Fuel sender resistance

[SeriesShorty]

How do you check the resistance of a fuel sender, empty and full? Alternately, does anyone already know the resistance of an ex-MoD fuel sender?

Thanks all!
J

[bkreutz]

I pull the sender out, connect an ohmmeter across the sensor connection and the mounting plate, then read the resistance with arm hanging all the way down, then read it with the arm all the way up. I can also watch the resistance change as the arm is moved slowly in one direction to see if there are any dead spots in the middle. I guess you could drain the tank, check resistance, then fill it up and do the same. (connections of the ohmmeter the same) Can't help you on the specs for that sender.
Edit: Forgot to mention, as the sender goes up (tank getting fuller) the resistance decreases. grounding out the sender wire will give you a "full" tank reading on the gauge. HTH

[siiirhd88]

The later trucks use a 240 to 33 ohm (nominally..) fuel sender and gauge. There are several aftermarket manufacturers that make compatible gauges.

Bob

[SeriesShorty]

Ok, I'm feeling like a total dork on this one.

I'm using my ohmeter to check resistance on a spare sending unit I have in my basement, but I seem to be getting exact opposite readings. I'm getting around 240 when full (arm up) and 30-ish when when empty (arm is down).

I'm checking Summit for gauges and they are showing plenty of gauges in this range...but they read 33 when full and 240 when empty...just the opposite.

Are the MoD trucks different? I think they use a different sender than the civvy trucks do.

Confused in The Lou (what's new)

[bkreutz]

Sounds like you have a reverse resistance system. Most fuel gauge systems us a decreasing resistance as the level goes up, but they can work the other way as well, just a matter of how the gauge is designed (Chrysler did it this way years ago) I'm not sure of the advantages or disadvantages of either system, I think they're just different. My 74 SIII is of the up=less resistance persuasion. Most manufacturers do it this way. (at least all the ones I can remember testing, Chrysler excluded)
One quick way to test your system is to remove the sensor wire from the sender on the tank, ground it. If the gauge goes to full, you have the decreased resistance to full type. If it's a reversed type, the gauge will read empty (assuming the gauge and wiring is good). Don't leave the wire grounded for an extended period, it could possibly damage the gauge. Hmmm, maybe that's why everyone now uses the decreasing resistance system, to facilitate testing the wiring and gauge, the other way kind of eliminates the validity of this test, one would have to introduce a resistor into the test circuit.

[Sputnicker]

I have read on this forum that the early "positive ground" Land Rovers have fuel gauges/senders that work opposite those of later "negative ground" vehicles.

[printjunky]

Agreed, no personal experience, but I read somewhere here that both were used by Rover.

[SeriesShorty]

Hmmm weird. Mine apparently does read in reverse then. But it's also a negative ground truck, not converted, this MOD came from the factory that way.

So I'm assuming if I put in one of the commonly available aftermarket fuel gauge that it would read in reverse. I guess that is better than nothing.

Any other ideas or suggestions from MoD owners that have switched to aftermarket fuel gauges?

Thanks,
Jen