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Adapting to Toyota adapts

Wednesday, August 27, 2014 - 07:00
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The U140/240 and 150/250 front wheel drive series transmissions (Figure 1) has had and continues to have issues related to the quality of shift transitions. Most times resetting the shift adapts after repairs, rebuild or the replacement of a transmission handles the problem. But then there are times when the vehicle is delivered shifting great only to return days later with abrupt or flared shifts. Reset the adapts and the shifts are back to being good again. On other occasions we have seen that resetting the adapts did nothing to correct the shift complaint.

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The loss of adaptation control becomes quite disturbing since years ago malfunctioning computers were the culprit. In fact I once authored an article called “Brutality” which spoke about the 2-3 shift being so brutal, it destroyed the C2 Direct clutches on just a road test after a rebuild. So it begins to make you wonder if the adaption issue is related to a bad computer. If losing the quality of the shifts days after resetting the adapts wasn’t disturbing enough, changing the computer and not fixing the problem is worse.

Years ago I was working on one of these problematic vehicles. It was a Lexus RX 300. I wanted to observe the 2-3 shift command at the ECM with a scope (Figure 2). In setting myself up, I made sure I had a good ground (Figure 3). When I went out on a road test, to my surprise, the longer I drove the vehicle the better the shifts became. When I took my ground away, I had slightly bumpy shifts almost instantly. Reconnected my ground and in short order it was shifting good again, no shift adapt reset needed.

Not too soon afterwards a Toyota Highlander came back with bumpy shifts (Figure 4). This time we pulled a wiring schematic right away. Most all Toyota and Lexus have quite a number of grounds for the ECM as seen in the example wiring diagram in Figure 5. The location of their ground points takes looking around on the back side of the engine (Figure 6). 

The factory manual for this Highlander shows a total of 7 ground wires going to two grounding points. One is referred to as the EC ground the other the ED ground.  Of these seven ground wires, 5 are for the ECM. The ED grounding point shows that it contains a total of 4 ground wires (with a 5th spliced into one of the 4 wires), while the EC shows two wires. The manual also provided their location. The EC would be on the right side of the surge tank while the ED would be rear side of the surge tank.

When I went to looking, I found the ED mounted on the rear side of the Acoustic Control Induction System on the intake air surge tank as seen in Figure 7. And wouldn’t you know it, the nut was loose.  A little cleaning and tightening was all that was needed to fix the shift transition issues this Highlander was experiencing.

These Toyotas and Lexus’s are notorious for have knock sensor issues causing a complaint of a no shift into high gear (4th for the U140/240 and 5th for the U150/250). Occasionally, when changing the know sensor these ground circuits may be unfastened to gain access to the sensor. If the grounds are not properly fastened after repairs, shift adapt issues follow.

Besides poor grounds causing flared or harsh shifts shortly after resetting the shift adapt, bad batteries can cause the same complaint. If it drains down or the amps are pulled down too far during crank, it affects the computer causing a loss of adaptation values.

Besides a good rebuild and resetting shift adapts, adapting to Toyota’s adaptations requires the verification of good ECM power and grounds to ensure a long lasting, good shifting transmission.

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