[DeTomaso] Temp Gauge Rise

Dave McManus dave at damardirect.com
Tue Apr 27 10:31:27 EDT 2010


Will, I noticed the same problem last year. Here is what SOBill has to say
about the problem: (I sent the attachment to you directly)

>From 2-23-09
Dave,
 
12V is applied to the temp gauge. The temp gauge divides this voltage and
sends part of it to the temp sender in the engine. The temp sender is a
resistor connected to chassis ground. Inside the gauge the divider circuit
and a light bulb are both connected to a terminal on the gauge case. The
terminal on the gauge case is connected to a like terminal on all the other
gauge cases ("daisy chained") and eventually connected to chassis ground.
 
When you turn on the headlight, you also turn on the lights in all the
gauges. The current drawn by all the light bulbs in the gauges will cause
the ground terminal on each gauge to be at a slightly higher voltage than
the ground terminal at the sender for that gauge. This difference in voltage
between the two ground points for the temperature gauge will cause the
temperature needle to move up scale.
 
See
http://www.panteraplace.com/Electrical/SOBill%20Veglia%20Temp%20Fig%203.jpg
for the details of the gauge circuitry. Just imagine that the ground
connection on  the right hand coil in the gauge (which is the ground
terminal on the gauge case) and the ground connection from the sensor (which
is on the engine, via the ground strap at the ZF) are at different voltage
levels. Elementary, my dear Watson.
 
So there you have it.
 
To solve this problem, connect a wire directly from the ground terminal on
the temp gauge to a solid chassis ground. The factory connected the gauge
grounds in a "daisy chain" because it was simple and easy. Each gauge should
have a direct connection to ground. In fact,  the lighting circuit should
have a separate power and ground from the sensing circuit ..... sort of like
the space shuttle does it.
 
Mike Drew suggests adding a second ground connection to the daisy chain at
which ever gauge has a single black wire on it's ground terminal. That fix
might work, can't hurt to try.
 
Attached are pictures of a test I did with my temp gauge and a one ohm
resistor.
 
Have fun,
 
SOBill Taylor
sobill at aol.com





-----Original Message-----
From: Will Demelo [mailto:wdemelo at cogeco.ca] 
Sent: Tuesday, April 27, 2010 6:31 AM
To: detomaso at realbig.com
Subject: [DeTomaso] Temp Gauge Rise

Another fellow on the PI board has reminded me of a similar problem I'm
having. My temp gauge rises considerably ( 15-20 deg) when I raise my
headlights. I was thinking it was induction in the wiring, but I beleive the
headlight switch only carries the current of the relay coil, which is
minimal.
Any suggestions?
Will




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