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MG MGB Technical - Tach

Any thoughts on why my tach starts out reading normal then after about 15-20 seconds it starts bouncing around?
I have the engine to frame ground strap right. Could it be that my battery cable ground is going to the body and not the engine?
Gm Gary

What year? The tach system changed significantly for the 73 model year.

Is it accompanied by a misfire? If not it's likely to be just a tach issue i.e. poor connections or an internal problem. If there is a misfire as well then an intermittent connection through the coil and points/trigger circuit will cause both. With the earlier tach two of the tach connections supply power to the coil, on the later tach power to the coil is independent but an intermittent short to earth on the signal wire will cause both.

All battery earths went to the body from the factory, and a battery/engine earth issue is more likely to affect cranking than anything else.
paulh4

It’s a 1968. My car is In perfect time and runs excellent. One guy said I might need to ground my battery cable to the engine and not the body.
Gm Gary

Electronic ignition? That can cause issues with the earlier tach that you have. Otherwise the 12v supply and earth to the tach could be making or poor connection, or it's an internal fault.

500,000 were built using the body for battery and engine earths, and if there is an issue with either of those (or the 12v connection) as I say it's more likely to be noticeable first with cranking. You can check it though, by connecting a voltmeter between the battery earth post and a good body earth and seeing what happens when you crank, and doing the same between the engine and a good body earth. You may need an analogue meter though to get a meaningful reading, ideally both should be under 0.5v. Also check the charging voltage isn't fluctuating when the tach is.
paulh4

I had a problem with the tach needle bouncing. The gauge guy recommended attaching a ground wire (there is a lug on the tach case).

The bounce turned out to be caused by a worn distributor shaft/bushing. It was a pertronix distributor (lousy quality) and the magnet had started scraping the reluctor. Replacing the distributor body and installing the existing Pertronix components solved the problem. Oddly, the apparent misfire was not noticeable as a poor running condition. The tach needle was bouncing more and more so correcting the fault was probably timely. The Pertronix distributor seems to be good for only 100K miles, at which point it becomes a throwaway.
Glenn Mallory

They have to be earthed anyway for the electronics as well as the internal illumination. If that fails the usual symptom is that the tach stops working when the lights are on. The bouncing was perhaps due to additional pulses rather than missing pulses, hence no misfire.
paulh4

This thread was discussed between 09/05/2023 and 28/05/2023

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