Tachometer troubles

My 72 911t just started having tach issues. When cold and first starting ...it works fine. After driving 20 minutes and or fully warmed up it decides not to work at all. No bouncing needle.... nothing. Next day it works again then slowly dropping to 0. Tried turning off and restart still nothing. Car runs great, points are new, battery is good, alternator charging at 13.9-14.2. Also I recently installed my rebuilt engine maybe coincidence?? All wires appear to be correct on the alternator including the ground strap.

Any time I have tech blues I always check the black wire/purple stringer and spade connector that goes to the distributor.... it can't hurt

The old circuit boards in the tachs can go bad. I would send it to North Hollywood Speeometer and have them install an updated circuit board.

Thanks guys. Just got home from work and fired it up and of course the tach works .... temporarily!!Off to North Hollywood it goes I guess.

The tach works through what EE's call a "one-shot" or monostable multivibrator-- it involves two transistors and a capacitor. One transistor is off and the other one is conducting, until a voltage pulse arrives from the points and turns the OFF transistor ON and the ON transistor OFF. When the OFF transistor turns ON, it starts conducting through a capacitor, which current flow induces a flux in the winding for the needle, causing it to bounce up. Successive pulses from the tach bounce it higher and higher, like a volleyball being hit repeatedly.

When the capacitor heats up it stops working, which is why you have no needle movement.

That's explanation #1. #2 is that your voltage regulator is kaput and the system voltage is going too high for your tach to work. When you turn the car off the battery voltage sinks. When you start it, the high voltage starts charging the battery, until eventually the system voltage is too high. That takes about 20 minutes.

So before you send anything out to North Hollywood-- ask yourself

1) what is battery voltage at rest measured at the terminals with engine OFF;
2) what is it with engine at IDLE; and
3) what is it at 3000 RPM?

post the numbers here before you mail anything and

GOOD LUCK!

Fantastic analysis!!!That's the explanation I needed. Just so happens I dug up my old good regulator last night and is sitting in front of me now. Diagnosis will start tomorrow. Hopefully this is a quick fix. Great advice and awesome sight!!!!