I have a MT 653 behind a Cat 3208 with ~43k miles suddenly won't shift into 5th gear with full throttle. Can get into high gear with part throttle. Was driving in high gear full throttle flat ground and suddenly shifted to 4th over speeding engine. Fluid is Dex III full on dipstick and not burnt Clean red in color. Any suggestions as to what went wrong?
Lots of things can cause this, First... check that the shift cable is free when in drive. If the pin on the shift arm is the slightest bit tight it will cause all sorts of strange problems. Also check the modulator cable for slop in the pins at the full throttle position. Get a manual and study it. It's all simple stuff but it all has to work together. Oh, knowing the make and model of the truck will help. Fords push the shift cable, Internationals pull. IIRC. Makes a big difference adjusting cables.
1990 ford C8000. I have the service manual. Going to check the modulator actuator tomorrow. the pin is tight on the modulator cable and the throttle linkages are tight and don't look to have moved (adjustments) recently.
You can accumulate a considerable amount of wear in 30 years. Push cables will shorten, pull cables will stretch. keep that in mind checking your adjustments. I had an outfit bring me two 653's about 10 years ago to rebuild on the bench. They were perfect inside, I resealed everything and told them when they reinstalled them to call me. Both trucks still had the same symptoms related to worn out shift cables.
Governor?
I have seen governor screens plug too. Actually had the little cone screen get pushed in the valve body in a 740.
Governor screen all plugged up. Cleaned screen changed fluid and filters, all good now back to normal. Thank You