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Transmission codes - simple tool and procedure
Posted to Technical Tips Forum on 8/6/2012 33 Replies

Hi All, Many aftermarket technicians such as myself are faced daily with the problem of the service information for certain vehicles that say you must use a "Breakout Box" or some other type of "special equipment" to properly diagnose a particular electrical circuit.

Most aftermarket technicians know EXACTLY what I am talking about when I say that you must "read between the lines" and figure out exactly what is being tested and why. Then you have to come up with an alternative method of performing the same test, in a different way, so you can get your customer back on the road.

My first purpose in this post is to share with you a "special tool" that is easy and cheap to make yourself and can save loads of diagnostic time. My second purpose is to show you a practical application (case study) of this "special tool" to show you some of its potential.

I'll start with showing you the tool [Handy Electrical diagnostic tool]. Not impressed? Let me tell you what it can do:

Have you ever tried to use a conventional test light to test for power and ground at a solenoid connector? At best, it is very cumbersome and hard to get a good connection.

The "test light" in the picture is fitted with flat GM weather-pack connector pins. This is my favorite one because it will fit many GM, Ford and asian connectors. However, I have about seven of these currently in my toolbox that are fitted with different pins (some male and some female) to fit different applications. I use them for everything from testing EVAP solenoid circuits at the solenoid connector, to window motor circuits.

The test light fitted in this manner doubles as a "drag test" tool to check for loose fitting connector pins (another reason I have several of them).

Now for the case study:

The Ford Explorer in this example came in with the check engine light on and a KOEO fault code of P0760, for which my Snap-On Verus rendered a description of "Shift Solenoid C Fault". (See area in picture marked in RED.) [1996 Ford Explorer XL, Engine/Propulsion Scan Data]

Reading through the service information, I found that it was one of those where the manufacturer's instructions were telling me to use a breakout box and a special adapter to fit the transmission connector. Then they gave a nice little diagram to show the correlation between the breakout box pins and the transmission vehicle harness connector pins. [1996 Ford Explorer XL, Transmission Drawing]

I also found that the manufacturer's information referred to the solenoid circuit as "Solenoid 3" instead of "Solenoid C". This may sound trivial to some of the more experienced technicians, but this can be really confusing to an apprentice. Sometimes it takes quite a bit of research just to make sure you are working on the correct circuit.

After reviewing some of the service information, I realized that I first needed to find out if the circuit fault was inside the transmission or if it was a vehicle harness or PCM failure that I was dealing with. I reasoned that the VPWR circuit to the transmission must be working. (Otherwise, there would be codes for ALL of the transmission solenoids, not just the one.)

So, to prove that it was inside the transmission, and to prove that the PCM was not defective, I did that "read between the lines" thing. I decided to disconnect the connector from the transmission and use my "special tool" to complete the circuit between the VREF and the Solenoid 3 circuit. (See areas marked in RED in picture.) [1996 Ford Explorer XL, Transmission Drawing]

I ran the KOEO self test again with the connection made this way, and Voila! The "special tool" illuminated for about 3 seconds during the test. Also, the PCM recognized the circuit through the test lamp and did not reset the code P0760, but it instead set codes for every solenoid in the transmission EXCEPT the "Solenoid C". (See area in picture marked in GREEN.) [Handy Electrical diagnostic tool] So, in one simple test, I verified that the circuit fault was an internal transmission fault. (It turned out to be the solenoid itself.) I also verified that the PCM was functioning properly and reporting circuit failures correctly!

I have found MANY uses for my "special tools". The application in this study is just one example. Hopefully you will be able to discover many others!

David from Missouri

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car Vehicle Data

1996 Ford Explorer XL 4.0L

Engine4.0 L
Trans4-speed Automatic (Electronic)