Car was in limp mode when it arrived. When scanned, the PCM, ABS module and BCM each showed an active code U0101 present for "Lost communication with TCM". The scanner would communicate with the modules that had the codes stored but he could not communicate with the TCM so it was decided to first check power supply and ground at the TCM connector.
The tech soon found there was no power supply to the TCM terminal 29 at the C1 connector, with the key on or engine running, indicating that the TCM was basically off. Wire diagrams showed a 15 AMP fuse(37) in the circuit between the relay and the TCM. The tech checked the fuse and it was bad so he replaced the fuse but the problem remained. Voltage was checked at the fuse with the engine running but there was no voltage present.
The transmission wire diagram showed a relay in the circuit before the fuse, so it was decided to unplug the relay to check for power and ground at the relay socket and if the power and ground was there, replace the relay, but there was no power to the relay.
A power distribution diagram showed that there was a DIODE-NO. 1 in the circuit before the relay and further inspection at the power distribution center revealed that the DIODE-NO. 1 (NAG 1) appeared to be installed backwards when compared to an identical diode in the Power Distribution Center (PDC) next to it. (The diagram on the top of the diode was turned around in the opposite direction) The NAG 1 diode was removed and then installed in the same orientation as the diode that was next to it and the problem was solved.
(Apparently it is still a mystery as to how or when the diode got turned around. The customer or his mechanic did not know anything about when the diode may have been previously serviced or replaced.)
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