KM 4-Speed No Reverse After Overhaul

Valve Body Xpress

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Problem

No Reverse after overhaul.

Possible Cause

One possible cause is that the pulse generators are swapped into the wrong holes. In other words, pulse generator "A" is in the hole for "B" and pulse generator "B" in the hole for "A".

Test Procedure

Put the transmission selector into Drive. Once the transmission engages, quickly move the manual lever to Reverse. Does Reverse engage now? If yes, swap the pulse generators.

Why

The TCM will inhibit Reverse if it senses road speed when put into Reverse. One pulse generator reads input speed and the other reads road speed. Both pulse generators are permanently attached to the same harness. When the pulse generators are in the wrong positions, the output pulse generator is now looking at transmission input speed or roughly engine RPM. The TCM thinks the vehicle is moving and inhibits Reverse. By selecting Drive, all the moving parts inside the transmission come to a stop, no speed registering on either pulse generator. By moving the selector quickly to Reverse, the gear train does not have time to spin up and you will get Reverse.

Related Unit(s)
Subject(s) Included
  • General Diagnostics and Theory

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