View Full Version : TR2/3/3A TR-3 Amp Meter

09-09-2008, 11:35 AM
Iím trying to figure out how to take a TR3 Amp meter and convert it to a Volt meter. I thought I read on the forum about someone doing this but I canít find the post.
So, does anyone have a dead or extra Amp meter they donít need? Iíll pay shipping and any reasonable cost for one.
I found a Volt meter that swings the right direction so I think I can install the works and re-decal the face.
Any advice will help.

Don Elliott
09-10-2008, 06:24 PM
Bill - I would have thought by now that our Electrical Engineer, Randall Young would have answered your question. If I remember correctly he wrote earlier (maybe you could do a search in the archives ?) that all you do is install the voltmeter after you remove the chrome bezel and curved glass from the ammeter you took out and then you install these on the voltmeter. Then you install the voltmeter you mentioned. How to connect it is not my bag.

09-10-2008, 08:27 PM
Bill, I'm not sure I understand what your intentions are.

Do you intend to replace the entire ammeter gauge assembly with a voltmeter gauge assembly, or swap the internals?

Two different animals, two different type of connections, two different tye of measurements.

Voltmeter can be hooked up with just one wire going to the gauge(provided the gauge is grounded) Usually with a fairly small gauge wire, compared to an ammeter wire.
Voltmeter gauge wiring an be accurate as small as 14 gauge wire, I prefer 12 gauge myself. Ammeter because it measures in series needs the largest gauge wire running through both connections on the ammeter. Hopefully 10 gauge.

Now both gauges should get one wire hooked up to the ignition switch(hot with key on). For the voltmeter that's all you need, but with the ammeter you need to hook the other terminal/wire up to a source voltage(battery, main fuse box, before fuses, starter junction, etc.. Remember the ammeter will be hot all the time.

09-10-2008, 10:30 PM
Sorry, I don't have any advice on how to convert an ammeter. I would probably just get a Smiths voltmeter (eg Stag or later TR6) and put a TR3 bezel and glass on it. But since my personal preference is for ammeters anyway, what I did was just add a shunt so I could keep the factory ammeter with my 60 amp alternator.

The Smiths voltmeter has two terminals (rather than being grounded through the case) so you'll also need to run a ground wire. Assuming this is for a TR3, the ground lug on the back of the panel will do.

To power the voltmeter, you can grab any white or green wire ... the green wire to the fuel gauge would probably be easiest if the wiring is otherwise stock. Of course you'll need to do something about the original wires to the ammeter, either connect them together in a safe (insulated) fashion or rewire to eliminate them.

BTW, the early Stag voltmeter has the needle pointing down like the original ammeter, tho it would still look somewhat out of place IMO.

09-11-2008, 06:51 AM
My intent was to swap the internals and make the gauge look as close to original as I can. Thatís why I want an old broken amp meter so I donít destroy my working one.

09-11-2008, 09:02 AM
I have stayed silent on this one but against my better judgment here I go.

Way back in the stone age pre digital days (well, in the transition to digital, when I was a young electrical engineering student) analog meter movements were all over the place. The theory is simple. The mechanical analog meter movement is actually a current measuring device. The typical one was a 50 milli amp movement (that is to say it was at full scale deflection at 50 milli amps. The "meter" (that is the box you held in your hand) was just a 50 mA movement with some stuff attached so that you could measure whatever - voltage, current, resistance etc. This "stuff' was mostly resistors as shunts etc. so that only part of what you were measuring went through the movement and the rest went through the shunts.

All this to say, the simple way to make the change is what you (and Randall)propose and swap guts. However one can take an amp meter and shunt it so that it measures voltage. I have not looked at the guts of our TR3 current meters, but if the movement in it is a low current movement then it could be modified to measure voltage. On the other hand if it is some esoteric movement that really does pass the full 50 amps then it would be more work then it is worth.

It would make an interesting "project" and/or article if one had the time. But since we all own LBCs who has the time. I think it is more fun working on the car or driving the car. Like Randall, my personal preference is for current measurement so I have not looked into this further.

If one wanted to I would suggest an old (pre 1975) text book on meter design. This was highschool stuff in the early 70's so there should be books still around for that.

I will go hide from the flames now :smile:

09-11-2008, 10:09 AM
but if the movement in it is a low current movementUnfortunately, it isn't. Instead it's a simple "moving vane" type movement, using a single turn of heavy copper wire to generate the magnetic field. There is no shunt at all, instead it takes a full 30 amps through the winding to generate full scale deflection.

There's also the problem that it's center-zero, which, even if you could rearrange it to read voltage, would mean that 11-15 volts (the range of interest) would be crammed into a very small movement of the needle, making it useless, IMO.

The case design is also totally different than the Smiths gauges (the TR3 ammeter was made by Lucas), which is going to make it difficult to transplant movements, IMO.

One alternative might be to come up with an electronic circuit to drive a TR3 fuel gauge. If you also repainted the face, it might look reasonably at home, at least to people who aren't accustomed to scanning a TR3 dash and seeing the ammeter in the middle.

09-11-2008, 10:53 AM
I like the fuel guage is a good idea. One could turn that into an expanded scale volt meter. Seems like a lot of work though.

I should have known the ammeter would have been so "simple". I get carried away too often.

09-11-2008, 10:56 AM
Actually a production idea. Kawasaki on the GPZ 550 I had years ago(okay decades ago, now) had this. Fuel gauge normally, but press and hold a button and you were reading voltage..

09-11-2008, 12:10 PM
For an interesting combination (very non-original) volt/amp meter, contact Sarastro on the Spridget side of this board. I believe he's an EE. He designed a combination gauge that uses an induction coil, a Hall Effect sensor, and a panel meter. Through the use of a selector switch you can see either current or voltage. Again, it doesn't look at all original, but he probably has experimented more with these instruments than anyone on this forum.

Here's a link to his web site:

Here's a link to his custom volt/amp gauge:

09-11-2008, 06:24 PM
You really can't convert your "ammeter" to a voltmeter, and measure anything meaningful in your car's electrical system. What yo can do is eliminate the ammeter circuit and replace the ammeter with a voltmeter. I believe others here have offered similar opinions.