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MG MGB Technical - tachometer mgb

I own a 74 mgb gt that I fitted with a 79 motor.
The tach did not work. I bought 2 other tachs and none work. Is there something different in the electrical power that prevents the tach to work? What is the best way to test a tachometer?

Francois
francois gay

Salut Francois.

Was the tach working before you installed the '79 engine?
Derek Nicholson

Francois,

the 1979 engine was fitted with an electronic dissy and an different tach that was used from mid 1977 on.

If you still have your old dissy, it will work with the old tach on the 'new' engine, but you should check wheter it's characteristics are O.K. for this engine.
If you do not want to change the dissy, you will nead a tach that aught to be connected to ignition+ and to the no.1 spade of your coil.

Ralph
Ralph

From 1973 on the *principle* of operation of the tach remained the same regardless of the ignition type (the tachs themselves only changed with the varying dashboards and instrument sizes). This was voltage pulses picked up from the -ve terminal of the coil and fed to the tach on a white/black. So a 79 engine should work in a 74 car and vice-versa. The tach also needs a 12v supply (from a green, fused ignition) and a ground (black) so check these as well. To check the continuity of the white/black, ground this at the tach with the engine running and it should stop the engine (incidentally makes a good immobiliser).
Paul Hunt 2

Derek, I don't know if it did.

Ralph, I do not have the old distributor, when I put the car together it had a Mallory double points and I just changed for a pertronix. Both would not make the dist. work.

Paul, the wiring I have has the green go through the voltage stabilizer. Maybe I should try with a direct green as the 1979 wiring.

Thanks all
francois gay

Francois - the green *feeds* both the tach and the voltage stabiliser with system voltage i.e. a full 12v. The voltage stabiliser was never used to feed the tach, only the ancilliary gauges. The tach always had a direct green from 1967 on, before that it was a direct white (unfused ignition).

The distributor should not make any difference as to whether the tach reads or not, although some after-market electronic triggers (used in place of points) can make it read high. If you have after-market electronic ignition i.e. an external module with its own trigger that could result in the tach not working, or maybe the tach wire isn't even connected.
Paul Hunt 2

Thank you Paul.

I understand the wiring better. I did the grounding test. It does stall the engine so that is good. I tested the wiring at the tach and also took power directly at coil and fuse box. Everything looks good but the 3 tachs I have do not work. One of them, my original one, the needle moves up to 200 RPM and stays there, no movement if I race the engine. I really think my 3 tachs are not good. I will try my friends dwell meter that shows RPM and see if that works.

Francois
francois gay

If the needle on at least one of them moves, even not properly as you say, then I tend to agree that all three are bad :o(

Paul Hunt 2

This thread was discussed between 14/03/2008 and 18/03/2008

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