C6 won't downshift under load
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C6 won't downshift under load
Hi guys,
I've got a 351w with a C6 that won't shift down to second if I want to accelerate or under load (like up a hill with a trailer). I replaced the vacuum diaphram - any other ideas would be much appreciated...
I've got a 351w with a C6 that won't shift down to second if I want to accelerate or under load (like up a hill with a trailer). I replaced the vacuum diaphram - any other ideas would be much appreciated...
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Freshman User
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kickdown doesn't work either, but shouldn't it shift without flooring it? not too sure about the carb. I've had the truck for four years and haven't replaced it but chances are pretty good that its a reman.
Senior User
Joined: May 2003
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From: Omaha, NE
Have you had a friend watch at the tranny when you open the carb (with the engine off of course)
to see if the kickdown rod is actually doing it's job?
Scott
to see if the kickdown rod is actually doing it's job?
Scott
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pretty sure its not. i thought the vacuum mod. was supposed to downshift with an increased vacuum signal (ie more load)....
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Hotshot
Joined: Feb 2004
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From: Marengo, Iowa
the downshift rod handles the pressure to the governor, as does the modulator, but it takes a lot for the modulator to kick it down, so they tend not to. If the downshift rod is not hooked up, the trans doesn't properly control the pressure, so when you are towing or such, it can actually contribute to trans failure as the clutches aren't getting squeezed hard enough.
Laughing Gas
Joined: Apr 2003
Posts: 837
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From: Minnesota
Am I missing something here??
What's the problem with manually downshifting ? It is a C6 isn't it, the transmission that when first introduced was billed as the automatic tranny you could shift manually?
Under load (towing or heavy hauling) I always shift Clyde manually. It gives me much more control than if I let the automatic run on it's own.
Years ago I had a job that involved extremely heavy hauling with 460's and C6's (GCVW >30,000#) and learned the best performance can be had with manual control; accelerating under load, passing, hill climbing and even learned to manually down-shift to use engine braking and save on both the truck and the trailer brakes. When hill climbing I downshift at the speed I want to "take the hill" at, passing I downshift before I start "putting my foot in it" and then only give it as much throttle as needed to do the job.
Mine doesn't readily downshift underload either ... I compensate
Roger
What's the problem with manually downshifting ? It is a C6 isn't it, the transmission that when first introduced was billed as the automatic tranny you could shift manually?
Under load (towing or heavy hauling) I always shift Clyde manually. It gives me much more control than if I let the automatic run on it's own.
Years ago I had a job that involved extremely heavy hauling with 460's and C6's (GCVW >30,000#) and learned the best performance can be had with manual control; accelerating under load, passing, hill climbing and even learned to manually down-shift to use engine braking and save on both the truck and the trailer brakes. When hill climbing I downshift at the speed I want to "take the hill" at, passing I downshift before I start "putting my foot in it" and then only give it as much throttle as needed to do the job.
Mine doesn't readily downshift underload either ... I compensate
Roger
Hotshot
Joined: Feb 2004
Posts: 11,697
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From: Marengo, Iowa
There's nothing wrong with downshifting manually, just that on this case, it wasn't doing it as it was supposed to, and diagnosing it to make it work proper, at least that's my understanding. I do the same on towing, just wasn't what he was asking about, it was in regards to the automatic feature, rather than moving the lever.
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ok, I guess my C6 is just a bit tired generally: I've found that after towing a trailer for an hour or so on the highway, the transmission has alot of trouble making it up grades and even starting up from stop (feels like it's slipping) Time for a rebuild, or is there still hope? Thanks, Dave
Cross-Country
Joined: Oct 2001
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From: Albuquerque, NM
If it's slipping, it is almost 100% sure it is time to rebuild. If you want to give it a go, it is definitely possible to DIY this job with a bit of investigation and tools. I did and my tranny is running great now. Plus, I KNOW how the thing works! A C-6 is pretty straightfoward as far as slush boxs are concerned, so it is a great one to start on. I used the TCI truckmaster kit for parts and had to buy a few additional hard parts too (I had a pressure diaphram that was absolutely shattered! I still wonder how it worked at all. Also, I had to replace the sprag clutch and I don't remember if that came in the kit or not. A few thrust washers were worn more than I was willing to live with, so they went too.), so that cost an additional $60 or so.
All in all, it was a great learning experience and I am very happy that I did it myself. If you do dive in, you would be well advised to get a digital camera and take a few shots as you go (especially when disassembling the valve body) so you have a sold reference when it goes back together.
All in all, it was a great learning experience and I am very happy that I did it myself. If you do dive in, you would be well advised to get a digital camera and take a few shots as you go (especially when disassembling the valve body) so you have a sold reference when it goes back together.