Driver said after his 3rd load of the day, truck wouldn’t shift above 3rd gear. Where should I start looking?
Check every inch of the trans wiring harness for any abrasions, very common issue.
The wiring looks okay so far. I was looking at the dash with the key on, and the gear indicator flashes from N to F. Does that point to anything else?
Clutch actuator issue, will cause N-F to Flash.
Should I pursue the SPN: 788 Clutch Actuator code?
Correct.
I can’t find anything out of the ordinary. Is it worth it to try to replace the gear selection sensor on the XY shifter?
That I can't tell you, More than likely you will end up at a Eaton certified shop.
This truck ended up going to Peterbilt dealer and got the clutch changed last year. It has been back probably 6 times for this same code and derate. They have changed batteries, added power to PTO dummy light circuit, and a bunch of diagnosing time. The last time it went in, they said to do a certain procedure for using PTO and quoted us $8,000 to replace TCM because the X-Y shifter is constantly trying to calibrate when key is off, which they told me indicated the TCM is going. The code only comes on when using a dump trailer. The new procedure for running the PTO, per Peterbilt/Eaton, is: Foot on brake, put in drive, turn PTO on, put in neutral, foot off brake. Then in a few seconds the PTO will kick on and box will go up. The procedure for turning PTO off is: Hit PTO switch off before anything else, foot on brake (which is when the PTO will actually disengage), then put in gear and drive away. After we brought it back from the dealer, I tested it on our dump trailer a bunch of times to make sure it worked with the new procedure and I had no problems. Today was the first day having someone actually run a dump trailer with it. He ran 2-3 loads with no problem, but on his most recent load, he said he did the exact same procedure and just as the box was coming down to meet the frame, it through the code. Typically it will derate to 10th gear max, but this time its 15th gear max. Is there something else we're missing? Could the TCM be the whole problem?
Correction: After talking with my driver, he said once he had the box in the air with the PTO still on, he would put it in drive and pull forward to get the rest of the load out (While the PTO was still on), then once the load was out, he would hit the PTO switch off, foot on brake, put in drive and go. Driver did the wrong procedure, but I'm still curious why it will throw a code for that.
Correction: After talking with my driver, he said once he had the box in the air with the PTO still on, he would put it in drive and pull forward to get the rest of the load out (While the PTO was still on), then once the load was out, he would hit the PTO switch off, foot on brake, put in drive and go. Driver did the wrong procedure, but I'm still curious why it will throw a code for that.