Re: Dual oil temp gauge probs-help

[Brett Van Sprewenburg <brett@netacc.net>]

>All,
>
>I rebuilt my center console this weekend to fix the theft damage from way
>back. Having climate controls again is SO COOL! Car cold-car hot-car
>cold-car hot-feet cold-shoulders cold-feet hot-shoulders hot!! What a great
>feature! Anywhoo, I also hooked up the stock oil temp gauge by tapping the
>Audi gauge I already had in place. I did this for two reasons, I've always
>been suspicious of the Audi gauge reading high and there is no use in having
>a gauge installed but not hooked up("Dude..why doesn't that gauge work?".
>The problem is that now neither gauge read correctly. The Audi gauge never
>gets higher than 80C and the stock gauge never gets higher than 80C. Is this
>because the tap somehow screwed up the resistance that gauges are expecting
>to see? Im a wiring moron mind you. Please let me know as I would to get
>them both working before ND, thanks.
>
>Shannon Fenton

As was previously explained the gauges are probably very simple, which
means they are calibrated to operate across the visible scale within a
certain range of volts.  The sender is a temperature
sensitive variable resister.  I believe the meter 'compares' the voltage
potential between it's 12v and ground, with the voltage potential between
the 12v and sender (G) line.  This difference moves the needle.  As
the sensor heats up, it's resistance either goes up, or down depending,
in a fashion that's calibrated to the gauge 'reading' it...what I mean
is that a certain temperature change will induce a certain change in
resistance which equates to a certain needle movement.  Not all gauges
or senders are matched to each other because of this.

Now, if you tap the line running between the sender and the gauge
with the 'G' line of another gauge, another path of current flow has
been introduced into the 'comparator' of both gauges.  There are now
3 paths (2 from each gauges view) of resistance, and which ever one
flows current more easily, will be the one used to set the needles.
One is the normal resistance through the sender, the other is through
the 'G' line of the other gauge.  This probably results in quite
inaccurate readings.

An idea I have that might fix it would be to put 12v diodes in
each of the 'G' lines for both gauges so that current could only
flow 'towards' the gauge unit, and not back towards the other gauge.

Anyone have any thoughts on my educated guessing and well intentioned
(and possibly incorrect) reasoning? ;-)

==Brett

 \/  '84 Scirocco (ITB racer 2B) | "Hot VW's, take two home. They're small"
\/\/ '88 Scirocco 16v (Show), '92 Passat 16v (Winter+) | - brett@netacc.net

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