There was once a time when transmission diagnosis was fairly straightforward: If it didn't shift, it was broken. You pulled it out, repaired the problem, put it back in - and the transmission shifted like new.

And no wonder - nearly everything that controlled transmission operation was inside the transmission itself. The only external controls might be a vacuum line and a throttle linkage.

Then one day, auto manufacturers came up with a great idea: They'd been using a computer to control engine operation for years - what if they turned control of the transmission over to the computer, too?

The marriage of transmission and computer system has proved a resounding success, providing better performance and adaptability than was ever possible before. But it also created an entirely new set of diagnostic challenges. Today a shifting problem may not be remotely related to the transmission itself. A single switch or sensor failure, all the way on the other side of the engine compartment, can be all that's necessary to put a unit into failsafe.

But, while the procedures for diagnosing these new units may be different, one part of transmission diagnosis hasn't changed: the importance of following a logical diagnostic procedure, from start to finish.

This program has been developed to address these new diagnostic challenges, by providing a logical procedure for diagnosing today's computer-controlled transmissions. It covers most of the domestic electronically-controlled transmissions in use today.

Some Transmissions Won't be Covered
The object of this program is to help you diagnose electronic transmissions without spending a lot of money on special diagnostic tools and equipment. Unfortunately, several units on the road today aren't accessible without making a serious investment in diagnostic equipment. For example, we won't be covering any OBD-II diagnostic trouble codes in this book. There's no way to retrieve or clear these codes from an OBD-II vehicle, without an OBD-II scan tool.

We also won't be covering Chrysler 41TE and 41LE units: There's really no way to check these units accurately without an up-to-date scan tool, equipped with the software specifically for diagnosing them. That's why you won't find these units covered in this program. That's the bad news.

The good news is that, using a scan tool equipped with the proper cartridge or software, diagnosing these Chrysler transaxles is relatively easy. The scan tool provides you with far more than just diagnostic trouble codes: It even helps the computer relearn shift timing information and provides clutch fill capacities.

So the real key to diagnosing Chrysler transmissions is having a good scan tool available. With it, you should be able to diagnose these units quickly and accurately.

Another helpful tool is one of the transmission shift testers and simulators on the market. These units will automate many of the tests we'll be covering in this program.

In this program, we'll show you how to perform electronic transmission and transaxle diagnosis, with a few simple tools you can build yourself. What's more, we'll even show you how to build them. Any unit that requires more extensive diagnostic equipment won't be covered here. This is strictly for transmissions that can be diagnosed without any special equipment. And that covers a lot of ground…

By following the procedures presented here, you should be able to analyze many electronic transmission problems, to determine whether the problem is inside the transmission, or caused by a failure in the computer system. And once you've diagnosed the problem, the repair is the easy part.

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