96-2000 Fuel gauge

I started a new thread so maybe we can figure out what's going on between the 96-97's and the 98-2000's. I'm going to post the information from AllData and hopefully working together through this thread we can figure it out. This first paste is for the 97 vehicles but when I looked at the 98's it's the same, they both should be 4 ohms when empty and 90 ohms when full. Please check ohms on your fuel gauge wire and post what you get here so we can move forward. Ignore the fuel pump balance module if you only have one tank.

The pointer of the Fuel Gage is moved by the magnetic fields of two coils. The coils are at right angles to each other. Battery voltage is applied to the E-Coil (empty) and the circuit divides at the opposite end of the coil. One path continues to ground through the F-Coil (full). Another goes to ground through the variable resistance of the level sensor in the Fuel Pump and Sender (Gasoline) or Fuel Gage Sender (Diesel) for vehicles with single fuel tanks. The Fuel Pump Balance Module averages the resistance for vehicles with two fuel tanks. When the tank is low, the resistance of the level sensor is low. A large current flow passes through the E-Coil and the pointer moves toward E on the Fuel Gage. When the tank is full, the level sensor's resistance is high. Less current passes though the E-Coil and the pointer moves toward F on the scale. With two coils operating the pointer, the Fuel Gage is not affected by changes in the system's battery voltage. For vehicles with single fuel tanks, the level sensor in the Fuel Sender produces a resistance of about 4 ohms when the tank is empty. The resistance is about 90 ohms when the tank is full. A short to ground in the sender or wiring will provide a fully empty indication. An open in the circuit will produce a fully-full indication. For vehicles with dual fuel tanks, the Fuel Pump Balance Module measures the resistance of both Fuel Senders. The module averages the resistance and produces an equivalent resistance reflecting the average level of both fuel tanks. Both power to the module and a change in the input resistance are required to produce a change in the output resistance to the Fuel Gage. The Fuel Pump Balance Module responds to a very low resistance (short to ground) in one or both fuel sender circuits by considering the affected tank(s) to be empty. A very high resistance (open circuit) in either fuel sender circuit will cause the Fuel Pump Balance Module to produce a zero output (both tanks empty) signal to the Fuel Gage. The Fuel Pump Balance Module equalizes level in the two fuel tanks. Fuel is fed to the Engine from the front (primary) fuel tank. When the level in the front tank decreases to about two gallons less than the level in the rear (secondary) fuel tank, the Fuel Pump Balance Module energizes the Fuel Pump Balance Relay. This starts the Balance Fuel Pump which transfers fuel from the rear tank to the front tank. When the level in the front tank increases to about two gallons greater than the rear tank, the Balance Fuel Pump stopped. This approach tends to empty the rear tank first.

BTW, from the looks of the diagnostic information the 98 should work just like the 97 without the pcm connected since the ohms are the same. The 98 pcm has fuel level input, fuel level output and a 5v sensor return. If we only knew what the pcm signal was reading. Maybe you guys could hook the old pcm up and check ohms and voltage through the pcm and also with the input and output wires connected without the pcm.

ohms on the 98 is 40 empty and 248 full.

ok i just pulled my truck in to the bay.started ohm ing stuff the gauge uses the old 0-90 ohms and on 98 the changed t0 the 40-248 ohms. i used my old sending unit unplugged pin 9 on pcm purple wire and attached a jumper wire to one leg of sending unit to gauge the other a ground and at an empty tank the gauge reads about a half tank and i slowly moved the sending unit arm to about a quarter and it read over full. ithen took my dvom and ohmed the sending unit out and it read 107. so thats what i came up with is that 98 and up need a 97 and older sending unit to work. i hope all this makes since. im bad at explaining things but it all works in my head lol oh and not for sure but i would say the pcm just changes the value out to work for the older head units

No, this is good and what we originally expected. AllData has a technical bulletin that says to upgrade to 40-250 ohm sending unit if using a 0-90 ohm sending unit starting in late 97. Hopefully a sending unit swap will take care of the problem and life will be great! Who wants to be a guinea pig? LOL

File In Section: 8 - Chassis/Body Electrical
Bulletin No.: 83-83-07
Date: March, 1998
Subject:
Fuel Level Sensor Replacement Procedure

Models:
1997-98 Passenger Cars and Light Duty Trucks
(Listed in Application Chart)

INFORMATION
This bulletin is being issued to clarify that, when fuel gauge diagnosis per the Service Manual indicates the cause to be the fuel level sensor, complete fuel sender assembly replacement is NOT required.
Available fuel level sensor service packages are indicated in the application charts and should be installed following applicable Service Manual procedures and those provided in the service packages.

