Econoline 150 hard time shifting.

My dads 1989 econoline 150 conversion van has a really tough time shifting into 3rd. All the other shifts are ok. The thing is when you first start it when its cold and you take it easy it shifts normally but if you accelerate normally it wont shift to third unless you are going over 35 and you give it a lot more gas. It has been doing this for years and i think it is just killing it.

One of the most common causes of a late/no upshift to 3rd is a leaking vacuum line to the transmission modulator valve (located on the RR corner of the trans).

This "tells" the transmission how hard you are pushing on the gas pedal. Disconnected = full throttle to the trans, so a split hose, mouse chewing, or maybe the hose just fell off. It runs from the intake manifold (varies by engine) down the RH side on top of the trans and connects to the modulator valve.