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C6 transmission thump into 1st gear

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Iain View Drop Down
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    Posted: 10-September-2020 at 11:21PM
Hi all,
 
Had the car tuned up, carb sorted on the rolling road etc, so obviously theres something else to look into now!
 
When downshifting, the C6 thuds into 1st gear from 3rd, but shifts smoothly down from 2nd gear into 1st. Upshifts are all fine, reverse is fine too. Never noticed it previously, but now I do, and hope it isn't anything too horrendous to sort out.
 
Fluid is red and smells fine (a bit sweet according to my nose), it is the older fluid type (F), which I have checked the level of and topped up with around 500ml (a pint for you lot!Smile) as it read a bit low. It has a dipstick that has been welded back together(!) and is a pain to get in the tube due to the tube curves and coolant hoses(the weld is not an issue as it is quite close to the top).
Have checked the level when hot and in park, but not convinced of the reading.
 
Would low and/or high fluid possibly cause this? And what other possibilities are there?
 
 
Many thanks,
 
Iain.
 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Big Bird Quote  Post ReplyReply Direct Link To This Post Posted: 11-September-2020 at 11:53AM
High fluid really takes a lot to be noticeable.
Low fluid usually slips and engages late.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Rockatansky Quote  Post ReplyReply Direct Link To This Post Posted: 11-September-2020 at 4:29PM
manually downshifting from 3rd to 1st?

is this with a manual valve body or is it still an automatic transmission?

my builder warned against manually downshifting on any kind of a regular basis, it's hard on the guts!

the clunk could be a sign that there's a planet set in the future for the C6? maybe Don 72 RS 351 can elaborate on the process that goes on internally to make the downshifts from each range to the next happen, my builder told me causing the band to apply to stop the drum is pretty tough on the giblets
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Iain View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Iain Quote  Post ReplyReply Direct Link To This Post Posted: 11-September-2020 at 8:45PM
Still fully auto, shifting itself in all cases.
Shifts fine in all other ways, just the 3 to 1 when stopping. If I slow down gradually, and it goes to 2nd, then 1st, it's smooth. Dread to think of a rebuild cost over here when you can get recon c6 units in the US for not much over 2k dollars. 

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Rockatansky Quote  Post ReplyReply Direct Link To This Post Posted: 11-September-2020 at 9:40PM
are you sure it's not the mount or U-joints?

not too long ago a 'bench job' for pretty much any rear wheel drive was about $500

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Iain Quote  Post ReplyReply Direct Link To This Post Posted: 11-September-2020 at 9:46PM
Will check it out and try to wobble stuff under the car.
Gearboxes are stupid over here, cars get scrapped as it's not worth the money to exchange or rebuild. Not in this case though! 

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Iain Quote  Post ReplyReply Direct Link To This Post Posted: 11-September-2020 at 9:47PM
Tick over is a little low too, not that I should affect this issue.  
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 72FordGTS Quote  Post ReplyReply Direct Link To This Post Posted: 12-September-2020 at 2:29AM
I agree with Rock, make sure you check the remainder of the driveline for slop.  Hows the differential for slop?

There is also the Ford Transmission forum if we can't figure it out:

Vince

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Big Bird Quote  Post ReplyReply Direct Link To This Post Posted: 12-September-2020 at 2:35AM
Do look at your engine mounts as well. Left side in particular.
Transmission mount, U-joints, and yoke bushing.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 72 RS 351 Quote  Post ReplyReply Direct Link To This Post Posted: 12-September-2020 at 9:34AM
Ditto, be sure that nothing else external might be causing the symptom. If you decide it's the trans, I'd check into a valve body kit before thinking of something worse or more expensive to do.

A VB kit is virtually always a great improvement to over all driveability. I'm not familiar with the C6 at all, the only automatics I've worked with are the four speeds, AOD/AODE/4R70W. There are lots of VB components and kits for those.

