'99 GMC 3500 fuel gauge issue

Discussion in 'GMC Truck Forum' started by BernieB, Jul 11, 2009.

  1. BernieB

    BernieB Full Member

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    #1 BernieB, Jul 11, 2009
    Last edited: Jul 12, 2009
    Greetings folks! I'm a newbie here (but not new to forums) and I'm hoping one of you might be able to help with an issue I'm having with my '99 GMC 3500.

    I picked the truck up down in Florida around this time last year, after buying it on eBay. It's a utility body (with strobes and light bar) 6.5L turbo diesel, and had only 70k on the clock when I stole... err BOUGHT it for $5,500. Good rubber, dual rear wheels, 15,000 GVWR, excellent condition in and out, heck, the seats weren't even very dirty and they'd just repainted the utility body inside and out, and I'm real happy with it. I figure it's just barely broken in at this point.

    When I flew down to pick it up the fuel gauge was reading just above 1/4 tank, and the guy I bought it from said he just put $30 into it (with diesel then running about $5 per gallon). I took it and filled it up, both tanks, it took 35 gallons, and noticed right away that the fuel gauge was reading wayyy above full. Like a full half inch beyond the full mark.

    According to the book it's supposed to have a 21 gallon front tank and 18 gallon rear (or maybe the other way around - I forget) and that jibes with the amount of fuel it took to fill it from the 1/4 mark that first day. The problem is, the gauge is reading so high that it's not dependable. I can get about 125 miles before it comes down to the "F" mark, and if I fill it up at the 3/4 mark it takes around twenty gallons. I've never actually run it "dry", and I start getting nervous when it gets below 1/2 because I figure the way the gauge is reading "off" then 1/2 actually equals about 1/4.

    I'd like to correct this problem as simply and easily as possible. Anyone know if there's a calibration screw on the gauge? If I pop out the cluster can I get in there to adjust that needle? Or does this all have to be done through the computer?

    If it needs to be done through the computer it'll have to wait. The speedo reads a couple miles per hour low (65 is actually about 62) and I'm planning on swapping out the 4:10 gears for a 3:70 set for better highway mileage. Once I do that I need to get it to the dealer to recalibrate the computer for the new gears, so I can have the fuel gague calibrated at the same time, if it's a "computer only" type calibration.
     
  2. stumpuller454

    stumpuller454 Well-Known Member

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    i would get the cluster flashed first, it could be bad, they are common to have problems, i beleive they are more affordable nowdays.
     
  3. BernieB

    BernieB Full Member

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    As I said, if that's what it takes it'll have to wait until after I swap out the gears on the rear. With what the local dealer charges for computer work I'm not looking to pay 'em twice. I appreciate the response though.
     
  4. BernieB

    BernieB Full Member

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    Just edited my first post because I realized that I'd forgotten to mention it's a 6.5L turbo diesel.
     
  5. stumpuller454

    stumpuller454 Well-Known Member

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    OH, I didn`t realize you hade the OBS like my truck, give me a minute to review my service manuals, mabe we can just remove the button on the gauge and move it....I thought you had the newer body style being a 99, my mistake, complete clusters are a problem with the 99 and up NBS.
     
  6. stumpuller454

    stumpuller454 Well-Known Member

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    alrighty, so are you up for checking some resistance readings? on both your tanks empty should read 0ohms, and full should read 90ohms. There is a connector on the drivers side frame rail near a transmission crosmmember (probly under drivers door) it should be a 6 wire plug, pink, light blue, dark blue/white,Light Green, Black/White, and purple. Connect a multi-meter or ohm-meter between terminals B and E (Light blue and Black/White. this should give you the resistance of the rear sending unit. checking between C and E will give front tank (Dark Blue/White and Black/White)

    we`ll go from here to further diagnose.
     
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  7. BernieB

    BernieB Full Member

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    #7 BernieB, Jul 12, 2009
    Last edited: Jul 12, 2009
    Ah...! Thank you sir! I won't get to it today because we're in the middle of a thunderstorm, but I'll crawl underneath the next dry day and see what I get. (Truck's too tall to get in the garage) Just topped off the tanks last night as a matter of fact. The needle was sitting just a hair below 3/4 and it to 23 gallons.

    BTW, where'd you find a service manual for this one? I used to be ASE certified, back when I had a shop in NYC in the 90's, but it's been about 15 years now since I was in the business. Looked for a manual for the truck but couldn't find one.
     
  8. BernieB

    BernieB Full Member

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    Yup, it's OBS with the mechanical gauges, not the electronic readouts, which was why I was thinking it might have an adjustment screw on the gauge itself. There's photos of it in my profile.
     
  9. stumpuller454

    stumpuller454 Well-Known Member

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    i have a contact that has a huge selection of automotive collectables, cars and literature, if you like i can give you his info, PM me if intersted.
     
  10. BernieB

    BernieB Full Member

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    I may do that... This truck has had a couple of minor electrical issues that I'd like to troubleshoot, so full wiring schematics would be a big plus. Simple stuff like intermittent wipers (I suspect a bad stalk) that sometimes refuse to shut off and a left headlight that sometimes won't come on until the high beams get cycled a couple of times, especially in cold weather (another reason to suspect that stalk).
     

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