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Tach Question

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Seik

Freshman Member
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Hello all,
The tachometer in my 1967 MG Midget is not working. It hasn't ever worked for me, nor has it ever worked for the previous owner. So it must have quit a while back, and no one bothered to fix it.

My question is this. The tachometer says Positive Earth on the face. However, my 67 Midget is negative earth. Could the car have been converted to negative earth, and whomever did it did not bother to switch tachs? Would something like that make the tach not work? If so, logic tells me buying a negative earth tach would solve the problem.

Also, on a peripheral note, there is a stray spade lead/wire by the coil, that when I connect it to the coil prevents the engine from starting. I'm not sure if that is from the tach or related or what (I have no idea what it is). /ubbthreads/images/graemlins/laugh.gif
 
Yes, if the tach has not been converted from pos to neg earth, it would not work. The direction of the wire at the pick up point at the back of the tach must be reversed. I also believe that there needs to be a change inside the tach itself. Use the search feature of this forum for "tachs" and "change pos to neg" and you might come up with the proper way to do it. Also two companies, Nosingers and MO Ma Manufacturing repair tachs.
 
There are 2 ways to convert a tach.
One is switching the wire/resistor inside a positive grd tach and swapping the white wire external loop.

Or if you can get hold of a later (smaller) negative ground electric tach from a 68-73 Midget of MGB, use those guts in the larger 67 tach housing and the face and needle are a direct swap. Those later tachs have the internal loop.
You will have to do some minor dremel work to the back of the original housing but it's a neat simple switch from + to - ground for the tach.
 
So if I get my hands on a '68 Midget RVI negative grd tach, would I still have to switch/reverse the external wire loop on the outside of the tach casing? The insides would be left alone?

Ebay

There is that on eBay. He says it is RVI, so it is current sensing. But you say the loop is internal?

Also, this one on eBay.
Ebay 2
 
If you really feel you must use the RVI tach then Google to find the conversion process. There are several sites that discuss this. Among other things there I believe there is a diode that has to be reversed. Most RVIs are positive ground but you may luck out and find one that is negative.

RVI tachs do not like electronic ignitions so don't bother buying (or converting) one if you have switched to Crane, Pertronix, or any other electronic system.

I have converted the RVI tachs for RVC movements (later voltage sensing), and I've converted some RVI tachs to use electronics from late model (non-Smiths) tachs. I greatly prefer the voltage sensing tachs which make a single wiring connection to coil (-).

The RVI tachs are wired in series with power to the coil. Without seeing your car and its wiring I can't comment on the specifics. However, if your RVI tach is wired incorrectly it's quite possible for it to act as a kill switch.

EDIT: The auction with the tach and temp gauge is good if you want to stick with the RVI. However, the temp gauge is toast since the line's been cut. It will cost more to fix than it's worth. The second auction is for the RVC type and it will be a better choice (IMHO).
 
Sounds like RVC is the way to go then. Greatly appreciate all the help. You guys never make me feel stupid for asking what are (probably) stupid questions. /ubbthreads/images/graemlins/tongue.gif
 
Good luck with your tach purchase. If originality isn't paramount to you there are lots of good VDO and AutoMeter tachs that show up on eBay. Obviously a Smiths RVC will look more original if you can find one with the right case size.
 
I went though the same process. Tried to convert mine to from pos to neg ground as the many instructions indicated. I'm just not that smart to handle those directions.

I am using a 7" VDO tach from Summit Racing. New item. Fits and works wonders. "dklawson" above is a VERY knowledgable person. He tried to help me out on the exact problem, I just couldn't figure things out and went for the 'easy way out' (enough money will solve all problems).
 
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