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MG Car Repair/1979 mgb usa ignition


when driving, the tach will sometimes fluctuate from normal rpm to zero and then back to normal. I lose power at the same time. Sometimes the tach goes to zero and I lose power for a few seconds,and then it starts running on it's own and runs perfectly. I have a relatively new coil. What's up? Thank you

Hi Don

There are several possibilities.  Since you have recently replaced the coil, I would start there:

1) Make sure that all the terminals on the coil are a good tight fit.  If you lose power to the coil, then the tacho will die at the same time.

2) Carefully check the white wire with a black stripe that goes between the coil and the distributor.  If it can touch anything en route, then this would ground the wire, causing an immediate momentary loss of power.  The tacho would drop to zero at the same time.

3) The power supply to the coil is a white wire.  There is another white wire that goes via the overdrive switch to the overdrive solenoid underneath the car.  This wire can dangle underneath the car and touch the gearbox cross-member.  Every time it touches, power to the coil is shorted out.  This results in a momentary loss of power and the tacho drops to zero.

4) You have an overdrive switch on the gearshift knob.  This has 2 wires going to it.  They are the same white wires and they can fray and touch ground.  Same result.

Let me know what you find.
Regards, Barrie

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Barrie Jones


MG Sports Cars from 1949 to 1980 - Including MGB, MGBGT V8, TF1500, TF, TD and modern Midgets. I also specialise in SU carburettors, Lucas wiring, suspension, steering & brakes.


I have owned my 1955 TF1500 since 1966. Technical specialist for the TD and TF with the MG Car Club T Register. Also owned 20 MGBs and currently own an MGBGT V8. Written 3 books on MGs and produced a DVD on how to strip and rebuild the TD/TF gearbox.

MG Car Club.

Barrie's Notes on the 1953-55 MG TF (author). /=====/ Barrie's Notes on the MGB (author). /=====/ Barrie's Notes on the MGB GT V8 (author). /=====/ The Essential Buyer's Guide to the MG TD, TF and TF1500. (author)

M Phil, C Eng, BSc (Eng).

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