Good C6 Line Pressure at 1000 rpm? And more... Stacked shift, Line pressure not changing at different vac levels

I have a C6 behind a 460 that is "stacking" (dumping?) shifts. It has a 2200 stall converter, vehicle has 3.70 gears.

I'm in 3rd gear by 12mph (roughly 2000 rpm) at just off idle.

Trans shop claims to have swapped valve bodies, changed governor springs and swapped modulator with no change in shift behavior. Not sure if they did pressure readings.

I set the modulator pin stickout to 0.675" at 15" HG. Tried both green (what was in there) and black stripe modulators with no change in driveability before borrowing a pressure gauge kit. Not sure if the 0.675" is correct, but it was a starting point as shown in TransGo instructions and elsewhere on the web. I did not feel any pressure/resistance when installing the modulator and pin. The pin length is 1-5/8".

I checked line pressure at 1000 rpm at the following vacuum points: 20", 15", 10", 5" and none (modulator open to atmosphere). I had the same results for all vacuum levels.

Park - 60 psi
Reverse - 70 psi
Neutral - 60 psi
Drive - 60 psi

There was a slight change in the pressure reading in Park and Neutral at 2000 rpm. Less than 5 psi.

Line pressure is at the low end of the "minimum" range charts I can find online. What is a "normal" expected pressure reading?

Engine produces 17" HG vacuum at an idle at operating temperature.

Manual shifting seems fine.

Any ideas on where to look for a problem?

The shop has done other work for me in the past. Not looking to throw stones yet.

The modulator only fits in one way, so I hope it is installed correctly.

Page 2 of TransGo SKC6 instructions mentions scale method or 0.675" stickout at 15" applied vacuum to the modulator.

How is the correct pin length determined? I just know that I have one, and it was in the transmission. There are 3 lengths, but the difference isn't much. The transmission was originally purchased from a rebuilder 20 years ago by myself. Never put many miles on it, current shop commented on the parts wear inside when I took it in for a re-seal that turned into a rebuild. Not sure if it was messed up originally (likely).

I am unsure if the valvebody is assembled correctly. Trans shop owner claims to have swapped valve bodies. Put a shift kit in what was in there, put same kit in swapped part, no change.

Modulator (both that I have) do move with applied vacuum.

What happens when you just leave the vacuum line off the modulator?

Line pressures are the same with the vacuum line disconnected from the modulator.

you should be able to remove the modulator and grab the valve in the case with needle nose pliers to see if it's stuck. If it's free it's time to pull the VB. Pin length is typically 1.70". modulator tension around 10 - 11 lbs

Is the pressure reading from the test port on the transmission affected by the pressure valve in the valve body?

How much pressure is generated by the transmission pump?

I can try pulling the modulator and letting the fluid drain out to see what's in there.

If the part in the valve body is not installed correctly, will that impact the line pressure?

My readings are at the bottom of the low end of the tables that I've found.

I haven't had time to mess with it yet. I'll have to let the fluid drain out of the modulator hole so I can check it out.

My vacuum modulator pin did not seem to touch the part in the valve body. 

A pin length of 1.734" gave me 12lbs of pressure before pin movement measured on a digital scale, and 0.030" of preload on the vacuum modulator as it is inserted. 

Pressure changes were not significant compared to before. 

60psi in park from 0 to 20" vacuum. 

I did see 5 psi change in reverse: 75psi at 0" HG and 70psi at 15" HG.

60psi in neutral from 0 to 20" HG.

60psi in drive from 0 to 20" HG. There was a very small change in the reading from 0 to 20, but it was only a few psi.

No significant increase in psi in P or N at 2k rpm.

The part in the valve body that the pin pushes against moves freely.

With your pressures being so low I would drop that valve body and do a pressure test .Modulator adjustments aren't going to raise your pressures 30-40 lbs. The three ports at the back of the transmission (where the valve body bolts up) will tell you if there is a leak and you can isolate the problem .Any old c-6 manual will show you how to do this .And if you don't have any leakage you could swap in another valve body.Doug

When the valve body is taken off of the transmission case, is there a gasket there?

