Exhaust Brake Interface With Allison Transmission

[shields]

I'm facing a situation where, in the process of addressing one problem, a second problem has arisen. I suspect the source of Problem #2 is the work related to Problem #1; but no one seems to have a clear explanation.

We have a 1997 Monaco Dynasty with an 8.3L Cummins engine coupled to an Allison six speed transmission. The coach also has a Pac-Brake exhaust brake.

Problem #1.

We've had two small transmission issues: (1) the shift from 2nd to 3rd was not a smooth as the other shifts, and (2) when the transmission downshifted to 4th up steep hills under heavy throttle, an indicator light saying "service" appeared on the transmission control panel. These small issues have existed for many months and I just got around to dealing with them.

We notified our mechanic who reported that the "service" code was a false report of the transmission being overfilled with fluid due to a malfunctioning sensor. As to the odd shift from 2nd to 3rd, he reported that the throttle position sensor and the transmission control module were not communicating properly.

We had the transmission fluid level sensor replaced, the throttle position sensor replaced, and the transmission control module tested by the manufacturer. The transmission control module was working properly but they upgraded it somewhat after the testing.

Problem #2.

Everything is back together and working fine, EXCEPT, now the exhaust brake is inoperative. When I depress the activator for the exhaust brake, the numbers on the transmission display panel start cycling rapidly from 1-2-3-4-5-6-1-2-3-4-5-6.

The technicians have replaced each of the three new components and rechecked their work. Nothing has changed. At this point everyone involved seems mystified.

Before the technicians start investigating further, I wonder if any of you FMCA members have ideas or suggestions.

Thanks so much!

Tim

[wolfe10]

Tim,

First, check for stored codes in the Allison ECU (using the shift pad).

If no problem found, I wonder if they messed with the wiring.The exhaust brake signal goes from the exhaust brake switch/throttle position sensor to the Allison ECU and THEN to the exhaust brake solenoid. So there is an IN and OUT wire at the ECU.

Only other item is the Allison ECU itself.

Brett

[shields]

Thanks Brett. It sounds like you and I are seeing things the same way; the source of Problem #2 is probably found in the work done on Problem #1.

Thanks again.

Tim

[renerbanville@aol.com]

Learning to post, may have answer to your problem. Rene.

[renerbanville@aol.com]

I had a problem similar to your's. MH 1997 Engine:Cummins M-11. trans. Allison HD4060R.

Allison dealer shop said NO fault code, go see Cummins/Freightliner dealer.

Result: lots of false code on engine ECM (electronic control module). ECM shot need new one.

Put new one in ($3800.00) all problems solved. Rene

[chucknewman]

I agree with everything said above, except I would add that on most Allison documentation I have seen, the exhaust brake signals go through a Vehicle Interface Module (VIM). It has several relays that perform voltage/current "interface" functions to and from the transmission and various external dry contacts in the coach (switches, relays, etc.). I mention this since it is another possible source of problems. The Allison ECU is silver and the VIM is black. Mine are side-by-side, but I have heard of many coaches having them many feet apart for some reason.

In addition, if you have a PacBrake exhaust brake, the air solenoid (on many designs) is actuated by a "exhaust brake relay" somewhere between the VIM and the air solenoid. I doubt that is causing this problem, but if your coach has this relay it is good to know where it is located.

Based on your current rapid number changing issue, I would query Allison and the Internet to learn what would create that symptom. But be prepared for a lot of "I don't know" responses.

I have seen very similar maladies after working on equipment. Each time it was improper connector mating (shorts or opens) or a loose crimped connector pin in either the plug or socket -- usually the plug due to improper unplugging technique.

If it were my coach, I would first look for error codes in both engine and transmission ECU's. Then check for proper chassis vdc at both ECU's and the VIM. Then check the exhaust brake relay line and note what is missing.

Of course, this requires diagrams, troubleshooting experience, and time. Most folks don't want to tackle this themselves.

Is your mechanic and his business Allison certified? If not, I would get another opinion from another Allison dealer. In fact, it's probably time for a fresh set of hands on this issue.

[shields]

Chuck, Rene;

Thank you both for the advice. The coach has been at an Allison shop and they reported that the Pac-Brake is now working properly. As Brett suspected, it seems one of the connections got goofed up in the course of the project.

I hope to get over to the shop within the next couple of days and check things over in person. I certainly plan on following your advice and checking for codes and getting more familiar with the VIM, ECM, etc.

Once again, thanks to you all. I'll let you know the final outcome soon.

Tim

[chucknewman]

Tim,

Take a pair of coveralls with you and have the dealer show you what connector/wire/component was wrong and why. For two reasons:

1. Sometimes (unfortunately) the repair is done in hast and fails again somewhere down the road. This will give you a great advantage if this monster returns.

2. You can give us now, and anyone suffering from the same issue in the future, a detailed treatise on what happened, where, and why.

Thanks.