First pull with 8.1/Allison

mitchntx · June 11, 2004

The first REAL distance I've pulled with the truck is happening this weekend. The first leg of the journey was to work and I'm leaving from here for about a 200 mile trip with 6000lbs of race car, trailer and misc. crap.

A couple of things are happening that I want to make sure are "normal".

In Tow/Haul, the shifts are monster hard. Normal?

In Tow/Haul, when I come to a long hill, the tuck pulls down about 500 rpms and then shifts down ... TWICE! It goes from 5th to 3rd. rpm goes from 1800 to 4000! and then it upshifts to 4th. Normal?

In Tow/Haul, trans temps drop 50 degrees. Normal?

Where do you guys connect your emergency break-away cable? I drilled a small hole in the wiring recepticle and connected it there.

snoman · June 11, 2004

I hate the way the Allisons shift myself from the ones I have driven, that is normal shift wise. I believe I would find a sturdier attachment point for my breakaway brake cable.

mitchntx · June 11, 2004

It's a stamped metal bracket, bolted to the bumper. The break-away switch is a plastic clip that slides between two contacts.

It takes a fair amount of force to remove it, but not near enough to pull that receptical bracket off the truck.

Give me a suggestion on where to attach it.

snoman · June 11, 2004

Can you attach it to bumber or hitch assembly. If it is ever put to task it will be pulled very hard suddenly until it shears and locks breaks on and you do not want the attachment point to fail first before brake "switch" shears and locks on.

Colorado Kid · June 11, 2004

In Tow/Haul, the shifts are monster hard. Normal?

In Tow/Haul, when I come to a long hill, the tuck pulls down about 500 rpms and then shifts down ... TWICE! It goes from 5th to 3rd. rpm goes from 1800 to 4000! and then it upshifts to 4th. Normal?

In Tow/Haul, trans temps drop 50 degrees. Normal?

Where do you guys connect your emergency break-away cable? I drilled a small hole in the wiring recepticle and connected it there.

The first several shifts in Tow/Haul should be quite firm. They'll back off a little as the tranny "learns", but they will always be firmer than in normal mode.

I believe what your feeling on the upgrade is a torque-converter unlock followed immediatly by a downshift to 4th and then by the converter locking back up. If it really is shifting 5-3-4 then it should be looked at. A technician riding along with a tech-II scanner can tell what it's really doing.

The cooler running is due to the torque converter being locked in gear 2 thru 5, instead of only in 5th. Running cooler is pretty much the whole point of tow/haul mode.

On my bumper pull car trailer I run the lanyard for the brakes through one of the safey chain loops on the reciever and hook it back to itself.

snoman · June 11, 2004

On my bumper pull car trailer I run the lanyard for the brakes through one of the safey chain loops on the reciever and hook it back to itself.

That should work nicely!

496 CUBIC INCHES · June 11, 2004

I don't use the T/H mode because of the way it shifts when "on". The only time I use T/H is when I'm going down a steep grade,and need the grade braking ability.

Try towing without the T/H mode engaged,just because you have it doesn't mean you have to use it.

Roofer · June 11, 2004

Mine doesn't shift 5th to 3rd, but mine might be different because of the diesel. I'm not sure because I haven't towed with a 8.1.

I put my break away cable around the bumper by the license plate and then back up and around the cotter pin on the pin running through the ball hitch.

KyleBst264 · June 13, 2004

GM Trucks Allison Transmission FAQ

Hope this helps.

linda · June 14, 2004

Q. Speedometer / Tires - What effects will big tires have on transmission operation?
A. The TCM calibration is based on factory tire size and rear axle ratio. Changing either of these parameters will have some adverse impact on transmission operation, including features like grade braking and shift stabilization. For instance, we have noted that when P285 tires are installed that the transmission generally does not shift to 5th range when towing heavy loads.

Thats interesting......

snoman · June 14, 2004

Q. Speedometer / Tires - What effects will big tires have on transmission operation?
A. The TCM calibration is based on factory tire size and rear axle ratio. Changing either of these parameters will have some adverse impact on transmission operation, including features like grade braking and shift stabilization. For instance, we have noted that when P285 tires are installed that the transmission generally does not shift to 5th range when towing heavy loads.

Thats also because it is pulling harder in all gears with load and engine vacum is lower than with same load than with 265's. It is the extra drag/load of tires that loads engine and prevents upshift not the 285 persay.

mitchntx · June 14, 2004

Thanks for the replies and the link.

I do believe I am feeling the downshift and a convertor unlock. I hadn't really thought about a locking convertor. Thanks for that.

I began using the T/H switch for grades as well. Works very nice. On a side note, trans temps didn't vary that much after the trans' temps stabilized. Ran at about 180* regardless.

snoman · June 14, 2004

Thanks for the replies and the link.

I do believe I am feeling the downshift and a convertor unlock. I hadn't really thought about a locking convertor. Thanks for that.

I began using the T/H switch for grades as well. Works very nice. On a side note, trans temps didn't vary that much after the trans' temps stabilized. Ran at about 180* regardless.

You really want to stay below 200 for max tranny life. Short spikes to 220 or so are acceptable.