The symptoms suggest (but not absolutely) that there may be a direct =
short somewhere in the sending unit, or that the float is stuck in the =
empty tank position.
If you want, you can test the fuel guage by connecting a wire-wound =
potentiometer (variable resistor, like a volume control) from your local =
Radio Shack in place of the sending unit. If you find one that is 0 to =
1000 ohms, it should work OK for testing. I don't know offhand what the =
specific resistance value range is for the M37 sending unit, but here =
are some rough estimates of typical resistance value ranges:
Empty Tank 20-50 ohms
Full Tank 200-500 ohms
NOTE: It could also be the OPPOSITE, depending on the vehicle:
Empty Tank 200-500 ohms
Full Tank 20-50 ohms =20
This also provides some basic information for testing the resistance =
through the fuel level sending unit. However, this would involve either =
lowering the fuel level or removing the sending unit to allow you to =
check the change in resistance as the float position changes. Again, the =
above values can vary from vehicle to vehicle. Basically, however, as =
long as the values change continuously as the float position changes, =
then the sending unit is probably working OK. Sometimes a fuel sending =
unit is intermittant, with gaps caused by debris or corrosion on the =
wire resistor windings or insufficient pressure between the contact and =
the wire winding.
WARNING! BE EXTREMELY CAREFUL whenever you remove a sending unit. I've =
said this before in other postings, but I'll say it again. When you =
remove the fuel sending unit or fuel filter from a tank, you are =
allowing air to enter the tank and creating a VERY volatile, explosive =
mix. You'll also have some exposed liquid fuel. Disconnect the battery =
first! Avoid making sparks or flames of ANY kind around the general =
area, including from any metal-to-metal contact! Even STATIC ELECTRICITY =
can ignite gasoline or gasoline vapors! In other words, if you remove =
these components, you do so at your own risk! Research this topic well =
before proceeding with it. Many people become complacent after working =
with fuel systems for a while, yet every year a bunch of people get =
burned or blown to pieces doing just this type of thing.
Anyway, here's another possible source for stuck or intermittant =
readings: The M37 fuel level sending unit has a gear-driven shaft that =
rotates a wire-wound potentiometer in the cap of the sending unit. It's =
not at all unusual for this kind of sending unit to jam up if there's =
any corrosion on the gears or shafts. Sending units in most other =
vehicles are simpler, with a potentiometer that is operated directly by =
the upper end of the float rod, and with only one pivot point. The M37 =
unit has three times as many places where friction is produced and where =
debris or corrosion can create a problem. Clearances are also more =
critical because of the gears and the way the float-rod gear is mounted.
As long as the mechanical parts are clean, there's usually no problem. =
However, there could be a corroded resistor winding, causing a break in =
the circuit. Or, the contact that rubs against the potentiometer winding =
might not be making good contact.
Hope that gives you something to work with...
Alan
(Salt Lake City, Utah)
-----Original Message-----
From: MJW71683@aol.com [SMTP:MJW71683@aol.com]
Sent: Tuesday, August 19, 1997 7:33 PM
To: mil-veh@skylee.com
Subject: [MV] M-37 Gas Gauge
I have a full tank of gas. When I turn on the ignition switch, the =
needle
goes only to Empty. If I disconnect the wire at the sending unit, the =
gauge
goes to Full. Does this mean the sending unit is bad or the gauge ?
=3D=3D=3D
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