Hot Off the Wire

My first appointment of the day was a customer with a 2001 PT Cruiser. Their complaint was, "It feels like all the gears are slipping, and a light is on in the gauge panel."

I started my evaluation with a fluid check and pulled the codes. The fluid level was normal, and the code I got back was a P0750 - solenoid error in the TCM. There were no ECM codes.

After clearing the code I started my test drive. The transmission preformed well, but I did experience what felt like a slight engine stumble when going up hills. It was intermittent, but it did occur twice during the drive. The check engine light also came back on.

I suggested that the customer allow us to conduct a Level One diagnostic evaluation and he gave me the ok, so I attached the scan tool and checked for additional codes. The only set code was the P070 for the TCM; again, no engine related codes.

I decided that my next step would be to do an ohm test of the four solenoids at the TCM connector. The 2001 PT has the TCM mounted inside the left fender well, which requires you to lift the vehicle and remove the tire and fender panels to access it. Thankfully, this also allows you to see the front of the transmission and check it for any leaks.

There was definitely a leak somewhere in this unit. It was covered in used ATF. On closer inspection, it appeared to have a solenoid pack leaking (among other possibilities).

The ohm test of all four solenoids was 1.7, which is well within the specification. At this point, I started to wonder if the L-R solenoid could be a heat-related issue. It very well could be causing the unit to go into limp mode (2nd gear) and maybe that was what the customer was feeling when "it felt like all of the gears are slipping"?

While I was doing the tests, the customer decided that due to the age and mileage on the transmission, he wanted us to stop our testing because he might want to just replace the whole transmission.

I stopped the troubleshooting process and assumed that either the customer would be back for a replacement unit, or would be back to allow us to determine the cause of the problem.

A few weeks later, the customer did return to our shop with a complaint of a 1-2 shudder. The check engine light was back on with the same P0750 code. There was also someone else?s replacement transmission installed in the PT.

I continued with the tests that I had started weeks before but had never got to finish. I disconnected the TCM and solenoid pack so I could test the L-R wire from pin 7 to pin 20 at the TCM. It tested good for continuity, but I decided to apply 12 volts to the wire just to make sure that there was no voltage drop. The wire was fine, with no drop I re-checked all of the connections and tightened the ones that were loose.

I reassembled everything and took the PT out for another road test. Everything was looking good until the temperature reached normal operating level. The PT began what is best described as a shuttle shift from 1 to 2, 1 to 2, and 1 to 2. The check engine light did not come on though.

It felt like engine stumble or maybe ignition fallout, but was now worse than it was during my initial test drive. Was this engine related, or something else? It felt like a shuttle shift, but he more I drove it, the worse it became. It almost felt like it was going into lock up after 2nd gear, and then dropping back out. Lock up; lock out, over and over again. The check engine light finally came back on and the same P0750 code was back.

I decided that it had to be the wire from the L-R solenoid even though it tested good. I replaced the wire from pin 7 to pin 20. With the new wire in place, I went for another drive. Everything worked perfectly. The customer left happy and so was I. I started trying to figure out how this could have caused the problem.

There must have been an intermittent short in the wire that only occurred when the PT was up to operating temperature. The tests I did on the wire were while it was cold. The wire passes through some very tight places to get from the engine compartment to the fender well and it can get very hot in there. The L-R solenoid does become the TCC solenoid after the switch valve moves in the valve body and commands the shuttle during the 1-2 shifts.