The Stovebolt Site Forums The Shop Area Driveline SM420 - Cannot find 3rd or 4th and is dirty

I am able to get into reverse, granny and 2nd gear with my truck but it will not find 3rd or 4th. Its not like it can't get into gear, there is no forward/back movement when searching to the right of 1/2. I saw some posts about checking the shifter to make sure its seated fine and when I opened it up it looks quite dirty. I would think the black stuff in the picture is not normal. Is there anything I can use to clean that up while its still in the truck? I saw an older post about using diesel or gas to soak it, is that viable?

Is there anything I can look/feel for from the top for a clue as to why 3rd and 4th have disappeared? I drained the oil and did not feel any bits of metal in the drain pan.

[img]https://www.stovebolt.com/ubbthread...umber/20825/filename/20200717_171651.jpg[/img]

If the shift housing was open to water getting in, it would be well to drain it to make sure all water is out. Looks like antique oil in there, anyway, so be good to drain and flush with diesel or kerosene, after you get it in gear, fun it for a short while with no load, and refill. I presume you had the top cover off, shift fork not in shifter groove. If not had the top cover off, shift rail could be clogged or rusted.

Ed

You've either got a locked up 3-4 shift rail, or the 3-4 synchronizer is jammed in the neutral position. Take the top cover off the transmission and see if you can move the 3-4 synchro with a pry bar. It's the one closest to the front. It looks like you've got a transmission overhaul coming up in the near future. Fortunately, most SM-420 parts are still available. Not cheap, but still out there in pretty good quantity.
Jerry

Awesome advice, thanks guys. Looks like I'll be dropping the tranny and taking a poke inside.

I had a similar problem with mine. Hotrod Lincoln is right on (as usual), I was lucky mine was from non use and had been drained years before being put up in storage. It had very dried oil on the synchro's and slide. The diesel and kerosene. Never gasoline as my father used to tell me. His theory from experience was that if anything that slides or has bearings the gasoline residue would not allow the grease to get between the race and cone for a little while. He probably learned this from his dealer Allis Chalmers manual's. Doc.

I had a similar problem with mine. Hotrod Lincoln is right on (as usual), I was lucky mine was from non use and had been drained years before being put up in storage. It had very dried oil on the synchro's and slide. The diesel and kerosene. Never gasoline as my father used to tell me. His theory from experience was that if anything that slides or has bearings the gasoline residue would not allow the grease to get between the race and cone for a little while. He probably learned this from his dealer Allis Chalmers manual's. Doc.

It took me a while to get the shift tower out but was finally able to get to it. The forks are not moving and it looks like the rods need a cleaning, should they move easily while the tower is off the tranny or is there a mechanism that releases them when its secured? I have side to side but not even granny/first anymore.

The shift rails have a series of notches in them, with spring loaded ball bearings that prevent the shafts from allowing more than one gear at a time from getting into mesh. With the shift box off and inverted, give the shift rails and the holes they slide through a good soaking with spray carburetor or brake cleaner, wait a day or two, then clamp the shift tower in a vise and try to move the shift lever. Don't get too energetic- - - -that top is cast iron and it could break with enough leverage.
Jerry

Great, thanks for the insight Jerry.