Shining Some Light on the Problem - Transmission Digest

Shining Some Light on the Problem

A 1996 Nissan 300ZX came into the shop last summer. Remember, I said it was summer, which affects this story later. The customer said the check engine light was on and that at times the transmission would not shift into fourth gear. I scanned the computer and found 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.

Shining Some Light on the Problem

R&R Tech

Subject: No fourth gear
Vehicle Application: 1996 Nissan 300ZX
Essential Reading: Diagnostician, R & R 
Author: Jerry Huerter

R&R Tech

  • Subject: No fourth gear
  • Vehicle Application: 1996 Nissan 300ZX
  • Essential Reading: Diagnostician, R & R
  • Author: Jerry Huerter

A 1996 Nissan 300ZX came into the shop last summer. Remember, I said it was summer, which affects this story later.

The customer said the check engine light was on and that at times the transmission would not shift into fourth gear. I scanned the computer and found 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; everything looked good. After talking with 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 had reset. I cleared the code and went for a test drive; the scan data was good and no code reset. After I re-inspected the wiring harness and connectors to make sure I had not missed something, 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 driven to work. While he was driving home from work on the highway that evening the check engine light came on and the transmission shifted into third gear. He said it stayed in third 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 fourth 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 on the highway. 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 it to set on any of my numerous test drives.

A review of the shop-manual information showed 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 that the TCM knows the shift lever is in park or neutral. When the transmission is shifted into reverse, voltage is switched to terminal 20 of the TCM; when it’s 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 fourth gear.

I disconnected the inhibitor switch and used a fused jumper wire to apply 12 volts to the harness connector of each 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 turned off. As I connected the jumper to the reverse circuit I noticed that it sparked.

Because the backup lights are fed from this circuit also, I thought this was why it sparked. 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 that the taillights were on and the backup lights were not on. When I disconnected the jumper the park lights and taillights went off. Thinking there might be a problem in the taillights 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 taillight connectors to the reverse-light connector. 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, causing code P0705 to set and inhibiting fourth gear.

I had performed my test drives 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. However, the owner of the car worked evenings and when he drove home needed the headlights on. That is why the code would set and the transmission would not shift into fourth 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 active only with the lights on the transmission would shift into fourth gear. By correcting the light-wiring problem I resolved the P0705 code and the customer complaint of no fourth gear.

Jerry Huerter, an ASE Master technician with L1 Advanced Engine Performance Diagnostics certification, is the head diagnostician at the Certified Transmission location in Overland Park, Kan.

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