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MG MGB GT V8 Factory Originals Technical - rev counter wiring/options

Hi All,

I can't find anything about ths in the archives.

Has anyone tried to use the electronics from the SD1 rev counter in the CB rev counter?

What is involved?

I'm using the SD1 box and it still has its wiring etc attached.

I must admit, I'm generally a bit confused about how to address the whole rev counter and speedo issue - what comes out of where and what goes into where!

As the SD1 had an electronic speedo as well, and if I swap the transducer out for a manual speedo cable drive won't I lose the electronic rev counter input as well??

Liam

Liam H

Just had another thought - the rev counter runs from the coil doesn't it ? Which means my two green wires in the gearbox are more likely to be for reversing lights?

Doh, doh, doh - why can't I match up these two haynes manuals.
Liam H

Yes, the two wires on your gearbox are for the reverse lights.

I'm looking at the Haynes manual wiring diagram for the V8 S model and it looks like your tach is hooked up with a white wire, a white/black wire and a black one. Ironically, the same as the wiring harness for a 72-74 MGB (at least the North American cars). The only difference I can see is the white/black goes into a ballast resistor then connects into the distributor. The white/black wire is the trigger wire for the tach and was connected to the negative terminal on the coil (which it shows in the wiring diagram). I don't see any reason why the SD1 tach shouldn't work in your car.

Sorry, can't help you with the speedo as I didn't use one in my car. What year of car did you use for the donor? I'll look through the manual and see what I can find.

Cheers
Simon
Simon Austin

This is an easy one.
Adjust the MG tacho'
Visit Leon Zak's website:

http://zaks.com/mgb/

<<<The only part you need is a 100K 10 turn pot they're available from Radio Shack, (Dick Smiths etc.) I wouldn't think that they are over a couple bucks.

The pot gets soldered on pin 4 and pin 7 of the only chip in the tach. There's a mark (usually an indentation in the chip case) on the corner of the chip that is pin 1. If you hold that so it's in the upper left corner, the pins are numbered 1-8 counter clockwise.>>>

Zacs site has lots of pictures for the electronicaly illiterate like me.
For my car it worked out about 20kohm. Since I don't have fancy electronic monitors, wave form detectors etc I simply left one of the wires loosely attached; I adjusted the resistor until I had the tacho reading about half, unhooked wire, check reading, reattached and fine tuned etc . Did this for a number of RPM's. The only glitch was that Dick Smiths sold me the wrong resistor (100ohms) to begin with, had to go back. This little resistor is blue, and about 20 by 6 by 6 mm. Cost $2.00, plus I had to buy a cheap $9 soldering iron, which has come in very useful. Took a couple of hours of not unpleasant fiddleing.
Peter

A better tach circuit

is the one you want
Peter

Liam, I used the innards of the SD1 tach & speedo & fitted them into the MGB inst. cases on my CB conversion. The dial diams had to be reduced by approx 1/8" & the cases needed a few extra holes drilled in them. The dials are made of perspex, so altering them was easy. Extra holes were also drilled in the dial faces for the ign. & hi beam lights. In my case, the speedo didn't need recalibrating as I used the same diff ratio as the Rover's HTHs, Barrie E
Barrie Egerton

This thread was discussed between 14/02/2006 and 17/02/2006

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