gas gage wire
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
The wire coming out of the dash harness is downstream of the gauge and isn't going to tell you anything useful. Crawl underneath the truck and disconnect the sending unit, and ground the wire coming out of the harness that goes to the sending unit. If the gauge pegs full with the key set to ON, the wiring is fine. When you do this check, test for ground with a multimeter. Don't just assume that rusted, grime-covered frame is going to give you a solid ground because it won't. It takes some probing. 9 times out of 10, the reason the gas gauge doesn't work is (A) the solder on the brass float cracks and the float sinks to the bottom of the tank, or (B) the crossmember on the frame has lost ground.
Freshman User
Joined: Apr 2009
Posts: 37
Likes: 0
From: Oshkosh, WI
The wire coming out of the dash harness is downstream of the gauge and isn't going to tell you anything useful. Crawl underneath the truck and disconnect the sending unit, and ground the wire coming out of the harness that goes to the sending unit. If the gauge pegs full with the key set to ON, the wiring is fine. When you do this check, test for ground with a multimeter. Don't just assume that rusted, grime-covered frame is going to give you a solid ground because it won't. It takes some probing. 9 times out of 10, the reason the gas gauge doesn't work is (A) the solder on the brass float cracks and the float sinks to the bottom of the tank, or (B) the crossmember on the frame has lost ground.
By "crossmember has lost it's ground"..... is that where the sending unit wire grounds to?
Postmaster
Joined: Jul 2001
Posts: 4,956
Likes: 10
From: MD
Its location is in front of the tank.
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MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
Not necessarily. The gauge can't peg full unless it grounds downstream of the dash, so it's grounding somewhere. If the wiring is grounding somewhere between the gas gauge and the sending unit due to broken insulation, the gauge will peg full. Without further testing, there is not enough information to determine the condition of your wiring or your ground. If your sending unit is just off, that could be a number of reasons, but it's usually the rheostat in the sending unit.
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Joe Kucinski
Fleet Mechanic
Joined: Mar 2004
Posts: 1,398
Likes: 17
From: Hesperia, CA
fmc400 is correct. Don't waste your time fiddling around with the dash or the wiring. Just climb under the truck and unplug the wire at the sending unit. If you ground and unground that wire and the gauge goes from full to empty then the gauge and wiring to the tank is okay. From that point on you just have to worry about the sending unit itself or the tank being grounded properly to the frame of the truck.
This is the most common problem with these trucks. This isn’t rocket science don’t complicate it more than is necessary.
This is the most common problem with these trucks. This isn’t rocket science don’t complicate it more than is necessary.
Postmaster
Joined: Jul 2001
Posts: 4,956
Likes: 10
From: MD
Not necessarily. The gauge can't peg full unless it grounds downstream of the dash, so it's grounding somewhere. If the wiring is grounding somewhere between the gas gauge and the sending unit due to broken insulation, the gauge will peg full. Without further testing, there is not enough information to determine the condition of your wiring or your ground. If your sending unit is just off, that could be a number of reasons, but it's usually the rheostat in the sending unit.
Because...when you get direct voltage at the instrumental cluster without the ground from the sending unit, the fuel gage will peg full as there is grounding going on already in the instrumental cluster. The ground from the sending unit is needed to get the correct reading from the resistance so the gage will vary readings.
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
My understanding is that the fuel gauge is basically an ammeter; the needle swing is proportional to the current going through the sending unit wire. A full tank has the lowest rheostat resistance, so the sending unit pulls the most current for the farthest needle swing. Anytime I've disconnected the ground at the sending unit, the needle is dead. I guess I'm not following what you're saying? Open to all ideas.
Postmaster
Joined: Jul 2001
Posts: 4,956
Likes: 10
From: MD
If you short out the sending unit plug with a paper clip, it will peg the gage full without the ground. Does it mean it is getting ground from somewhere else? I've always used this method to determine the fate of the gas gage on the cluster which has worked for me in the past. Your replies made me question myself if I'm doing it right or wrong.
I don't have a degree in electrical engineering, so forgive me if I'm talking out of my ***.
I don't have a degree in electrical engineering, so forgive me if I'm talking out of my ***.
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
Do you mean to say that if you remove the plug from the sending unit, and short the two contacts inside, the gauge pegs? This is what should happen. The black lead coming from the pigtail grounds to the cross member. The sending unit body itself is not guaranteed to be grounded. The sending unit ground comes from the pigtail.
Fleet Mechanic
Joined: Mar 2004
Posts: 1,398
Likes: 17
From: Hesperia, CA
You guys sure like to complicate something that is really simple. If the wire to the sending unit in the gas tank is unplugged then the circuit is open so no current can flow. So how is the gauge going to move? If the gauge shows empty with the key off. (No voltage) Then it would have to show empty with the wire unplugged from the sending unit. (Again no voltage) The sending unit is nothing but a variable resistor that changes value with the float position. You plug in the sending unit and the gauge now has current and starts moving. Enclosed is the wiring diagram for the gauge.