View Full Version : Ammeter conversion

09-10-2016, 01:00 PM
As you know, the 64 ammeter takes the full amperage from the alternator at the gauge on route to the passenger side kick panel fuse panel. Since I have a modern 100 amp alternator, I have re-wired the car with a modern fuse and relay box under the hood, and that feeds the remainder of the OE wiring through a 40 amp fuse. I'm using the relays in the box to control all my high amp circuits, so my OE dash controls, for example the headlight switch, are controlling those relays instead of switching the full amperage circuits.

The ammeter needle never moves now. I suspected that this would occur, because I have very little amperage going through it. But now that I've had the car completed for a while and am driving it, I'd like to get it working.

I just learned that the factory system was changed over to a safer, remote shunt in 65. I've been considering converting to a voltmeter but I'm intrigued by that.

Does anyone have a 65 or 66 wiring diagram that shows this circuit? Or a method to convert mine to a remote shunt?

09-10-2016, 03:01 PM
Steve, I do not know if the 1965 or 1966 wiring diagrams in the TRL shows that or not, but you might take a look at them there, if you have not done so.

09-10-2016, 04:08 PM
The 66 is shown on diaphragms 22 and 23. Wire #654 and 655 come out of the gauge, through the main disconnect then splice together at the hot side of the starter relay, through wire #37. Unless that short #37 is a shunt, I'm not sure how that works.

09-10-2016, 04:41 PM
If I'm not mistaken wire 37 is a fusible link. It goes from the junction block to the starter relay.


11-05-2016, 09:28 AM
While changing out my odometer, I've taken out my instrument cluster with the troublesome bolt-in wires to the ammeter for the last time. Cut them, making the necessary wiring changes at the fuse panel. I decided to convert my ammeter to a voltmeter. I'm looking for a vendor who can do this with my core.

11-06-2016, 09:44 PM
Steve is this link any use to you?

11-07-2016, 12:01 AM
Steve, you're an engineer so I know you will understand...

ALL D'Arsonval movements (http://www.engineersedge.com/instrumentation/electrical_meters_measurement/darsonval_movement.htm) in meters are volt meters. We normally connect the amp meter around a resistor to get a sample of the voltage drop across that resistor as current goes in either direction. Self-centering meters normally have two opposing springs so the needle rests at zero in the middle. One spring needs to be removed so the needle falls to one side.

Knowing the meter is already a volt meter, we now want to sense between let's say 10-volts (basically a dead battery) and 15-volts (an overactive charging system) with 11.5 to 13.5-volts being the 'norm'.

Easy as pie... Simply make a voltage divider much like a rheostat where one resistor is connected to pos and the last resistor is connected to neg. Your meter would be connected to the MIDDLE of those two resistors just mentioned and the other post connects to NEG (body chassis). The gauge feed comes from your key switch (like the radio).

The meter takes very little current (less than a milliamp) at full scale so the resistors will be low-wattage but high resistance. What are the values? that depends on the meter movement you are using and the precise current it draws at full scale. The rest is easy. Total resistance allows enough current to bring your pointer to full scale at 15.5-volts and the low end resistor allows enough current to bring the pointer to 10-volts. It's a divider, but not equal on both sides because 0-9 volts are useless to a car.

There are many LED displays in the form of a bar, on the market. Low voltage is represented by red, normal uses green and high voltage is red or yellow. Others show real numbers in 8-segment displays. I have one plugged into my cigar lighter at all times. - Dave

The hardest part to this job is the art work on your gauge face. The paint and font should match your other gauges. It needs to show, "VOLTS" with a normal range scale like your Temp gauge does.

11-07-2016, 11:55 AM
Steve, you're an engineer so I know you will understand...

...Only enough to be dangerous! I'm a civil engineer with a life-long mechanical hobby. The other main branches of the engineering tree, electrical and chemical, are at best vaguely familiar.

My dad is a retired electrical engineer and when he would explain something to me, if it took more than a minute, he would resort to: "it's magic". I did eventually learn basic power systems- how to renovate a house or a car, for example. My son is a chemical engineer. Both are way smarter than I ever was.

That being said, I did take "electrical engineering for civil engineers" in college, actually Aced it, not because I learned the material so well but because it was a sophmore class of about 150 that many were close to flunking so the grades were on a curve. :o

Yes I do understand the basics of what you are talking about. It's the details that I'm having a problem with. If you want to do a specific write-up then this would be an excellent opportunity. I can supply test results.

Regards to the artwork, I plan on leaving the face as is. The face has left (D) and right (C) hashmarks. Ideally I'd like to have the needle centered at 12.6V and 14.4 at the right-center mark.

11-07-2016, 01:29 PM
...If you want to do a specific write-up then this would be an excellent opportunity. I can supply test results...I would love to do this for the forum but not right now. I have wayyyyyy too much on my plate with major jobs stacking up and not many good-weather days left. Right now, I'm into next year.

