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While our editors traverse the country to find the best content for those magazines, we find other oddities related to the old-car hobby that we really had no place for - until now. With this blog, we're giving you a behind-the-scenes look at what we see and what we do during the course of putting out some of the finest automotive magazines you'll ever read.

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How and Why to Replace an Ammeter with a Voltmeter in a Vintage Mopar

Posted October 07, 2021 4:00 AM by dstrohl
Pathfinder Tags: Plymouth Road Runner voltmeters

It happened on the way to Chryslers at Carlisle. "The car was running great," Rowland George said of his 1971 Plymouth Road Runner. "About three hours in, the ammeter began to bounce a tiny bit. I went to get fuel, and when I came off the exit, the car completely stopped. I went to start it again and smoke came out from the ammeter area under the gauge cluster." No amount of fiddling with the burned wires would get the Road Runner to start again, so Rowland called for a tow truck and made his way to Carlisle.

As he told his story to his Chrysler pals, he found that he was far from the first muscle-era Mopar owner to suffer from the dreaded ammeter failure. Long after many other carmakers had switched from ammeters to voltmeters, Chrysler continued to use the former - even into the late Seventies in its trucks - and the gauges became notorious as a common point of failure.

It's not that ammeters didn't once have their place. In the days of low-output generators and relatively untaxed electrical systems, the ammeter - a device that monitors the electrical system's current and relays whether the system is charging or discharging - arguably does a better job of alerting the driver to faults in the system than a voltmeter. But the ammeter also had its faults, primarily arising from the fact that all system current has to flow through the ammeter for it to work. That means, at its most basic, wiring from the generator or alternator, through a bulkhead connector, through the alternator, on to feed the rest of the electrical system, back through the bulkhead connector, and back to the battery.

As the wiring ages, its electrical resistance increases. As the output of the alternator increases, so does the amount of current going through the ammeter. Both create additional heat, often in the workings of the ammeter. It also doesn't help when carmakers use shunts to tune the ammeters, adding even more heat right at the gauge. At some point, it proves too much for the ammeter, and when it gives up the ghost, it takes the car's entire electrical system down with it.

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Re: How and Why to Replace an Ammeter with a Voltmeter in a Vintage Mopar

10/07/2021 7:59 AM

So why can't you have a remote shunt with shielded leads to the meter movement?

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Re: How and Why to Replace an Ammeter with a Voltmeter in a Vintage Mopar

10/07/2021 7:10 PM

Because that might make sense.

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Re: How and Why to Replace an Ammeter with a Voltmeter in a Vintage Mopar

10/10/2021 11:34 PM

I'd say it probably did have a shunt but it was internal shunt.

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Re: How and Why to Replace an Ammeter with a Voltmeter in a Vintage Mopar

10/07/2021 11:51 PM

Using remote shunts would have made sense as they had been in use since Edison's day when a voltmeter read the PD across a resistance and the meter was calibrated to show the inferred current, but you can't expect horseless carriage makers to be smart technologically.

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Re: How and Why to Replace an Ammeter with a Voltmeter in a Vintage Mopar

10/21/2021 5:09 PM

Most analog meters have a galvanometer movement,which measures current to move the coil to indicate voltage,so in a sense,all meters are amp meters.The difference, of course,is the small amount of current needed by the voltmeter to measure implied current,due to it's high internal resistance.

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