You have undoubtedly heard the phrase “Everything is not always as it seems”? That can hold true to many things in life. Harry Houdini made a lot of money creating illusions and misconceptions. And the field of fixing transmissions is an all too common problem area for misconceptions, that can cause you to lose money.
Nothing is more frustrating to a rebuilder at the bench than having a unit placed on his bench, tear it down, and find absolutely nothing wrong with it. I refer to that as “diagnosing with a half inch impact.” (Figure 1, impact wrench)
An example of this is an F150 with a 4.6 engine and a 6R80 transmission. The complaint was “slips on hard acceleration”. It was test driven and sure enough, on hard acceleration it would break free and spin up. Stall testing gave the same result. It was noted that the pass thru connector was leaking, so that was addressed first. No change. During the scan, there were some misfire codes observed. The decision was made to send the truck to an engine drivability shop for diagnosis.The tech there scanned it and noted more than one cylinder with misfires in history.An inspection under the hood found a low cost, aftermarket ignition coil pack. It was determined that the plugs and all the coil packs should be replaced with OEM coils. When it was done, it was driven and performed perfectly! No spin ups or breaking loose. What was happening was the misfiring coils were creating EMI, or electromagnetic interference. Now in this case, the unit was not pulled before the real problem was found. But there are similar circumstances where the transmission ended up on the bench first. Not good. (Figure 2 coil 1, Figure 3 coil 2)
The EMI was causing the computer to play havoc with the solenoids, so it was neutralizing.
Looking in the ECM side, it was noticed that the ambient temperature read -40F. Since this was in Arkansas in the summertime, it was very unlikely that this was accurate. Spending some time researching found that the sensor for that PID was on the driver’s side mirror. Yes, replacing the mirror resulted in the transmission working correctly! Apparently, the computer will inhibit shifting to a point, with temperature that low. Again, this vehicle was fixed without the unit landing on the bench. But it certainly could have ended up the other way.
Certainly we’re all aware of the TCC issues on older Ford trucks with an E4OD that have a burnt out or missing brake light bulb, or LED bulbs substituted, but how about a 2001-2003 VW Jetta that won’t move and has no codes? Bad bulbs or a bad brake switch will cause this. And your scanner will show that neutral is being commanded! And while you are replacing the bulbs, be sure to check for any corrosion that could cause excessive resistance. Clean or replace, as necessary. (Figure 4 socket)
These are just a few of many examples that can cause you, the transmission technician, to go down the wrong path and grab the impact gun. Instead, you need to be grabbing test lights, multi-meters, scopes and wiring schematics! You will be more productive and make more money!