Mopar Action On-line – Tech Questions
Todd Branstetter, Nowata, OK, 1972 Plymouth Satellite Sebring Plus 440
I have read many of your excellent articles, and bought a few things off eBay from you. Anyway, my question is about ammeters and your article from 1999 titled, “Taking Charge: Electrical and Charging Tips and Tweaks to Keep Your Mopar Current”. Maybe the answer is in the article, but it is not clear to me. I will rewire my car such that the alternator goes directly to the starter relay. My question is, “Why can’t I still use my ammeter?” The modified wiring would look the same — to me — as my 1985 Ramcharger. It has a low resistance 10 AWG wire running from the alternator directly to the starter relay, and functions as a shunt. The ammeter is hooked in parallel with this circuit. No, the needle never moves much, but it is hooked up. Is there some big difference in the 1972 ammeter and the 1985 ammeter that would prevent the 1972 from functioning? Would it blow up? Oh yeah, I plan on getting a voltmeter, but I hate the thought of a (totally) dead gauge in my Rallye Dash.
Todd, the later (late-70’s – ’80s) ammeter is a totally different design. In fact, as you discovered, the design was pretty screwed up: the meter movement was only about 1/10th as sensitive as it should have been and winds up being pretty well useless. But hooking the earlier, internal-shunt ammeter up as you’ve described would result in virtually no needle movement. Interestingly, most serious ammeters and voltmeters of the “analog” era were really just very sensitive ammeters. if you look at a lab-type panel meter that reads 0-1000 VDC or 0-100 DCA, it will usually say, in very small print somewhere, something like “FS=1mA”. Which means it’s really a one milliamp meter movement, with internal shunts and / or series resistors to get the required full scale reading accurately.
I haven’t researched this – it might be possible to re-design an earlier stock Mopar ammeter so that the internal shunt’s value is drastically increased (by a factor of, I’d guess, at least 100), or, possibly, have no shunt at all. The devil is in the details – calibrating it. You also need to be very careful with the wiring, so, if a fusible link opens, all current doesnt pass through your new ammeter. Then yes poof! That’s why the way to go — and the way the factory wound up going, too — is the voltmeter.