C6 shifting late/hard

[Ky85]

Burners,

Would you willing to help me compile a checklist for troubleshoot?

The c6 in my 85 feels like it lags, feels like it has to hit a higher rpm than previous years before shifting. And when it does shift it sorta 'kicks into it.

My initial idea is to pull the filter off, check and refill. Then start checking vacuum hoses and pump.

Could this be in anyway related to fuel system? Air system?

Appreciate any advice!

 

[MadMac]

Had the same unexplainable issue: "Required to rev high in order to get kicked like an angry mule, even worse when cold."

Best response I'd rec'd after a bunch of people drove it: "likely an installed shift-kit". Don't know that helps you much, but something to keep in mind as you mull the reasons...

 

[Ky85]

thanks! the guys at the Zone said try this additive to see if it's mechanical or fluid related...friction modifier or something.

 

[Brian VT]

There's an easy adjustment to try. Give me a bit to find it.

 

[Brian VT]

Mine shifted late and hard when I got it.
On the top of the Vaccum Relief Valve there's a cap you can pull off and then adjust it.
Ford didn't intend for it to be adjusted but the fine folks here turned me onto it and it worked perfectly after I found the right spot of adjustment.
Here's what Big Bart said:

"Ford says you can’t service the VRV if out of spec. (Can’t move it enough on the IP to or from to get a good shift point.). That is not true, you can carefully take off the plastic cover on the top that covers the top and the vacuum ports. Pull straight up lightly and it snaps off. Now there is a nut under the head of the VRV valve. Only the nut turns, the head and base do not, but it moves the head up and down. Be careful not to break your VRV. Now turn the nut one turn and test drive. If things get worse go two turns the opposite direction. Things get better turn 1/4 turn and test drive again. Continue to turn 1/4 turn in the direction of improvement till it shifts as you like."

The thread with my trials and tribulations:

Too much vacuum

86 f250 xlt lariat 6.9 c6 I believe I am getting to much vacuum My shifts are very early 8 and 12 mph and my Cruze just floors it I went to adjust my vrv to find out that it was already maxed to less vac Any help would be appreciated

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[HammerDown]

I agree with BrianVT ^^^
For me those symptoms was the VRV vacuum switch mounted to the side of the injection pump, it controls the C6 shifting.
There is a sequence to setting it up with a specific gauge-block.
It started shifting high and hard then it wouldn't shift at all unless I back off the throttle a little... that was my problem... installed a new one, adjusted it as required and all was good.

 

[Brian VT]

Yup. That's the adjustment I was referring to.
I just adjusted it by feel until it shifted where I liked.
It's super easy to do once you get the old/brittle cap off. You can adjust it on the side of the road in a few seconds when doing test drives. Then put the cap back on after you get it where you like it.

 

[Ky85]

I agree with BrianVT ^^^
For me those symptoms was the VRV vacuum switch mounted to the side of the injection pump, it controls the C6 shifting.
There is a sequence to setting it up with a specific gauge-block.
It started shifting high and hard then it wouldn't shift at all unless I back off the throttle a little... that was my problem... installed a new one, adjusted it as required and all was good.

that's what it does! if I let off fuel it will shift.

thanks this gives me something to sink my teeth in

 

[Brian VT]

I don't remember mine needing me to let off to shift. Maybe it did?
But the VRV adjustment is free and easy to try so worth a shot before looking into other causes.

 

[gandalf]

Heres another transmission adjustment, for those who are hardcore about this sort of thing. This was written by cdnsarguy many years ago, posted in Dieselstop. I've never tried this, so can't vouch for it, but it may answer some questions.
**********************

Manual lever position lever not in park or MLP voltage out of range.

This is adjustment info for C6 and E40d just read it all the way through first......

Automatic Transmission Shift Cable Adjustment

1 Raise vehicle on jack stands or hoist. (I'd block well and scramble under)
2 From inside the vehicle, place the select lever in drive. Hang a three pound weight on the selector lever ( six pack still full)
3 Remove shift cable from the transmission lever ball stud.
4 Pull down the lock tab on the shift cable body.
5Position transmission lever in drive position. This is three detents from the front-most lever position with the first position counting as one.
6 Connect the cable and end fitting to the transmission lever ball stud.
7 Push up on the lock tab to lock the cable in the correctly adjusted position.
8 Remove jack stands and lower vehicle from hoist. Remove three pound weight from column shift select lever.
9 After making adjustment, check for Park enguagment. Check the transmission control lever in all detent positions with the engine running to make sure correct detent/transmission actions. Readjust if necessary.
CAUTION:
Under no circumstances will it be permissible to adjust the cable in any position other than D for the C6.
Now all should be ok if it is in PARK and everything works, remove blocks. (drink warm beer)


Notice:
These instructions are also applicable to the E40D, however the shift lever must be in OD (overdrive) for the E40D transmission.

 

[Brian VT]

thanks this gives me something to sink my teeth in

Another cheap/easy thing to check or replace is the vacuum modulator.

