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Username Post: TH400 Governor Valve Body Headache        (Topic#376317)
1983G20Van 
Super Senior Member
Posts: 3998

Loc: Hurst, Texas, USA
Reg: 11-13-02
02-02-25 11:16 PM - Post#2877606    

Hoping to find someone that really knows the TH400s valve body mechanically and hydraulically that might have some insight. I am trying to adjust the governor to allow for a ~5,500-6,000 rpm WOT automatic 1-2 upshift, I can manually 2-3 myself because it is seldom I make a 2-3 WOT upshift with a 3.08 gear as even 4,000 rpm is something like 85-90 mph. I bought a NOS L82/TH400 Corvette governor that should have been setup to shift about 5,500 with the lighter weights. Tried it stock, there is as much as a 1,500+ rpm difference in the RPMs it wants to shift at on its whim. Drives me nuts because sometimes it is 4,500, sometimes it 5,500 and other times it will hit the 6,000 rpm fuel kill. I put the stock 1987 TBI 305 governor back in and it shifts the same point every time for 2 weeks now. Just far too early, like 3,800-4,000 rpm. I verified the kickdown solenoid gets power everytime and re-adjusted the throttle linkage so it allows for actual WOT and the vacuum modulator should not be building much vacuum at all. I even looked at the hydraulic flow and it looks like in Manual 1st the 1-2 shift valve should have full pressure on the downshift side of it regardless. I do not understand how a new GM governor creates this issue but a 100k+ mile used unit has the same shift points. I just want the thing to automatically shift 5,300-5,600 in Drive or Manual low, looking impossible to do. Possibly related it will not 2-1 downshift even with the shifter in 1st until it is under like 10 mph. No 2-1 par throttle or WOT kickdown at any speed either. Even rolling 10 mph it stays in 2nd. That being said the modified and Corvette governor both allow a part-throttle and WOT downshift up to maybe 25 mph, better but still nowhere near correct for a vehicle that will easily run 60+ mph in 1st gear. Problem with that is with the TH400s 1st gear and 3.08 rear gear it is already like some other transmissions 2nd gear and accelerating from 10 or 20 to 60 in 2nd gear is like accelerating in 3rd with other setups. Neither governor will allow the unit to make a Manual downshift to 1st over about 10 mph. My train of thought is that perhaps this 1987 unit that was behind a TBI 305 has a spring or two somewhere in it that is giving it this characteristic to comply with some kind of drive by noise decibel limit or emissions requirement GM contended with in the late 80s.

I think that is why I like the computerized transmissions like the 4L80E. If I cannot get this figured out, I think I will bolt a 4L80E into this before long and swap the rear end I have from a 92 G20 that has stronger 30 spline axles, 3.73s and a functioning G80. Might have to do something about that gov-bomb though before that rear end goes in.

Here it is with the TBI governor, automatic "Mercy Shift" in Manual 1st gear. As I mentioned before seems perfect with the old governor aside from no 2-1 kickdown or manual downshift. Swap in the NOS high rpm governor or the one I modified with lighter weights and springs and for some reason it hates life. I bought the GM calibrated governor because I felt maybe the modified old governor was acting up. I don't get it, maybe I need to put a shift kit in it with their spring changes here or there to correct something else the valve bosy is doing. I do not get why the RPM for manual low "mercy shift" would change unless the governor is sticking, but it is new and the plunger valve inside operates smooth as butter. Then again this may be why guys just swap them to a manual shift, reverse valve body when they race them.

~4,000-4,200 rpm manual low automatic upshift. Seems like it is exactly where GM would have made it for the TBIs powerband. Ignore the fan clutch stuck on, that is a problem for a different day. I know it is causing acceleration to drag a bit, but I feel unrelated to this problem, because the engine still pulls easily to the 6K fuel cut-off even with the fan drag when the higher rpm governor fails to make it upshift in time. This cammed Vortec 350 engine makes peak power at 5,500 and carries great power to 6,000 hence the reason the shift point needs to be raised and it needs a 2-1 downshift even at higher speeds when you want to accelerate hard. Some people are probably why bother, but I am tired of a 400 hp 350 V8 accelerating about like the stock ~180 hp TBI 305 that was in it before because it cannot get into the RPM range it makes its best power at or bouncing it off the limiter and having to lift to let it shift via modulator vacuum increase. Literally the 104,000 mile stock TBI 305 ran about the same way because its torque curve matched the shift points better.

https://youtube.com/shorts/hSR8kKJkMk4

This was from a previous attempt I made to adjust a factory governor with weights and springs. It would either shift early or bounce the 6K limiter, only occasionally shifting at the desired ~5,300-5,500. I was initially shooting for 5,300 then later had it on a chassis dyno and saw it carries power well to 6,000 rpm so revised that target RPM a bit, hence the 5,500-5,800 range I am shooting for now. But even then with the 5,300 rpm target it would sometimes not shift even at the 6K fuel kill.

https://youtu.be/UjEKGehu1M8?si=sUdaYAC HqfSi95M0

1983 G20 Van, 350 TPI, Ported 906 Vortecs, Edelbrock 3817 Base, ASM oversize runners. Reed Custom Roller cam, 700r4, 12 bolt with 3.08 gears, Doug Thorley Tri-Y headers, true duals


Edited by 1983G20Van on 02-03-25 12:03 AM. Reason for edit: No reason given.

 


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