Peterbilt Automatic Allison Transmission Information Please?

Pretty much works more or less the same as an automotive automatic.

Allisons are a "true" automatic in the traditional sense. They have a torque converter and gears are changed by the engagement of different combinations of clutch packs inside the transmission.

The majority of heavy truck automatics are, more or less, just a regular manual transmission with a clutch and a whole bunch of electronics and actuators bolted on to shift the transmission with a computer.

 

I found that my truck can have buttons to switch between auto and manual mode, economy and power mode, but those buttons can be disabled in the ECM if the company dictates or if the truck came from the factory with them disabled. Depending on the size and type of company getting ANYONE to approve making a disabled button active again is like getting written permission to speed from a cop.

 

this manual may help explain M.

To my understanding the exhaust brake is linked to the accelerator. You can turn it off or on via switch on the dash. Foot off the gas and exhaust brake starts or it doesn't.

If the exhaust is on and the auto is still on original factory setting (which are adjustable), the box will start maximum braking to achieve the lowest gear as to stop. Ie from 6 to 2 usually. You can have this set 6 to 4 or 3 also.

We found the problem with these settings is if you want 3 on decent, and you pass the calibrated factory set revs, it will change up. I think you can change those factory revs perimeters a little, but you can't lock it in a gear as its still in auto mode.

So, I had my setting adjusted (auto service shop) to leave the auto in chosen gear (any) and let the exhaust brake do it thing, and not trigger down changes. You now have to be aware however, you may be sailing along in 6 and get off the gas with exhaust brake on and think it will start to down shift, well it won't. You NOW have to manually down change through the gears. The box will eventually automatically down change when speed has become to low for it to operate safely so that side is covered. But it won't automatically start slowing for you on your behalf. You can punch those arrows to second if hard braking is deemed and it all start working like original factory setting though.
This setting gives you better flow of speed and gear selection, better flexibility and overall braking, plus ability to lock it in a chosen gear.

This open setting also allows you to set a gear for decent to ascent and it will stay there. You now have to be aware however, it will hold that gear even in redline as you have taken away the boxes ability to think for you.

You must trust your driver to be paying attention to manually downshift. Or they will be working the brakes hard and increase chances of fade, as they are not manually helping the truck to slow with gears along with the exhaust.
And still needs to keep an eye on revs if you are lightly braking it down a hill when the gear is almost but not enough. It has become a manual and if you don't slow it or change to an appropriate gear, just like a manual it may detonate.

To confuse you more, you can also just press D and let it behave just like in car. Up and down the gears but no braking. This is help full in light traffic or slower applications.

We found this approach gives far more control and flow overall. Before hand in original factory setting often dropped a gear just at the wrong time before a coroner, or went up a gear when you prefer it didn't. Plus you could not lock a gear, or the gear you were happy with suddenly changed because you went 1 mile faster.

We have found a correctly working exhaust brake is a absolute must to have the box working at its best.

If Alison has a failing in my view, is that they are only a 6 speed. If they were 7 or 8 they would be perfect in every way I feel. We like it a lot.

 

This is not meant as disrespect to you at all driver. But why would a company turn you loose in a truck without explaining how to operate that transmission ? That's how people get killed. They need to show you how to shift that transmission manually so you can come off a mountain safely.

 

By the sounds of it, very few have ever actually driven a truck with an Allison.

 

I never worked somewhere where to company does more than give you truck keys, fuel card, etc and says "this is your dispatch".

 

Yeah I know what you mean. But the part where he said he only 2 months experience is what alarmed me.

 

Yes probably true, but if you drop over the edge in a manual in the wrong gear, you have to make the same decision pretty quickly. An auto is no different.

In the factory setting you will soon work out you have the wrong gear, because as speed increases it will keep changing up. Get the gear right and it will hold. The downside is it may not be as fast as it safely could be.

When my exhaust brake was sounding like it was working but it wasn't, we had to descend in 1st at 12mph as it was the only gear that held. Painful.

Get the brake sorted and we could descend at 22mph for most part in second using factory perimeters. Any faster and it would jump to 3rd and would not hold, so we had to keep lightly applying brakes to keep it in second.

Get the factory setting changed to open, and we could manually lock it in second and hold 25mph and still be well under redline.

Living on mountain I am regularly entertained by drivers in manuals or auto descending to fast in the wrong gear. Brakes smoking, going to fast, eyes wide open, some go over the edge. Cars or trucks doesn't matter.

I imagine however as a professional driver, driving in snow, heavy rain, heavy traffic, or tight spaces is no different to hill climbs or descents. Its a learnt skill that once understood becomes second nature and all part of the day.

 

the Allison has a couple other things that keep it from perfect

you can lock it in gear from downshifting- only up shifting. I know this because I am running an aux transmission behind the Allison and I need this capability to shift on the fly and the Allison can’t do this even with programming.

The other is issue is the gear spacing for running heavy down hill - there are speeds that are not doable - you either get 25 or 40. This is an issue when you reasonably heavy.
When you combine the 2 issues together it can be frustrating for sure.

 

Um not sure I follow, if I lock it in a gear it will not upshift or down shift, unless I manually change it, or unless the revs get so low for that gear it may cause damage to the box. This is in the open setting I have been talking about. You have get it programmed at by an agent. The Mode option does not do this.

Yes I agree, if it has one or two more gears it would bridge that gap between gear spacing better.