TH400 Governor - Is this my problem?

[mcintyrederek]

Hi,
1986 C10 with aftermarket 350 crate motor and TH400 transmission, 2.73 rear end.

Everything runs/shifts great.  The vacuum hose to the modulator was replaced, and I had to "turn in" the screw inside the modulator to lengthen the period of time in between shifts.  Before I turned in the modulator screw, I'd be shifting from 1 to 2 to 3 before I even hit 25 MPH.

The problem is down shifting when slowing down or coming to a stop.  I'm in 3rd, running 55, and then slow down to make a turn..  Many, many times even at 15 to 18 MPH, I'm still in 3rd.  It will finally shift back into 2nd below 15 MPH or so.  I just don't like making a turn, and then taking off in 3rd at 15 MPH.  I have done a little research on the governor but not sure what I'm doing.  Does this behavior mean the springs are too loose?  Should I just buy a whole new governor assembly?  I have seen them on Ebay for about $30. 

Your suggestions?  Is this difficult to change, and are there any special tools or procedures needed to change it?

Thanks.

[rich weyand]

No special tools.  The governor is behind a cap on the side of the transmission, which itself is retained by a spring clip. 

Rotate the spring clip off to the side.  Remove the cap.  A couple ounces of ATF will drain from the chamber.  Pull the governor straight out.  Insert the new one, engaging the gear on the end as you do.  Replace cap.  Rotate retainer spring into place.

[mcintyrederek]

Sounds simple enough. 

So, with the symptoms I described, the governor is the problem? 

The springs are worn?

[rich weyand]

The way to test the governor is with WOT shifting.  For lower throttle settings, shifting is determined by both the governor and the modulator, which provides reduced pressure to the governor.  This reduced pressure is easier for the weights and springs in the governor to overcome, so the transmission shifts earlier with more vacuum (less engine load) and later with less vacuum (more engine load) due to the modulator action. 

When the kickdown switch is on, though, the modulator is bypassed and full fluid pressure is provided to the governor valve.  This will be the latest shifts you ever get: at wide open throttle with the kickdown cable pulled.

So, if the WOT shifts are fine, then the governor is not your issue.  The modulator is either malfunctioning or misadjusted.  If the WOT shifts are early -- the transmission shifts and then acceleration goes down because the shift was too early; the pull should be smooth through a WOT shift -- then the governor needs to be modified.  If both shifts are early, modify the weights.  If the 1-2 and 2-3 shifts are too close together or too far apart, modify the springs.

Read this whole thread: http://www.gmsquarebody.com/forum/archive/index.php/t-1107.html

[mcintyrederek]

Thanks for the good information and the link.

I have never tried the vehicle in WOT due to fear in tearing something up.. The engine seems to have a tremendous amount of torque on the low end.  Even in 2nd gear and running 15 to 20 MPH and the engine only running 2800-3000, I'll break loose from the road and the rear end is all over the place.  So, I've driven the vehicle granny style since I've got it running, and I'm not looking for performance..  Someone told me the engine is an "RV Engine" but it idles rough like a complete turd and the comp. is 10.3..  So I'm not really even sure what I've got in the first place..

At any rate, I understand more about how the governor and vac. mod. work together now after reading several articles, and I think I can handle the modification.

Thanks again.

[rich weyand]

Governors are cheap and easy to replace.  They are also easy to screw up when people modify them, or replace them with a "TH400 governor" that is generic: i.e. not the correct one for the vehicle.  Given the other mods to your vehicle, I would suspect someone tried, less than successfully, to modify the governor for some reason, or replaced it with one that was intended for a different, probably lighter, vehicle.

I would probably start with buying a stock governor and putting it in so I had a known starting point.  Make sure to get the correct part number for your truck; the governor was how GM was able to build and inventory a bazillion identical transmissions, yet have each one tailored to specific vehicles when installed.  Pretty slick, actually.

I *think* this is the one you want.  This is listed for TH400 in CKs in the 1978 parts book.  I don't have the book for your year.  Note that in 1978, the TH400 was only used in a few vehicle combinations, not across the CK line, so I can't be positive this is the correct part number for an 86.  Maybe someone on here has the 1986-1987 parts book and can look it up for you.
http://www.ebay.com/itm/GM-HYDRA-MATIC-TRANSMISSION-GOVERNOR-8627445-OEM-NIB-/360373792820

While I'm on the subject, the 1986-1987 parts book would be a very good thing to have.  You can look up the exact correct parts and then search by part number.  Not cheap, but saves a lot of grief.
http://www.ebay.com/itm/GMC-1985-86-CK-Light-Truck-and-1987-91-RV-Parts-Illustration-Catalogs-NOS-/251538201270?pt=Motors_Manuals_Literature&hash=item3a90d85ab6

BTW, on the TH350, the governor cover is retained by the spring clip, and has an O-ring seal.  On the TH400, the governor cover is retained by four screws in a square pattern.  There is a paper gasket for that cover, which will probably die when you remove the cover, so you need one of those as well.

[mcintyrederek]

Sounds good.  I can see where I will probably need a new gasket when I replace or modify it.

I am not sure this is the same TH400 that came with the truck.  It might be..  But it originally had a 4.3 V6 in it.  Like I said earlier, someone took that out when it wore out at 200K miles and replaced it with this 350 crate motor.  The truck came with a TH400 for sure, because there is an orange wire and kickdown switch at the pedal.  The rear end is a 2.73 which I verified by tire turns vs. tailshaft turns, plus it plainly says it on the glove box data.

So, if someone replaced the governor or replaced the entire transmission that came out of something that had a 3.55 or 3.73, then that governor would not be correct for a 2.73 vehicle?  If I understand how the governor works, it senses tailshaft speed.  So, grabbing a TH400 out of the junkyard without knowing what rear end it was designed to drive means you need to know what governor goes with your rear end and change it..

Thanks for the info.

[rich weyand]

Yep.  GM changed the governor depending on axle ratio, vehicle weight, engine size, etc.

Get the parts book so you can find the right one.  You will use that book again and again.