1998 gmc c2500 5.7 4l80e Tachometer reads high speedometer reads high
 

Hey guys, I'm new to the board and i've spent weeks searching google and various boards to try and troubleshoot my problem.

I've got a 1998 GMC Sierra 2500 5.7 4l80e ext cab 2wd with 102,000 miles. And I bought the truck 1000 miles ago from my work, where it was the mechanic's run around truck.

My problem is that at and idle, the tachometer reads at 2000 rpms and the speedometer rests at 0. When I put it in gear, the rpms drop down to 1800. And when i start driving with a light foot, the rpms race up to 4-5k and selects the next gear. and if i'm climbing a hill i can bury the needle at 6k! When i first started driving i couldnt believe how quiet my new v8 runs at 5k rpms :D.

Now for the speedometer, at 55 I'm actually doing 40, and at 85 i'm doing 60 this is tested with the gps off my phone (pm me if you need any advice for a good app to use as a speedometer :cool:)

Also the transmission has problems keeping it's mind up on which gear it wants to be in (hopefully related to my tach and speedo problem)

So it's not throwing any codes, but i read at 100k they advise replacing vss sensors, both input and output (check) and The mechanic at work replaced ignition switch the day before i bought it (i believe him because he broke the steering column cover which gives me a pretty sweet view of the shinny new plastic and the key's brand spankin new!)

Could it be a bad ground to my PCM? Or possibly a stepper motor on my cluster? (the needles don't jump, they are smooth all the way through their motions, it's just the rpms go up WAY too high and fast)

If there is already a post on this, please send me in that direction!

So despite there not being a check engine light, I took the truck up to a local NAPA and ran an obd 2 scan and it blew a c0281, c0265 and a c0237. The last code is an erratic rear speed signal. In the morning I'm going to crawl under the truck and check the connection and follow the wire to make sure there's no damage. Hopefully this is my problem!

I unplugged the sensors and my connectors look fine, and i pulled the sheilding off the harness and i'm not finding any frayed wires, I turned the key to on, without the truck running and i'm getting 4 volts to the input vss and 3 volts to the output sensor. Then for curiosity purposes i turned the truck on with only the output sensor plugged in, And it fired right up and was still showing 2.2k on the tach!?