Weird Tach Behaviour
 

Okay as the title says my seems to be jumping to high RPMs with any change in engine tune. It has happened 3 times so far in the past two weeks. I have seen it jump from idle (600 RPM in park) to 1000 - 1500 RPM and from 2000 PRM driving to 5500 - 6000 RPM and it does it for a split second before returning to normal. There is no change in engine sound or tune when it happens. I think it has something to do with the dash, but no other gauges are effected. Is there a way to test this?


BTW, the tach jumps so fast that the first two times it happened I though it was me and didn't take it seriously.

Year/make/model?

Add that to your signature for future reference so people are able to help you better. Thanks!

2000 chevy blazer 2wd

I'll be watching this thread. I've got the same thing happening on my 2001 4wd.

I got a 2000 LT 4x4... this has happened to me once

Just as question; where does the truck get its tach signal from?
I know the computer displays what it sees on to the dash, but what is computer referencing?
Is it the crank sensor, the cam sensor, or something else?

All of the modules in the vehicle are networked together. They communicate with each other using classII serial data. The PCM, (Powertrain Control Module) supplies the BCM, (Body Control Module) with RPM data, and the BCM supplies the IPC, (Instrument Panel Cluster). The IPC commands the tachometer. You'll need a bi-directional scan tool and a qualified technician to diagnose the problem.