Early 70’s Ford Truck w/Dana 20 Xfer case
The problem is that when I put the truck in 4W Low, it will sometimes slip out of gear into 4W Neutral. It happens more often when climbing hills than driving on level ground. It hasn’t ever slipped out of 4W High. It is drivable the way it is, but it’s annoying. If there were an easy fix I’d do it. If I have to remove the xfer case, that seems like a big job, so probably not. What do you think?
The manual suggests these four as possible reasons
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Shifting poppet spring weak.
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Bearing broken or worn.
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Shifting fork bent.
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Improper control rod adjustment.
Which is the most likely problem? What is the “poppet” for and where is it located? Can anything be done to diagnose or fix this problem w/out removing the xfer case from the vehicle? What about the control rod? Can it be adjusted with the xfer case installed? If so, where is that adjusted?
Does it happen if you use 4 hi? How fast are you going?
Never slips out of gear in 4W Hi. (By “slipping” I mean the xfer case gear from 4WL to 4W-neutral, the xmission remains in gear ok).
When it slips in 4W Lo, it occurs when I’m going slow and uphill. Dirt jeep roads I’m talking about. The steeper the hill, the more likely it is to slip. 2-10 mph.
I have noticed that only Barkydog has responded. Take the following for what it is worth since I do not have much information on the Dana 20 Xfer case or 1970s Ford pickups. If you do not have the shop manual for this truck, visit your local library(s) to see if there is a Mitchel or Motor manual that covers this truck.
Regarding the poppet spring, this would be the spring behind the ball bearing that drops into the detent of the shifter shaft the moves the fork and low/direct sun unit. There usually is an external bolt that holds this spring in place. It would be located along the axis of the transmission and rear drive yoke. But you would need the breakdown of the transfer case to know which bolt to access. If you wanted to test the power of the detent, remove the linkage coming from the cab and try to move the low/direct shifter arm by hand to feel how sharp the detent entry is and how hard it is to get it out of detent.
Regarding the control linkage, you will have to access a service manual as each vehicle and each transfer case is a different case. You might have someone move the lever while you put your hand on the shift arm and see if you can feel the shift shaft going into detent as the control is moved. While you are down there inspect any bushings in the control linkage for damage.
One final idea. Inspect the motor and transmission mounts to make sure that the engine/transmission/transfer case is not moving excessively and effectively pulling the shifter arm out of the low detent into neutral.
Good luck on this.
@Researcher I checked the motor and xmssion mounts this past weekend per your idea; they are ok. I do have a shop manual which has a diagram and procedures for troubleshooting and rebuilding the xfer case. I’m sure I could fix it if I took the xfer case off the truck and had it sitting on the work bench. But that’s too much work for what is really an annoyance, rather than a serious drivability problem. I think I’ve heard other people with this problem just use a stiff bungee cord to hold the gear shift lever in place. I can always use that as a back-up.
I wasn’t sure from your comments: Do you think it is possbile to fix the poppet/spring/bolt (assuming that is the cause) while leaving the xfer case installed on the truck? Or do you think I’d have to remove the xfer case? The shop manual isn’t clear if that mechanism is accessible from the cab (from above) or not. It appears that it isn’t accessible from the cab, just from looking at the diagram; i.e. I’d have to remove the xfer case. But it is hard to say. In any event, thanks again for taking time to post your comments.
It is possible that you might be able to replace the poppet spring from below. Read through the rebuilding procedure. At some point there will be a mention of taking out two bolts with steel ball and spring. These will be the detent spring and balls for 2/4 wheel selector shaft and the reduction/direct selector shaft. They should be in the vicinity of the shifter arm attached to the linkage to the cab. The bolts could be going up, down, or sideways into the transfer case. It would depend on clearances as to whether you can get access to them with the transfer case in place.
For my information is the shifter in this truck linear i.e. you push or pull one lever in line to get 2WD high, then 4WD high, and then 4WD low?
You might try the bungee cord idea but that will accelerate wear on the sleeve and fork of the high/low slider.
It is one lever, yes, but the shifting pattern is sort of “J” shape: upper left 2WD , lower left 4WD HIGH, lower right N, upper right 4WD LOW. It’s the 4WD Low position that is balky, doesn’t hold in place, slips into N.
I’ll take a closer look at the assy’ diagram, thanks.
You might grab the front output shaft yoke and shake it. If there is any lateral play there is wear in the bearings and if driven in 4-low the gears will likely be damaged soon.