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Some Tips for Diagnosing Chrysler's EATX SystemPosted 4/20/2003By Todd Plourde
In 1989, Chrysler decided to throw a little fun into their transmissions by the fully electronic controlled transaxle. This system is still used today on most of Chrysler's front-wheel drive vehicles. The electronically controlled transaxle (EATX) system consists of a transaxle control module, a solenoid pack, several sensors and switches and finally, the serial bus. The transaxle control module (TCM) is programmed to monitor more than sensors. It gathers information from the bus and combines it with TCM sensors and switches for shift points and diagnosis. Electronically controlled transaxle systems are capable of learning the customer's driving habits. A customer with a heavy foot will cause the TCM to learn to shift late and hard. On the other side, the TCM will learn to shift early and soft for someone with a light foot. The EATX system also watches for a request for vehicle speed control and when it sees a request, it adds "grade hunting." This alters the shifting patterns to help prevent excessive shifting and variations in vehicle speed on those hilly roads. When diagnosing a Chrysler electronically controlled transaxle, make sure any recent fluid flushes or changes were filled with the correct fluid type. ATF+4 is compatible with all transaxles except on 1999 and earlier minivans, which can have TCC chatter. So continue to use ATF+3 on them (per Service Bulletin No. 21-006-01). Check for codes or lack of codes in the transmission, body controller and powertrain. It's also a good idea to record the clutch volume index (CVI) even if you feel there is not an internal problem. When you erase the TCM's codes, you also erase the CVIs stored in the TCM. If the situation arises where you need to look at CVIs, remember that if you erase them, they will not be there. Clutch volume index is the transaxle controller's way of monitoring the efficiency of the clutch packs. Every time the vehicle is driven, the TCM measures how long it takes each clutch pack to completely fill up and stop slipping. Worn friction material or leaking clutch pack seals increase the time it takes for a clutch pack to fill and be able to grab. In this situation, CVIs will appear on the scanner with high numbers for one or more of the clutches. Numbers that are too low might indicate insufficient clearance inside the clutch pack, which would cause it to grab too quickly or possibly never fully release. Next, check the park/neutral information. On vehicles equipped with a transaxle range sensor, make sure they are reading correctly all the time. If you have an intermittent limp-in operation with a pressure switch fault, it could be caused by incorrect input to the TCM. The controller is constantly monitoring this input and if it thinks the gear selector has been moved to another gear, it expects to see a change in one or more of the pressure switches. If it doesn't, a code will be stored and you might end up in limp-in. When the transaxle is placed in limp-in on these vehicles, you will only be able to get two gears: second and reverse. As I mentioned, fluid quality is very important in these transaxles. In many instances, I've seen a number of problems caused by a fluid change with incorrect fluid. And most often, it causes torque convertor clutch problems. If you get a vehicle in with a complaint of surging or bucking while at a steady speed above 40 mph but no problems when accelerating or decelerating, test drive to determine if it is related to the torque convertor. If it is, ask the customer if they have recently had the transmission serviced or flushed. On the older EATX systems, the TCM doesn't have great on-board diagnostics. The problem is that the controller doesn't set a code, so therefore you won't have any direction to go in your diagnosis. First, do a visual check of the TCM for heat sink fins. If there aren't any, then it's the old style and should be replaced - not just for diagnostic purposes but for shift quality and longevity. Also check the Safety Shutdown relay and its circuits. When the TCM energizes the relay, it expects to see an output from it but if it doesn't it is supposed to set a DTC. The older controllers tend not to set any codes when it is intermittent. If you've covered all the basics - fluid, fluid quality, codes and CVIs - then the next step is to start checking electrical circuits. Just give us a call if you need a hand diagnosing any Chrysler system!
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