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MG Midget and Sprite Technical - tachometer fluctuations

Our 1968 Sprite has the inductive loop (on the back of the tach) signal tachometer. It was fluctuating wildly so we had it serviced and restored by Nisonger. It worked much better but only for a short time. Nisonger provided a fancy rewiring schematic to eliminate some known original wiring high resistance connections in the tachometer run. I did the rewire and that made no difference. I spoke to the tachometer mechanic at Nisonger in excruciating detail and he claimed it could be the condenser or the coil but he felt since the tachometer starting behaving properly (no more jumping) at about 4700RPM it was not the Tach. So, I took the 12 volt coil out of my old jeep and installed it and the Tach issues were all immediately resolved! Should have stopped there but I ordered a new (proper?)Lucas coil from Moss. I installed that and the tachometer is acceptable if it doesn't deteriorate but it is no where near as steady as with the jeep coil. The ignition system is stock with points and condenser as original. Anybody else experienced this or have suggestions? The tachometer specialist told me the inductive tachometers are very sensitive to stray signals and things like high output coils. It could just be that the coils Moss is selling are a bit hotter than original???
Suggestions appreciated

Brett
BEW Brett Wright

Can you find another Jeep coil?
Dave O'Neill 2

Well, as an update, I ran the car 15 miles on the new Lucas coil and it is acting up very similar to the old coil. I hooked up the jeep coil and things are much better. I am going to drive it a while to make sure that it does not deteriorate and then if all is good, I am going to mount it down. I really don't get it though. The jeep coil says it is for non ballast resistor applications, I recall. The jeep coil is quite new and available from Walcks so I can replace it.
BEW Brett Wright

I had this many, many years ago (1970's) when I fitted electronic ignition and the tacho became very intermittent. I reckoned it was because the transistor ignition cut down the current going down the feed to the coil. So I took the tacho apart and sussed out the white wire to the coil carries all the current and it is coupled to the tacho by looping a few times around a part of the tacho circuit thus forming a transformer. I made the white wire about twice as long and looped it twice as many times so increasing the coupling and it worked. Full function restored.

It could be a similar thing with all the components ageing and getting less efficient causing the intermittent working.

Rob
Rob aka MG Moneypit

Rob,
Interesting you say that. The Nisonger mechanic said I should try just passing the wire instead of looping it at the tachometer. Similar but different direction solution. I could mess around with that in both directions and see if it makes a difference. I am beginning to like my coil change as a solution. The tachometer is quite steady with the jeep coil except in engine braking if that tells anybody anything. Hard to believe there hasn't been a lot of collective experience on this problem.
Brett
bew wright

Id say check all your ignition connections ... pull it all apart and give it a good cleaning

Btw... auto zone sells the exact same coil that moss sells so heads up
Prop and the Blackhole Midget

This thread was discussed between 16/08/2016 and 17/08/2016

MG Midget and Sprite Technical index

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