gas gauge fix
I, too, had the mystery gas gauge that would stick, or work from full to 1/2, etc. I went to battle with the gas tank and over the course of three weekends, here's what I learned. If I knew what I was doing at the start, the whole process would have taken about 4 hours or so.
BTW, I have an 87 silverado blazer 305 TBI.
All directions (front, back, left, right, etc.) are in relation to the truck (front is where the engine is, driver sits on the left, etc).
Check the Gauge:
(1) The Fuel Pump / Gauge sender is connected to the tank by two wires running down the left frame. One wire is the pump power, the other the sensor wire. The tank itself is grounded to the frame (another seperate wire) completing both circuits.
(2) Find the junction of the wire. Mine was just forward of the rear axle. Disconnect it while someone watches the fuel gauge with the key on. If your engine was running it will stop (no power to fuel pump).
(3) With the circuit open the gas gauge should run fairly quickly past full and peg out at max.
(4) Try grounding the circuit to the frame (the end of the wire that runs towards the front of the truck, not the one running to the tank). If you ground the fuel pump, nothing should happen to the gauge but you may pop the pump fuse ... don't forget to fix that.
(5) With the circuit grounded, the gas gauge should run fairly quickly to empty.
(6) if the gauge in the dash is working, Proceed.
Removal of Gas Tank (the haynes manual is decent, but here are some gotcha's):
(1) Run the truck out of gas. gas is heavy, especially when you are trying to support the tank with your head. Disconnect the battery.
(2) Place jackstands under the tank straps 1/4 of the way back from the front of the tank (front of the tank). Leave the Jackstands about 1" below the tank. Place a floor jack under the rear of the tank, about 1" below it.
(3) Loosen the tank straps. Open the tailgate and use a deep socket. Check that the jackstands are holding the front of the tank before you remove the nuts. The tank should drop about 1" until the jackstands catch and the back of the tank should lean down a little further. Be gentle, the tank is fairly easy to bend.
(4) Disconnect the fuel filler hose from the fitting at the body panel. Watch out for dribbly gas. This would be a good time to stop smoking. This filler hose can be a real pain in the ass, because you have to bend it forward to get it inside the frame member to drop the tank. try to be gentle.
(5) Disconnect the vent line from the fitting at the body panel. Mine was a straight metal pipe with hose at either end. You can't bend it forward to get it inside the frame because it hits the fill fitting, and you cant bend it to the rear because of limited space. I ended up pulling it straight out (horizontally, left to right if you're standing behind the truck) and pulling the metal pipe out of the hose fitting at the tank. Be gentle, you don't want to screw up the head of the sender.
(6) WIth the fill pipe bent forward, lower the rear of the tank with the jack just enough to get an arm up to the sending unit. It's time to disconnect the fuel lines. There shouldn't be any pressure in the system, but there is gas in the lines, so have lots of rags ready. Did I mention this would be a good time to stop smoking?
(7) Disconnect the fuel lines, and remember the order. Be sure to support both sides of the fittings with wrenches/pliers while you loosen them.
(8) Pull the sending unit wire towards the rear of the truck (it goes over the cross frame member) so it doesn't snag anything.
(9) Disconnect the ground wire. I had to cut mine and put connectors back on.
(10) The tank should drop freely now, just be gentle on the fill hose.
Removal of the Sending unit:
(1) The haynes manual is pretty good.
(2) Clean the top of the tank first !! There will be tons of mud there.
(3) Remove the retaining ring with a screw driver and hammer (try to keep everything basically grounded to prevent sparks ... i.e. rest your forearm on the tank while you hold the screwdriver by the metal shaft. Hold the hammer in your other hand with a couple of fingers touching the metal head. It's not perfect but it's better then an explosion).
(4) Check all of the orings (the fat one at the head of the sending unit and the small ones on all of the gas line fittings). Fuel leaks suck, if the orings are at all questionable, replace them.
(5) Remove the sending unit ... be gentle and dont bend anything.
Testing the sender:
(1) Hook a multimeter up to the sending unit. It shouldn't be too hard to figure out where.
(2) The resistance of the potentiometer should vary SMOOTHLY between near 0 at empty and 100 ohms at full. If this part is broken, you may be able to take the sending part off of the pump/sender assembly and work it over (clean the contacts, check the coil of little bitty wires for breaks etc).
(3) I looked closely at the coiled wires inside the potentiometer and noticed from the wear that the float and arm wasn't moving it's full travel. The float was getting stuck on something. Granted, I had to drop the tank 3 times to figure this out.
Adjusting the float arm:
There is a rubber box like device (a square box with no lid) attached to the bottom of the tank to keep the gas from sloshing too much. Put a flash lite in there and look around (don't use a match). The float arm on my truck would work from a full tank all the way down to about a half tank, then rest on the top edge of the rubber box. The gauge would sit just above a half tank until I ran out of gas. I finally, after dropping the tank 3 times, figured out that the float needed to be moved closer to the sender/pump unit to clear the rubber box.
Hold the entire sending unit vertically as it would sit in the tank. Move the float to the full stop and see how high it goes relative to the unit. Let the float settle to the empty stop and see how low it sits. The objective now is to make small bends in the arm to get the float closer to the unit (so that it will clear the rubber box) while preserving the vertical range of motion (so that full means full on the gauge and empty means empty). I made a small "Z" bend in the float arm. Support the arm on both sides when you are making a bend so that you don't put any stress on the sending unit.
Testing the free motion of the arm:
After you think you've got it, be sure to test it before you remount the tank (or you'll have to drop it again ... murphy's law). I tried this several ways. First, I would very carefully set the pump/sender unit back in the tank without putting the retaining ring back on. Then tilt the unit away from the float and listen for the float to drop. If it drops, it was hung up. That method wasn't sure enough for me (after my third try at the tank) so I decided that since I had just dropped a bunch of crud through the hole that tank needed to be flushed anyway.
I'd make a small adjustment, set the unit in the tank, hook up the multimeter, and fill the tank up with water. As the tank fills you can watch the resitance change. I then used a shop vac to suck the water back out and make sure the float didn't hang up. After a few iterations, everthing was working correctly and it was time to reinstall. I finished flushing the tank, and used a shop vac to blow air through the tank (an electric leaf blower would work too). Be sure the tank and sender are completely dry!!! A warm sunny day helps alot.
Reinstalling:
As much as I hate to say it, installation is the reverse of removal. having friends helps. Be patient. I found that the easiest way was to attach the straps towards the front, then put the tank into the straps with the front of the tank as far up and into the compartment as possible and the rear of the tank resting on the ground. get the tank sitting correctly in the bends of the straps. Support the front of the tank with jackstands right under the straps (this keeps the straps on the tank). Then use a floor jack to lift the rear of the tank. Lift it enough to reconnect the gas lines and get the fill tube bent inside the frame. Dont forget the ground wire. throw the sender/pump wire over the frame member and reconnect it. Jack the rear up a little at a time as you fiddle with the fill tube. Reconnect the fill tube and vent.
After everything is re-hooked up, get under the truck. Support the rear center of the tank with your forehead and use your hands to put the strap bolts back through the holes near the tailgate. Have someone start the nuts on the bolts and then climb out and tighten everthing down. Viola!
Go fill your truck up and watch the gas gauge work!
