another part of the harness I got out the trusty DVOM and found that the charging system was working fine. Full 14.7V at the battery with the engine running, 12.7 with the key off.

All other gauges and accessories working fine, no apparent other damage.

SO, totally disgusted, I put the car away and now this year, having finished what needed to be done on my boat, (another LOOONG story), its time to address this issue.

So last nite I again pulled down the dash and using the DVOM I get continuity across the two Ammeter terminals and I also get full system voltage (12.7V) from either terminal to ground.

I have invested significant time researching this problem both here and at the NCRS site (archives) plus the AIM and various Chevy wiring diagrams I have accumulated and nothing I've seen definitively addresses this issue.

Here's the questions:

1/ Given the tests I performed at the rear of the Ammeter, is it at all possible that the Ammeter is actually bad, yet gives continuity across the terminals?

2/ Is it possible that I can get a good voltage reading from either terminal to ground because the ammeter is picking up system voltage thru some other part of the harness and will show voltage but be unable to display amperage, dur to internal frying?

3/ Is there some other fusible link I missed that fryed and allows 12.7V to the back of the ammeter yet will not allow an amperage reading? Does that even makes sense?

4/ Both fusible links at the horn relay are good, no apparent damage there.

5/ One really sharp mechanic I know thinks the ammeter's internal coil could have been fused in the mishap and is showing continuity but cannot function correctly to display amperage flow. Is this possible?

Where the heck do I start, given the test I've already performed, and what should I look for?

I'm not really electrically challenged, but this one is well beyond me, obviously.

Help.

rgds,

Leprechaun

 

First are you sure that it's an ampmeter? I'll bet it is a voltmeter. Since you did say that you have 12 volts accross the meter terminals, there are two things that can be bad.

1. A voltmeter is actually ampmeter (typically milliamperes) with a series resistor.

2. Either the resistor is open or the meter movement coil is open. If the series resistor is open you can calculate the resistor value from the votage drop accross the meter movement at full scale (it may be something like a 1 milliampere meter movement) with a variable bench power supply with current limiting.

Anyway the first step may be taking the gauge apart. I hope this sorta explains some of you question.

 

Thanks, but maybe I wasn't really clear. It is definitely an Ammeter in a '72.

Also, I didn't say I got 12V across the terminals, I got CONTINUITY across the terminals and got 12.7V from EACH of the terminals to ground.

What do you think? Look at the gauge or keep looking for more burned links in the harness? Every other link I've found, other than the orginal 2 I fixed test fine.

rgds, Leprechaun :confused:

 

Do what we electrically challenged mechanical engineers do; keep fixing things by elimination until you have gone through all the stuff that wuz smokin' an' arcin'. :D

Actually, Lep, I'm with your mechanic: an ammeter is a pretty sensitive electrical instrument. I doubt very seriously that you can strike an across the terminals and NOT do some internal damage. I would never be that lucky anyway. I think you are probably looking at an ammeter restoration or repair. Now you know why all the manuals tell you to disconnect your battery before you even change the spare...only kidding. :D

Congratulations on your completed restoration. I have been working on a 70 about the same amount of time, and yours is indeed a great accomplishment.




[Modified by Chuck Sangerhausen, 8:17 PM 8/30/2001]

 

OK since it is definitely an ampmeter we have a sensitive ampemeter per same example 1 milliamp meter movement and it has a shunt resistor (very low ohms like less than 1 ohm) accross the meter terminals (inside the meter). This allows 59.999 Amp to flow through the shunt and 1 ma through the meter itself to read full scale at 60 amps or whatever it's rated at.

I would say that you cooked the coil in the meter movement when it flashed. The shunt is still good that's why every thing still works.

It may be time to break out the wallet and buy a used ampmeter.

Joe

 

Thanks guys! I "Aquired" another ampmeter yesterday and will try it. Too bad its the earlier green color, gotta get a white one to be right for the car. Anyway, I'll post results later today. rgds, Leprechaun
:rolleyes:

 

Yep - swapped out the ammeter and Voila! The new ammeter works like a champ. You guys were right.

:cheers:

Mr Wrencher or anybody else - want to swap your incorrect white-faced meter for a correct (For you) green-faced one?

rgds, Leprechaun