Available fuel level sensor service packages are indicated in the application charts and should be installed following applicable Service Manual procedures and those provided in the service packages.

Allen an Jon at nelson performance got my fuel gauge working by putting in the stock pcm back in. But for some reason when accelerating it will dip down, when I let off it will slowly go back to were it should be. This isn't under heavy acceleration, just normal so I don't think its from the gas moving around in the tank.. it does it even when full.

Good deal, It would be nice to have only 1 pcm though. Can you read the output from the pcm in ohms and voltage? I know there's a 5 volt sensor return but I want to see what C1 pin 9 fuel level input and C3 pin 13 fuel level output are. Just ground one side of the voltmeter and prick the wire going to pin 9 and 13 to see what they show and please post it. Make sure the ignition is on......................LOL:)

Thanks

Quote:

---

Originally Posted by NITROUS

Good deal, It would be nice to have only 1 pcm though. Can you read the output from the pcm in ohms and voltage? I know there's a 5 volt sensor return but I want to see what C1 pin 9 fuel level input and C3 pin 13 fuel level output are. Just ground one side of the voltmeter and prick the wire going to pin 9 and 13 to see what they show and please post it. Make sure the ignition is on......................LOL:)

Thanks

---

Ill do that sometime next week, when i have access to some of those tools.
Were is C1 pin 9 at? LOL i have no idea how to read were the pins are.. :doofus:

awesome post im excited to see this one pan out having no gas guage sucks and i really dont have room for 2 pcms. so i hope we can get everything figured out here.

Quote:

---

Originally Posted by ChevyLee

Ill do that sometime next week, when i have access to some of those tools.
Were is C1 pin 9 at? LOL i have no idea how to read were the pins are.. :doofus:

---

Blue connector = C1
Red connector = C2
Clear connector = C3
Black connector = C4

If you unplug the connectors you can read the pin numbers on them. You'll need to turn the ignition on and read the C3 female pin 13 connector itself first (not the pcm) then plug it back in and read C1 male pin 9 on the pcm (not the connector).

perfect, thanks checking tomorrow :)

so whats the outcome on this one guys? any solution ever been made for keeping just 1 PCM?? its about that time where i need to fix mine lol and am looking to know what works. thanks all.

use a 97 sending unit

Hopefully I am not out of place with this post. I do not have a Gen3 engined 98-2000 OBS truck. I have installed a Gen 3 PCM (12200411) in my 98. The 97 and down fuel gauge does not use the PCM. On the 98-2000 OBS trucks C1-9 will go to GEN3 C2-38---OBS C2-26 will go to GEN3 C1-23---OBS C3-13 will go to GEN3 C2-54. There are about 40 98-2000 OBS trucks that have installed the 12200411 PCM and ALL of their fuel gauges work. I hope this helps. There is no need to run 2 PCMs on these conversions.

Jeff

Quote:

---

Originally Posted by beater6.0

use a 97 sending unit

---

any idea where i can pick one of these up???

parts store or a dealer then u need to bypass the pcm if i rember right its two wires

Quote:

---

Originally Posted by beater6.0

use a 97 sending unit

---

Ya, but if you do that you will lose the fuel tank pressure sensor. The Evap system will not function properly without it. If you are not using the Evap stuff then it wouldn't matter.

Jeff

98 doesn't have an FTP sensor...pretty sure a 97 doesn't either.

Quote:

---

Originally Posted by GasGuzzler

98 doesn't have an FTP sensor...pretty sure a 97 doesn't either.

---

Did a '96 have one?

i think the 98 does have one. but isnt a nelson harness stand alone

They pretty much don't until 2000 or so (some 99 half tons do). The 2000 model 2500's don't either and even some of the mid 2000's GMT800 body 2500's don't have it.

Ooops...I stand corrected. I see now that some 98's have one too. I looked in a different place in service information and it didn't list it. Hahaha there is reference to this sensor on the 97 too but no replacement instructions. Boy I love depending on faulty and imcomplete information from GM to make a living. :badteef: I'll go out on another limb and say the 96 does not have one.

hmm well im going to check all my connections first to make sure its not a bad ground or a bad connection, then i guess i will try a new sending unit from a 97.

