Well guys,
Here is what I have learned about my none-functioning gas gauge. I used
a digital multimeter and got the following readings: Limiting coil (Empty
side) in gauge=28 ohms; operating coil (Full side) in gauge=32 ohms; tank
sending unit 1 ohm at empty, 32 ohms at full with a linear resistance in
between. Note: the gauge must be disconnected and disassembled to get these
readings. Across the two terminals you should read 28 ohms (Limiting coil
value only). Note: I got the same readings on two different gas gauges!
Here is how I have determined this simple, yet challenging unit works.
I used the small diagram in the shop manual for reference. Please correct
me if I go wrong (I know you will).
As the fuel level in the tank increases, the resistance of the sending
unit increases, but the total circuit resistance decreases (operating coil
and sending unit are electrically in parallel). When the total circuit
resistance decreases, the total circuit current increases. Since the
sending unit resistance went up, the current through the sending unit
decreases, causing increased current through the operating coil. More
current means a stronger magnetic field which pulls the needle toward the
full position.
After reading Bob Adler's answer today, I tried what he suggested. I
connected my battery charger + side to the gauge input and the - side to the
gauge case. If I am correct, that is the equivalent of disconnecting the tank
sending unit and I should have seen a full indication. No luck, the gauges,
both of them, jump to empty. Does this mean both of these gauges are bad in
exactly the same way? It appears the operating coils are open, and the needle
will never be piulled towards full.
Help, I'm about to spend $45 on a rebuilt gauge if no one else has been
down this road. These are the things that make me wish I would have kept my
VW bug. Sorry for the long post.
Mike Klepp
'48 3100
Wichita Falls, TX
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