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SM420 Stuck in First Gear

69Willys

69Willys

Las Vegas, NV
2026 Sponsor
I was diagnosed with cancer and the Jeep's been sitting for a while. I've finished all my chemo and radiation and am feeling a lot better, so I decided to start the Jeep. She fired right up with a jump, but I can't get her out of first gear. She's been sitting in first this whole time on my inclined driveway. I don't have an e-brake, so I use the granny gear and a wheel chock. Everything was working perfectly when I parked her (haha...we've probably all said that line at some point!) I should mention that it rained HARD a few times and the top was off, but I can't see much water getting inside. The clutch is working fine--I rolled her forward a bunch of times and tried to shift, but no luck. Any ideas how to troubleshoot this?
 
Chock a wheel.
Shift transfer case into neutral.
Continue pushing/pulling Jeep for evaluation.
 
Can you drive it in 1st gear for a little bit and see if it breaks loose?
More than likely the problem is in the top of the trans, not the gears.
 
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Drain some oil out and see.
No harm in running it with the transfercase in neutral after the sample is replaced until the transmission warms up.
 
69Willys said:
I should mention that it rained HARD a few times and the top was off, but I can't see much water getting inside.
Click to expand...
Water will run down the shifter and freeze in the transmission. Warm it up.
 
Just got back from the Mayo Clinic in Phoenix. This was my first opportunity to crawl under the Jeep. What i found doesn't look good. Sure enough, those couple of rainstorms were the problem. Obviously, I need to replace this oil. Should I try to flush the case somehow? Will getting fresh oil in there cure this? Thanks for any help!Water in Oil.jpeg
 
If you can pull off the bodywork enough to acesss all the bolts on the shifter cover....Block your wheels.

Take set of channelocks, put it over an around the shifter retainer, push down while turning 1/4 turn to the left. It will pop up and then pull off the retainer and shifter.

Take off all the bolts, tap the housing and pull off the shift tower. Figure out what gears don't move and make them with a dead blow hammer. It may be your shift tower...

If that works, fill it with ATF and or Diesel, put the shift tower back on and drive it around the block. Drain it out and fill with GL4.

If that doesn't work, pull transmission and repair... Others may have different ideas, but the SM420 is robust. They are hard to destroy.
 
Here is an old thread of mine about the SM420
47v6

Thread 'SM 420 new synchros and bearings'

This is not a rebuild, just a refresh. I have only fixed one other transmission, a t-90. I used the universal manual for that and a chilton book that just copied a lot of that service manual.

I am using this manual for my reference. http://kinzers.com/don/landcruiser/transmissions/

I hope to not destroy this transmission and I really hope to make it work again after I take it apart. The reason why I am dissembling it is because it grinds going into high gear. Double clutch and its ok, but something is wrong. I think its the brass synchronizer cone.

I like to learn. I would...
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Damn...in all my excitement to drive the Jeep again, I kinda forgot that I'm not supposed to be messing around with chemicals right now. It was all good when they told me that I can't be the one to pick up after the dog, but I guess that applies to diesel, gasoline, ATF, etc., for now. It felt so good tinkering again...like I was getting my life back to normal. I even unwittingly siphoned some gas from the tank to prime the carb (not by mouth). I have a damn trach tube in my neck right now and all vapors and fumes go directly into my lungs. Well, I need to do what the doctors say...it seems to have worked so far. Luckily, my son lives nearby. He will help me.
 
69Willys said:
Damn...in all my excitement to drive the Jeep again, I kinda forgot that I'm not supposed to be messing around with chemicals right now. It was all good when they told me that I can't be the one to pick up after the dog, but I guess that applies to diesel, gasoline, ATF, etc., for now. It felt so good tinkering again...like I was getting my life back to normal. I even unwittingly siphoned some gas from the tank to prime the carb (not by mouth). I have a damn trach tube in my neck right now and all vapors and fumes go directly into my lungs. Well, I need to do what the doctors say...it seems to have worked so far. Luckily, my son lives nearby. He will help me.
Click to expand...

Yikes! Health is more important than the Jeep.
 
So, I've had a couple of great days! Today, my son came over and we fixed the Jeep! We removed the transmission top cover and got the shift rails moving. Then we got the transmission in neutral We drained all the sludge, put a couple quarts of diesel fuel in the case and sloshed it around by rocking the Jeep side to side. We let the diesel sit for an hour while we went and got some GL4 90W. Drained the diesel fuel and refilled with GL4. Took her for a drive and all went well. I'll do another drain/fill in a while--there was definitely some rust inside. But she's fixed! Thank you all for your help!

And...2 days ago, my oncologist told me that, as of now, I'm cancer free! There will be future testing, and I still need an operation, but I've moved from the treatment stage to the surveillance stage! Happy Days!

Forgot to add...the transfer case oil was good!
 
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