Allison Transmission Problems

David E. Damouth

9 April 2010

During a trip through the Rocky Mountains in October 2008, we experienced unusual problems with the shifting in our Allison 3000 MH transmission (in a 2002 Winnebago Journey 32TD). The problem wasn't fully resolved until the summer of 2009.  I have compiled this document from notes written at various times during that period.

The Problem:  While accelerating at full throttle on an uphill entrance ramp to the expressway, I felt an unusual lurch in the rig, and the transmission warning light came on in the dashboard console. I soon discovered that the torque converter lockup clutch wasn't working, so the engine was revving higher than usual and engine rpm was changing with power level rather than road speed.  We exited in Glenwood Springs to check this out. The VMSpc engine/transmission monitor showed a transmission error at about the time this happened, but gave no useful information - something like "unknown component".  I also found that the transmission would not shift to neutral or reverse.  VMSpc showed the shift (showed N when I pushed neutral on the transmission control panel).  But the console display still showed "6", and it was indeed still in gear.  This transmission state is often called "limp mode" - a built-in protective behavior for some types of Allison transmissions when a serious problem is sensed. Transmission temperature was normal.

I shut the engine off, and then restarted. The warning light went off, and the transmission shifted normally between N, D, and R.  But as I accelerated up the ramp back onto the expressway, the rig again lurched as it shifted (3 to 4?) and the transmission warning light came on again, indicating limp mode.

We continued driving in limp mode until the next exit.  Nothing else appeared to be wrong. VMSpc showed that transmission fluid temperature was about 160 F. - somewhat higher than usual, but still within the "normal" range.  The engine was revving higher, fuel mileage was down, and the engine was running a bit warmer (around 190 coolant temp).  After exiting, I again shut off the engine and restarted, and the error went away.  And again, recurred as I accelerated onto the expressway.

I again drove to the next exit.  This time, I shut off and restarted, then accelerated very gently onto the highway, taking a couple of miles to get up to 60 mph.  No problem - the transmission worked normally.  I engaged the cruise control, accelerated gently to 62 mph and continued without problem.  I experimented a bit.  At one point, on a downgrade, I let the engine brake engage fairly forcefully.  No problem.  And on one modest upgrade, the transmission down shifted from 6 to 5 at near full throttle - again no problem.

Later, on the way up a long slope, the transmission started malfunctioning again.  We pulled into a view area part way up, and when we pulled out again, with hard acceleration on an upslope, the transmission seemed to shift into neutral briefly during the 3d to 4th gear shift, then went into limp mode - with the engine revving about 200 rpm higher than normal, and the transmission oil temp running about 163 F. (It had been nearer 135). Again we exited, then shut down and restarted to reset limp mode.  I checked the transmission fluid level with the keypad (it was normal).  By accelerating gently, particularly at the shift points, I got up to 62 mph in 6th without triggering the malfunction. Later, the same thing happened again, I think in a 4th to 5th shift - the engine raced briefly, and the transmission went into limp mode.  Later, under full-throttle in 5th, the engine raced (not sure if in neutral, or just the converter clutch released). But it recovered when I quickly backed off the throttle, without going into limp mode.  We completed the day without further transmission problems by making sure that shifts occurred with only part throttle (less than 200 hp on the VMSpc).

During this period, the VMSpc recorded the following error codes (each of these occurred several times):

Codes stored and displayed on the keypad:

The motorhome then sat unused for two weeks at our destination.

Initial Diagnosis:  During this period, I telephoned Lake City Truck, the Allison franchise in St. George, Utah.  I spoke to their Allison expert, who asked me a bunch of good diagnostic questions, then said that his initial guess was that a valve was sticking in the valve body.  Assuming that this was the problem (pending further diagnostics when I brought the rig to their shop), he would have to remove the valve body. Not a big deal, except that replacing it required a set of gaskets, and he had just used his last set. It would take a few days to get a gasket set shipped in.  When I indicated a reluctance to wait around, he said that it was probably best to drive it back to Denver, and that this wouldn't cause any damage if I continued to baby it by shifting only at relatively low power as I have been doing, which allows more time for the sticky valve to move.

I also spent some time on the Internet, searching for diagnostic information and asking my Internet friends on the "Diesel-RVs" Yahoo Group for advice and suggestions. The "sticking valve" diagnosis still seemed plausible, but several people suggested that faulty wiring or a bad sensor could also cause these symptoms. Driving it back home, where diagnosis and repair could be done without time constraints, seemed like the best option. Both the manager and the technician at this shop were very helpful and cooperative. They were quite willing to come in and work on the rig immediately, on a weekend (at overtime rates, of course).

On the trip home, the transmission actually improved a bit - the 4-to-5 shift became almost normal both directions. 3 to 4 was still a big problem. And letting the exhaust brake downshift was an even bigger problem (a violent lurch).  As it turned out, I rarely used the exhaust brake, and still didn't do a whole lot of braking - the steep downs weren't very long and didn't have sharp curves, and there wasn't much traffic, so I could usually just put it in fifth (occasionally 4th) and let it run, braking only if it threatened to exceed the engine rpm redline.

On the next day, the transmission continued to work well as long as I didn't use the exhaust brake.  I went up the steep eastbound grade to Eisenhower Tunnel mostly in 3rd gear, backing off the throttle to keep it from upshifting. I was still faster than many of the trucks, and the traffic was light, so this wasn't a problem (other than taking a little longer than usual). Coming down, I was again able to just let it roll in 5th most of the time.  I would occasionally manually shift down to 4th, and in 4th, I could occasionally use the exhaust brake for brief periods when the rpms were high enough so that I knew it wouldn't downshift. I kept the transmission in "mode" most of yesterday and today, and it seems to result in fewer automatic shifts under some conditions. 

Back home, winter was approaching.  I spent more time on the Internet, asked for recommendations as to the best Allison repair facility, and didn't get around actually doing anything until spring.

The Fix:  Stewart & Stevenson seemed to be the repair shop of choice.  I took the motorhome to Stewart and Stevenson and gave them a written narrative about the way the transmission was misbehaving under specific driving conditions. One of the technicians was sitting in the office at the time and said that my written notes would be very helpful and that he wished all his customers would do that. His initial guess was a bad speed sensor.  I hope he's right, but it doesn't feel right to me. I still favor the sticky valve theory.

A few days later, I picked up the supposedly fixed motorhome and took it for a test drive. They had replaced two speed sensors.  It behaved ok until I got into the mountains:  Going up the long I-70 grade to Genesee it failed in the same way as before: I noticed a little slipping between gears even at partial throttle; down shifts were harsh (although not as bad as before); and on an automatic downshift on a steep downhill with exhaust brake on, it illuminated the "check transmission" light and went into limp mode.  I recorded the code (5454).

Back to Stewart & Stevenson for another few days.  This time they said that they found "some electrical problems - battery" and that they also reprogrammed the transmission, and said that it was shifting much better.  They were quite apologetic about not getting it fixed the first time.  This time, there was no charge - no paperwork at all.  I drove it hard, up to Idaho Springs and back, and could not trigger a fault.  Downshifts are smooth, upshifts were ok except a couple of times in town when I noticed some over-revving between shifts (3-4 and 4-5, I think).  Later, on the way back, after driving hard, I could not reproduce that behavior.  So maybe it's fixed.

As I write this, a year later, there have been no further problems.

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