I have a 91 4.9l C6 trans. that shifts good into 1st and 2nd(both from D or Manually) and the shift points seem to be ok. However, I have to drive 65MPH and manually shift from 2nd to D to get to 3rd. Also, after driving, the truck will go into reverse hard(doesn't have reverse until driven and 3rd is achieved) then I can shift in and out of reverse and the shift is normal. Any help with this from some C6 experts.
thanks

 

I'm no expert but have rebuilt my own transmission and took some time to study the parts, how the went together, and how everything functions. This certainly sounds like a problem. Since I don't know your technical level or experience, I'll try to explain things simply.

First you should know that the C6 transmission operates using hydraulic pressure. It pumps ATF through lines and veins in the transmission to shift gears.

The up-shifts are controlled by a combination of vehicle speed and engine RPMs. The vacuum modulator is what "detects" the engine speed and does this by hooking up a vacuum line to a spring-loaded diaphragm that is set to a specific tension. You can see in the diagram below that when the vacuum is low, the spring pushes the push rod into the transmission which closes the throttle valve. When vacuum is high, the pin is pushed back into the vacuum modulator which opens the path for ATF to flow through the throttle valve cavity. The vacuum modulator is pretty cheap and does need replaced occasionally, especially if the diaphragm breaks and it starts leaking out fluid into the vacuum line. It is installed on the outside of the transmission at the rear of the transmission.




The vehicle speed is "detected" by something called a governor. It is a weighted valve thingy on the output shaft of the transmission (inside the tail housing). As vehicle speed increases, the centrifugal force increases which opens/closes the valves in this governor.


Governor


These two components are what allow the automatic control of shift points in the transmission by controlling the flow of fluid to/from the valve body. The valve body has a series of valves (which are basically spring-loaded pistons) that can be opened/closed to allow fluid to flow to different places in the transmission. For our purposes here, we're only concerned with the valves that supply hydraulic pressure (using ATF) to the pistons. These pistons compress the clutches and steels inside the clutch pack to engage the different gears (forward, reverse, etc).

While the vacuum modulator might need to be replaced, to me, it sounds like you might have some debris that was floating around in your ATF (usually chunks of clutch material) that got stuck inside a valve somewhere, whether in the governor or the valve body. I'd recommend taking apart your transmission, removing each valve one at a time, cleaning it up, and re-installing it in the same hole in the same direction as it came out. Also check the springs to make sure they are not broken. You'll also want to replace the transmission fluid with clean fluid.

Still, if it were me, with such an old transmission and having obvious problems with the clutches coming apart and getting valves stuck, I'd probably just rebuild the whole thing replacing all seals, gaskets, o-rings, clutches, steels, fixing any broken/worn parts, and installing a shift kit (if it hasn't already been done). You'll save yourself a lot of time later.

 

Thanks for the info Braggs!
I have replaced the modulator valve with a black stripe one and dropped the valve body. I believe it's a new one(all the valves were tight but not sticking). The governor is something I haven't looked into yet, but I suspect this is were my problem lies. I just wanted to get the reverse/3rd gear working until I can build a bulletproof AOD and put in her.

By the way: My brother has an 86 F350 Just like yours! One of the best diesel motors ever built. I have been looking at z5 to replace the BW T-19 for him.

 

Mr Braggs,
I adjusted the mod. valve and now is shifting normal. Thanks for the info, ordered a tailhousing gasket, bushing, and seal kit and new fluid. Think my problem is solved! Again, thanks for the help.
matt

 

Glad to know you got it working!

 

That's a VERY good write up! Kudos!

I have one small correction. The upshifts are controlled by vehicle speed and engine load, not RPM.

Engine vacuum is proportional to load. For example, if the engine is at 2000 RPM at half throttle you'll have some engine vacuum. If you suddenly press the go pedal to the floor vacuum will drop to 0 and the engine will start to accelerate, but right at the moment the pedal is floored the engine is still at 2000 RPM with zero vacuum.

 

Thanks. I often read responses that say "rebuild time" without any explanation as to why. While it may be true, I figure a little better explanation might be more helpful to understand why to do something.