Allison MD3060 serial connection


New Member
My '94 Vogue with Cummins 8.3 300 mechanical and Allison MD3060 trans.

The trans has a "service indicator" showing and a fault code of 66 which is serial communications failure. I had the local Allison shop look at it and they said it was not seeing the TPS.

I only have a barebones and barely readable schematic on this orphan coach and cannot determine how all the components wire together so I can do some trouble shooting. I am not about to pay Allison $90.00/hour to trace wiring harnesses around the coach.

I bought the Allison Troubleshooting Manual and I have learned a lot.

The trans works OK the way it is but it runs on it's own default setup. It won't go into 6th until I get over 65 MPH and there is some "flare" in the 4-5 shift on a hard pull. I'm hoping that getting this corrected might help my mileage.

The TS manual says that code 66 is bad serial communications from the engine ECU, but I don't have one.

I did some digging (had to remove the front TV) and found that the Vehicle Interface Wiring (VIW) connector which contains the serial interface pair, does not go anywhere and never has. It's just coiled up at the trans ECU.

I talked to the Allison folks in Ocala, FL but not much help over the phone.

BTW the tech did tell me to cut the ground wire (wire 144) to the ECU, said it was a mod from many years ago. I did that but no joy. Hope he is right.

Anyway, it seems to me that since Vogue did not connect this cable to anything that there is probably a way to tell the trans that there is no serial connection to the engine. It's a two hour drive to the nearest Allison facility and I want to do as much as I can before going there.

I have only had this coach a few months, the problem has been there all along. I talked to the previous owner and was told that he did have some trans problem. The ECU may have been replaced, all he knew was that it cost $1000.00 to fix I can see that the ECU was not properly reinstalled, just left loose.

Any help greatly appreciated.

Len Silva


New Member
Re: Allison MD3060 serial connection

because you don't have an electronic engine you have a TPS on the throttle linkage. That is the most common problem with older models. Check to see if it is un hooked, sticking or just bad. Good luck


Senior Member
Re: Allison MD3060 serial connection

Sounds like the ECU is expecting to see an engine to transmission communication link. Since you do not have an electronic engine, the problem is likely that the calibration that was put in the ECU when it was replaced is the wrong program. Sounds like Florida DDA in Ocala should be able to contact Allison TAC with the calibration identification number that's in your ECU and get some help in putting the right calibration in it.

The flare and shift points you are experiencing is because the ECU turns off adaptive control and assumes wide open throttle (no shift point modulation) when it is not seeing any throttle signal.


New Member
Re: Allison MD3060 serial connection

That's the conclusion I have come to. I now have the Trouble Shooting Manual and have verified that the TPS is good. All I need now is to be able to speak intelligently to the Allison folks and you have helped with that.