1998 GMC C7500 7 liter v8

I have a medium duty truck with air brakes that has some sort of issue with the abs. It has a standard 16 pin obd2 port rather than a 6 or 9 pin found in most larger trucks. I don't believe it's using a heavy duty truck protocol because of the dlc pin placement but I am not able to achieve communication with my scanner. I have 12volts to pin 16 and good grounds at pins 4 and 5. I also have wires to the #1, #6, #7, and #15 pins. I am assuming the #1 and #6 pins are proprietary lines GM used. My check engine light comes on when the key is turned to the on position but cycles off. The truck starts and runs with no issues. My abs light stays on. I checked the voltage at pin 7 which I am assuming is the k line and I get almost 70 volts with the key in the on position with engine off. When I disconnected the abs module the voltage dropped to 0 volts but I still can't establish communication. The only thing I can think of that could introduce that much voltage to the system is something shorting in the ignition system but the engine runs smoothly and I still don't see how that much voltage could even possibly get to the k line with key on engine off. Any ideas??

Is there any voltage present if you go from battery neg to engine block? Have you used this Scan tool on the truck before? I'm assuming you see that 70v with engine running? How about koeo?

Nissan Technician - ASE L1 Master Tech

This is gonna sound backwards, but... check fuses. I've seen it before, when a module or component is missing a 12V supply, and ends up backfeeding higher voltage into that same feed circuit when the fuse is popped. I don't understand it, but it happens. :silly:

Mark.A wrote: I checked the voltage at pin 7 which I am assuming is the k line and I get almost 70 volts with the key in the on position with engine off. When I disconnected the abs module the voltage dropped to 0 volts but I still can't establish communication. The only thing I can think of that could introduce that much voltage to the system is something shorting in the ignition system but the engine runs smoothly and I still don't see how that much voltage could even possibly get to the k line with key on engine off. Any ideas??

I, recently, saw a 70 volt squarewave on a communication line. :woohoo: After pulling my hair trying to understand it, I realized that channel B of my Pico was on a X10 setting, instead of X1. :whistle: Not that this is your problem, but if you are using a scope, it's something to check.