[DeTomaso] headlights vs temp guage

[SOBill at aol.com]

Peter,
 
O.K. I've ignored my wife's request.
 
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_ 
(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.
 
Have fun,
 
SOBill  Taylor
sobill at aol.com  



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