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1998 Fuel Gauge sorta working but not

Landon730

Landon730

Active Member
Messages
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186
Location
Greenwood Arkansas
My 98 has never had a working fuel gauge ever since I got it and changed the fuel pump. At the time I had bought a fuel pump off Facebook marketplace that was actually for a blazer but was brand new and it was all I could afford. It worked just fine although it was a couple inches shorter than the stock pump. I put 3000 miles on that pump without a working gas gauge (which sucked). The rear end went down in it and I decided to take the flatbed off for a easier time to change the rear end. I bought a new stock fuel pump of rockauto and put in it. I thought something was wrong with the sensor for the gas gauge on the old pump. No such luck. I have traced all the wiring and can't find any problems with wiring. All the schematics show a pink wire for fuel gauge but I have a blue with black stripe. Gauge used to stay on E and never moved at all. Now with the new pump when I turn the key on it goes to F then kinda flutters back down. It stops at 1/4 (where it should be) for a second or two, then flutters back down to E. Its like a fluttering/buzzing. Ill take a video when I get out there today to finish the rear end.
 
Not having a schematic to look at-
Pull the fuse. Or disconnect batteries.
Get an ohm meter on it. Measure the ground wite connection back to battery ground. Do same at gauge. Then unplug connector at tank. read signal (power) wire from sensor connector to the gauge.

One thing I learned in electrical- assume nothing. If it isn’t working, assume everything is bad and start at the beginning. Once you find a problem, fix and test it. Then continue through the circuit until all is verified. This is because often, one problem creates another.

Unless chasing down opens and shorts is you favorite thing to do and you want to do more of it in a couple months starting over from step 1 again.
 
Will L. said:
Not having a schematic to look at-
Pull the fuse. Or disconnect batteries.
Get an ohm meter on it. Measure the ground wite connection back to battery ground. Do same at gauge. Then unplug connector at tank. read signal (power) wire from sensor connector to the gauge.

One thing I learned in electrical- assume nothing. If it isn’t working, assume everything is bad and start at the beginning. Once you find a problem, fix and test it. Then continue through the circuit until all is verified. This is because often, one problem creates another.

Unless chasing down opens and shorts is you favorite thing to do and you want to do more of it in a couple months starting over from step 1 again.
Click to expand...
Figured something new out. When the truck is running it stays in between 3/4 and F
 
I'm guessing this is a gasser? If so it is a very ODD fuel gauge arrangement. It should be a purple or purple with white stripe wire that runs to the PCM, then a purple wire running from the PCM to the actual fuel gauge. You'll find lots of posts about the fuel gauge on these buzzing or fluttering. Many pull the cluster out and add fresh silicone dampening fluid to the actual gauge air core motor, but the actual problem is a failed PWM buffer in the PCM output.

The sender in the tank works from 240 ohms down to 30 ohms iirc(240 at full, and 30 when empty, but I may have that backwards). The PCM takes that reading, and then converts it back to a 0-90 ohm output for the actual gauge. They did this to stop the gauge from fluctuating when the fuel sloshed, but they ended up complicating a simple design on the 98-00 black box PCM equipped trucks. So the age old test of unplugging the sender and looking for the gauge to go all the way up, then grounding it to send it to empty does not work on the PCM controlled circuit. You have to ohm the sender circuit to the PCM, then check the PCM's voltage output to test the 98-00 fuel gauge circuit for gassers(diesels retained the old style 0-90 ohm direct connected sender).

If your gauge problem is in the PCM, there is a couple of ways around this. But diagnosing this circuit is not easy. Most scanners won't even see the fuel level reading when scanning it.
 
THEFERMANATOR said:
I'm guessing this is a gasser? If so it is a very ODD fuel gauge arrangement. It should be a purple or purple with white stripe wire that runs to the PCM, then a purple wire running from the PCM to the actual fuel gauge. You'll find lots of posts about the fuel gauge on these buzzing or fluttering. Many pull the cluster out and add fresh silicone dampening fluid to the actual gauge air core motor, but the actual problem is a failed PWM buffer in the PCM output.

The sender in the tank works from 240 ohms down to 30 ohms iirc(240 at full, and 30 when empty, but I may have that backwards). The PCM takes that reading, and then converts it back to a 0-90 ohm output for the actual gauge. They did this to stop the gauge from fluctuating when the fuel sloshed, but they ended up complicating a simple design on the 98-00 black box PCM equipped trucks. So the age old test of unplugging the sender and looking for the gauge to go all the way up, then grounding it to send it to empty does not work on the PCM controlled circuit. You have to ohm the sender circuit to the PCM, then check the PCM's voltage output to test the 98-00 fuel gauge circuit for gassers(diesels retained the old style 0-90 ohm direct connected sender).

