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MG MGB Technical - Crane XR700 and Tachometer

Can anyone advise me how I can get the tachometer to work with a crane xr700 electronic ignition. So far, I have used two tachs, they wiggle for a micro second and then never move again.

I am using a +/- 3 ohm coil, and do not have the voltage drop resistor connected.

A shop advised me the tach would not work with a standard hookup, and wanted $200+ for their electronic circuitry.
James Huggins

James, I don't remember what year car you have. I have a 74 B with the set up you describe, my tachometer works fine. My coil is a stock Lucas 3 ohm coil and the tachometer is hooked just as it was with points. I had a 68 GT several years ago and couldn't get the tachometer in it to work properly with the XR700. I tried 3 different used Tachometers, they would all work but the rpms weren't right so I think I had some bad tachometers. I converted to a later electrinic MGB tachometer on the 68 and it worked OK, maybe less than 100% accurate. Seems I remember you posting on this before?

Clifton
Clifton Gordon

Thanks Clifton,

I posted something like it before, this is a continuation. Finally got back to the subject.

Question: How is yours wired that is working. When I got the car, everything was apart.

James
James Huggins

The 74 has an RVC 1410/00AF tachometer. It has a white/black wire from the negative coil terminal to the tachometer. As I recall it plugs into a bullet type connector. It's exactly the same as it was with points. The tachometer also has a green wire coming into it, there should be 12 volts on the green wire.

As I said I had an earlier car with the the white wire going through the tachometer and had trouble getting the tachometer to work properly, I tried 2-3 tachometers, they worked but they we not even close to being accurate. I switched the tachometer to a RVC 1410/00AF. To switch them I removed the white wires from the old tachometer and plugged them together to provide power for the coil. I ran a white black wire from the negative coil terminal to the tachometer signal input. It was working fine when I sold the car about 3 years ago.

Clifton
Clifton Gordon

I have a 68 GT with the XR-700 and the OE Smith's RVI 1433/00 tach. I have not had any trouble with it affecting the tach's indication of RPMs or with any significant misrepresentation of actual RPMs. But, then, that has just been my luck.

On my car, it wired up as below:

Black to chassis ground (- grnd battery)
Red to stock coil (+)
Yellow to coil (-)
Molex connector to trigger unit within the distributor.

If you don't have a copy of the instructions they can be had as a PDF file at http://www.cranecams.com/pdf/90002000a.pdf Page 16 does have instructions for modifications necessary to some Smith's tachs.
Bob Muenchausen

Bob, I think the problem I had on the 68 GT was that the tachometers I tried all needed repair. They all worked but had big errors compared to a dwell-tac. One wouldn't go over 4k rpm. As you know they can be adjusted on the back but I couldn't get them within limits with the adjuster.

Clifton
Clifton Gordon

Are you aware they make an adapter for proper operation with some tach? Mine works fine with the OEM tach but it's a '79...
CD Dewey

Thanks everyone for your replies.

Bob, that is the way mine is wired. Doesn't that mean there is switched voltage thru the tach from the switch as well as the XR700. White wires to bayonet male and female on tach to primary of internal coil.

My coil is an Intermotor (UK), blue case with yellow and white band on top. It appears to have about 3 ohms resistance. Car has more power without resistor, and coil does not get hot.

I also ordered a new crane (per crane), same thing, no help.

The tach did work, although not accurately when I got the car running, then quit. Hasn't worked since. Replace tach with used one from EBay, same thing.

I really think I have two bad tachs.

James
James Huggins

This thread was discussed between 20/05/2007 and 23/05/2007

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