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Fuel gauge or sending unit reading wrong?

stoney126

stoney126

1/2 ton status
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Jun 2, 2005
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Hey all the fuel gauge on my blazer is pretty inaccurate. It reads full when it is but drops to half after a few trips it drops to half. Then sometimes it drops to E when I know there is no way its empty. Like today was showing E for a good part of the day the bumps up to a quarter tank. So Is it the fuel gauge or is the float in the tank giving me the bad readings?

Now there has been a hole cut in the body over the fuel tank so im sure the PO has done something to the sending unit or replaced it.
 
test the guage by jumping the guage wire/s at the tank. at full voltage guage should peg full right. confirm that then change the sender.
 
ok will do. Do i need to pull the sender to test it? also are these expensive? please say no:D
 
no don't pull the sendr to test. just disco the wire or wires. I don't know how many you've got 1 or 2. but ground the power wire direct and the guage should peg instantly. take it off and back to E instantly. no hesitation no jitters. full sweeps right now.
 
stoney126 said:
ok will do. Do i need to pull the sender to test it? also are these expensive? please say no:D
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Pull the gauge wire off the sender and ground it, if the gauge pegs full the gauge works and the sender is your problem. The senders are 349.95 from Classic Chevy truck parts (87-91 blazer and burb).
 
4X4HIGH said:
Pull the gauge wire off the sender and ground it, if the gauge pegs full the gauge works and the sender is your problem. The senders are 349.95 from Classic Chevy truck parts (87-91 blazer and burb).
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wow 349:eek1: Guess ill be playing it by ear for a bit err eye I suppose:D Can you repair the sender or clean it up?
 
While checking can grime cause poor reading on the gas gauge? Mine is a two wire setup. Looks like the ground is soldered to the sender lid, and was covered with grime. Perhaps I should clean the connection at the frame? Not sure if this relates but when I had say a quarter tank and I turn On the head lights It reads half a tank:doah::haha:
 
stoney126 said:
While checking can grime cause poor reading on the gas gauge? Mine is a two wire setup. Looks like the ground is soldered to the sender lid, and was covered with grime. Perhaps I should clean the connection at the frame? Not sure if this relates but when I had say a quarter tank and I turn On the head lights It reads half a tank:doah::haha:
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That sounds like a grounding issue. There is a wire that is soldered to the sender and should bolt to the frame rail (usually driverside) make sure that ground is CLEAN and TIGHT.

There should be two wires going to the top of the sender as well, one is for the electric fuel pump and the other is for the gauge. The gauge wire is the one you want to ground out to test for a faulty gauge.
 
ok Ill check clean and tighten the ground at the frame. Besides the ground wire I just have one other wire (pink) that goes over a single prong on the sender cover. Mine is a diesel if that matters
 
ok once I pull the pink wire the needle climbs past F. Once I ground it direct it goes to E. Then when I put it back on it goes back to a quarter tank
 
dude, its a crap sending unit. They are none too reliable. but luckily they're not $350, i just got a brand spanking new one for $134 from Advanced Auto with new mesh filter, o ring and lock ring. Now have fun dropping the tank to install :D. Easiest to either run it dry and then change or chop a hole in the floor of the back of the truck over the sending unit. and while your in there check the tank baffle to ensure its not busted loose and destroys a perfectly good sending unit. . . . . yes it happened to me.
 
I was gonna drop the tank to do the shackle flip but the PO already cut a whole in it so I guess Either way would work:doah: How do I check the Baffle? Does it make a difference On sending units if its a Diesel?
 
i think the output/return/breather hoses on top of the sending unit are different for diesel and gas but i'm not sure. The baffle is a plastic tub that sits in the bottom of the tank. Its held down by 4 studs and grommets. This thing keeps gas flowing to the pump when its 1/4 tank or less. when the tank is almost empty shine a light down there and make sure that the baffle is still attached and its not cracked around the studs (my first one just came completely off and was floating around beating the s*** out of my sending unit, got a used tank and its cracked so im just gonna use fule safe epoxy or something and glue it). Thing is that when the baffle comes off the tank is trash, noone can re-attach thhe stupid thing. I've heard that if you dont use it in the tank i'tll cause the fuel pump to fry out more often. I have a buddy thats attempting the same thing so we'll see how that works
 
Yep, can't go wrong cleaning/inspecting all the grounds, making sure the battery cable isn't shot internally, which tends to be a "hidden" ground issue.
 
multimeter set to resistance from the fuel gauge sender (little vertical pin) to the ground connection on the sender. should be between 0-90ohms . 0 being empty, 90 being full.

Mine had been getting worse and worse over the last month or two. it used to work great, lately its been empty, 1/2 or full, nothing in between... then for a week it's empty or full... and now it's just full

$70 later (4 connection senders are more expensive), and about 15 minutes, it works perfect.

15 minutes is NOT typical, I have a bed made out of planks of wood, so a couple bolts and I'm staring at the top of my tank.
 
If the ground cable is brand new, pretty unlikely it's bad. Usually they or other grounds that are redundant will get hot in operation if they are problematic. Bad grounds usually show up as problems elsewhere though, as only a few grounds tie the entire truck together.
 
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