Strange reoccuring tansmission problem

[netminer]

Hello all. I have a 96 Sidekick Sport. I recently transplanted a 1.8l motor out of a 99 Esteem. After figuring out i had 2 ground wires not connected and corrected the issue the car did not give me any trouble for about 2 days. Now what it is doing is at random times the transmission will not shift down into first gear automattically and gives a CEL. I pulled the code from the ECM and it says P0753 (shift solonoid 1-2 electrical). First thing I did was to put a new tranny filter and fluid in. Problem keeps popping up. secondly I checked the wires in the harnesses for breaks. None found. What is strange is I can pull the grfound wire off of the battery to reset ECM and then all will be fine for a couple hours then the problem comes back. Also when the problem exists I find it is possible to shift the transmission manually. When I put it in first it will downshift to first . When I put it in second it will upshift from 1-2. When I go from 2-D it will upshift fine and vise-versa when I downshift manually. Any advise would be greatly appreciated.

[jtgh]

will not shift down .

ok , your basic tranny is ok, and can be driven with the TCM removed. (unplugged)
4speed only.
drive hard in manual mode, as the FSM states. no slips , basics are ok.

but down shifting, that takes brains.
IIRC:
the TCM must detect the Throttle cut (kick down fail) end to end must be good.
and TAIL shaft speeds dropping (tachomter on tail shaft, VSS#2 tranny )
TPS cut. (it sees that too) LOOK at TCM inputs, see them. Any can be bad.

Engine speed and load is communicated from ECU to TCM. or using todays parlance.
PCM to BCM (power train control module to body controller.)

the TCM(BCM) sees your cut fuel and begins to down shift.
there is a engine braking section in the fSM on this tranny, diagnosis.
there is chart in the FSM that shows the rules for the shift points up and down.
rpms, etc.

THIS IS an electronic shifted tranny , and is missing many of the parts that a 3sp would have.

THE TCM is either not seeing the trasition or the solenoid is not obeying the down shift.

my insticts say its not a bad solenoid.
that the tcm just has to blame something for the failure to drop and cant guess correctly. Par for the course on TCMs.
May be the TCM discovers it is LATE to shift. can cant understand why.
bad inputs? intermittant Inputs, then bingo the are good.


im no tranny expert but i understand the brain a bit.

Id make sure the tranny is missing no ground straps and the battery to body grounds are all perfect.
that is my take on this tranny.