P0750 – LR solenoid circuit
P0888 – Trans relay output always OFF
Transmission in Limp Mode
MIL ON, fault codes always present. Codes return when cleared after key cycle. Both faults indicate a voltage problem. The P0888 can lead you to the cause of both fault codes, so I started there.
I first went to control side of relay. There no trans relay control signal from TCM. TCM sends 12 volts from PIN 15 (light green) at TCM connector to PIN G4 at trans relay. The relay has a permanent ground on PIN G5. The 12 volt signal to the trans relay was not present. The ground was good.
With this information I went to the TCM to see if the 12 volt was present. If it was this would indicate an open circuit between TCM and junction box. The 12 volts signal was not present at TCM.
Next up I checked all powers and grounds to TCM. I found PIN 11 (light green / black) missing battery voltage when ignition is switched ON.
I went to fuse 12 (10A) in fuse block. Voltage was present and fuse was good.
At this point I ran a jumper wire from fuse 12 to PIN 11 at the TCM. Cycled the key ON. The trans relay activated, no fault codes present.
To go a bit further I located splice S115. Voltage was present. At this point I advised the shop to run a new wire from splice S115 to PIN 11 at TCM.
A test drive after repairing the open circuit confirmed the fix. No fault codes present and the vehicle shifted normally.