Tach jumping all over

[olebusman]

 Ok guys, here's the problem. My setup is a 4106 with a 6V92ta and a Bill Teal V730 tranny setup and the original 50dn alternator. I have been chasing this problem for months. The tach, driven off the relay terminal, does not read right. I have a newer delco regulator. As I start the engine the alternator ramps up slowly as normal. As I turn on the high idle the tach goes to high, then starts jumping between 1500 and 2000 rpm. I have been through cables, both ground and power. I get 14.2 volts at the batteries when its running. All is good. I can't make up my mind if its a bad tach or something in the alt. I don't know how much voltage should be on the relay terminal. It seems to be 5.60 to 6.3-4 depending on engine speed. Has anyone had a bad diode that caused this problem. This problem started when a transpo v1200 regulator went bad. I'm tired of this problem. Any ideas?  thanks  olebusman

[Geoff]

I have never liked alternator driven tachometer pickups​.  They are always off.  I don't know what caused your sudden problem, but two magnets on the crankshaft or cam pulley will give you 100% accurate readings with a sensor.  Also, if your flywheel housing is dry and drilled out for a ring gear tooth sensor you can get 100% readings from it.

--Geoff

[akroyaleagle]

Try putting a load on it. Turn on the lights, it may settle down.

[mikelutestanski]

Hello.   It could be a diode    The only sure way to check that is to use an oscilloscope to read the pulse train . The pulse are the diodes firing at half wave rectification.  The diodes filter the alternator pulses to flatten the waves into a more DC signal.  The R terminal is taken out of the middle of the diode configuration when the pulses are greater.  The bigger pulses are read by the tacho.  The faster the rpms the more pulses... 
  So  the R terminal is pulsing and your tach reads the pulses.. SO if the problem started when you changed the regulator is makes sense that the new regulator may be involved  with the pulse signal....
   SO.  The wire from the R terminal  and a good separate ground.  Are both needed to complete the circuit.   ( what I am suggesting is to isolate the circuit consisting of the tach and the R terminal. Making sure nothing  is adding pulses to the wires).   
   I would check the wiring to the new regulator to make sure no wires are running parallel to each other.  Pulsing wires should cross at right angles to minimixe crosstalk....
  I installed a tach and am using the R terminal for at least 20 years.. been ok so far.. The only problem I had was getting the dip switches set correctly to display the proper reading...   

FWIW.  Just some thoughts...
    Regards and happy busssin. 
    Mike

[olebusman]

  Yeah Mike, I finely checked the frequency  with a hertz meter. In talking with Precision meters as I was checking we came to the conclusion that my 20 yr old tach is bad. Time will tell.  Thanks for the reply.  Geoff, I don't need precision just a good indication that stays the same. I set my no load speed with my optical tach.    akroyaleagle, I tried that many different ways including running the ac off the alt. none of it made any difference and I run with my headlights on for safety. Thanks for the replies everyone. Olebusman