Author Topic: 2.73 to 3.42 and TH400 governor change?  (Read 1538 times)

Offline mcintyrederek

  • Registered Users
  • *
  • Posts: 198
  • Rookie
2.73 to 3.42 and TH400 governor change?
« on: September 18, 2015, 01:15:42 PM »
I have a 2.73 open carrier rear end in an '86 C10 with a mild 350 and TH400.

The vehicle up-shifts fine.  I can adjust the modulator to delay the time between shifts upon takeoff to my preference.  However, the DOWNSHIFT is not operating as well as I think it should.  I can slow down for a "coasting turn" and hit 15 to 20 MPH and I'm still in 3rd.  Below 15, It will shift into 2nd.  Slowing down even further, it finally shifts into 1st at 4 to 5 MPH.  It seems to me I need to be down-shifting MUCH sooner.  Running 15 to 20 MPH in 3rd gear with a 2.73 can't be good on anything. 

Question #1 - what do I need to do with the governor to correct this?  Change springs, weights, or both?  How do I know what direction I need to go?  Heavier or lighter weights?  Springs?  Do governor springs get weak and is this a symptom of this?

Question #2 - I really think the truck will behave better all the way around if I change to a 3.42 set.  When I do this, the governor will definitely have to be changed, and so will the speedo gear.  Is there a set standard of what governor to use with a certain gear ratio, and if so, where can I find a chart or information?

Thanks much.

Mack.
1986 C-10 Silverado 350 CI 260 HP, Edelbrock 1405, TH400, 2.73's - rescued from the junkyard - Farm Truck, daily driver!

Offline rich weyand

  • Senior Member
  • ****
  • Posts: 1391
Re: 2.73 to 3.42 and TH400 governor change?
« Reply #1 on: September 19, 2015, 12:48:41 AM »
Springs change the gaps between 1-2 and 2-3 shifts.  Basically, you can move the shifts closer to each other, or further apart.  Heavier springs move them closer.  I once put heavier springs in mine and turned it into a powerglide, it would double shift 1 to 3 and 3 to 1, with no second whatsoever.

Weights move both shifts up or down.  Heavier moves them down, lighter moves them up.  You can change one at a time to get half-steps.  Small changes move a lot.  One size heavier weight in mine moved both WOT shifts down 500 rpm.

Bear in mind, the governor moves the WOT upshift points.  If those are fine, it's the modulator that needs adjusting or replacement.  So mod the governor to give you the WOT upshifts you want.  If you are already there, then leave the governor alone and mod the modulator.

Rich

"Working Girl": 1978 K-10 RCSB 350/TH350/NP203 +2/+3 Tuff Country lift

Offline mcintyrederek

  • Registered Users
  • *
  • Posts: 198
  • Rookie
Re: 2.73 to 3.42 and TH400 governor change?
« Reply #2 on: September 19, 2015, 09:40:51 AM »
OK.. thanks..

Should disconnecting the modulator simulate the governor changing at WOT ??  If I am understanding right, the modulator will override the governor when changing up-shifts.  If the vacuum is HIGHER to the modulator, meaning I'm driving easy, the modulator will change sooner at low RPM.  If the vacuum is LOW, simulating heavy throttle, then the modulator will change at a higher RPM.  If the vacuum nearly disappears (WOT) then the modulator pretty much does nothing and the governor does the changing.

Is this correct?

Is this what would happen if you disconnected the modulator altogether?
(for test purposes only, of course)

Mack..
1986 C-10 Silverado 350 CI 260 HP, Edelbrock 1405, TH400, 2.73's - rescued from the junkyard - Farm Truck, daily driver!

Offline rich weyand

  • Senior Member
  • ****
  • Posts: 1391
Re: 2.73 to 3.42 and TH400 governor change?
« Reply #3 on: September 20, 2015, 05:47:57 PM »
Not quite.  The modulator regulates the fluid pressure to the governor, which is a valve that connects to the shifting mechanism.  The governor has a piston valve that the weights have to push down against that fluid pressure.  The higher the fluid pressure from the modulator, the higher the governor has to spin to push its valve open.

The modulator provides more pressure at low vacuum than at high vacuum.  So low vacuum, more fluid pressure, higher spin of the governor required, later shift.

The kickdown cable/switch bypasses the modulator entirely, providing full pressure to the governor.

The problem with disconnecting the modulator is that you will be driving around for extended periods at high rpms in low gears before it will shift.

All you have to do for testing is make sure the kickdown is adjusted properly, whether cable or electric, and then hold the throttle pedal down all the way.  The rpms you get shifts at will be the highest they can be.  If those shifts are where you want them for WOT, then the governor is adjusted correctly to your taste.
Rich

"Working Girl": 1978 K-10 RCSB 350/TH350/NP203 +2/+3 Tuff Country lift

Offline mcintyrederek

  • Registered Users
  • *
  • Posts: 198
  • Rookie
Re: 2.73 to 3.42 and TH400 governor change?
« Reply #4 on: September 21, 2015, 04:33:25 PM »
Thanks for clearing that up.  It's a TH400 and the kick-down cable is on the accelerator pedal, and it does work.  I have energized the kick-down solenoid manually with 12 volts and a test switch to make sure.

For the mean time I'll tighten up the vacuum modulator screw a few rounds and see how that affects partial throttle shifts.  This is likely what I need to adjust anyway, now that I understand how these things work together.

Appreciated,

Mack.
1986 C-10 Silverado 350 CI 260 HP, Edelbrock 1405, TH400, 2.73's - rescued from the junkyard - Farm Truck, daily driver!

Offline VileZambonie

  • Global Moderator
  • Senior Member
  • *****
  • Posts: 17777
Re: 2.73 to 3.42 and TH400 governor change?
« Reply #5 on: September 21, 2015, 04:41:36 PM »
You need to adjust the switch for the detent solenoid on the pedal assembly

Sent from my SCH-S968C using Tapatalk

,                           ___ 
                         /  _ _ _\_
              ⌠ŻŻŻŻŻ'   [☼===☼]
              `()_);-;()_)--o--)_)