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Fuel gauge


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By Richard - Tuesday, December 11, 2018 2:05 PM
The fuel gauge on my 1971 Dodge D300 ( see page 73 of 2018 Show Time) is inop. I took the lead from the sending unit to the dash gauge and grounded it --dash needle started to climb quickly toward full. Next pulled SU from the tank to check float --perfect. Then took SU to a good battery, and ran test light from + to terminal on the SU and ground the SU to the battery. Dull light in test light, but when I raised the float, the light got brighter and brighter. Took SU hack to truck , hooked factory wire that goes to gauge to terminal on SU, then grounded SU to tank. NOTHING Raised and lowered the SU arm--nothing. So now what should I do, check, etc??? Thanks R PS The SU is about a year old and was said to be matched to the
Dodge gauge. It never worked all that well and stopped about the beginning of the summer. With a small factory tank and a very thirsty BB Mopar with a 4 bbl, knowing how much gas is in the tank is important!!!
By Geoff Weeks - Tuesday, December 11, 2018 3:43 PM
Check the grounding of the tank itself. If all else fails, run a separate grounding wire from a known good ground to one of the sending unit hold down screws (if screws are used) or to the tank itself somewhere, like if there is a tab of metal that you can drill and tap that will not cause a leak (not into the liquid side of the tank.
My guess is they expected the tank mounting straps to ground the tank, but a little rust and you lose the ground.
By junkmandan - Tuesday, December 11, 2018 4:08 PM
Richard................I can cure the thirsty part with a '97 D 350 I just acquired from a club member who just went in an assisted living facility . It has some fixer-upper issues setting since 2016, but you'd be pleased with end result !
By Richard - Wednesday, December 12, 2018 12:13 AM
Geoff-- At 0330 this am I woke up for a second or two and thought,Is the tank grounded??? Hmmm, will check that out today. Danny Boy Had a 1996 D250 4x4 with Cummins and 5 speed in about 2000--2005. I fitted a GearVendor to that truck that I bought from a guy who was trading in a truck like mine --he removed the GV with all parts before trade. In 2005 I sold my 96 with GV to a club member. This past Jan. I asked how the Dodge and GV were doing --he told me he removed the GV "a while back" as he was headed home one day with about 150 miles to go , when the GV started to make a loud clicking noise. He pulled into a parking lot and removed the rear driveshaft, put the transfer case into 4wd, and drove home on front wheel drive! SO, I bought the GV with the "loud clicking" and used it as trade-in on a re-man unit from GV, and that is now in my 1971 Dodge D300. Took the Spicer 3 speed out --it was tired, noisy, and leaky. Sold the Spicer, as is, to another club member. Funny how this stuff" moves about our club! Love the GV
By junkmandan - Wednesday, December 12, 2018 2:43 PM
One of our club members with a '60s era IHC has a GV in his platform dually with 6 cylinder engine and likes it . Slick, but not cheap .
By Richard - Thursday, December 13, 2018 2:59 AM
Checked ground of tank --OK Then I cleaned, sanded contact areas of sending unit/tank. I also used "the gauge that doesn't lie" (dip stick) to see how much gas is in the tank --about 1/3 full. Reinstalled sending unit and the dash gauge moved up to ALMOST 1/4. so, I'm leaving it at that. I JUST bought a new take off , round , 33 gal tank with all brackets, etc that will fit perfectly on the Dodge frame rail under the flatbed. ACTUALLY, when I bought the Dodge for the first time in 2003 as a Cab&Chassis, it had an Aux. tank mounted on the frame, but I removed it as the straps were rusted as well as the tank. The transfer valve is still in the cab floor by the drivers feet.
By luckydog - Sunday, December 23, 2018 1:10 PM
As I recall, the gauges in your truck should be powered by 5V. There is a voltage regulator/limiter on the back of the instrument cluster- it plugs in to the printed circuit board. You need to see if you have a 5V feed going to the sending unit from the gauge. Because the old regulator is a mechanical unit, you need an old school multi-meter to check this. A modern digital multi-meter will just keep bouncing around. I remember somebody used to make a solid state voltage regulator to replace the old one. Looking on one of the Mopar forums may help you find if they are still available. Also, the voltage regulator will affect all the gauges unless the problem is on the circuit board itself. If you need any more ideas or help, send me a message and I'll dig through my old notes. Merry Christmas!
By Richard - Monday, December 24, 2018 1:53 AM
Luckydog, Thanks for the response. I have replaced the strip/mechanical voltage reg for the instruments with a solid state unit. I was having trouble with all the gauges and the electronic unit cured them. However, the fuel gauge never read correctly as I noted below. My next step is to measure the ohms from the sending unit through the range of the float movement. I will get to that in due course, however, the Dodge is essentially in "dry dock" until Spring. The "good news" is that the gas tank is in the cab behind the seat, not under the truck, so removing the sending unit is a bit "fiddly", but easy to do. I have a chart somewhere giving the ohms reading for Mopar sending units of my Dodge's vintage. Just got to find it! I have mounted the round 33gal. Japanese tank, as used on Izusu, Hino, etc on the Dodge frame, under the flat bed. fits perfectly behind the cab and inf ront to the rear spring hanger with about 8 inches clear on each end. Looks and fits great, even if I do say so myself. Just have to plumb it in, through a 2 way valve, and I'll have about 25 more usable gals. of gas to feed to the thirsty 383 Mopar.
By luckydog - Monday, December 24, 2018 1:27 PM
Then it sounds like your sending unit is either bad or needs to be adjusted. Once you get the sending unit out, slowly lift it through its range and see what the gauge does. My experience is that even an NOS sending unit would need to be bent or tweaked at least a little. I went through this with my White last summer. It took several removals/installations before it read full when the tank was full and empty just before it was empty. It certainly sounds like you covered everything else. You'd have to do the same thing if you hook your gauge up to the auxiliary tank.

As for your 383 being thirsty, a good tune up may help. Aside from carb adjustments, make sure both the vacuum advance and centrifugal advance are working. A hole in the vacuum advance diaphragm will stop it and create a small vacuum leak off idle. Many of these distributors never got the required oil under the rotor, causing the centrifugal advance weights to seize. Since the truck 383 is fairly low compression it may run better with the vacuum advance hooked up to manifold vacuum instead of ported vacuum. You may already know this stuff but I thought I'd through it in just in case.