MGB & GT Forum

Removed an ammeter and the associated wiring that was in my car when I bought it and wanted to fill the hole in the dash with a voltmeter. Bought a new Smiths voltmeter from Holden Classics in UK. There was nothing on their website about not being able to fit the gauge in a positive ground vehicle so assumed it would work in my car. Gauge is made by an outfit called "Caerbont" and is very nice quality (see attached).

Got a question about fitting this gauge to my '66 GT that is positive ground. When I opened up the package the instructions state "gauge to be fitted to negative earth vehicles only" (see attached). I examined the gauge and both terminals are isolated from the case (see attached). When I tested on the bench with a battery I could get no continuity through the case or through the mounting studs in either direction. So it is unclear to me why this gauge cannot be mounted in a positive ground vehicle and the warning in the instructions a bit of a mystery to me. Plan was to feed the negative terminal of gauge from the ignition switch and put a low amperage fuse (2 amps) in that line. Positive terminal on gauge from ground. Any reason why this shouldn't work?

TOBIN: YES, very ODD. I would have thought that a 'VOLTMETER" was of the D'Arsonval, permanent magnet type. and would normally work for either polarity. Possibly, it das an internal, steering diode, or surge protection. Give them a call, that would be the proper route to take. Good Luck.

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IN ALL SUBJECTS.: For those who believe, no proof is needed. For those who don’t believe, no proof is possible.

"Strive for perfection in everything. Take the best that exists and make it better. If it doesn't exist, create it. Accept nothing nearly right or good enough"
Sir Henry Royce
co-founder of Rolls-Royce .

"I take no credit for my own, of another man's fame".
Joe Baba 2021

Kind of curious, since I have a tuneup (Homemade) box, That has both a Tachmometer and a volt meter, on it, and I can use it on + or - ground cars.

Kind of curious, since I have a tuneup (Homemade) box, That has both a Tachmometer and a volt meter, on it, and I can use it on + or - ground cars.


Interesting. I have had no problem using a modern strobe light with tachometer and dwell meter on my car but I always use an external 12V power supply rather than the car's battery. Dwell clip goes to points side of coil (positive on my car) and it gives me inverse dwell readings. So I get readings of say 30 which I can convert to actual dwell readings by subtracting the number on my dwell meter from 90. So 90-30= 60 degrees of dwell.

Hi there,

I Think that you will find it's OK to connect the "Volt Meter Only".

BUT DO NOT CONNECT THE LIGHT.

The "Polarity On The Light Is NEGATIVE EARTH".

I had a "Similar Issue With A Tacho", when young & "Inexperienced".

All was fine, until I "CONNECTED THE LIGHT".

Ross H.

PS. Are you mounting it to "Metal Of The Body, or Plastic Etc"?

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Any day above the ground is a great day, especially "Driving Miss B".



Edited 1 time(s). Last edit at 2023-08-27 02:16 AM by Tar Trekka.

Body of my gauge is metal but some of them are plastic. As I said above, I could detect no electrical continuity between case or mounting studs and 2 input terminals. So as long as both input terminals remain electrically isolated from the case and mounting studs, I should be OK to mount in steel dashboard. Hopefully the inline fuse will ensure that if the case becomes conductive because of an internal short, the wiring will be protected.

Response from Caerbont:

As long as you have a volt meter with 2 terminals then connect the gauge connection 2 to battery positive and the other to battery negative it will work OK.

Later versions with a 6 way plug or negative earth only.

Regards

Peter Wilkinson
Software Engineer
Direct Phone +44 (0)1639 732243
Caerbont Automotive Instruments Ltd

His gauge does not have an internal light. It is a push-in type from the harness. According to the picture.

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IN ALL SUBJECTS.: For those who believe, no proof is needed. For those who don’t believe, no proof is possible.

"Strive for perfection in everything. Take the best that exists and make it better. If it doesn't exist, create it. Accept nothing nearly right or good enough"
Sir Henry Royce
co-founder of Rolls-Royce .

"I take no credit for my own, of another man's fame".
Joe Baba 2021

Correct! No internal light. Light supplied with new gauge has a plastic base, two leads, and a conventional bulb and slides into a brass tube attached to case. Since case is electrically isolated from terminals, I should be able to reuse the light that was in my ammeter. That one has a single lead and relies on grounding through the gauge case.



