i have a 1985 6.9l idi 4x4 XLT Lariat Extended Cab LB with a C-6 my problem is it only likes to shift when i let off the throttle it shifts hard when i do this at times any ideas would be appreciated thank you for the help

 

Could be the torque converter or the VRV on the IP. Try turning the VRV on the side of the IP toward the front of the truck, drive it and see if it's better. If not, then turn it back a little. If neither of those help, then you might need to adjust the diaphragm cap castle-nut in the VRV, but that's dangerous. If you break the VRV, you can't get a replacement. I'm working on a replacement solution for the VRV, but the plastics I'm prototyping is not holding up to the stress an IDI puts out. Maybe in a few months if I can get the suppliers to improve their plastic quality for a reasonable price.

The other culprit would be the torque converter. Also, you might have a vacuum leak somewhere.

 

Please explain how the torque converter could possibly cause this. Get as technical as you like, I fully understand how an automatic transmission shifts.

The other likely cause is that the vacuum modulator is bad. The modulator is near the rear of the transmission, near the bottom. It has a vacuum line attached. Remove the vacuum line and see if there is any oil inside the line. If it isn't totally dry inside, the modulator is leaking and needs to be replaced.

 

Thank you for the help guys I'll try adjusting the VRV and check the modulator Tuesday I know the modulator has different color stripes any idea what color I need also do the gas trucks have a VRV if so what are the year ranges that would fit my truck

 

A torque Converter can cause very similar symptoms. While not likely, it could have sprung an internal leak due to corrosion, or have bolts loose. Both would cause a less than ideal shift. Loose TC bolts while a light hammering, would make a late C6 shift feel like your internal organs are re-organized. Most likely it is a vacuum issue however. As I doubt the TC is going to be stripped on a shaft or anything like that to cause a massive slam that the OP is questioning about. Gummed up valve body could cause this, add that to a TC that is over full and frothing the fluid and bam, a ram hammer shifted C6.... again, all very unlikely and would require a "perfect storm" of failures. Possible though...

Two things here, a C6 is not a modern transmission. If you are romping on it, then let off, get ready for a fun shift to follow. It is a mechanical transmission, driven by your foot... and some vacuum control. I do not believe that gas vehicles have ever really needed a VRV, although some of the complicated British and German vehicles have a similar device due to the fact they have vacuum controlled headlight wipers and such... But your vacuum system is artificial in nature, so a vacuum pump must create the vacuum your diesel does not. This also means the vacuum works a bit different compared to gas vehicles which lose the vacuum they create when they floor it, we actually build more. Since we are spinning the pump faster.... This means we need to regulate the vacuum and ensure that we match the functionality the gas engines would have since our diesels are more or less prototypes. We "borrowed" gasoline engine transmissions and had to add things to make the Diesel engine play nice with it's unique set of characteristics. Keep all of this in mind, as your transmission should shift well, but it will NEVER shift like a modern computer controlled transmission. If under normal driving conditions, the shifting of your C6 feels as it should. Then you should consider it functioning properly. I personally own a 1985 F-250 XLT Lariat Explorer "A" Variant. It has a C6 and shifts great, I personally rebuilt it.... But if I romp on my pedal and get the motor all wound up, then let off in a rapid fashion. I could expect a very fun shift from first to second, followed by a soft shift to third. It is just the fact the Transmission can only account for so much. All of that being said, the vacuum control at the back of the transmission is most likely the culprit if shifting is extreme or routinely delayed.

 

Thank you for the information it is very helpful

 

VRV is diesel only. before you go messing with the VRV, check the amount of vacuum your vacuum pump is pulling. any less than 19 inches and the trans is going to shift funny. any less than around 17 inches and your power brakes are going to act up.
vacuum pumps go bad and cause all kinds of interesting issues when they are dying

 

Good point checking the vacuum pressure first. Maybe give the tranny fluid level a check too. Sometimes I get ahead of myself.

 

Thanks for helping a guy out I checked the transmission fluid it's full and looks good I'll have the vacuum pump checked on Monday before I mess with the VRV or modulator

 

Check your vac hoses too. A pin-hole leak could reduce the vac capacity or cause the pump to degrade prematurely.

 

I'll do that as soon as I can could have a hole for sure I've had the truck since 2015 I have never change them I randomly lost my cruise control coming back from town one day in 2018 cruise control worked going to town lol thank you for the help

 

If your cruise is odd, and your shifting is off, your brakes are losing power assist, and your climate control is not performing right, then those are all signs that your vac pump is losing potency, or there is a leak somewhere.