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Allison Transmission Stuck in 3rd Gear

stuck in 3rd gear on allison transmission

It was a rainy and windy day and all of a sudden there were two buses with transmission shifting problems. It’s not a surprise since any kind of wiring problem pops up when it’s wet and sloppy on the highways. When oxygen contacts an open electrical connection the chemical reaction begins and that green corrosion that all mechanics love rears it’s ugly head.

The module below is called a vehicle interface module. The heat generated inside this module caused the design to be open to the atmosphere for cooling which produces the green corrosion build up. The relays inside the module effected by the corrosion will click on and off making certain circuits come alive with the ignition key turned off. I’ve seen this happen before my eyes. One instance was the back up lights stayed on with all circuits shut down.

The First Sign of an Electrical Fault

Electrical signals get crossed and the main objective of electrical current is connecting to a good ground. If there isn’t one available because of high resistance due to corrosion the circuit will naturally find a better ground elsewhere which causes a back feed to another light or component and turns it on. If you turn a signal light on for instance and the back up light illuminates instead… most likely you have a bad ground. On the other hand if there is an open circuit which we experienced with our trans problem there will be no action on that circuit.

The Transmission “Shift By Wire” Problem

Getting back to the transmission issues…. one bus was stuck in 4th gear while another bus was stuck in 3rd gear. It was a coincidence that two buses were acting up the same day but it just indicated that there was a wiring fault somewhere in the harness. The electrical on an Allison transmission is intimidating since it’s mostly white wire and numbered with the circuit designation. The diagnostic software would not communicate because of the wiring issue so it was a hands on repair which meant physically inspected every strand of wiring in the transmission harness.

The wire shown below is a piece cut out of the transmission harness that we found after stripping off the convoluted loom that caused the wear marks on the insulation. This takes a long time to develop but once it hits you’re looking at a lot of time searching for this type of problem. The fault codes will not tell you nothing because everything has gone south and all you see is communication problems and a transmission that’s stuck in gear.

clog in allison transmission

Electrical Loom Adds To The Problem

The plastic loom wrapped around the wiring harness is convoluted or ribbed so it bends easily. The design causes the loom to wear into the wiring insulation and expose the wiring as shown in the photo above. You have to strip the loom off of the wiring harness and physically check for wear and corrosion.

We repaired the faulty wiring and road tested the bus and luckily it was on a rainy day. This ensured us if we missed something it would show up. The mechanic gods were kind that day and both of our buses passed the road test. Experience is a great teacher and when problems come up that happened in the past it’s nice to know exactly where to start.

NOTE: One extra step we’ve been doing to help prevent the loom rubbing through the wiring insulation is wrapping friction tape around the harness. This tape is made by 3M and looks similar to hockey tape made with a cloth like material. It works and prevents this type of failure from happening again.