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MG TD TF 1500 - Electronic tachometer

Please see email and response below. My tachometer is very unreliable (soon to be reconditioned) and I'd like to use a cheap tach from the local auto parts store in the interim. Has anyone had any experience hooking up an electronic tack meant for the - ground system to a TD with + ground? Thanks. Jud

Bosch Response: The gauge was designed and tested for use on 12 volt negative ground systems. No testing has been done on positive ground applications so we cannot state with certainty that the gauge would operate properly if installed on your application.
Bosch Automotive Service Solutions

From: Jud
Sent: Monday, February 11, 2013 7:39 PM
To: S2 AMER Actron
Subject: Positive Ground

Can I use a Sunpro Supertach in my Positive Ground MG TD? I've had one in my '77 F-100 for many years and am very pleased with it.
J K Chapin

Cheap tach = what have you got to lose?
P.W. Lester

Any suggestions on which wire goes where?
J K Chapin

See if you can find a used Tach. from a '62-'68 MGB, it would be + ground.
David Werblow

Why not make it a negative ground vehicle? Unless you happen to have an electronic fuel pump it shouldn't be too difficult.
J E Carroll

If you have a smart phone there is an app that gives pretty accurate speed. I place mine in the glove box behind the wheel. 54 TF

Peter Dahlquist

I've got a positve ground TD. I wired my accessory (power)jack so that the positive lug went to ground(this is for my Garmin). I don't think the tach will care how your car is wired as long as the + lug sees + and the - lug sees -.

Said the non-auto electrician, Mort
Mort Resnicoff (50 TD-Mobius)

67 was the last year for positive ground MG tachs. Found in Spridgets as well as B's . The case is a ground point and reverse polarity will ruin the tach. If my memory serves me...and it doesn't so much any can get the signal from the coil.
L E D LaVerne

Jud, does that tach use a clamp-on sensor that goes over a plug lead, or does the sensor lead clip onto the coil or distributor? If it's a clamp-on you should be able to simply connect the power leads directly onto the + and - terminals of your battery. Bud
Bud Krueger

If it gets its signal from a plug lead and the tach gets the polarity it wants, then I think you'll be OK.

If it's like the tachs of my youth and gets its signal from the points to coil lead then I'm afraid you will likely toast the tach.
J E Carroll

My fancy expensive digital engine analyzer does not work on positive ground cars. When I set the dwell on the positive ground MGs I have to use the cheap analog Actron engine analyzer I bought at Sears when I was in high school.
D. Sander

The tach on my F-100 taps onto the distributer so I guess it would be toast on the + ground TD. I'll look around for one that clamps onto a spark plug wire.

I reverse wired a cigar lighter and run the Garmin off of that to get a good speed reading.


J K Chapin

Jud, I think I just googled the instruction manual for a Sunpro Supertach. Forget about trying it with a positive ground system. You green wire goes directly to the distributor. It's not a clamp-on system. Bud
Bud Krueger

Bud, thanks. Jud
J K Chapin

I woul not expect it to get toasted as long as the supply power gets the right polarity. The signal input reacts to the contact switching on and off (which means 12 or 0) and you have that in any configuration, be it pos or neg ground. It should also read correctly, since it measures the frequency of ONE of the switching slopes (i.e. On or off) and ignores the other to be independent of swell.

Mike Fritsch

Forgot- of course on pos gnd the usual care about isolating the tach housing from the car chassis still apply, or at least something will get toasted....
Mike Fritsch

This thread was discussed between 20/02/2013 and 21/02/2013

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