Yes. I made sure to check the correct fluids on the forum. Does the D1 auto tranny have an overdrive solenoid? If so, my brother is considering that. We're going to outsource the diagnostics, either way. My bro isn't a tranny guy. I'll update as soon as I get the results. Thanks, Spike.

Juan

 

D1 tranny is not electronic. No solenoid. Shop manual on it is in the RAVE, the factory set of manuals for the truck. From page 422 of 873 in the workshop manual:





On my 97, this happens at about 53 mph. So when my offspring drives about at 45, it does not go into overdrive. You see a drop of 300-500 rpm. In the shop manual they tell you certain valves that may stick and cause this. New fresh fluid could certainly help, as well as solvent poducts like SeaFoam transmission additive. The manual shows how to use a pressure gauge to evaluate transmission.

SYMPTOM 9 - No 4th gear ’D’ selected
FAULT - 4th to 3rd downshift valve stuck or
3rd to 4th upshift valve stuck

FAULT - Clutch brake 4

SYMPTOM 10 - None or harsh engagement of
direct drive clutch
NOTE: The direct drive clutch will only
engage if 4th gear is engaged at 40 to 45
mph.
Carry out road test pressure check with gauge
connected to torque converter.
oe Normal pressure
FAULT - Failed direct drive clutch
oe Pressures correct but remains high
FAULTS POSSIBLE -
Direct drive clutch and torque converter
control valve stuck
Hysteresis valve stuck

Direct drive clutch control valve stuck

 

Thanks for the info, Buzz.