I've worked on quite a few of the 98's and they do have the FTP sensor and it's also in the Alldata cluster schematics per California emmisions. The problem with the fuel gauge on the 98 is the fuel sender is 40-250 ohm and goes directly to the PCM C3 connector and out the C1. There's also a 5v return for the FTP sensor. The 97 is 0-90 ohm and goes directly to the gauge. I'm running a 2000 Denali 120mph cluster in my 97 and it works fine. The only way for the 98 to work is to change the sending unit out to a 96 or 97 and start from there. Sorry I didn't tell you this i the PM but I was thinking 97 Wtfan18.

wtfan18, go buy a sensor and unplug your on top of the tank. If you have enough wire plug in the new sensor without dropping the tank and test it to verify the gauge by sliding the rod up and down the resistor.

he will still have to bypass the pcm conn. only two wires dont rember the colors though

Thanks Lance I already admitted by dumminess :doofus:

No, you actually know quite a bit and I didn't see your edit. .:melissa: :LOL:

well after almost running out of gas again last weekend i decided enough is enough. i was about to break down and buy a 97 sending unit when i realized the donor parts truck that my sending unit and fuel pump are out of is a 97 and not a 98 like i had thought so i dont need to buy a new unit i just need to know how to bypass the pcm and get my guage working! anyone have any way they can describe what needs to be done here? i really just want to fix the issue. it sucks not knowing how much gas i have...

If your truck is a 98 then the stock 98 fuel pump/sender module WILL WORK your fuel gauge. From your original Black PCM connectors--98 C1-9 Purple wire will go to 02 C2-38. 98 C2-26 Black wire will go to 02 C1-23. 98 C3-13 Purple with White stripe wire will go to 02 C2-54. It will work. The 02 PCM #12200411 has been installed into dozens of 98-2000 GMT400 (Old Body Style) trucks.

Jeff

Quote:

---

Originally Posted by Lextech

If your truck is a 98 then the stock 98 fuel pump/sender module WILL WORK your fuel gauge. From your original Black PCM connectors--98 C1-9 Purple wire will go to 02 C2-38. 98 C2-26 Black wire will go to 02 C1-23. 98 C3-13 Purple with White stripe wire will go to 02 C2-54. It will work. The 02 PCM #12200411 has been installed into dozens of 98-2000 GMT400 (Old Body Style) trucks.
Jeff

---

right but i have a donor fuel pump and sending unit from a 97 gmc sierra 5.7liter vortec because i had a 4.3 pump before and it wasnt enough, will your method work with a 97 sending unit and 97 fuel pump or just the 98?

I don't know the answer to that one. I don't know if the 97 sender/pump module is different than the 98---I'm pretty sure it is. I know the 98 and up fuel gauge is a PWM deal that gets its info from the PCM.

Jeff

yeah, the interchange of 97 and 98 sending units has fixed many fuel sending unit issues. The 97 allows you to feed the signal directly to the instrument panel instead of going through the PCM if I remember correctly. Never had to do this myself but a little research on here will lead you down the right path.

I had to research this when I was attempting to get my 1996 GMC fuel gauge working. The '96 & '97 are analog signals/gauges, via one sender wire directly from the sender/fuel pump module in the tank, to the firewall C-100 connector, and it works. 1998 & up are different. Make sure all your grounds are good. They are very important. All GM engine block grounds are solid black wire. All chassis ground wires are black with a white tracer, on the entire truck.

Quote:

---

Originally Posted by Wtfan18

right but i have a donor fuel pump and sending unit from a 97 gmc sierra 5.7liter vortec because i had a 4.3 pump before and it wasnt enough, will your method work with a 97 sending unit and 97 fuel pump or just the 98?

---

The 4.3 fuel pump was'nt supplying enough fuel volume or pressure for your 5.3? Another little characteristic I discovered about our fuel pumps, is they won't deliver enough fuel, if you run the fuel tank level below 1/4 to 1/3 full. Always keep plenty of fuel in the tank. The pumps rely on the fuel for cooling. If the pump is'nt submerged, it overheats, shortening its lifespan, & burning up the pigtail wiring harness thats plugged into the top of the fuel pump along the way. This in turn burns up your fuel pump relay under the hood. My 4.3 fuel pump/sender module supplies sufficient fuel to my 5.7 LS1. Wondering why yours did'nt.