I have a 85 GMC 1500 w/ 305 & TH350. When accelerating slowly it upshifts early between 2nd and 3rd. When accelerating rapidly it upshifts normal. I checked the engine vacuum. It think it was about 17"-18". I am currently installing new heads, so the vacuum should improve once that work is complete. The trans oil and filter were recently changed. The pickup was parked for about 5 yrs and the trans oil was low the 1st time it was driven. Any suggestions would be appreciated.
There is also a adjustment screw in the vacc port on the modulator. clock wise is harder shift, counter clock wise is softer shift. If tranny fluid runs out of the mod or the steel line to carb replace the modulator
Thanks guys. I installed the new modulator but haven't tested it yet. No oil came out of the original modulator when I disconnected the vac line. Seems like it shifted OK before so I won't adjust the screw for now.
Post by reiners81chevy on Aug 6, 2007 17:04:01 GMT -5
Yea alot of people dont like lock up converters. As for the early upshift, if the modulator doesnt fix it try looking at the lines themselves. I've had the problem with my dad's truck before with the lines jsut being rotted out.Hope it helps. Hopefully its just a simple fix.
I ended up getting the trans rebuilt by the local trans shop when I had it out during an engine swap to a new goodwrench 350. It is a turbo 350C which has the lock up converter. The trans shop replaced the input shaft and torque converter. The electrical and vacuum components for the lock up converter are all hooked up as per original because I wanted the lock up converter for improved fuel economy. The new engine has better vacuum and all new vacuum lines which I think improved the shifting.
After a few adjustments of thescrew inside the new vacuum modulator it seems to shift OK, but still seems to lock up kind of early when accelerating slowly. The trans guy rode in it one day and said that is normal for the 350C and is why lots of people don't like the way they shift.
I still have an intermitent problem that I haven't figured out when towing on the highway. Sometimes when I slow down from highway speed to say 20mph and then accelerate again, the trans seems to stay locked in high gear. You really have to step on it to get to down shift. I assume it has something to do with the lock up converter. Guess I need to go see the trans guy again. Chris
Post by hobbytrucker1966 on Dec 31, 2007 17:09:47 GMT -5
you can put a switch on the lockup converter i have one on my turbo 350 and what a difference turn it on when on highway for fuel economy when im not stompin on it and turn it off when cruising in town
Post by hobbytrucker1966 on Dec 31, 2007 17:13:59 GMT -5
by the way if you dont know where to put the switch theres a vacume line on the firewall that goes to an electrical connection on the drivers side i took the vacume line off and just hooked up the switch to those wires and kept everything all out of sight
Thanks for the info. Maybe I will try installing the switch. The new engine and trans have about 6000 miles on them now and the shifting seems to have improved over time. The lockup feature is nice on the hiway especially when towing my boat as it is like having a 4th gear. Chris