Edited 1 time(s). Last edit at 2023-08-27 10:10 AM by FulfordMan.

I HAVE TO ASK" Did you test the polarity with a 'FLASHLIGHT Battery? AAA< AA< C or D are all 1.5 Volts DC, They will deflect the needle, and tell you if the polarity is correct, but NOT enoygh amperage to dame it if NOT. I have a bench poweer supply tha is variable, from about 1 VOLT to 24 Volts, for testing electrical components. (Home made, and is about 30+ years old)., Cheers.

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IN ALL SUBJECTS.: For those who believe, no proof is needed. For those who don’t believe, no proof is possible.

"Strive for perfection in everything. Take the best that exists and make it better. If it doesn't exist, create it. Accept nothing nearly right or good enough"
Sir Henry Royce
co-founder of Rolls-Royce .

"I take no credit for my own, of another man's fame".
Joe Baba 2021

Tested with a 12V motorcycle battery. Also confirmed that gauge is quite accurate with a multimeter

Just for fun. DID you try a flashlight battery, to see the results ??? For other people to see. I also use a AA Battery to test car stereo speakers, Putting the speakers leads across a AA or any 1.5V battery, produces a '"Scratchng Sound"", = GOOD Speaker.

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IN ALL SUBJECTS.: For those who believe, no proof is needed. For those who don’t believe, no proof is possible.

"Strive for perfection in everything. Take the best that exists and make it better. If it doesn't exist, create it. Accept nothing nearly right or good enough"
Sir Henry Royce
co-founder of Rolls-Royce .

"I take no credit for my own, of another man's fame".
Joe Baba 2021

I'm not sure how this fits into your problem but it might. I have a positive ground car as well. When adding an accessory gauge off any kind you need to check the light. Most gauges like MGs have a bulb and a socket that plugs into the back of the gauge. Usually it has one wire in the center. The light socket is metal and grounds to the case of the gauge that is then grounded to the car body. So even if you isolate the body of the gauge from the body you still have the problem with the light shorting. Speedway motors and others have gauge lights that are plastic and have two leads that do no ground to the case that will fix this problem. You just plug them is as normal and wire both leads as needed. So I assume your gauge has a light plumed into the back. If it has one wire and the socket is metal then it is part of the electrical chain and the case is grounded. My only hopefully helpful tip is be careful that the gauge light polarity doesn't contradict the gauge polarity. Good luck.

Thanks for the tip. I have determined that the input terminals and internal mechanism of the gauge are electrically isolated from the case and studs. So no need to isolate the gauge case from the dashboard, it already is. So it should be no problem to reuse the original light bulb I had for the ammeter in the same hole in the dash. In this situation the light would rely on a ground through the case. So the light will be grounded through the case but the gauge will not. Does this make sense? A bulb with plastic base and two leads as you describe was included with the gauge so I could go that route as well if necessary.

A single-wire light bulb as found in the MGB. "Dash Lights" RED w/Wte stripe, work for either Positive or Negative ground They GROUND at the gauge body. (Whether that is + or - Voltage there.) They are NOT polarity conscious Any after-market gauge you buy, is going to have a PLASTIC, TWO wire holder, NOT because of polarity, but because of convenience, NOT sure of HOW a light bolb socket is going to short out on the gauge body, POS or NEG. Mybe I am misunderstanding your post. .

HERE. Very simplified drawing but it shows the relationship between the lightbulb in the electrical circuit. Battery voltage switch from negative to positive has no effect on the operation of the lightbulb. The gauge itself of course is polarity conscious and must follow the positive negative inputs.

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IN ALL SUBJECTS.: For those who believe, no proof is needed. For those who don’t believe, no proof is possible.

"Strive for perfection in everything. Take the best that exists and make it better. If it doesn't exist, create it. Accept nothing nearly right or good enough"
Sir Henry Royce
co-founder of Rolls-Royce .

"I take no credit for my own, of another man's fame".
Joe Baba 2021



Edited 2 time(s). Last edit at 2023-08-27 07:23 PM by oleanderjoe.