Tachometer Glitch on '03 Duramax

[JohnnyR]

Has anyone run into the following:
My 2003 GMC 2500 HD Duramax/Allison seems to show approx. 150 RPM more for any given speed when ambient temp. is less than 50 degrees. On a morning like that, I can start it up and take off down the highway and the tach will read 2150 RPM at 70 mph even after the engine reaches operating temp. and transmission locks up in 5th gear. Normally it would register 2000 RPM at 70 mph. Even if the outside air temp. rises, it will keep reading the higher RPM (2150) if I don't stop and kill the engine. If I do so, restart the engine, then the tach reads a consistent 150 RPM less at any speed,e.g. 2000 RPM at 70 mph.
My 2001 Duramax/Allison never exhibited this. Has anyone run into this problem.

[mackin]

I think what you will find is this is normal ... What happens is the Allison will not shift into OD till the sump tempature raises...It is to assist in tranny warm up...Normal operating procedure...

MAC ;)

[JohnnyR]

Mackin
I'm familiar with the sump temp. requirement. The tranny temp. will come up to 160 or so and this tach glitch still remains unless I stop and restart the engine which I assume resets everything. Really got me buffaloed. As I said, my 2001 never exhibited this.

[Gasman1075]

Isn't the 150-200 RPM the torque converter lockup?
When its cold out mine won't lockup for about 10 mi.

Walt

[TBC]

I think the lack of torque-converter lockup is what is happening. My 2001 reads high until after I driven far enough for the trans to warm up. The colder the ambient temp the longer it takes.

Tom Cobb

[Burner]

I "HEAR" you Johnny. What you'er saying is that no matter what the outside temp becomes your truck will show 150 rpm higer than normal, provided that it was a clod motor started below 50 degrees. Maybe there is a signal change? Compensation factor is 50 degrees...maybe?

Burner -----> :D