Temperamental Tachometer

[JohnnyR]

My 2003 Duramax/Allison exhibits a higher reading on the tachometer if it's less than 60 degrees or so when I first crank it up. Tachometer will read about 100 to 150 RPM more across the board versus when temperatures are warmer at initial startup. Even more confusing is that if I stop, kill the engine, and restart after it's reached operating temperature; it will read at the lower RPM numbers across the board. But if I just keep going and don't kill and restart, it will continue to read 100-150 RPM higher at idle or highway speeds. Torque converter is locking up so that's not it. Lower RPM numbers are 650 RPM at idle and 2000RPM at 70 MPH. Higher numbers are 750-800 at idle and 2150 at 70 MPH.

Have noticed that it can be below 60 degrees ambient, but if the trucks been in the sun all day, it will probably read at the lower RPMs. Dealer was unable to explain or find what does it.

Has anyone else experienced this?

[MIKE MOG]

No but I have a 2003 and I will let you know when I drive it again. Is your truck a 2500 with a quad cab and a 8 foot bed. I ask this becuse I have a noise and between 52 and 64 mph and no dealer can find it.
Mike

[JohnnyR]

Mike
Mine's a crewcab w/shortbed (6.5 ft). No noise but describe yours.

[More Power]

Engine rpm is varied by the ECM for a variety of reasons - cold start/warmup, A/C request, etc.

If you think there's something out of the ordinary, I'd compare the tach to what a Tech II scan tool is seeing.

MP

[JohnnyR]

More Power,
I can see where the ECM might vary the idle RPM for different conditions, but I wouldn't think the RPM would change at a given road speed, say 70 MPH. Since the tach is electronic, could I have a sensor that's temperature or moisture sensitive that's not working correctly but resets when I stop and restart the engine?

[Jim Brzozowski]

Johnny R, How do you know if your speeometer is not also compensating, its electronic too. I think off the top of my head that voltage variation may have something to do about it, but I really don't know. Hope someone figures it out. I just hate when I can't figure something out.

[More Power]

The torque converter remains unlocked till the trans warms up. This produces about 150 rpm or so more rpms while cruising with the TC unlocked.

Once reaching the desired temp, the TC will lock, and the tach will drop accordingly.

MP

[JohnnyR]

Even after the transmission/engine warms sufficiently and torque converter locks up w/corresponding RPM drop, tach will still read the higher RPM until I stop and restart engine if when I initially started it, it was idling at 800 in lieu of 650 RPM.

I checked speedometer against a handheld GPS unit this past weekend. Speedometer seems unaffected or is consisitent; RPM is just 150 higher for a given speed when the thing acts up.

Darned thing has me skunked.