Testing a fuel sending unit

[68_Step]

When testing a fuel sending unit (90 ohm), should it read from 0 to 90? Or a range of 90? I tested mine tonight and the ohm reading was 20 to 110. Is that ok? Or am in the market for new sending unit?
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[ray_mcavoy]

It should read 0 to 90.

Are you connecting your ohmmeter directly to the sending unit? Or are you doing your test through some of the truck's wiring? If you're measuring through the wiring, a bad wire or connection might possibly be the source of that extra 20 ohms.

[chengny]

Fresh batteries in your VOM?

They are usually "out of sight out of mind" but for whatever reason, when they get weak resistance (and voltage) readings rise

[68_Step]

Originally Posted by ray_mcavoy

It should read 0 to 90.

Are you connecting your ohmmeter directly to the sending unit? Or are you doing your test through some of the truck's wiring? If you're measuring through the wiring, a bad wire or connection might possibly be the source of that extra 20 ohms.

Directly to the sending unit, we dropped the drivers side tank yesterday.

Originally Posted by chengny

Fresh batteries in your VOM?

They are usually "out of sight out of mind" but for whatever reason, when they get weak resistance (and voltage) readings rise

Hmmmmm... About a year old, I will change them and retest.
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[68_Step]

Originally Posted by chengny

Fresh batteries in your VOM?

They are usually "out of sight out of mind" but for whatever reason, when they get weak resistance (and voltage) readings rise

Replaced battery in multimeter and still reading 20 to 110. Hooked up long test leads to it so i could watch fuel gauge as I moved the float. With float all the way down (empty) it reads half full. Gauge pegs out at full way before float is at top of range.
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[chengny]

Here is the GM diagnostic flow chart for a faulty fuel gauge:



It calls for a special tool but you can read between the lines.

Simple wiring diagram:





And general info - note the bold type:


Description and Operation


Gauge failures are often caused by defective wiring or grounds. The first step in locating trouble should be a thorough inspection of all wiring, terminals and printed circuits. If wiring is secured by clamps, check to see whether the insulation has been severed, thereby grounding the wire. In the case of a fuel gauge installation, rust may cause failure by corrosion at the ground connection of the tank unit.

VARIABLE VOLTAGE TYPE
The variable voltage type dash gauge consists of two magnetic coils to which battery voltage is applied. The coils act on the gauge pointer and pull in opposite directions. One coil is grounded directly to the chassis, while the other coil is grounded through a variable resistor within the sending unit. Resistance through the sending unit determines current flow through its coil, and therefore pointer position.
When resistance is high in the sending unit, less current is allowed to flow through its coil, causing the gauge pointer to move toward the directly grounded coil. When resistance in the sending unit decreases, more current is allowed to pass through its coil, increasing the magnetic field. The gauge pointer is then attracted toward the coil which is grounded through the sending unit.
A special tester is required to diagnose this type gauge. Follow instructions included with the tester.

[68_Step]

Chengny- thanks for the troubleshooting info. I saved them
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