QuestionI have a 1987 chevy v20 350 3/4 ton 4 wheel drive with tbi. The transmission was replaced 2 yrs ago, and has about 20k miles on it. I think it is a thm 350 , or more likley the new numbers for this model. It has a vacuum mechanism on the passenger side and a one wire conection on the driver side.
Yesterday while the truck started to shift irradically. Not slipping, more like shifting late. The truck would rev very high before shifting from 1st to 2nd, 2nd to 3rd. The speedometer worked fine. When the shifts did occur they were very hard( b/c the truck RPMs were so high at the shift) When I made it the two miles home, the check engine light came on . It gave two codes, 33 MAP sensor, and 45 oxy sensor rich. I let the truck sit for a few hours and started fine, but ran rough. One other thing, when the truck started to shift wrong, I heard a load buzzing. I opened the hood and it was coming from two electrical conection boxes located on the passenger firewall. I don't know what these boxes are. They are left of the blower assembly and conections. Side by side. wires plug in from the bottom. each has four wires that run back into the wire harness. After the truck sat, these no longer mad any noise.
So, question is , what is causing the shift problem? Thanks for any help you can give me. MJS.
AnswerHello Michael,
I am not sure, but I would look for vacuum leaks, considering the MAP code. Map measures vacuum.
Make sure full manifold vacuum gets to theat modulator on the transmission. And if you connect a vacuum hose to that modulator, and suck on it, it should hold a vacuum, not leak.
Check the tranny fluid level also.
Van
