C6 Yoke Lube
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Fleet Mechanic
Joined: Mar 2004
Posts: 1,398
Likes: 17
From: Hesperia, CA
C6 Yoke Lube
1977 F250 460 C6
I have this clunk when I start accelerating from a stop. The clunk happens after the truck is moving and before shifting into intermediate. I think it happens at about five to 10 MPH. The clunk isn’t there every time you take off so it surprises you when it happens. I checked the driveline. The U-joints are almost new and checked ok. The center support bearing was shot so I replaced it. The rear yoke slides real good. The tranny yoke slides real good, but it was very difficult to get it started onto the tailshaft. Once I got it on there it slid up and down the shaft rather smoothly. The yoke seemed very dry. Most trannys leak fluid when you pull the yoke out of the tranny, but nothing came out. Doesn’t the C6 tailshaft/yoke assembly get lubed from the tranny?
The only other problems with the tranny is that it runs quite hot when towing (in excess of 240 degrees) and the 2nd to 3rd shift is sometimes extremely harsh when towing with a heavy load. The kick down lever is missing and I think that is attributing to the harsh shift. Down shift is smooth in all ranges.
Does anybody have any suggestions.
I have this clunk when I start accelerating from a stop. The clunk happens after the truck is moving and before shifting into intermediate. I think it happens at about five to 10 MPH. The clunk isn’t there every time you take off so it surprises you when it happens. I checked the driveline. The U-joints are almost new and checked ok. The center support bearing was shot so I replaced it. The rear yoke slides real good. The tranny yoke slides real good, but it was very difficult to get it started onto the tailshaft. Once I got it on there it slid up and down the shaft rather smoothly. The yoke seemed very dry. Most trannys leak fluid when you pull the yoke out of the tranny, but nothing came out. Doesn’t the C6 tailshaft/yoke assembly get lubed from the tranny?
The only other problems with the tranny is that it runs quite hot when towing (in excess of 240 degrees) and the 2nd to 3rd shift is sometimes extremely harsh when towing with a heavy load. The kick down lever is missing and I think that is attributing to the harsh shift. Down shift is smooth in all ranges.
Does anybody have any suggestions.
Elder User
Joined: Feb 2004
Posts: 909
Likes: 9
From: San Luis Obispo, CA
I experienced the same clunk in a '80 F-150 with a C-4 trans. It had the same kind of consistent clunk you describe and I replaced U-joints with no improvement. Below is my theory, but I never took the truck apart again to confirm it.
Even if the yoke seems to slide smoothly when unloaded, there may be a light wear step at the end of where the splines mate. Under the torque load of starting things rolling, that almost imperceptable step keeps the yoke from sliding, but suspension "squat" changes the geometry enough to load up the yoke axially. Once the initial accelleration is over, the torque is lower, and the yoke moves with the thunk you can hear. (This whole analysis also applies to dry-spline conditions. Even though ATF lubes the yoke, it is a good idea to lube the spline with some grease as there is no mechanism to encourage the ATF to go inside the splines. This is a cheap thing to try.)
New Spicer yokes are around $75, if it bugs you enough to spend the $$$. A cheaper way to go would be a junk-yard yoke would likely have enough difference in its wear pattern to get rid of the clunk. (I decided I could live with it, especially as it was my Mother-in-law's truck!)
I would be interested to see what other people have to say about clunks. Please report back if you solve this problem.
Even if the yoke seems to slide smoothly when unloaded, there may be a light wear step at the end of where the splines mate. Under the torque load of starting things rolling, that almost imperceptable step keeps the yoke from sliding, but suspension "squat" changes the geometry enough to load up the yoke axially. Once the initial accelleration is over, the torque is lower, and the yoke moves with the thunk you can hear. (This whole analysis also applies to dry-spline conditions. Even though ATF lubes the yoke, it is a good idea to lube the spline with some grease as there is no mechanism to encourage the ATF to go inside the splines. This is a cheap thing to try.)
New Spicer yokes are around $75, if it bugs you enough to spend the $$$. A cheaper way to go would be a junk-yard yoke would likely have enough difference in its wear pattern to get rid of the clunk. (I decided I could live with it, especially as it was my Mother-in-law's truck!)
I would be interested to see what other people have to say about clunks. Please report back if you solve this problem.
