I recently acquired an '82 Chevy C20 with a 6.2L NA. The original transmission was a 700R4, but someone along the line has replaced it with a TH350. It shifts fine from 1st to 2nd, but seems like it slips really bad between 2nd and 3rd. If you shift it into 2nd at a stop and then pull it into drive at around 35-40mph it seems to do fine. At first I thought the tranny was just going out, but then started thinking that maybe it's a vacuum problem since it shifts fine manually. I've never dealt with diesels and I'm not really sure how the modulator gets it's vacuum signal. The vacuum line goes up to one side of a switch near the throttle lever and then the other side of the switch goes to a T. One leg of which goes to a vacuum resevoir and I haven't found where the other leg goes yet. I'm just lost on where to even start diagnosing the problem. Any help would be greatly appreciated.
Thanks in advance.