[DeTomaso] Pantera Tach Shows 0 at Cold Idle

[Mike Reilly]

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

-------------- next part --------------
   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)
       ([1]https://youtu.be/yIFtsp0ljzk)
     * Example of revving the motor when cold
       ([2]https://youtu.be/kWaRfGfj2HU)
     * Warm running and every thing works fine
       ([3]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

References

   1. https://youtu.be/yIFtsp0ljzk
   2. https://youtu.be/kWaRfGfj2HU
   3. https://youtu.be/X1RY4VwmSmA