Date:         Tue, 21 Feb 95 12:11:41 PST
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         David.Kao@Eng.Sun.COM (David Kao)
Subject:      Re: Weird '81 westy gas guage

Larry MacPhee wrote:
>>
>>When we bought our '81, it had a malfunctioning odometer (sticky) and gas guage 
>>(wildly erratic).  
>>
>>Symptoms:  Fill the tank and the guage reads full.  Drive a bit and the guage 
>>dips to 1/2 full, back up to 3/4 and after driving a bit more down to 1/4 and 
>>sometimes towards empty.
>>
>>Partial Diagnosis:  Since we replaced the guage and are still having identical 
>>problems, the guage was not the problem.  Duh!  We also replaced a little 3 
>>pronged gadget on the back of the clock which the junkyard guy told us was fuel 
>>guage related.
>>

I picked up a temp and a fuel gauge for Volvo from a junkyard a few weeks
ago. I did some experiment to them to see how they work. I think the fuel
gauge of Vanagon works at the same principle as Volvo's.

I found if I apply 6 volt DC to both terminals of the gauge the needle
slowly swings to the full position. I also measured the current that went
through the gauge. It was about 200mA. The current is used to heat up the
thermo element inside the gauge. When it is heated its length is increased
and pushes the needle upward.

To debug your fuel gauge problem you need a VOM (volt-ohm meter). Set the
VOM to 12V (or a range greater than 12). Connect both VOM terminals to the
terminals of the fuel gauge. Fill up fuel tank and start the engine. Reverse
polarity of the VOM (swap the terminals) if the meter goes to negative
reading. Now when the gauge reads full your VOM shoud read around 6Volt.
While you drive the van and the gauge reading fluctuates as you described,
look at the VOM reading to see if it also fluctuates. If the VOM reading 
also fluctuates then the gauge is fine. The problem is either on the 3 pronged 
thing or on the fuel sending unit. If the VOM reading does not fluctuate
like the gauge from 6volt, then 3 volt, then 1.5 volt... The gauge
is faulty.

The little 3 terminal thing in the back of the gauge assembly is a voltage
or current stablizer of the gauge circuit. It's a mechanical thing trying
to do an electronic job and it often does it poorly. One of the 3 terminals
on it gets 12V dc from battery. Another is ground and the 3rd is the regulated
output. I believe the output should be kept at about 6volt. Use the VOM
the check the output. If it fluctuates like your gauge then it is the part
that is defective.

If both the gauge and the regulator are fine then the fuel sending unit is
to blame, end of world, disaster I think. 

David Kao
dkao@eng.sun.com