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MG MGB Technical - tachometer out-car won't start

My tachometer is out of the car for a rebuild. I could not start the car today, is it because the tachometer is wired through the starter system somehow?

If so, is there a way to by-pass this problem, or am I stuck with a dead car until the tach is back?

Thank you.

Craig C. in Florida
cwcolby

Hi Craig.

If it is the type of tacho that has a current loop pickup then removing it will effectively disconnect the ignition system.

If it is the current loop type then bypassing it by connecting the two white wires together should get the revs back again.

Don
Don

Should make no difference, current or voltage type, unless you have a voltage one and have shorted the white to ground.
Stan Best

Craig - It depends on the year of your car. The early cars have the wire from the ignition switch looped through a small link that plugs into the back of the tachometer and then on to the coil. Since there is no break in this wire, removing the tachometer on these cars have no effect on starting the engine. Sometime later, the loop was placed internally in the tachometer and the wire from the ignition switch connects via a bullet connector, with the wire going on to the coil connecting to a second bullet connector on the back of the tachometer. In this case, removing the tachometer breaks the connection from the ignition switch to the coil. So, depending on the tach that is in your car, either Don or Stan could be correct. If you have the later tach in your car, you can get back on the road by making up a small jumper to connect the two ends of the white wire from the ignition switch to the coil together. Good luck - Dave
David DuBois

Thanks All.

Car is 1968, has 2 white wires with bullet connectors, 1 male and 1 female. Both connect to counterpart connection on back of tach.

Sounds like I have a later type tachometer from the descriptions.

So, bottom line, is it going to be safe to connect the 2 white wires and start the car?

Thanks again.

Craig C. in Florida
cwcolby

Hi all.

Thanks Dave, I had forgotten about the early variant.

Yes, if you make up a proper link wire (so that it can't accidentally short to earth) you should be back to normal.

I think you will also have a 'stray' green wire that needs to be made safe from shorting, and you will need to do something to make the tacho instrument light safe.

Don
Don

Craig, connect the two white together and start'er up. Same set up on my 71 midget.
J.T. White

Up to 72 used the current pulse tach and the two whites for the tach pickup must be joined together for the car to run. Of these up to 66 used a contiuous white loop and an external pickup on the tach, and unless this loop is cut the car should run. After that there were the two separate whites with bullet connectors Dave mentions, and linking these toether should allow the car to run. The should be male and female bullets so should just plug tohether.

From 73 a voltage pulse tach was used which can be removed without affecting the running of the car.
Paul Hunt 2

This thread was discussed between 27/05/2006 and 29/05/2006

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