I had the problem of the 5 speed shifter popping out of first gear on my D6. I was unconvinced when everyone here told me it was probably because of the slop in the main clutch and linkage to transmission lock mechanism.

So I did a test with the tractor stopped, engine stopped too....

with transmission in first gear (my problem gear), I engaged the clutch and then pulled on the gearshift to try to get it to pop out into neutral......it did not, no tendency to move toward neutral at all. So I knew I had a functioning lock on that shift rail and the problem was elsewhere.

Next, I kept up my hard pull on the gearshift and simultaneously pushed the clutch lever forward. There was a lot of slack in it and about time all the slack was taken up, suddenly the gearshift slipped into neutral. My solution was actually pretty simple, I just adjusted the linkage between clutch lever and lock lever on the side of the transmission. That saved a lot of reworking of the internal clutch release mechanism where most of my slack seemed to be.

When I got it right I knew it was right because I repeated my test and the clutch popped out (released) just befor the transmission unlocked and allowed the shifter to move out of gear.

I would try a similar test before I did much tear down on your transmission. It’s possible the reverser gears on yours have enough wear that they tend to shift under load unless the lock is secure. As I said mine was first gear and while I’m sure it has some wear, once the lock was functioning properly, I never had any more trouble

I finally got back to work on my D6C. I found a large chip in the interlock shaft at the uppermost plunger position. All the springs were intact and I checked the top two plungers. Both were free in the bores and free of rust. I'm puzzled as to what would make the plunger bind enough to chip the shaft since I found no evidence of it being stuck. Any ideas or insight would be greatly appreciated. I've attached a picture, I think. and

With the help of the local Cat house I managed to locate and purchase a shaft. Had to get it from a company called Vintage Parts. The only one in stock of course. I've read the service manual and it looks like there is a keeper that would retain the shaft for the foward/reverse fork on the transmission side. I'm wondering if the keeper could be pooched and have let the shift shaft backlash the plunger into the interlock shaft to chew the chip in it? Also ordered a spring to go over the plunger and a new spring for the shifter linkage. The latter is to double verify the correct dimension on the shift link adjustment. Any ideas or suggestions are greatly appreciated.

Well, spent the biggest part of the day working on the D6. I put the new shaft and a new seal in the interlock housing. I double checked all of the plungers and put them back in place. Of course fifth gear was the best, so it got moved to the direction position. The direction plunger got a new spring. After feeling the tension of the new one, the old was a might weak. I put a new spring in the shift linkage. The old one was most surely shot after comparing them side by side. I mentioned earlier on that the shifter had a lot of slop in it. When I took everything apart, it just fell off the shaft, the bolt wasn't tight and the woodruff key holding it had a bunch of wear on it from that. After a good bit of wrestling around I managed to get it to tighten up and bite fairly well. Now on to other things involving the clutch brake but that's a for a different post. I'll have to wait to get the rest of the tractor reassembled before I can try it to see how well I did, got the throttle linkage to repair, looking for a decelerator to put on it, have to finish the fab work on a new battery box, mine sits right over this floor panel and was rotted to pieces, and waiting on the alternator rebuild so I can put it back on.

While getting adjustment on my clutch brake correct, per Kittylover's helpful message, I learned I couldn't get the interlock to release from reverse where it was locked to allow me to reach down in the belly. As I said over in my post about the brake adjustment, I had not previously moved the adjustment from where the prior owner had it. I had to lengthen the linkage out a good bit to get the interlock to disengage and then work somewhat backward to get brake shoe adjustment. That leads me to ccjersey's post in this thread. I'm in the process now of getting the rest of the linkage freed up to get the flywheel clutch lever back to spec'd adjustment. I believe the misadjusted brake has led me into problems I've found in the interlock. No telling how long it was used like that. I've spent plenty of time working on dozers, but this is my first direct drive tractor, so I'm learning as I go. Hopefully I'm getting it sorted out.

any progress to report? any idea how much you had to lengthen the interlock linkage? does the 15.22" spec seem to be close to what is necessary?

-only had my dozer 3 years and adjusted the clutch brake once...but it needs more attention, i'm sure the linkages need to be reworked to take out the slop and interlock linkage corrected as well. I notice the throw of the forward/reverse shifter lever is not 100% consistent so I'm pretty much in the same situation...learning as i go too

I've got the interlock lever at 15.25 or so, based on a regular tape measure. Most of my adjustment had to be done one the flywheel clutch and clutch brake linkage. Got the brake in adjustment, now I've got to free up the adjustment to get the flywheel clutch lever back in the spec. Need around
3/4 of an inch at the handle to meet the
book spec. Hopefully have that and the sticking throttle control sorted out by the end of this weekend. I think everything being out of adjustment so far is what has built the interlock problem, i.e. the chipped shaft.

A simple spring replacement on the interlock shaft linkage made a big difference from the old weak one that was on it. As I said before, I replaced the worn pins in the yokes of the linkages and got decent results with no other adjustment. I'd like new yokes on all of it, but that's going to be too pricey. I've got a bit more to do to it before I can give it a testing. If it's right it's got plenty of terraces to push down.

-a new spring on the interlock shaft linkage, great idea
-three quarter inch on the clutch brake lever sounds right
-new yokes would be nice but the old ones could be easily improved, welding, JB Weld, re-drill, grinding etc..

Yeah, going to try some welding and redrilling on a couple and see how that turns out.