• Welcome to The Truck Stop! We see you haven't REGISTERED yet.

    Your truck knowledge is missing!
    • Registration is FREE , all we need is your birthday and email. (We don't share ANY data with ANYONE)
    • We have tons of knowledge here for your diesel truck!
    • Post your own topics and reply to existing threads to help others out!
    • NO ADS! The site is fully functional and ad free!
    CLICK HERE TO REGISTER!

    Problems registering? Click here to contact us!

    Already registered, but need a PASSWORD RESET? CLICK HERE TO RESET YOUR PASSWORD!

1998 Fuel Gauge sorta working but not

Landon730

Active Member
Messages
238
Reaction score
136
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.
 

Will L.

Well-Known Member
Messages
8,870
Reaction score
8,981
Location
Boulder City Nv
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.
 

Landon730

Active Member
Messages
238
Reaction score
136
Location
Greenwood Arkansas
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.
Figured something new out. When the truck is running it stays in between 3/4 and F
 

THEFERMANATOR

FRANKENBURBAN
Staff member
Lead Moderator
Messages
9,398
Reaction score
3,037
Location
ZEPHYRHILLS FL
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.
 

Landon730

Active Member
Messages
238
Reaction score
136
Location
Greenwood Arkansas
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.
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
 

THEFERMANATOR

FRANKENBURBAN
Staff member
Lead Moderator
Messages
9,398
Reaction score
3,037
Location
ZEPHYRHILLS FL
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,
 

Landon730

Active Member
Messages
238
Reaction score
136
Location
Greenwood Arkansas
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,
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
 

THEFERMANATOR

FRANKENBURBAN
Staff member
Lead Moderator
Messages
9,398
Reaction score
3,037
Location
ZEPHYRHILLS FL
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
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.
 

Landon730

Active Member
Messages
238
Reaction score
136
Location
Greenwood Arkansas
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.
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?
 

THEFERMANATOR

FRANKENBURBAN
Staff member
Lead Moderator
Messages
9,398
Reaction score
3,037
Location
ZEPHYRHILLS FL
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?
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.
 
Last edited:

Landon730

Active Member
Messages
238
Reaction score
136
Location
Greenwood Arkansas
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.
Ok thanks probably gonna be putting the second tank back on
 
Top