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MG MGB Technical - 68 mgb tach

My tach will not work and I was wondering if anyone knew how to test it on my workbench.
FYI All connections test ok in the car
2 white wires one green wire and one ground plus the light. Thanks
Bill Mason

I've not done it, but you can test it using a battery charger, although that's normally to check the calibration.

If it doesn't work at all then there are two simple tests you can do to see whether it is the tach or the wiring that is causing the problem.

The first test is to simply make sure you have 12v between the insulated spade on the back of the tach (white on early cars, green later), and the case. No voltage, then it won't work, of course.

You don't say the year, but if you have the pre-73 tach with bullets on the back for the white wires then simply unplug one of them while the engine is running. If the engine stops then you know the trigger signal is going through the tach pickup, so if tjhe tach isn't registering it's the tach that is faulty. If the engine *doesn't* stop, then the ignition supply to the coil is bypassing the tach, hence no trigger signal.

If you have the early tach with the external pickup, and a continuous white coming out of the harness, through the pickup, and back into the harness, then you can't do the second test unless you cut the wire. Short of that the only way to make sure that wire isn't being bypassed, is to remove it from the pickup (noting carefully which way it runs!), and disconnect the white from the coil SW or +ve, and provide another temporary wire from the ignition switch via the pickup (wired correctly!) to the coil SW or +ve. If still no tach, then assuming you *have* wired it correctly, then the tach is faulty.

Of course if the tach or harness has been fiddled with, and it hasn't worked since, then it may be wired through the pickup incorrectly, so have a look here to see how it should be.
PaulH Solihull


The link shows all about the early current sense tacho , the one with the external wire loop.

Herb Adler

Since the title of the thread is "68 mgb tach" I assume he has the later tach....
Rob Edwards

"Since the title of the thread is "68 mgb tach"

Ah yes :o)
PaulH Solihull

Now that I have finished all MY HONEY DO LIST
It was time to again check out the tach
Thanks to all that answered and as Paul said check the green to black wires for current and found there was none. So I hooked up a jumper wire from the -ve coil to the spade on the tach and it worked.

I relize that this bypasses the voltage stablizer.
Will it cause any problems later on ????
Bill Mason

No, the voltage 'stabiliser' is for the thermal fuel and temp gauges only. As originally provided it switches 12v on and off about once per second to average 10v over time, which wouldn't be ideal for a tach :o)
PaulH Solihull

This thread was discussed between 24/11/2012 and 02/12/2012

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