“ Complaint: The "check engine" light is on and the transmission goes into the limp-in state (starts in third gear). I had the tech check the ground going to the governor pressure sensor, the 5-volt reference and the voltage for the signal wire. ”

Complaint: The "check engine" light is on and the transmission goes into the limp-in state (starts in third gear).

I had the tech check the ground going to the governor pressure sensor, the 5-volt reference and the voltage for the signal wire. When he tested the circuit, the voltages and ground were reading normal. When he monitored the voltage with the scan tool while driving the truck, he found the voltage went high as he approached a stop sign. Since the ground and reference voltages checked good, he replaced the governor pressure sensor and the variable force solenoid. In spite of this effort, the problem was still there.

I asked the tech to check the wiring from the transmission connector to the power train control module (PCM) for a short to voltage. No problem was found with the wiring but he did notice the voltage on the scan tool would go to 7 volts when he stepped on the brakes. I suggested he cut the wire at the PCM and check to see if the voltage was coming from the vehicle side or the PCM side of the wire when stepping on the brake. He found it was coming from the PCM side. After he replaced the PCM, no change.

The brake switch input to the PCM and the grounds were OK when he stepped on the brake. But he noticed on the scan tool that when he stepped on the brake that the governor voltage changed and also the downstream O2 sensor voltage went high. He disconnected the O2 sensor and the voltage still did the same thing.

I suggested that the technician unplug the brake switch and check the input on the O2 signal wire at the PCM. The voltage read normal. I had him reconnect the brake switch and cut the O2 sensor signal wire at the PCM. The voltage on the harness side of the wire still went high but the governor pressure read normal. He test drove the truck and found that the transmission worked normally. He traced out the wiring and found that there was a shorted trailer wiring harness, causing the wiring above the fuel tank to melt and get into the downstream O2 sensor signal wire. The shorted O2 sensor signal wire was feeding back through the PCM into the governor pressure sensor circuit, causing its voltage to go high, which caused the code and the transmission symptoms to occur.

After some research, I found that the Jeep truck engine controllers (JTEC) have three circuits that can do the same type of thing. There is a shared microprocessor between the inputs, and under normal conditions this is not a problem. The processor can share time with no problems, by the charging and discharging of a capacitor. When one of the shared inputs gets shorted to system voltage, the capacitor doesn't fully discharge. After it switches to the other input, it discharges the remaining voltage into that circuit. This will cause the JTEC PCM to see a higher than normal voltage on the shared circuit, causing possible fault codes and symptoms in that circuit.

If the downstream O2 is shorted to voltage, it can set a P1763 governor pressure voltage high and/or a P0138 downstream O2 sensor shorted to voltage. If the speed control switch signal wire is shorted to voltage, it can cause a P0123 throttle body sensor (TPS) voltage high and/or a P0121 TPS that does not agree with MAP sensor. If the upstream is shorted to voltage, it can cause a P0108 manifold air pressure (MAP) sensor high and/or a P0132 upstream O2 sensor shorted to voltage.

The JTEC PCMs were used in Dodge truck and Jeep vehicles from 1996 to 2002 and all share these same traits. A side note on the JTEC PCMs: If the 5-volt reference that feeds the three wire sensors is shorted to ground, it will lock the PCM. This will cause no communication with a scan tool, no "check engine" light operation and a no start/no fuel pressure/no spark/no fuel injector pulse situation. The 5-volt reference feeds the MAP sensor, throttle position sensor, crank position sensor, cam position sensor, vehicle speed sensor, governor pressure sensor and in some cases, the oil pressure sensor.

Tom Vadnais is an Identifix DaimlerChrysler, Mitsubishi and Hyundai specialist. He is ASE master and L1 certified with 29 years of diagnostic and repair experience..

This information is provided by IDENTIFIX®. IDENTIFIX® resources cut diagnostic time and provide repair solutions that increase the shop's bottom line. From Repair-Trac pattern failure quick fixes, to Diagram-Online wiring diagrams by fax, to the Repair Hotline staffed by 32 master techs who specialize in diagnosing complex problems by phone or fax, IDENTIFIX® helps techicians fix more cars in less time. For more information on IDENTIFIX, call (800) 288-6210, 8 a.m. - 6 p.m. Central Time. www.identifix.com. © 2011 IDENTIFIX. All Rights Reserved. © 2005 IDENTIFIX. All Rights Reserved.

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