This short guide is will hopefully help you diagnose simpler Allison Transmission problems. Allison Transmissions have been around for a hundred years and have set the industry standard for durability. The odds are pretty good that you’ll never find your way here, but if you did lets get this figured out!
Limp Mode on Allison 1000
Modern Allison Transmissions come with a limp mode installed. This is a different shift table that allows the transmission to protect itself from catastrophic failure. You should always assume that this isn’t “bogus” and try and get your transmission. This limp mode locks the transmission into third gear, and keeps the torque converter from locking up. It also keeps the transmission from going into reverse! Keep that in mind. You don’t want to get stuck somewhere that you can’t back out of.
The limp mode is often activated by things an excessive of slippage by the transmission. Here are some common conditions that can trigger a limp mode situation.
Towing: If you are towing a heavy load (especially up a steep grade) you can often activate the limp mode. This is certainly can be an inconvenient time for this to happen. Third gear should pull a load fast enough on the highway, but it can be challenging to get a load moving.
Excessive Heat: If the transmission overheats, that can cause it to slip as well. If you are noticing this problem during hotter weather, you may be able to keep it from happening by installing a transmission cooler. They can be found cheap on Amazon and don’t require a lot of mechanical knowledge to install. Heat issues are the number one killer of this transmission. Do your best to protect the investment.
Performance Tune: Often, when people get an engine tune they don’t realize it should really be a driveline tune. Even just a little bit extra slip put into the tune, and an Allison Transmission can have problems. So if you’ve gotten a tune recently and you’ve found your way to this page try asking if they could take the tune back to the factory specs. It should solve the issue for you.
If you’ve experienced one of the issues above, it may very well be time for a rebuild. It could be a false positive. Either way, you’ll probably want to clear the trouble codes. The best way to do so is to get a code scanner and scan the code to wipe it. It will go away after enough engine on/off cycles where the problem is no longer recognized.
Common Allison Transmission Problems
Modern transmissions are computer controlled, but if you are working on your tank (they made transmissions for them in WWII) or your classic vehicle you may wish to take a look at the vacuum lines. If it’s shifting to early and bogging, you may have it in the wrong port. If it feels like it is shifting toward the engine red line you could have it hooked to the wrong port.
If you have an Allison Transmission in a GM Truck from the mid 2000’s there is a service bulletin for the TCM module. GM sent a letter to customers advising them to bring it in and get it replaced. The going rate for one of them is right around $250, but GM should still be on the hook for it if they never replaced it. http://www.dieselplace.com/forum/57-gm-transmissions/41-allison-oem/223645-allison-transmission-problem-finally-addressed-gm.html.
Amazon has a pretty good compatibility chart on their TCM listing if you would like to check it out. Here is a link to that: https://www.amazon.com/gp/product/compatibility-chart/B005OVLAXQ?ie=UTF8&Make=Chevrolet|47&Model=Chevy II|2002&Year=1968|1968&i=175&vehicleId=1
Neutral safety backup switch issues are another common Allison Transmission problem on the GM Duramax versions. If the D doesn’t light up on your gear indicator, this could be the issue. It is part number 29540479. If you think this may be the problem, you should read up on that more here: http://www.duramaxforum.com/forum/transmission-shop/39968-allison-shift-problem-cured-thanks-all.html
Make sure that you are performing the scheduled maintenance in order to get the most out of your transmission. Most GM automatics come from the factory with non-synthetic fluid. You should make sure that you are using Allison approved fluids and changing them when they recommend. It’ll help you prolong the life of your transmission. If you do that you may never even have Allison Transmission problems!!!