Dan no gasket between the valve body and case .That is when you need to do an air test whenever you remove the valve body from the case.Doing this air test will tell you if you have a leak in either the forward clutches direct(third) or low reverse that could be the source of your low pressures .Your transmission shop would be able to do such a test.Haynes has a good manual that shows the location of each port for the three clutches I mentioned you would use a rubber tipped air nozzle and listen for leakage .I would do this and if all is good with your air test go to your valve body the air test is easy after you remove the valve body . Good luck Doug

The owner of the trans shop called me on Friday 7-30 and told me that his tech support told him to order a modulator pin kit and a couple of different modulators.

Not sure what that will do, as I made a pin with preload and the pressure did not change much at all, but I'll give them the opportunity to follow through with trying to "make it right".

So, I won't mess with the valve body until after they do their instructed procedures. Should happen by this coming Saturday.

I did copy/paste a couple of the above replies and sent them off, and I also told him what I did with the new pin, preload and results.

How does one tell if there is leakage between the valve body and the case? Assuming that an air test comes out OK? The shop claims to have done an air test, at least for function. Not sure if anyone listened for air leakage.

If the shop did an air test they would have heard it .And if they did such test and their is no leak I would imagine your problem is in the valve body It seems you have covered the modulator and its pin and valve .A c-6 with no shift kit is good condition should have 90 psi in park if my memory serves me correctly .As far as leakage between the valve body and case that is possible but most mechanics know to tighten those bolts up good .Doug

Yup to all of that.

However, I'm kinda stuck with letting the shop follow their diagnostic procedures until either a solution is found or the other alternative. I am glad that they are making the effort to fix the problem.

Unfortunately, it seems that actually getting good referrals for a local performance transmission shop, even in the Chicago metro area, is difficult. I have one, so far, and I haven't discussed anything yet with them. That shop may not be interested, for all I know.

Ordering one "online" is probably even more questionable. Any vendor can simply say "it worked when it left here"... a couple of big names have some not so good reviews, and there are the YouTube teardowns of "built" transmissions that are junk.

There is one shop that comes up recommended across several forums, but I really don't know yet. It'll probably be 2500 or so between the trans, converter, shipping and core charge.

The trans shop called, and it was a valve body problem. Vehicle had been there for 5 business days.

Something about the two halves not lining up properly at the 2-3 shift point location and bleeding off pressure (what I was told, not sure about accuracy). The valve body, or at least part of it, did not match the case number. More than likely, it has been this way since I purchased the rebuilt trans 20 years ago.

The owner did an air test and found/heard air leakage. I have seen references to an air test on the case, not sure about an air test on the valve body of a C6 (have seen the procedure using a plate for an AOD valve body).

Trans shift points are much better.

Shift "firmness" and "speed" is so-so (1-2 is better than 2-3), and not sure about what kit was used.

I'll have to hook up the pressure gauge for giggles. Really just wanted to drive it around for a bit.

Based on the description of what was done, I doubt that I would have found it from just looking at the valve body parts.

So, it got interesting. And more expensive

All along, the final intent was to install the Gear Vendors Overdrive unit that I kept from a previous vehicle. I was also going to install a Stage 4 valve body that I had purchased from Broader Performance last year.

When I set out to do this, the overdrive assembly was hitting a brace that is under the cab of the truck. I would have had to cut out almost all of the brace to make room for the OD unit.

Major suckage. Did not foresee this issue.

After calling Gear Vendors, I elected to try their "shorty" kit. Guess what, this requires a new and custom transmission output shaft... that has to be installed into the transmission that was just rebuilt.

More major suckage. Major suckage x 2. But I was already too far into it with new parts that could not be returned.

I decided to not go with the place that did the trans last time. It was quite difficult to get a recommendation. Many tales of woe.

I discuss the work with the recommended shop, and there were no issues, just bring the parts. He would even install the valve body.

Upon disassembly, I got bad news.

The band and drum were shot, and there were only 4 clutches. Most likely trashed during the valve body pressure bleed and "troubleshooting".

In the just rebuilt transmission, the same trans that should have been beefed up a bit. They knew what it was behind.

More money. More parts.

Gear Vendors parts installed nicely, did not have to modify that brace. Final length with the shorty kit and OD unit is really close to a stock trans. Unfortunately, this is a no going back install.

In the end, the trans now shifts as I was expecting, maybe a little harder than expected. It'll chirp on the 1-2 shift.

Trans guy was really happy with how it all turned out.

Still have to do the electrical portion of the OD setup.


Shop to recommend: Certified Transmission Specialists in Streamwood, IL