Vac Modulator Color Stripe

OK what color stripe for teh modulator? some say balck, some say green. what about the pin? does it get color matched too?

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[Jesus Freak]

As Columbo, Matlock, or Perry mason would say........,"there is just one more thing......." Check your actual vacuum pump, they do wear out and get weak. Thus your gradual demise.

 

[david85]

Glad you guys caught him before he dropped the pan! Got scared for a moment as I was reading.

The C6 uses high vacuum to produce early, soft shifts, and low (or zero) vacuum for late, hard shifts. Full throttle = late and hard. Light throttle = early and soft. It works well with gas engines that use manifold vacuum to manage shift behavior, but it also makes it fail safe if the vacuum line breaks off. In the case of a diesel, a dead vacuum pump or VRV would cause the transmission to think your foot is always to the floor.

Basically, this is a C6 in limp mode.

 

[Cubey]

Glad you guys caught him before he dropped the pan! Got scared for a moment as I was reading.

The C6 uses high vacuum to produce early, soft shifts, and low (or zero) vacuum for late, hard shifts. Full throttle = late and hard. Light throttle = early and soft. It works well with gas engines that use manifold vacuum to manage shift behavior, but it also makes it fail safe if the vacuum line breaks off. In the case of a diesel, a dead vacuum pump or VRV would cause the transmission to think your foot is always to the floor.

Basically, this is a C6 in limp mode.

Hmmm. Well my RV's current vacuum pump is getting weak, as was discussed in another thread, making shifts later (but not hard). But maybe it's still enough vac to prevent that.

When it lost the alternator belt in 2019 and I limped to a truck stop, I don't remember it acting badly, but then again I might have been manually shifting since I had no power brakes and I had the hazards on, going 40-45 in a 65 zone for a few miles to the truck stop. Stomping the brake with both feet due to a few stop lights toward the end. I know it wasn't shifting hard though, I would remember that for sure if it had. But again maybe I was manually shifting and managed to avoid it by fluke.

 

[Big Bart]

It is very hard to believe when your VRV valve on the IP pump is not working or the transmission modulator it supplies vacuum to is not working. That it would cause a late and very hard shift but it does. But if you let off the gas at higher rpm it will shift.

1) Check to see if your vacuum pump is working and if you have any vacuum line leaks. Even if your vacuum pump is pulling 18hg instead of 22hg that can change the VRV adjustment.

How to test and replace a IDI vacuum pump with a grove pulley.

Recently while towing a loaded a trailer I came off the freeway off-ramp to find out my power assist brakes were not working. Fortunately the light was green, there was no one in front of me and I could just take a left at the end of the on-ramp and slow down to a stop with the 40% braking I...

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2) Check to see if the vacuum modulator on the tranny leaks atf when you pull the vacuum line off it. If it does, its bad, and replace it.

3) Now that the above is verified working, it’s time to adjust the VRV system. Ford had a very hard to do exercise to set your VRV. I did not find it worked well unless you have a brand new VRV and a néw vacuum pump. It basically suggested with the IP cam open .55” from full throttle and vacuum applied to the system, you set the vacuum to a specific hg using the VRV adjustment. But it’s not very easy or effective in my opinion. So I suggest you use one of the below ways.
a) The VRV is the plastic device on the drivers side of the IP. It has 2 screws (Maybe three.) That when loosened let it rotate towards the window or the radiator. As I recall moving to the window caused a earlier shift and towards the radiator a later shift. Try moving it towards the window as far as it will go (1/3 of a turn) and tighten it and test drive. Did not improve? Now loosen and turn it towards the radiator tighten and test drive. Still not right, the VRV is out of calibration and not where you need it to be Releasing the vacuum. So loosen the VRV and put it halfway between the window and radiator travel. We will set it in middle so it can be adjusted later.
b) The VRV has a plastic cap over the head (The head has the two plastic vacuum ports.) Remove the cap that sits on top of the head of the VRV by pulling it up and away from the valve, it just pops off. Now just under the head of the VRV is a plastic nut but it has 4 indents so it does not look like a regular nut but it is a nut and it turns. This is how they were calibrated at the VRV factory. The nut turns and as it raises and lowers the head on the valve. (The head does not turn, just goes up and down.) Turn it 1/2 turn clockwise and test drive. Turn it a 1/2 turn clockwise and test again. Do that for about 2 complete turns. Did not get a good result, try the other direction. Now go back counter clockwise 2.5 turns. Then a half turn 3 more times. Likely by now you are finding where it shifts great. If not your VRV could be toast. Nick Pisa is making them but not sure of how good they are.
c) If you have an aftermarket tranny modulator (The little vacuum device on the tranny that hooks to the VRV via a vacuum line.) they have a small screw inside the vacuum port hole that also can adjust the shift point like the VRV. But if yours is older and is a Ford one, it will not adjust. But if your VRV or your modulator is bad, adjusting it is not going to fix the issue.

Let us know what you find.