Silverado w/ high voltage at TPS/ EGR sensors

Fraxinus · July 28, 2014

Hi All,

Hoping for some help with a situation that's got me pretty stumped.

The truck is a 2000 K2500, 5.7 liter, 4x4

The symptoms started near the end of the day last week very abruptly, after it had been driven on & off all day. Idles very high, very jerky moving from park into gear- likely idle related- & will 'bog' if quickly accelerating. No apparent correlation to bogging & what gear the truck is in.

Check engine light was on (not sure if when it came on- one of our staff was driving it that day), and pulled codes:

P0122: TPS Circuit Low Voltage
P0404: EGR Open Position Performance

If it wasn't for the codes, my first thought would be MAF sensor or fuel pressure issues. However, starting with the codes, I've been poking things with a multimeter. Results:

Power to TPS: Should read 4.5-5 volts, I read 11.8 volts
Ground: Should read 11-12v, I read just a hair over 12
TPS output: Should read .5-.9v at idle, with a steady increase to 4.5-4.9 wide open. I read 1.4 v at idle, and then a mostly steady increase to 9.5 v moving the throttle fully open. There is a repeatable 'spasm' at around 2.0 v where it briefly reads .6 v before continuing up again gradually.

Tested the voltage on the EGR & got a similar response.
The pintle position sensor is reading 1.52 volts, should be .6-.9. Which would suggest there's some carbon blockage.
However, the input from PCM reads 11.37 volts, and it should only be 4.5-5 (same as the TPS pretty much).

At this point, I pulled the EGR valve to rule out anything physical- looks good, pintle functions fine. Cleaned it for good measure (& the MAF while I was dinking around). No change.

The next step was assuming I had a problem with the PCM- swapped it out with one in a '99 Silverado we have, condition is exactly the same.

I've checked my major grounds (grounded engine to frame, engine to battery), no luck.

This is where I'm at. Thinking a ground problem? Anyone have wiring schematics for this truck? Do the TPS/ EGR share a ground that could be bad? What about the PCM- where does that ground to?

Any help/ advice/ troubleshooting tips appreciated.

Thanks,

-Tony-

Fraxinus · July 29, 2014

Update if anyone is interested, I've narrowed my issue down some.

When I pull the 50 amp blower fuse the issue appears to be resolved. The voltage to the EGR & TPS sensors normalizes, and when I start the truck it runs great. That's the good news.

The bad news is that from there I pulled each plug in/ around the blower box that I could find, one at a time, trying to pinpoint the source. None of the plugs or relays that I pulled made any difference. Which leads me to one of two conclusions:

-the issue is within some component that I haven't been able to see a connection for. I pulled the glove compartment to access the wiring back there. Have definitely disconnected the blower box, the main relay, and 4-5 other plugs in that region. Any other ideas?

-there is a short somewhere along the line to the blower box, after the fuse, with the wires leading to my 2 5v sensors. Any ideas on the best place to start looking? Any wiring diagrams that would help?

I've been having a hell of a time searching online for model/ year- specific wiring diagrams. Which range of years should work for this truck?

Thanks,

-Tony-

5thGearTapped · August 9, 2014

Have a short to power. Most likely inside a harness....somewhere. Pull the 50A fuse, do a continuity test between load side and your sensor reference wires that show high voltage. Based on what you've found so far it'll probably show a connection. If so, the 50A blower fuse only draws power on high (theres a 25A that feeds the lower settings)....so if you can live with the other 4 speeds leave it out. Or cut the ends of the wire and run a new one. It goes to the relay at the blower housing.