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If you read these pages on a regular basis then you’re familiar with our late model street/strip project car, the Maulin’ Magnum. This 2007 Dodge SRT8 Magnum wagon is pretty highly modified; Magnuson supercharged stock long block, upgraded rear end with 3.55 gears, OS Giken limited slip differential and Driveshaft Shop level 5 axles rated at 1400 horsepower, Westminster Transmission upgraded NAG1 transmission, higher stall convertor, etc. Towards the end of summer last year our data logs at the race track indicated a problem with our transmission’s shifting. In our December, 2012 issue we covered the upgrade and rebuild that Westminster did to our NAG1 which cured one problem. The other problem was traced to our stock OEM transmission control module (TCM).
In a nutshell, the TCM is the computer that in concert with the engine computer, called the PCM, controls the transmission. Late model cars are complex at times and in addition to the stock PCM and TCM we piggyback a Diablosport Trinity hand held programmer on top of them. For performance builds like ours we discovered that the stock TCM is a bit too limiting, it’s just not capable of as firm and as quick shifts as the Mopar replacement TCM can produce. So we upgraded to Mopar part number P5155177.
We spoke with Martin Wilson, Head of Performance Planning for Mopar about the Mopar TCM. We also spoke with experts at Diablosport and other companies as we wanted to clear up some myths floating around the Internet regarding this part.
The TCM in a late model Hemi car or truck not only tells the transmission when to shift up or down and how long to hold a gear, it also adapts to your current driving style and adjusts its shifts firmer or softer based on whether it thinks you’re cruising around or hammering around. And therein lies our first complaint with the device. Its adaptive behavior is based on how you’ve been driving in the semi-recent past, not on how you want to drive right now. So if you’ve been cruising on the highway and arrive at that favorite mountain road or at the race track and start to drive in a spirited manner, the transmission is behind the curve. It takes a while, up to ten minutes of driving, for the adaptive tables within the TCM to update and adjust. This translated in our first foray’s to the drag strip with ever changing shifting behavior over the course of a day of racing after arriving at the track at the end of a long freeway trip. This made the car hard to predict and difficult to dial-in for bracket racing. It was also just annoying at times on the street.
We learned to mitigate this issue to a large degree using our Diablosport Trinity hand held programmer which is permanently hooked up to our OBDII port under the steering column and is mounted on our A-pillar in an Auto Meter dual gauge pod using Diablosport’s Trinity gauge pod adaptor. The Diablosport devices (Predator, inTune and Trinity) allow you to “Clear TCM Adaptives” which wipes the tables clean and reverts to baseline tables, which is useful for getting consistent performance at the race track. So that short coming in the OEM TCM was addressed.