Have 1990 e-350 van with 7.3 diesel.Trying to sort problems with tranny.First found {teca]power relay was intermittent replaced this now OD light goes on and off properly.Truck seems to shift fine but at certain steady speeds it acks like it is being held back,and does not act fully engaged.I checked voltage out of throttle position senser around 1 volt at closed and 4.1 at full throttle is this good voltage.The van has only 43,000 miles and in great clean shape.Not sure what to check next.Thanks

 

Count how many upshifts you get from starting at a standstill up to freeway speed - it's a 4-speed transmission but you should see 4 upshifts thus acting like a 5-speed trans, the extra upshift is the converter locking up between 3rd and 4th gear. When the converter is free it really feels like something is dragging the truck back, there is a good 500 rpm difference under load between the converter locked and free. Also crawl under the van and remove the transmission harness connectors one by one and clean them and grease them (dielectric/tuneup grease) real good and plug them back in, more often than not a simple bad connection is what causes the converter acting strange.