Transmission type for 2500 HD 4x4 8.1 gas

Denis S · March 19

I recently purchased a 2001 2500 HD 4x4 with the 8.1 gas engine. While greasing the driveshaft, I noticed a spin on filter for the transmission and the words Alison on the tyranny pan.. It's a 4 speed automatic.. I thought Alison transmissions were either 5 or 6 speed.

Which model Alison transmission do I have?

CamGTP · March 19

The Allison 1000 is a 5 speed transmission in 2001.

The 4L80e is a 4 speed transmission but those obviously won't say Allison on it and they don't have external transmission filters.

Denis S · March 20

So which transmission do I have? The shifter has the standard PRND321, and I have a tow/haul mode button on the end of the shifter. Is it a 5 speed, but I'm only using 4? I'll test drive it later and verify the shifts.

CamGTP · March 20

From all the information I have and what I you can find from GM on those trucks, The 8.1 came with a 5 speed Allison transmission.

davester · March 20

Yeah, the 8.1 got the same transmission as the Duramax, at least in the pickup trucks...

jake111 · March 21

13 hours ago, Denis S said:

So which transmission do I have? The shifter has the standard PRND321, and I have a tow/haul mode button on the end of the shifter. Is it a 5 speed, but I'm only using 4? I'll test drive it later and verify the shifts.

The 5-speed Allison has PRND321 on the shifter. Press/hold the T/H button for a few seconds and you should get a D-with-slash on the cluster display, aka overdrive lockout. Press/hold the T/H button for a few seconds to re-engage 5th. They used this instead of a "4" on the shifter.

When you count shifts it may feel like it's a 6-speed, but one of them is the torque converter locking up.

Denis S · March 22

Thanks for the info. As you said, it's a 5-speed. I have 4 upshifts, at 2500 rpm, and then the torque convertor lockup. So it is the Alison 5-speed. I'll play around with the T/H mode to get familiar with that function.