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Re: fuel gauge problems

To: Will Zehring <wzehring@cmb.biosci.wayne.edu>
Subject: Re: fuel gauge problems
From: "W. Ray Gibbons" <gibbons@northpole.med.uvm.edu>
Date: Thu, 27 Apr 1995 12:32:03 -0400 (EDT)
On Thu, 27 Apr 1995, Will Zehring wrote:

> question:  I assume that what the sending unit really is is a variable 
> resistor (Yes/no?).  

Yes.

>Hence, what the gas gauge really is is an ampmeter 
> (yes/no?). 
Yes.

 Can I just hook up my trusty radio shack amp/volt meter to the 
> lines and make sure that the signal is a steady one (i.e. that the needle's 
> dance is not due to wildly fluctuating signals from the sending unit)?  

Yes, if it is really a volt-ohm-ammeter and you do it right.

What 
> do I read?  

Well, the *first* thing to read might be the F****** manual.  Sorry 
Will, who could resist a setup like that?

Amps?  What scale?  Comments?  Diplomas?

Assuming that the system follows the good ol' standard british wiring
colour scheme, there should be two wires to terminals on your guage.  One
ought to be green, if memory serves--that's the power side.  The other
should be green with a black tracer.  (You can tell someone who is really
over the top in this hobby--when he has the color code memorized, there's
no hope.) If there's any doubt, or if the colors are not as stated, look
at the back of the guage.  One terminal may be marked "T" for tank. 
Disconnect the wire to the tank, and connect your trusty VOM between it
and a good ground.  Set the meter on ohms, with a range 0 to 100 or so.
(Be sure the meter battery is good--i.e. meter reads 0 with leads shorted
together.)

The resistance should be between about 0 and 70 ohms (reasoning by analogy
with a sprite sending unit).  Whatever it is, it should be fairly steady. 
If it jerks around like your gauge did, then observe:  does it twitch
between something reasonable and 0?  If so, you've an intermittant short
to ground somewhere.  Does it twitch between reasonable and infinity? 
Then you have an intermittant open circuit.  If it is steady at something 
between 0 and 100, and not either 0 or infinity, and if it changes 
gradually as you use gas and suddenly when you add gas, look elsewhere for 
the problem.

Good luck.  Intermittant electrical faults are the funnest kind.  I bet 
you 50 cents it's a bad ground.  It is almost always a bad ground.  

> I suppose it is time to either (1) ignore the whole thing and fill up every 
> 200 miles or (2) get the gauge fixed.

You could always get a T-series gauge.

> 
> Will "and my oil preassure gague is sticking, too" Zehring

At zero??????  GOOD luck.  Actually, you don't need to worry about oil 
preassure, but oil pressure, that's a whole different kettle of worms.

   Ray "Gonna write more questions now" Gibbons  

Dept. of Molecular Physiology & Biophysics
                Univ. of Vermont College of Medicine, Burlington, VT
                gibbons@northpole.med.uvm.edu  (802) 656-8910


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