Help with C6 diagnosis please

[Garbage_Mechan]

Got some pressure tests done and the good news is we didn't find anything wrong. Bad news is we didn't find anything wrong either. Here is what we found:

Control pressure is 60 at idle in neutral. It ramps up proportionally with throttle and rpm as high as 180. Trans does have a Transgo SK-6 shift kit installed long ago (mostly for better manual downshift control) I does not have any major pressure drops during shifts. The TV pressure ranges from 0 at idle sitting still to 80 wot moving down the road. It seems to be very proportional to vacuum.

To recap the issue:

The only change to a perfect shifting low mile trans was to rebuild the engine, and R&R the valve body to replace the shift shaft seal.
The issue is early soft shifts. In fact with the VRV adjusted properly, at light throttle it will shift into 2nd at 10 mph so soft you didnt know it shifted. At mid throttle, about normal daily driving acceleration it will shift into 2nd at about 15 to 20 and feel like it is slipping a little. At zero vacuum, it shifts at part throttle about like it should with 15" vacuum, that is fairly crisp and on time. It does kick down.
There is no metal or clutch material in the oil. Remember we have ruled out VRV adjustments because a vacuum gauge tee'd into the vacuum modulator line shows 18" vac at idle and proportional control down to 0 at WOT. And the TV hydrauilic pressure follows the rise / fall of vacuum proportionally.
My problem now is lack of experience to know where to look next and what suble things the gauges are teling me I don't know to look for.

 

[Garbage_Mechan]

So we dropped the valve body and looked at everything one by one. Looking at each valve for sticking / scoring and for out of place parts or broken springs.

The only thing we found was a stripped screw on the end cover for the valve body. Off to get a 10/24 helicoil set in the AM. This could be it but it is thin.

 

[typ4]

That is a strange one, I have done hundreds of them and this is a new one.

 

[Garbage_Mechan]

Tore the valve body down and checked every valve for scoring or sticking, springs for breakage etc. NOTHING. Did find one stripped screw on end plate of VB, and helicoiled that, some of the screw locations did seem to be mixed up. Had a very complete diagram of springs and valves. everything was in place correctly. Back together now all inspected, and torqued carefully. Hopefully solved, we'll see!

 

[Garbage_Mechan]

Sitting at Seattle airport for 6 hours on the way back to work in Montana so I have time now to update this a bit.

As I said above, went through the valve body with the precision of a surgeon. Not my first complex VB, just most are on Caterpillar or stationary equipment. Really never found a smoking gun. The only 2 issues were a stripped screw on the end plate of the VB and a few of the screw lenths were mixed up. Nore were bottomed out though.

Got it back together and road tested. At first I thought it was back to square one. Then I adjusteed the vacuum regulator valve outside of the specs to a much lower initial vacuum setting and things started to come around.

I might say having a vacuum gauge tee'd in and visible under the hood makes VRV adjustment much more scientific.

With the "correct" VRV setting shifting was super early and soft to the point of slipping (back to the original issue)
Moving the setting down to 14" of vacuum at idle took the shift points up to 2500 rpm and it would "hang" and not upshift until letting off the throttle just a bit. Sneaking up on the adjustment 1/2" of vacuum at a time at 15 1/2" of vac the shifts are still at 2500 at anything above light throttle, but no hang. This is how it was before I tore it down. It needs bit more refinement but I know the trans is OK and that's what I really needed. In keeping the VRV set this way, the TV pressure stays up to 75 to 100 psi during normal driving. Not sure of the implications of that, but the shifts are fairly crisp. Would like more rpm at say 3/4 throttle, but if needed flooring it and activating the kickdown brings it to 3300 rpm. Time ran out with another must do project yet to do so this is where it is for now.

 

[OLDBULL8]

You should have NO less than 21Hg of vacuum from the VP for it to work properly.

Did you by chance check the Intermediate band adjustment? Heres how, back off the nut, tighten bolt to 14 Ft lbs, back off exactly 1-1/2 turns, tighten nut, they recommend a new nut be used.

 

[idiabuse]

I grew up on C-6 transmissions, Early on I realized the smallest issue with the valve body will toast clutches in a hurry with the C-6
So I looked into the difference of the reverse manual and OEM valve body.
At 22 years old I thought to myself if I want to use my C-6 more than work on it I will use these Reverse manual Valve bodies that have 2 or 3 valves/springs in total than the OEM with a dozen or more springs and valves. 20 years later I have been using my C-6 a lot lately and it works better than the IRS year to year.

Nice to be able to just leave it in high gear all day long and just drive, saves a bunch of fuel cause you are driving nice and easy.

Javier

 

[Garbage_Mechan]

Javier, I appreciate your comments but I'm not interested in a manual VB. This trans has lasted very well so far, thruck has 500,000 miles and trans has only been done twice, second time recently, and worked well untill the repalcement of the seal on the shift shaft.


Old Bull, band adjustment has been double checked. As to the vacuum, the vacuum source from the pump is great, the inches of vac I described is what is supplied by the VRV to the modulator measured by the vac gauge tee'd into the line.

Please don't take my tone as unappreciative, just limitied time to type.

I grew up on C-6 transmissions, Early on I realized the smallest issue with the valve body will toast clutches in a hurry with the C-6
So I looked into the difference of the reverse manual and OEM valve body.
At 22 years old I thought to myself if I want to use my C-6 more than work on it I will use these Reverse manual Valve bodies that have 2 or 3 valves/springs in total than the OEM with a dozen or more springs and valves. 20 years later I have been using my C-6 a lot lately and it works better than the IRS year to year.

Nice to be able to just leave it in high gear all day long and just drive, saves a bunch of fuel cause you are driving nice and easy.

Javier

 

[jayro88]

Did you by chance check the Intermediate band adjustment? Heres how, back off the nut, tighten bolt to 14 Ft lbs, back off exactly 1-1/2 turns, tighten nut, they recommend a new nut be used.

This comment is of great interest to me since I recently installed a temp gauge and adjusted my band. According to all the searching I have done the procedure for adjusting the band on a c6 (per the Ford manual I could find) was to tighten to 10 ft/lbs (not 14lbs) and back it off 1.5 turns. Did I miss read something or was I misinformed? If I was please let me know so that I can readjust things on m c6.

 

[OLDBULL8]

Jayro88: You are correct, it is 10 Ft Lbs, it's 14 N-m. Gotta get my glasses changed.

Garbage-Mechan:

Please don't take my tone as unappreciative, just limitied time to type.

No problem. Just thought it might have been overlooked.

I even put the front pump in backwards on a 57 Plymouth, what a mess that made. Took 3 days just to clean all the aluminum shavings out.

 

[OLDBULL8]

Maybe these could help.Could you have left one of your BALLS out?

Edit: Oh crap,the modulator pic is upside down, ain't gonna change it, just print and flip it.

 

[typ4]

another issue i thought of is vac settings dont mean anything if the modulator has been changed. just adjust it for the shift timing and feel you want and if the line pressures are in spec or higher , run it. Replacement mods are made who knows where and well you know the rest.