Fuel guage not working

[ilyafish]

So this is a problem the bus had when i bought it, i just decided to tackle it now....the fuel guage doesnt work. every now and then it will jump back and forth, but usually just stays about 1/4 of the way up.  What are the possible problems, and how do i diagnose them?

[Hartley]

Veggie Oil acids eat and gum up fuel gauge sensors.

The senders for MC9 tanks also can get stuck on the fill and pickup tubes inside the tank. The universal sensors suck and condensation in the tank corrodes them in no time.

Someone sells a sealed sensor that will work, But I don't remember where I saw them.

Besides You can't use the last 50 gallons to "Empty" should be at the 50 gallon level.

I really hate re-priming detroits on the road...!!!!

[Blacksheep]

Re-priming is easy if you carry a 1 gallon sprayer and install an a/c schrader type valve on your secondary fuel filter!
I just found this out and hope never to have to use it but better to have it and not need it than to need it and not have it!

Ace

[buddydawg]

Fuel Guage?  What's a fuel guage?  I still use the dipstick that Dallas gave me as a bus warming gift.....

[NJT5047]

You can easily check for a bad sender by removing the center lead from the sending unit and grounding the removed lead to the metal part of the fuel sender.   Have someone watch the guage for movement.  Once it's traveled toward the peg, remove the wire.  Don't allow the gauge to remain in the pegged position. 
The guage should move to full when the sender lead is grounded.  If it moves, replace the sender and you'll be good to go.  If the guage  doesn't move, look for a loose ground wire from the bulkhead that is typically attached to one of the sender mounting screws.  The tank and sender must have a patent ground in order to function.
If all of this fails to excite the guage, you may have a loose guage power lead, open sender lead, or bad guage. 
The sender wire can be checked by grounding  the dash end and check for continuity to ground.    If it doesn't go to ground, bad lead.  It isn't shorted or the guage would be indicating full.
I'd put my money on the sender.  MC9s have the fuel sender positioned above the fuel level and they'll rust like a bandit.  By a good quality sender.  I've had one fail within one year of install.  The second unit,  Which cost a good bit more than the first unit has functioned will for several years.  The good unit came from US Coach.   I believe about $70 bucks?   That may not be accurate...?   
The sender is too easy to replace.  Just be sure that the correct resistance range for your guage is used. 
May be stamped on the sender.  Or, just tell the sender supplier what brand dash gauge you are using. They can figure out the rest. 
HTH, JR

[belfert]

As others have said, the float assemblies can corrode.  My fuel gauge was totally inaccurate (Showed 1/8 tank when it was 1/2 full.) and when I removed the float assembly it was all rusted to heck.  I put in a capacitive fuel sender, but it stayed on full way too long so I adjusted the screw for full and now it stays on full the time.  I was sent instructions to test and fix it, but it has not been a priority yet.  I can easily see the fuel level when I open my tank so I can get a pretty good idea where I am mileage wise.

I have a 150 gallon tank and I have usually tended to fill at 75 to 100 gallons just based on where the cheapest diesel is and how many miles my trip is.  That said I have been down to 25 gallons and I don't worry about it.

[belfert]

Someone asked me privately where I got my capacitance based fuel sender.  I figured everyone might want the answer.  www.centroidproducts.com  I think I paid around $60 shipped.