Hot off the Wire - Transmission Digest

Hot off the Wire

My first appointment of the day was a customer with a 2001 PT Cruiser. The complaint was, “It feels like the gears are all slipping, and a light is on in the gauge panel.” I started my evaluation with a fluid check and pulled the codes. The fluid was at normal level, and the code pulled up was P0750 for a solenoid error in the TCM. There were no ECM codes.

Hot off the Wire

R&R Tech

Subject: 1-2 shuttle shift, code P0750
Vehicle Application: 2001 PT Cruiser
Essential Reading: Diagnostician, R & R 
Author: David Finley

R&R Tech

  • Subject: 1-2 shuttle shift, code P0750
  • Vehicle Application: 2001 PT Cruiser
  • Essential Reading: Diagnostician, R & R
  • Author: David Finley

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

I started my evaluation with a fluid check and pulled the codes. The fluid was at normal level, and the code pulled up was P0750 for a 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 the vehicle was pulling hills. It was intermittent, but it did occur twice during the drive. The check engine light did come back on.

I suggested that the customer allow us to conduct a Level 1 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, there was no engine-related code.

I decided that my next step would be to do a resistance 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 panel. 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 resistance test of all four solenoids showed 1.7 ohms, which is well within the specification. At this point I started to wonder whether the low/reverse solenoid could be a heat-related issue and cause the unit to go into limp mode (2nd gear) and whether that was what the customer was feeling – “feels like all of the gears are slipping.”

While I was doing the tests, the customer decided that because of 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 the customer would be back either for a replacement unit or to allow us to determine the cause of the problem.

A few weeks and miles 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 low/reverse 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 there was no voltage drop. The wire tested good; no drop. I rechecked all the connections and tightened the ones that were slightly loose.

I reassembled everything and headed back out for a road test. Everything was looking good until the temperature reached normal operating level. The PT began what is best described as a shuttle shift between first and second gears. The check engine light did not come on.

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. The more I drove it, the worse it became. It almost felt as if it was going into lockup after second gear and then dropping back out – lockup, lockout, over and over again. The check engine light finally came back on. The same P0750 code was back.

I decided that it had to be the wire from the low/reverse 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 was happy and so was I. I started trying to figure out how this could have caused the condition.

There must have been an intermittent short in the wire that occurred only 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 wheel well, and it is very hot inside the engine compartment. The low/reverse solenoid does become the TCC solenoid after the switch valve moves in the valve body and commands the shuttle during the 1-2 shifts.

David Finley is the head diagnostician at Certified Transmission’s retail location in Bellevue, Neb., and is also an experienced transmission builder.

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