2002 gmc ABS on , no codes , no speedo

jempey · February 17, 2019

hi , driving me crazy , 2002 gmc 2500hd , auto trans , 4wd , 6.0l , app 250,000 km , 4dr xcab , long box , factory 2" lift , . my ABS lamp is on , there are no codes , no speedometer , ABS lamp behaves normal , comes on , goes out , comes back on. originally had a vss code , so i installed a non gm vss , still the same , when i drive 1/4 block the park and brake light come on , they go out after restart and only come on when moving , no speedo , i sent the ABS module away to be repaired , twice , first time it came back it was the same but wouldn't recognize that i had unplugged the vss as a test , the module will recognize either front abs sensor wires disconected , code for them , and then recognize that i plug them back in , i checked the fuses , i took the wiring harness from the vss to the pcm and verified that the two wires from the vss are ok , everybody i ask says that it is impossible to have an ABS lamp on and no codes, the module repair man told me i had a cheap code reader so i had a professional shop reader diagnose the codes , ($ 200.00) same thing , ABS on , no codes available , driving test showed no output from vss , put the old original vss back in , same thing , i bought a used complete ABS unit yesterday , stripped the module off , plugged into my truck , the ABS lamp went off ,finally , truck running or shut off . so i went for a test drive , no speedo still , and then the ABS lamp came back on and the park/brake lamp too , no codes still , park/brake lamp off when restarted but ABS still illuminated . also replaced the vehicle battery , i'm working outside in canada and it's about 0F , not much fun with those little t20 module bolts.they want $750 canadian dollars at the GM local dealer for a new module . is it possible the ignition switch is haywire ? is there a vss speed buffer in this truck ? and will it hurt my trans to operate like this , it shifts hard like a race car , been 2 mos without using the truck , thanks , jim. i should also mention that i cleaned and verified the 2 ground points under the cab and the ground under the battery on the frame, ran jumpers from battery to engine, frame , firewall etc, no better, unplugged pcu and visually checked the pins for corrosion , truck was from british columbia and has minor corrosion , almost none, no road salt area , . we fixed a pontiac vibe that had no speedo , ABS lamp wasn't on , researched that the two front wheel sensors ran to the abs module to make the speedo work , no vss, we replaced the ABS module and all is working , i thought similar situation here but this truck has a vss, the vss wires run directly to the pcm . where they go next i don't know. i purchased a cd of a factory service manual , but it has 23,000 pages and no index that i could find , if i had the proper page numbers to search perhaps i could do more , so if you can give me the page numbers it would help a lot, also it's very slow to load as too much data .

Edited February 18, 2019 by jempey
additional info

andrewlolney · February 18, 2019

Sounds like you have the 4l85e for that year. I know there is 2 VSS on that tranny front and rear of the tranny- and they need to able to communicate with eachother. I noticed you talk about changing one. Did you do both? Also I would reset the Throttle position sensor for the hard shifting issue. The TPS and vss's all communicate to eachother to calculate load and line pressures for the tranny. I dont know much about this issue but to me almost sounds like a faulty cluster- but that doesnt explain the hard shifts. Personally I would probably drop the pan change all of the electrical components (shift solenoids, Tcm, wire harness etc.) inside the tranny plus swap in a new filter. That will just rule out internal electrical components. The kits for these trucks are under $200 and fairly easy to replace. It's just dirty and time consuming. Hope you get it figured out! Good luck.

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jempey · February 18, 2019

thanks , but i have the manual transfer case which does not have the second speed sensor , the speedo doesn't work somehow due to the speed sensor ? or the ABS module ? and as there is no speed input to the computer the truck is shifting hard as the computer doesn't know what speed the truck is moving at , trans oil and filter is new fairly recently , i need to know why the ABS lamp is on , but does not throw codes ?????