Duramax Diesel and Allison Transmission help

[Gunny4Sawx]

I have an 05 Duramax 6.6L with the Allison 5 speed automatic. It is the next generation LLY motor. Lately on longer trips the tachometer needle "bleeds down" and ends up below zero RPM. The towing setting seems to work with the downshift "kickdown" when I pump the brakes. Does anyone know if there is a sensor or pickup that might need to be cleaned? Does the reading come from the transmission or engine? Thanks for any help on this one.

[Z16Monte]

Not sure about diesels, but tach signals usually come from the computer via the crank sensor.

[Uncle Salty]

You probably have a code stored even though it may not have triggered a warning lamp on the dash. If you know someone with an advanced code reader, you could check for any codes.

[Fourcats]

Most any auto parts store can run/clear codes for you.

[mike634]

I had problems with the speedo and tac on my 04. There is a known problem with the stepper motors on these gauges. I know GM extended the warranty on mine because of it.

Good luck and Happy Birthday Marine

[NJ Rider]

If you go to a site GMTnation, they give you a step by step with pictures on how to replace the stepper motors.

[towboattrash34]

I'm a Dmax guy, but no nothing about Trans stuff.

Go here

http://www.duramaxdiesels.com/forum/index.php

Or here

http://www.dieselplace.com/forum/index.php

Or here

http://www.duramaxforum.com/forum/

[towboattrash34]

And the expert is Mike L here give him a buzz.

http://inglewoodtransmission.com/jm/

[Gunny4Sawx]

Thanks to all for your responses. Looks like I've got some reading to do, I'll try to post the results if/when I find out the cause.