I would need the meter to run my own current tests and then determine the voltage divider resistors. As I said, one meter spring must be removed to send the needle to 'D' when power is removed (or key is off). Full scale current must overcome the remaining spring tension. Without knowing full scale current, it's impossible to give you the math for sizing your resistors. - Dave

11-07-2016, 08:57 PM
I bought a cheap voltmeter at A's and dissected it. From the positive side, it has a resistor and diode then a coil with about a thousand windings, and the needle with round permanent magnet centered in the coil. The axis of the coil is 90 degrees to the pivot shaft. A second stationary bar magnet acts at the "spring" to "center" the needle, adjusted all the way left.

This is different than the OE ammeter that has a U shaped shunt passing through a steel loop. Its permanent magnet is U shaped and acts to center the needle in the gauge.

11-14-2016, 08:45 PM
An important lesson to learn about these cheap little voltmeters- don't take them apart too deeply. If you get to the point where you can take the needle out of the pivot, if you have bear hands like mine you'll never get it back together without breaking at least one section of the delicate winding wire.

11-14-2016, 08:50 PM
I ordered a different meter on line- A sunpro CP7985. $8 from Amazon. The mechanism is the exact same as the Bosch unit that I bought for $17 locally at Advance. Except unlike the Bosch unit, the Sunpro swings in the correct direction (the pivot is at the top). Although it's an easy job to reverse the wiring the self-centering pointer is not so easy to reset, because, as I have written in the previous post, the winding spool is difficult to reassemble without breaking delicate wire.

11-14-2016, 08:51 PM
Here's the "guts" of the Sunpro unit before electrical disassembly.

11-14-2016, 08:55 PM
Rough assembly of the voltmeter on the left, OE ammeter on the right. Do not cut into the OE plastic- simply cut the metal top bracket as shown. Do most of the plastic surgery on the Sunpro, because if you mess it up, a replacement is only $8 and two day shipping away.

11-14-2016, 08:58 PM
The first part of the modification involves removing the diode-resistor-potentiometer as a unit (and remember which side it gets resoldered onto). Then trim the plastic to fit into the OE plastic retainer. One important part is to get the pivot in the same place as the OE ammeter, and I measured and marked this carefully.

11-14-2016, 09:02 PM
Take the lense out to dry fit the voltmeter. Then replace the lense along with the metal bracket piece that you cut from the ammeter. I trimmed a little bit too much plastic, but if you do it just right the small "ears" on the Sunpro will rest on top of the OC retainer, then when you screw the lense on the screws will bottom out on the Sunpro piece.

11-14-2016, 09:06 PM
Here's the bottom of the unit after resoldering the electical together. I tweaked the potentiometer just enough to center the needle right in the middle with the battery that is installed in the car, engine not running.

Copious amounts of "Goop" automotive glue, which is a favorite of mine since it is heavy bodied, acts as an electrical insulator, and bonds just about everything. There's a little unused solder tab on the potentiometer that I used to secure into a 1/16" drilled jole, my only modification to the OE plastic.

11-14-2016, 09:08 PM
Completed assembly. I used a thinner bodied all purpose glue to attach the metal OE bent needle to the Sunpro straight plastic needle, using tweezer to set it and adjust it.

11-14-2016, 09:13 PM
I may have to fabricate a little bracket to confine the min-max swing of the needle so it doesn't get snagged on the lense cover. I'm going to let the glue harden before I start tinkering with that.

11-15-2016, 08:56 PM
Actually, the base of the needle pivot on the voltmeter is a tiny bar magnet that can be turned to adjust the pointer 0 mark. So I set that at the D line on the ammeter face then used the potentiometer to set battery voltage right on the middle hash mark. Adjusted, in and done.

12-01-2016, 04:01 PM
does the guage come out easily without pulling the whole dash out?

12-01-2016, 04:45 PM
The lense of the instrument comes off when you remove the single screw on the lower part of the bezel. I think that you can get to the large wire terminals at the rear if you remove the clock panel. The wires are #10 size or so, ring terminals secured by 3/8" drive nuts. Then you remove the instrument itself by removing the two screws from the front. Disconnect the battery first!

You'll need to dead end the large wires and locate their terminals at the fuse panel.

To power the volt meter you should use the radio circuit. You can find those wires by removing the metal panel at the driver's side of the console.

12-02-2016, 01:56 AM
The lense of the instrument comes off when you remove the single screw on the lower part of the bezel. I think that you can get to the large wire terminals at the rear if you remove the clock panel. The wires are #10 size or so, ring terminals secured by 3/8" drive nuts. Then you remove the instrument itself by removing the two screws from the front.

alright thanks, i had the faces off when i pulled the dash and know they all looked roughly the same, which is why i asked. i need to pull the two middle gauges

12-02-2016, 08:59 AM
The other three gauges can be removed from the front, but you'll still need to reach up behind the dash to remove the wires (or through the clock assembly opening). You have two wires for each gauge plus the bulb assembly. Remove them, service then replace individually so you don't get the wires mixed up.

The bulb assembly can be pushed out from the front. To replace, remove the bulb and use needle nose pliers to pull it back into the hole, then replace the bulb.

There is no + and - on the gauge wires, so they can be reversed and they will still work properly.

12-02-2016, 10:16 AM
Cool, Thanks for the advice