[DeTomaso] Pantera Tach Shows 0 at Cold Idle

[Mike & Elizabeth]

Hey Mike, good to hear you out there.  Sound's like a grounding issue with
the tach but beyond that I'm afraid I'm of no help.  Have you tried any
other switches such as the fan or wipers to see if you get the same
reaction?

Hope you had a good Christmas and are up for a better New Year.  Stary Safe
Mike Thomas

-----Original Message-----
From: DeTomaso <detomaso-bounces at server.detomasolist.com> On Behalf Of Mike
Reilly
Sent: Tuesday, December 29, 2020 3:21 PM
To: detomaso at server.detomasolist.com
Subject: [DeTomaso] Pantera Tach Shows 0 at Cold Idle

Hi all.   I've been having a weird tach problem for a few months now.  When
I cold start the car the tach sits at zero but will respond when I give it
gas and go to 2000 RPMs for example.  When I let off the gas it goes back to
zero yet the car is idling fine at around 900 RPMs.  Now when I push up on
both window switches then the tach will respond and read at idle speed but
when I let go of the window switches the tach goes back to zero.  However,
once the car is warmed up then the tach works fine and shows idle speed etc.
without ever going down to zero unless the motor is turned off.  If I
restart the car when warm the tack works fine.  I think somehow I'm not
getting enough voltage to the tack when the car is cold.

Videos of the problem:

  *   Cold start and playing with the windows (you can hear me describing
what I'm doing near the end of this video) (https://youtu.be/yIFtsp0ljzk)
  *   Example of revving the motor when cold (https://youtu.be/kWaRfGfj2HU)
  *   Warm running and every thing works fine (https://youtu.be/X1RY4VwmSmA)

Jon Haas rebuilt my tach as I thought that might have been the problem but
it was doing this behavior both before and after Jon's rebuild.  (By the way
he does great work on rebuilds and found some cracked resistors in my tach
that were about to fail).  I also converted to his new alternator to replace
my stock rebuilt alternator thinking that would be the issue but the tach
idle problem remained.  This new alternator has a built in voltage
regulator.

I measured the various voltages coming to the tach and per my comms with Jon
these are correct voltages when the engine is running:

  *   Voltage at the tach (+terminal) light blue wire = 14.84V
  *   Voltage at the +coil wire = 14V
  *   Voltage at the +Duraspark = 14V

My Pantera is #7366 and is a 74L running a DuraSpark electronic ignition.
My battery is strong, new and kept on a trickle charger

Any ideas as to why when the motor is cold the tach won't register at idle
speed but does once the car is at operating temp?   It's also strange that
it will come back to life when cold if I push up on both window switches
when the windows are already closed thus adding some load to the car's
electrical circuit.

Thanks, Mike Reilly