View Full Version : Converting ammeter to voltmeter - issues and solutions

07-16-2011, 09:33 PM
About two years ago we equipped our 84 Stars and Stripes with a very nice stereo system including two MTX amps, 4 cadence component 6 inch speakers and tweats and 2 12 inch subwoofers. My 37 amp alternator could not keep up and so we upgraded to a 120 amp alternator. It seemed to work fine for awhile, but eventually we began to notice a couple of funny voltage issues and some alternator whine in the stereo as well.

To attempt to monitor/solve the problem I decided to do some clean up on the grounding to the stereo to ensure no ground loop issues and I decided to install a voltmeter in place of the factory Faria ammeter. I found a brand new voltmeter that looks almost identical to the ammeter except for the color of the meter measurement lines in the face of the gauge.

So I go to install the new voltmeter. When I do, I notice first of all that the wiring on a ammeter is somewhat different than the voltmeter. The ammeter has one more wiring post than that of the voltmeter. After some testing with my ohmmeter for continuity I figured out what all of the wires were and what they were doing.

For reference, the voltmeter basically has three total posts for wiring (a positive and a negative and a post for the light in the gauge). The ammeter on the other hand has four total posts including the post for the light in the gauge. It should be noted that you do not want current running to the voltmeter all of the time the same as you would the ammeter as the voltmeter will eventually slowly drain your battery because it is registering voltage which is always present as opposed to amperage which is not pesent unless current is flowing. The voltmeter should run off of the ignition switch wire that activates in the accessory position of the key.

I finally figured that the main large gauge red wire running to the ammeter is the hot/constant wire from the baterry. There were three other orange wires running to the ammeter that you need to hook up directly to the main red wire as they carry the power/constant battery source to many other critical component including but not limited to your ignition switch. Two of the orange wires are larger gauge and one is a smaller gauge.

Okay, after doing this I began looking for a wire that is hot (approx 12 volts) when the key is in the accessory position. Oddly enough, every last wire I tested in the dash and surrounding area was reading between 8 and 9 total volts. I then tested the voltage directly off of the accessory side of the ignition switch and it too was 8-9 volts but I was getting a true 12 volts from to the battery side of the ignition switch. I then realized that the ignition switch was 27 years old and filled with corrosion which equals resistance and therefore less voltage.

I replaced the ignition switch with a new one and now have 12 volts to all wires in the dash that are supposed to be hot when the switch is in the accessory position. I have also noticed how much brighter all of my gauge lights are now that I have the full amount of voltage that I am supposed to have when the engine is not running and the switch is in the accessory position. Ran a new wire directly from the side of the ignition switch that is hot when the switch is in the accesory position and now I am reading the proper amount of volts and do not have to worry about the gauge draining the battery.

Problem solved. Just goes to show that just because it hase been working for 27 years, and still seemingly working, that it may not be working perfectly. It seems that I have also solved my engine noise problem as well. Hopefully the other funny voltage issues I have been having including a tachometer that has been giving some intermittently odd readings will be solved too.

Hope this helps someone else out.

07-21-2011, 02:06 PM
I also have an 84 S&S, but have a three position ignition switch. Is your switch original? . I am about to replace it, Can I go to the 4 position switch with Accessory. I have an issue running the stereo directly to the battery so I can use it when the engine is off. Not planning on switching to a voltmeter at this point.
Any pictures?

07-21-2011, 03:33 PM
Great job markismm. That write up will be used many times for reference in the upcoming years.

I guess my working ignition switch om '77 should be replaced as well.:rolleyes:

07-21-2011, 05:33 PM
Mine was the original, but I replaced it with a four position switch. I did not use the accessory switch though because I already have a separate toggle switch I use to control my stereo installed right above the ignition switch. I did not want to have a useless toggle switch, so I am not using the accessory position in the new switch for my radio.

07-21-2011, 05:34 PM
I'll get some pics up soon.