Hello,
I am new here and also new to diesels and Allisons.

I just purchased a '99 GMC W4500, turbo diesel, mitsu. engine, 128,000 miles.

How do you determine what type of trans I have in this truck ? Where are the markings located and what am I looking for ?

My problem is:
The check trans light comes on when I coast (goes out if I shift to neutral, then comes back on once I get up to speed and back off)

Any ideas on this

Thanks to any that answer.

Dave J

As I believe the Allison 1000/2000 Series transmission was not released until the following year for medium-duties it probably has a 3000 Series transmission. Does the truck have a mechanical or push-button shifter?

There should be an identification tag on the transmission (passenger side near the rear) that has the make and serial number. That would be a good place to start providing it's still there. If not, see if you can take a picture and post it here.