It's Summer

Written By - Jerry Huerter, Diagnostician

A 96 Nissan 300ZX came into the shop last summer. Remember, I said it was summer which affects this story later.  The customer stated that the check engine light was on and at times would not shift into 4^th gear.  I scanned the computer, and it had a P0705 inhibitor switch code.  I cleared the code and went for a test drive with the scan tool connected; inhibitor switch data was correct and the code did not reset.  I then put the vehicle on a lift and inspected the wiring and connectors, and everything looked good.  After talking to the customer we decided to replace the inhibitor switch.  The customer picked up the car and said he would let us know if it acted up again.  Two days later the customer returned with the same complaint.  He said he would leave the car with us for a few days so we would have time to do further testing.   Again, I scanned the computer, and the code P0705 was reset.  I cleared the code and went for a test drive; the scan data was good and no code reset.  After re-inspecting the wiring harness and connectors to make sure I did not miss something, I found everything checked OK.  Then I test drove the car off and on for two days and the code never reset.  The customer came and picked up the car, and we told him if the check engine light came on again to let us know what type of driving he was doing and where he was driving.  He said OK.

The very next day the customer was back at the shop with the check engine light on again.  He explained that he had left our shop and drove to work.  While driving home from work on the highway that evening the check engine light came on, and the transmission shifted into 3^rd gear.  He said it stayed in 3^rd gear the rest of the way home.  When he drove his car to the shop the next day he said the check engine light was still on, but the transmission did shift into 4^th gear.  I checked the computer for codes, and the P0705 was back.  Again I cleared the code and took the car for several long test drives down the highway, but no code reset.  Now I was stumped.  The customer can get the code to set within hours of leaving the shop, yet I can never get the code to set on any of my numerous test drives.

Upon reviewing the shop manual information it shows that the inhibitor switch gets 12 volts from fuse 19, then as the shift lever is moved to the park or neutral position  the inhibitor switch switches this 12 volts to terminal 19 at the TCM so now the TCM knows the shift lever is in park or neutral.  When shifted into reverse, voltage is switched to terminal 20 of the TCM, and when shifted into drive voltage is switched to terminal 18 of the TCM and so on for the rest of the gear positions.  When the TCM sees no voltage or voltage on one or more of these circuits at the same time it will set a P0705 and inhibit 4^th gear.

                                   (See wiring diagram from ALLDATA)

I disconnected the inhibitor switch and took a fused jumper wire and jumped 12 volts to the harness connector of each one of these circuits and watched the scan tool as I wiggled and pulled on the wiring harness trying to see two circuits on at the same time or one that turns off.  As I connected the jumper to the reverse circuit I noticed it sparked.  I thought this sparked because the reverse lights are fed from this circuit also.  As I connected and disconnected the jumper wire I noticed a light at the front of the car turning on and off.  With the jumper connected to the reverse circuit I looked at the front of the car, and the park lights were on. I then went to the rear of the car and found the tail lights on and the reverse lights were not on.  When I disconnected the jumper, the park and tail lights went off.  Thinking there maybe a problem in the tail or reverse lights, I started checking in that area. 

I opened the hatch and saw that the trim panel for the rear lights had been removed and was lying in the back of the car.  The wiring and connectors for the rear lights were pulled down and hanging.  I printed a wiring diagram and started checking the wires and connectors in the rear lighting location.  I found that someone had connected one of the license plate light connectors to the reverse light connector, so with the headlights turned on it fed 12 volts to the inhibitor switch reverse circuit.  When the lights were on and the shift lever was in drive the TCM saw 12 volts on two circuits at the same time which caused code P0705 to set and would inhibit 4^th gear.  My test drives were performed during daylight hours with no headlights turned on so the code would never set for me. I even drove this car home from work one evening and back in the morning but because it was summertime I didn’t need the headlights turned on.  However, the owner of the car worked evenings and when he drove home needed the headlights turned on. That is why the code would set and not shift into 4^th gear every time he drove home from work or at night. In the morning the check engine light would still be on but because the code was only active with the lights on it would shift into 4^th gear.  By correcting the light wiring problem I resolved the P0705 code and the no 4^th gear customer complaint.