Allison limp-home mode

[Tom]

One of the things I learned at the Allison seminar today ....

If an Allison transmission develops a serious problem it will still attempt to get you home, or to the nearest shop. As a last resort, the transmission will drop to fluid coupling and allow you to limp home. Performance will be poor because of the slip and, if you try to put pedal to the metal, the transmission will likely overheat. However, when it goes into limp-home mode, do not pull over and turn the engine off unless you want to stay in that location. Once you turn off the engine or shift to neutral, the transmission will not re-engage.

I learn something new every day.

 

[Gary RV_Wizard]

Even your 'Burb has a limp home mode, Tom, though it is primarily an engine thing there.  There was a time when vehicle power trains shut themselves down to avoid serious damage. Then some people sued because they had been stranded in hostile places (either climate or people) with no ability to move to a safer place.  I think most of us would say "Damn the engine - I'm getting out of here" if lives were in danger.  Bad press as well as expensive, so the "limp home" concept became pretty much de rigeur.

Nice to hear that Allison has extended the concept to purely transmission functions, though.

 

[Tom]

Even your 'Burb has a limp home mode

Didn't know that Gary. Guess I should read the manual  :-[

I think most of us would say "Damn the engine - I'm getting out of here"

I agree. The Allison guy told us about one guy who put pedal to the metal and drove 850 miles in limp-home mode. He burned up the transmission.

 

[Gary RV_Wizard]

Didn't know that Gary. Guess I should read the manual  Embarrassed

It may not even be in the manual, so you need not bother hunting for it.  Basically, the engine [aka powertrain] controller will do everything possible to keep the drivetrain operating at some level, even though performance may be truly awful. It may create an artificially rich fuel mixture, shut down the a/c, limit top speed, etc. but will not decide to shut itself down just to save its own life..

 

[John From Detroit]

I read the manual yesterday... It's not in there.  So,  thanks for posting it folks,  I have an Allison on my rig

The engine on my rig (Vortec 8100) also has a "limp in mode" (Note I'm chaniging to "limp in to nearest service center" instead of "limp home" and this IS mentioned in it's manual.. (Which I also read yesterday.. Had lots of time and lots of coffee yesterday)

Now, Detroit Diesel (Penskie) endinges... When the check engine light comes on STOP NOW... My brother took like 10 seconds to pull off the road... He only had 8

 

[Tom]

I'm sure different manufacturers call it different things John. Allison specifically calls it limp home, both in my owners manual and in their talks. As I indicated in another message, someone interpreting this as 850 miles to home burned up his transmission. So it's obviously intended merely to get you to a safe place where service can be rendered. I haven't found an equivalent statement in my Cummins owners manual, although it describes a progressive reduction in operation in the event of a problem, leading eventually to a Stop engine light coming on.

 

[John From Detroit]

As I said, I did not see any thing about "limp home" when I read the Allison manual, however in both the Allison and Workhorse manuals I did see "Drive to nearest service center" more than once

I changed it to "limp in" though I can see where "limp home" works too, just the transmission's home, not yours (Allision authorized service center)

 

[Tom]

As I said, I did not see any thing about "limp home" when I read the Allison manual

I suspect you have the 1000MH or 2000MH transmission in your gas coach and so would have a different manual than the 3000/4000 families and the wording might be different. OTOH the 1000MH/2000MH may not be designed to run under Check trans conditions. I just got back from the Allison shop and picked up the latest (5/2005) manual for the 3000MH & 4000MH transmissions; It still says Limp home and still clearly means a short distance.

Since your manual doesn't reference it, I would advise against running your coach under Check trans conditions unless told otherwise by Allison.

 

[John From Detroit]

It says Drive to nearest service center... it simply does not use the words "limp home" that I noticed.

Otherwsie, it appears there is no difference.

I do know this... Sweet trans, good engine too (Vortec 8100) darn thing can perform if it has to, though normally I tend to baby it

(If you look at instant fuel consumption baby it runs from about 5mpg to,  well it gets right up there when you are going down hill

But if you floor it. that five turns into a point five real fast)

 

[Tom]

You're probably right John. The Allison guy didn't differentiate between their various xmission families when describing the ability to keep going after a problem. I'm just not sure if he realized there was a 1000MH owner in the room. My understanding is that these transmissions are virtually bullet proof. Automotive electronics have come a long way since I built my first electronic ignition for my car.