If your gauge problem is in the PCM, there is a couple of ways around this. But diagnosing this circuit is not easy. Most scanners won't even see the fuel level reading when scanning it.
Click to expand...
The blue wire completely threw me off. Yes it have a 350 vortec (sadly wish it was 6.5 lol). Is the PCM the thing in the frame rail with 2 connectors going to it? The blue wire i'm talking about runs down to it and plugs into it
 
The PCM is the powertrain control module, most call it the engine computer, but in your case it does alot more than just the engine. It's the black box mounted on the drivers fender, under the hood, near the fuse panel, with 4 big plugs going into it,
 
THEFERMANATOR said:
The PCM is the powertrain control module, most call it the engine computer, but in your case it does alot more than just the engine. It's the black box mounted on the drivers fender, under the hood, near the fuse panel, with 4 big plugs going into it,
Click to expand...
I know which one your talking about now. There’s one green circuit broad that snaps into a black plastic box on the frame rail that has two plugs that plug into it. The blue wire from the fuel pump goes to it
 
Landon730 said:
I know which one your talking about now. There’s one green circuit broad that snaps into a black plastic box on the frame rail that has two plugs that plug into it. The blue wire from the fuel pump goes to it
Click to expand...
Is this a dual tank truck, or was it at one time and now only has 1 tank? I'm not certain howthe fuel gauge works on a 98+ dual tank truck. I know diesels and earlier trucks had a balance module mounted on the frame rail much like you describe that controlled the transfer pump and fuel gauge.
 
THEFERMANATOR said:
Is this a dual tank truck, or was it at one time and now only has 1 tank? I'm not certain howthe fuel gauge works on a 98+ dual tank truck. I know diesels and earlier trucks had a balance module mounted on the frame rail much like you describe that controlled the transfer pump and fuel gauge.
Click to expand...
Yes it used to have a rear tank that I took off. I'm actually wanting to put it back on for extra fuel. Could it be the cause of this?
 
Landon730 said:
Yes it used to have a rear tank that I took off. I'm actually wanting to put it back on for extra fuel. Could it be the cause of this?
Click to expand...
Yes. Dual tank trucks use a balance module to control the rear tanks pump to transfer fuel forward, and take both tank fuel readings to output 1 gauge signal output. You can't just leave 1 sender open, it causes the balance module to flip out. I don't know if the pcm is even involved in that system, or how it works on a 98+ gasser.

The only thing you could try is to install a 40 ohm resistor across the wires for the transfer tank sender. Your gauge will never show full since its only showing the fuel level of 1 tank. Otherwise you could try bypassing the balance module and hooking the 1 sender straight to the pcm, but I'm not sure if that would work.
 
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THEFERMANATOR said:
Yes. Dual tank trucks use a balance module to control the rear tanks pump to transfer fuel forward, and take both tank fuel readings to output 1 gauge signal output. You can't just leave 1 sender open, it causes the balance module to flip out. I don't know if the pcm is even involved in that system, or how it works on a 98+ gasser.
Click to expand...
Ok thanks probably gonna be putting the second tank back on
 
THEFERMANATOR said:
Yes. Dual tank trucks use a balance module to control the rear tanks pump to transfer fuel forward, and take both tank fuel readings to output 1 gauge signal output. You can't just leave 1 sender open, it causes the balance module to flip out. I don't know if the pcm is even involved in that system, or how it works on a 98+ gasser.

The only thing you could try is to install a 40 ohm resistor across the wires for the transfer tank sender. Your gauge will never show full since its only showing the fuel level of 1 tank. Otherwise you could try bypassing the balance module and hooking the 1 sender straight to the pcm, but I'm not sure if that would work.
Click to expand...
I can't find ANY information about these secondary tanks. All I can find is the replacement sending unit. I need the plastic piece that splices into the vent hose where the supply hoses from the secondary tank hook up-but I'm having no luck. I can't find any other trucks or diagrams with the same setup. Do any of y'all have any ideas?
 
Last edited:
Ok, but you asked for it- haha
Leaving everything stock for solving such a problem is a wonderful idea. When the factory over engineered, fluffy and lace covered, hold your hand cuz your not s big enough boy to cross the street alone system just isn’t doing it for ya- trade in power windows for a crank window and do things manual!

Buy two aftermarket fuel metrum rods- Leroy sells some.
Wire them to a switch and an aftermarket gauge or no switch and 2 aftermarket gauges. Then manusl valves to go from tank1 to tank 2. Done. There are plenty of late 60’s and 70’s trucks driving around here that still run that and have no issues.

Variation:
My hummer is 1 tank stock, newer ones have 2 tanks and an elaborate system like yours. Forget that nonsense! The smaller tank gets plumbed to the main tank with a lift pump on a switch. When my main goes low, flip a switch to fill the main. If problems arise with aux tank, so what, rig still drives, fix it when time & $ allows.
 
Will L. said:
Ok, but you asked for it- haha
Leaving everything stock for solving such a problem is a wonderful idea. When the factory over engineered, fluffy and lace covered, hold your hand cuz your not s big enough boy to cross the street alone system just isn’t doing it for ya- trade in power windows for a crank window and do things manual!

Buy two aftermarket fuel metrum rods- Leroy sells some.
Wire them to a switch and an aftermarket gauge or no switch and 2 aftermarket gauges. Then manusl valves to go from tank1 to tank 2. Done. There are plenty of late 60’s and 70’s trucks driving around here that still run that and have no issues.

Variation:
My hummer is 1 tank stock, newer ones have 2 tanks and an elaborate system like yours. Forget that nonsense! The smaller tank gets plumbed to the main tank with a lift pump on a switch. When my main goes low, flip a switch to fill the main. If problems arise with aux tank, so what, rig still drives, fix it when time & $ allows.
Click to expand...
That’s where I’m at right now. Truck still runs and drives good it would just be nice to have everything working right again. Thanks for the help I’ll start looking up and buying parts
 
I have no clue what removing the float signals may do computer wise. Might be you have to add a resistor in its place to fool the computer?
 
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