peculiar transmission behavior

[chevyh20]

hey guys ,
 i just got my blazer back on the road yesterday after a year and a half off the road. One of the jobs i did to it was replace the transmission with a used one that i bought on craigslist. The owners said that it only had 6000 miles on it. when i bought it they had it stored under a tarp outdoors.
So anyway yesterday it worked great on my maiden test drive. shifted just the way i expected it to. Ok, here is the peculiar behavior, today for a while it started to shift late and hard. I had to push it to get it to shift. But after i got it up to highway speed with these hard shifts, it started to behave normally again. tonight it was back to behaving well again.
Any guesses as to what caused this? is there any maintainance  i should do? ie get the tranny flushed? thanks, jeff

[Lt.Del]

VSS
what year vehicle do you have.  Computer controlled?  Type of trans?

My 4l80e has a VSS on it and mine was doing the exact same thing and I found out that two wires going to one Vehicle Speed Sensor were stripped of some coating, thus, touching wire to wire bypassing the VSS.  Hard shift to first after high rpms.  thens sometimes it would work just fine if i pulled over and turned engine off and waited a minute.  

http://forum.73-87chevytrucks.com/smforum/index.php?topic=12080.msg90690;topicseen#msg90690

[chevyh20]

i forgot to mention , i have a 700r4 tranny.  i have a 89 blazer. vss? does my year have a vss? my speedo does seem to be off. the axles have been changed on this. wondering if it might be a stuck valve. that caused the bad shifts, then free'd up?

[topp]

Yes you have VSS.... but not as complicates as SgtDel's.

My thought is buying off CL, never believe what the seller says.

[Lt.Del]

your speedo isn't tied to your computer like my 91.  I think 91 was the only year (73-91) in this era that the speedo is not mechanical.

I would begin by checking out your VSS....check wires and remove it---they do go bad and they can become dirty, giving false readings to computer to switch gears.  

When mine was acting up, sometimes it would shift extremely hard into 2nd and just stay in 2nd no matter how fast i drove it. Sometimes it would stay in 1st and after pushing the eingine she would slam into 2nd.  Needless to say i didn't drive it like that too much for fear i was causing some damage.

google "vehicle speed sensor" and  700r4 and see what you find.

[jaredts]

If you have a 700R4 I think the transmission's only function controlled by the computer is the lockup converter, so I wouldn't think any computer inputs like vss would cause you a problem other than tc lockup at highway speeds.  I'm thinking the tv cable or valve just got stuck when you gave it gas and then later freed up.  On the other hand I'm no transmission guy, so ??

[VileZambonie]

Jaredts is correct. The 700R4 is not controlled electronically. I agree check the TV valve operation and change the fluid and filter if you haven't already done so.