C6 deceleration down shift?

[RossCo]

Ok gents....here it is. I have an 85 F250 6.9IDI 4x4 with a C6. I have noticed over the past couple days that the truck is down shifting upon deceleration. Almost like a new truck does when it in tow/haul mode. Any ideas what might be causing this? At first I was thinking vacuum pump but, my brake pedal feels just fine. So I kind of ruled that out. Whatcha think gents? Any ideas?

 

[franklin2]

It upshifts during acceleration from a stop ok? Not early and soft or late and harsh?

 

[RossCo]

I haven't notice anything odd in the up shifting. When I rebuilt this trans about two years ago I did install a shift kit so the shifting is quite snappy. None of the shifting is violent however. Even when it does this weird down shifting thing it's a smooth transition.
I keep thinking its some kind of vacuum problem but, there again every scenario I can think of would cause an up shifting or rather (none) shifting issue.

 

[franklin2]

Only thing I can think to check is the kickdown linkage. I have had that stick in the activated mode all the time and cause strange shifting problems.

 

[robw]

^ That's my thought, the C6 is a simple beast.

 

[RossCo]

Checked the down shift linkage. Everything appears to be ok there. Externally at least.

 

[Garbage_Mechan]

It is a vacuum issue. I don't recall the specifics, but I was experimenting with the 86 and got it to do this and was thinking it would be cool to be able to make it do it when decelerating with a load but was concerned there would be long term effects on the trans.

I believe zero vacuum on decell results in downshifting. The way most of these C6's are set up you would likely not notice too much difference on accell with zero vacuum. On the 86 I have everything set tight to get it to shift as late as I like. Could almost forget the vacuum valve on the throttle.

 

[RossCo]

Hmmm, 0 vac on decel.. I would have thought that would cause stiff brakes. Either way, one way or another I'm going to have to throw the vac gauge on the system and test the vac pump, vac regulater valve (dear God I hope not), and on from there.

 

[franklin2]

A little summary on how the c6 control system works on these diesels.

On a gas engine they use vacuum (engine load) and road speed (governor inside the transmission) to determine the proper time and firmness of the shifts. Of course a diesel doesn't have vacuum, so they took vacuum from the vacuum pump, and ran it up to a gizmo on the side of the IP where the throttle is attached. The gizmo is basically a variable valve, controlling the amount of vacuum to the line that leads down to the c6 modulator located on the rear pass side of the trans in relation to throttle position.

This variable vacuum valve on the throttle is known to give problems from time to time. I would inspect the vacuum lines and if you want to check vacuum levels, I would tee into the line after the valve leading to the trans modulator.

 

[typ4]

Check for cracked vacuum lines or caps on the vacuum tree., also there may be a HVAC line on the heater control supply that cracks and leaks.

 

[RossCo]

Alright, well had just a minute to hook up a vac gauge on the system. I hooked into the line that goes to the HVAC system and comes directly off the vac tree. Drum roll please..... 5 in hg.....So definitely some kind of Vac issue. Anyway I only had a moment. So tomorrow full checkout of the vac system. I'll let you all know what I find.

 

[RossCo]

Well got to the bottom of the mystery/rabbit whole. Started the truck this morning and test the vacuum directly off the pump.

You must be registered for see images attach

As you can see plenty of vacuum there. So I started plugging each line one by one at the vac tree. Low and be hold I have a leak in the brake booster. Not the out come I wanted but at least I found the problem. Next on the agenda hydro boost upgrade or just remove and replace.

 

[franklin2]

So, it probably wasn't deceleration like you thought, but when you touched the brake pedal? I will have to remember that one.

 

[RossCo]

It definitely did do it even if I didn't hit the brake pedal. I tried it with and without using the brake just in trying to diag. I'm sure that using the brakes definitely made the problem more pronounced.