[DeTomaso] headlights vs temp guage

[Rich]

I used to have this problem on my car.
I don't know why it does it like Bill explains, but I had a mercedes that 
had about 30 black wires that all came together behind the dash to ground 
everything.  I thought well maybe they know something I do not.  So I 
reground the council, speedo gage cluster and the solenoid box under the 
dash and the problem went away.
Rich
----- Original Message ----- 
From: <SOBill at aol.com>
To: <MikeLDrew at aol.com>; <peter-kovacs at sbcglobal.net>; 
<detomaso at realbig.com>
Sent: Friday, February 20, 2009 11:58 PM
Subject: Re: [DeTomaso] headlights vs temp guage


> 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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