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I want to have my original ammeter converted to function as a voltmeter. I see some companies on-line that offer that service, but I want to know the company is reputable before I send my gauge to them. Does anybody have a recommendation based on personal experience? Thanks.
 

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Never heard of anyone doing that. Ammeter measures Amps which is limited by the gauge to only thirty amps. In old cars with generators and early external voltage regulators that was sufficient since the alternator only out put 37 Amps at full charge. The charging device was just that a battery charger, and the car ran off the battery all of the time. This is why your head lights grow dim turn yellow a ta stop light as the voltage dropped down to eight volts when everything was discharging and no power was coming from the alternator.

Modern cars have transistorized voltage controls that maintain constant voltage and run everything on the car off of the alternator. The battery is aboard only to start the car. This was done not because GM liked the more expensive charging system, but because computers require a rock steady voltage supply to work.

So a modern one wire alternator will always out put 12 volts at 110 amps or more. It has one wire as it doesn't rely upon the car to regulate voltage, making power as required (up to it's rated power out put) from idle to full RPM. Because the amperage is so high an ammeter to measure it is much more expensive (requiring shunts to handle the power), which is why in the mid seventies the factory switched over to cheaper voltmeters as the power levels of the alternators rose (when they introduced the internally regulated SI series).

Voltmeters are wired in parallel and ammeters are connected in series. So all of the power in the car has to run through the ammeter; which drives the cost up, as the amperage rises. It also explains why there isn't a conversion but a full replacement of your old ammeter with different voltmeter: with the old ammeter just bridged electrically, and the voltmeter sensing the voltage flowing through the wiring. So they won't keep your old meter only the face of the gauge, with a new meter replacing what was behind the face plate. Hopefully they will silk screen the old face plate to read voltage instead of amps or even that will be useless.

Big Dave
 

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There are companies that will sell you a conversion voltmeter for a Chevelle so they are probably available for Impala's too.Here's one from GroundUp for a Chevelle :

I'd try to find someplace that sells a similar part instead of sending your Impala's part to someone you're not familiar with. Try Shiftworks first they might already have a voltmeter that fits your cars dash.
 
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