1. Welcome to PlowSite. Notice a fresh look and new features? It’s now easier to share photos and videos, find popular topics fast, and enjoy expanded user profiles. If you have any questions, click HELP at the top or bottom of any page, or send an email to help@plowsite.com. We welcome your feedback.

    Dismiss Notice

Gas Gauge Pegging Out Over Full

Discussion in 'Ford Trucks' started by All_Clear, Nov 15, 2005.

  1. All_Clear

    All_Clear Senior Member
    Messages: 206

    I'm not to sure how many have a problem with this but this is the only info i have been able to find on this problem. My Truck is an 89 Ford F-350 crewcab 460 Dual tanks and has the same problem.

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    OK, I have a problem with my gas gauge.

    This problem occurs regardless of which tank I am using, front or rear.

    When the fuel level in the tank I am using drops below about 1/4 tank, the gauge works ok. If I have more than 1/4 tank of fuel, it reads more than full. It pegs all the way over on the full side.

    Since it does this with both tanks, I thought it might be a problem with the gauge itself. Does that sound right? Or have both of my sending units failed in the exact same way?

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    It could be either.

    Ford uses 75 ohm fuel senders. So a full tank would be 70-75 ohms, empty would be 0-2 ohms.

    Now there are two problems with this.

    First, many aftermarket replacement senders use the same element, in different housings, thus saving manufacturing costs. Many of them use a 90 ohm element. So if your tank's have aftermarket senders, "full" will be "way past" the full mark.

    Also, because the more resistance there is, the higher on the scale the needle goes, the aging of the wires, poor grounds and so forth, add to the resistance. The fuel sender is grounded at the tank - so the stuff between that ground, and the ground on the firewall for the cluster, is a lot of potentially rusty stuff. The tank hole plate it mounts to, the tank itself, the tank straps, the frame, and the body bolts/ground strap under the hood.

    It's not uncommon for the fuel gauges to drift as the vehicle ages. To test this, run one of your tanks to "E", essentially empty, but not empty enough to stall the engine and plug the fuel filter. Then disconnect the battery, pull the cluster, and measure the resistance between a good ground on the firewall, and the wire in the connector that's for the fuel gauge. Flip the tank switch to each tank, one should have a very small resistance depending how empty the tank is, and the other should be 70-75 ohms if it's completely full.

    If you do have "correct" readings on your ohm meter (testing at the cluster connector), then it's the gauge itself (unlikely).

    If you have incorrect readings at the cluster connector, you can drop one of the tanks and measure at the fuel tank itself. If it's correct there, then you know it's ground related. If it's incorrect at the tank, then you know it's the sender itself.

    This is why garages will charge some money for this... measuring it at the tank is difficult because most people have hands that don't fit between the bottom of the bed and the top of the tanks to disconnect and reconnect the connector. I was able to do this on my side tank, but not the rear tank. My hands are "medium" I guess !?

    I had the bed of my truck off for part of the summer doing body work so I decided "screw it" and replaced both pumps and senders, so I don't have to think about failure in either tank for years to come. I hate spending money that I don't need to, but I hate taking out gas tanks even more.

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    This guy seems to know what he's talking about. Does/Has anyone had this problem and how did you solve it. Seems like alot of work and I too dont want to drop the tanks if i dont have to. But if this is the only way to go about fixing it then so be it. Like to hear a first hand expierence.

    Thanks
    All_Clear
     
  2. 75gmck25

    75gmck25 Senior Member
    Messages: 119

    The older Ford fuel gauge troubleshooting guides had a simple procedure.

    - Disconnect the wire that runs from the gauge to the sending unit and turn the ignition on. You can probably access the wire at the tank switch under the dash.
    - Ground the sending unit wire, connecting it through one of the specified reference resistors. For example, with a 75 ohm resistor it should read empty; with a 2 ohm resistor it should read full.

    I don't know the exact values for the reference resistors, but 2 and 75 ohm should be close enough.

    If the gauge passes that test, then the problem is in the wiring or the sending units. Grounded wiring would cause the problems you describe, but usually it will constant read full. Since it affects both tanks the same way, the grounded wiring may be in or near the tank switch.

    Bruce
     
  3. LINY Rob

    LINY Rob Senior Member
    Messages: 478

    I had the same problem with my 99, a new sender and it was back to normal
     
  4. MickiRig1

    MickiRig1 PlowSite Veteran
    Messages: 3,617

    I would go with ground / wiring problem. A new gage is expensive! The last time I looked they were like $75 or so in LMC Truck Catalog. (Get a junk yard one) The tank switcher picks which low pressure pump is running and the sending unit involved with that tank. Check your grounds,my diagram shows the ground is supplied to the tanks by the pig tail that's plugged into the tank plate. If I remember right that ground is on the engine ,trace the wires that come off the relay. There should be two relays side by side in a holder by them self,one is for the engine CPU and one is the fuel pumps.
     
  5. All_Clear

    All_Clear Senior Member
    Messages: 206

    Well all signs are pointing to faulty ground or both senders have/are going bad in the same way (not sure about that tho)

    I pretty sure its not the gauge itself besides i'd just find a used cluster if it were that easy. The gauge works fine under 1/4 tank anything over that it pegs out.

    Will see about tracking the grounds first. Then go towards the senders.


    Thanks for the input guys.


    All_Clear
     
  6. MickiRig1

    MickiRig1 PlowSite Veteran
    Messages: 3,617

    I chased a fuel pressure loss problem for 2-3 years. I pretty much know that year Ford's fuel system forward and back.
    Clean and Dielectric grease all the grounds you can find. Bad grounds can do weird things like feeding back thru stuff for grounds. You will be surprised how many the truck has. You may even have to replace both pig tail connectors on the tanks.
    Course they say 1 in 10,000 parts are bad right out of the box,I always seem to be Mr.10,000.
     
  7. All_Clear

    All_Clear Senior Member
    Messages: 206

    "Course they say 1 in 10,000 parts are bad right out of the box,I always seem to be Mr.10,000."

    Haha I know what you mean.

    Will check out those grounds once i get a free weekend. Watching the kids durring the week and still doing gutter cleanings on weekends, doesnt leave much time for tinkering on the truck. But i wanted to do some searching/research since my last dual tank ford never had this problem.

    I'll let you know when i figure something out or have more questions LoL

    Thanks
    All_Clear