The C6 is a strong trans and Ford worked it out ages ago. But I'm sure you can find suggestions for what can be upgraded just through the pan. I will need the same advice myself with my new 73, the C6 is supposed to be newly rebuilt. But I like to do a VB kit as it makes shifting quicker, and that alone makes it last longer.
Don
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Post Options Post Options   Thanks (0) Thanks(0)   Quote californiajohnny Quote  Post ReplyReply Direct Link To This Post Posted: 12-September-2020 at 6:03PM
i did a shift kit in a C6 years ago for a buddy it was a bit more involved than say for a TH350 or TH400 but nothing too difficult Wink
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Post Options Post Options   Thanks (0) Thanks(0)   Quote aquartlow Quote  Post ReplyReply Direct Link To This Post Posted: 13-September-2020 at 1:19AM
The "thump" could be a few things. What is your idle rpm(too high and the trans can make the downshift  feel a bit more abrupt). Governor may need cleaning and possibly a valve body valve and/or spring issue. A C6 really needs a shift kit of some-sort installed, I have used a B&M Transpak in my C6's with good results, a Transgo kit is said to be the best out there for a C6. Here is a link to a Ford transmission forum, great guys there.    https://www.tapatalk.com/groups/fordtransmissionforum/index.php
 Hope this helps.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 72 RS 351 Quote  Post ReplyReply Direct Link To This Post Posted: 13-September-2020 at 3:25AM
TransGo was my favorite for the AOD VB kits. They were much more involved(more parts and things to do), the instructions were excellent. Other brands were few in the 80's when I began learning the AOD and VB kits. The other brands were very sparse with parts or instructions, they required very few changes, parts or holes drilled, minimal instructions. The results were poor also, harsher shifting at low throttle settings and clunky shifting overall(down shifting included).

Things have changed over decades, for sure, and I don't know what other brands have done since about 2000. I liked the driveability improvements a bunch with TransGo VB kits, everything was better not counting the WOT shifts. The kits allowed adjustments(choice) of the level of WOT shifting.

I would tend to try the TransGo kit if nobody else suggests something amazing from another brand.
Don
73 Ranchero GT 351C-4V 3.70 gears, to build a keeper
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Iain Quote  Post ReplyReply Direct Link To This Post Posted: 13-September-2020 at 9:40AM
Thanks for the replies guys.

It didn't do the thud today on the way home, but I did some good 'zooming' around this morning! Quick acceleration and harder than usual braking. Will check under the car later this week for anything odd. 

I think the diff is due for refurb too, as I left a '1' instead of an '11'!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 72 RS 351 Quote  Post ReplyReply Direct Link To This Post Posted: 13-September-2020 at 12:08PM
That's something we don't often do enough, drive it hard occasionally. That helps to work out the bugs, find them, and kill them.

I haven't worked on a 9" in decades, I do remember how heavy the chunk was though. I've got a 4.11 LS in my older truck, and a 3.70 in my new one. I'll have to swap them assuming I sell the old truck and rear with it. The 4.11 is too much for a typical C6 truck unless the owner really needs that. I did it to use with an AOD and a mild 351C-2V. Things change, now I think a 3.50 is about the right gear with an over drive.
Don
73 Ranchero GT 351C-4V 3.70 gears, to build a keeper
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Iain Quote  Post ReplyReply Direct Link To This Post Posted: 23-September-2020 at 8:35AM
Had a good root around, didn't find anything obvious, aside from one starter motor bolt was half out!
Several small leaks here and there, but nothing 'clunky!' 
The search continues....... 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Rockatansky Quote  Post ReplyReply Direct Link To This Post Posted: 26-September-2020 at 4:06PM
i just happened across a good teardown & inspect vid




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Post Options Post Options   Thanks (0) Thanks(0)   Quote Iain Quote  Post ReplyReply Direct Link To This Post Posted: 19-October-2020 at 5:11AM
Seem to have stopped the thump into 1st gear. I upped the tick over and it has gone. Would this mean the vacuum to the box was too high, and it has lowered slightly as the flaps are a touch more open?
I know about the modulator, but have not looked at it to see whether it is adjustable or not.
Tickover was too low anyway, but I don't know how low as my tach works when it feels like it! Winter job that one! 
Was getting concerned in case it was box out time. Not yet it seems! 
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