When cold the tacho indicates the cranking speed of the motor then indicates idling and driving revs accurately. When the alternator is hot, on cranking, the tacho needle leaps up to 2500-3000 revs while the engine is still cranking. It then resumes an accurate reading of the rpm. It does not fluctuate while driving and steadily indicates engine revs. I diassembled the alternator, cleaned sliprings, and blew body out. Fitted a new regulator, but problem persists. Battery and charging voltages are OK Any ideas would be appreciated . Thank you

That's a difficult one. They can be notoriously fickle. The tacho on my original TM used to play up in wet weather. Sometimes it would work intermittently and sometimes not at all for several days after rain. I haven't had any tacho problems with my EFI TP sedan or wagon but the carby TP sedan also had intermittent problems with its tacho. I eventually found the cause, a loose module in the distributor. It is unlikely the cause of yours is the same but might be worthwhile for you to read my first post and post #13 in this thread: http://www.aussiemagna.com/forums/showthread.php?t=101284&highlight=module Otherwise I'd suggest checking for loose or deteriorated wiring at the distributor.

Only had this problem once in my carby TP. Just before the car cut out altogether. Turned out the HT cable from the coil to the distributor popped right out of the coil. So possibly you have some HT arcing somewhere?

annoying but .. the first gens are getting on

tacho gets its signal from the dissy ..check its connections

or you can take the dash apart and replace the tacho .. seperate items. in that dash

ones I owned ..happens on carby never any EFI :nuts:

The pulses from the ignition timing are used to drive the fuel relay in the cabin. There is a connection from the starting circuit to bypass the fuel relay when cranking. It is possible there is something amiss with the fuel relay. Cant think exactly what but your problem seems to be cranking related.
The tacho itself uses a LM2917 integrated circuit and this device converts a pulse train into a DC current which drive the tacho galvanometer coil. It is hard to see how the tacho electronics could cause this problem.
I did have a problem with the tacho on my TP. The needle was jumping around intermittently and ultimately the problem was found to be a faulty connection to the actual galvanometer coil. This fault could not be repaired. I got a Sigma tacho and adapted it to fit but I could not advise anyone to try it cos the sigma and Magna tacho galvanometer units are physically DIFFERENT. Only a motor car manufacturer could invent a way to make two things that are the same to be different. Thank the lord carmakers dont make roller bearings.

Thanks to you four 1st Gen experts for your suggestions and help. It is much appreciated. I discovered a high energy feed back in the primary circuit momentarily after the first ignition impulse. Suggested a faulty condenser. Changed same and now no further problem. Still does not explain why problem occurred only when the distributor was hot. !st Gens! Why do we bother? Thanks again