Still going through my new to me '52 GMC 150. Manual says to drain and refill the tranny every 10,000 miles and I have no idea when it was last done. Drove the truck about 10 miles to get the fluids nice and warm. Picked up 6 pints of 85W-90 gear oil at FLAPS, got out the oil catch pan and pulled the plug on the bottom of the SM420 tranny. Nasty, butterscotch colored stuff came pouring out. I assume it was water and gear oil. How does water get into this transmission? DPPO (that's Dumb Previous Previous Owner) used to tow his boat with this truck. He would have had to back it pretty far down the ramp to get water high enough to get into the tranny. And where would it get in? The water has been in there for a minimum of the 2+ months I've owned it and likely for the 2 years the DPO had it. How likely is the tranny to crater soon? Anyway, drained the old stuff and put in new. May be my imagination, but it sounds a little better.
mine was the same way - tan coloured goo. since mine was an old grain truck and spent it's life on the farm i'm at a loss as to how it became mocha but I think the old girl appreciated the oil change as she shifts great and is quiet.
Water in tranny Bill Its very likely that the former owner has launched his boat at a peer and the tranny has taken in water that way. Transmission oil is a type of oil wich is very hygroscopic , means it attracks water very easely. I`ll suggest that you clean out the tranny with plain parafin or diesel and check the top and side cover gaskets Good luck Martinius.
I hear top loader 4 speeds are bad about the windsheild leaking in the middle, the water going down the dash and onto the shifter, then down into the tranny.
Mine was like this too. After about 4 times of draining filling, driving, repeat. It was clean and clear. Not sure how the water got in, but its good now.
Condensation in the colder climates, the older tractor manuals discuss this problem and mandated the fluids be checked at regular times. That may be the reason the manual for the G talks about checking every 10K miles.
Tranny Lube What Russ said ~ As mentioned , this simply means you're going to have to drive it Bill , then when it's nice and hot , drain it and allow to drip an hour of three before re fitting the drain plug and re filling it . I assume you've already bought a 1/2" NPT thread MAGNETIC drain plug..... Even if it's bad from the water , these trannies are dead simple to overhaul , they're just heavy buggers and will bite or cripple you if they get the chance , remember that . Is it a Muncie SM420 top loader 4 Sp. or a Saginaw 3 Sp. side loader ? .
It's a 420. I drove it about 15 miles in 80 degree weather before draining. I left the plug open for 2 hours.
my personal "rule of thumb" Whenever I buy a vehicle, I always assume the previous owner didn't do ANY maintenance besides oil changes. Tranny and rear diff fluid change, coolant flush/change and brake fluid change are a bunch of relatively inexpensive things that most certainly were NOT done by the seller. Once they are all done, you now have a baseline to work with. While this applies to modern iron in my case (as I have yet to pilot Penny under her own power), it is sound advice that applies to all iron, regardless of vintage.
Old sm420's My money says the tranny condensates moisture in the Gulf Coast climate we share. If the tranny has been driven into standing water, I suspect you would have found some water in the bottom of the case. The nice caramel goo is likely a sign of an old truck tranny with not much recent use or recent oil changes. After you refill the box, drive it a while and see if it whines or grinds. If it grinds when you shift, double clutch when shifting and it will likely shift quietly even with the wear. I drove one like that for a while. Good luck. Jim
Jim, it shifts nice and clean. It is real quiet in high gear, so I'm guessing the input and output bearings are in good shape. The spur gears (1st and reverse) are very noisy, but I'm comparing them to helical gears in more modern transmissions. 2nd and third aren't too bad.
Tranny Fun The good news is : Muncie 420 parts are cheap and easy to find , cheaper yet if you use used gears . FWIW : the milky god indicates the water and gearlube have mixed , not that it's been sitting . I too , always assume the DPO / DPM didn't do anything or did it wrong . Oil and grease remain the Cheap Mechanic .
More fun Bill If you need a 420, I can likely assist. This has been a really entertaining discussion. I can't wait until you open the pumpkin to see what lives in there. While you have the tranny apart, I can find you an Aggie sticker to install on the side cover, my compliments. Enjoy Jim
The whole rear axle (from late 60's GM car) was installed very recently by the shop in Humble that ripped off the DPO so bad. They actually did a good job on the rear end. I pulled the fill plug and stuck my finger in. Proper level and no milk. Don't need no Aggie stickers.
Bill, We did a 40 Pontiac coupe that had never been near a lake but had set in the garage of a high dollar home in Plano for 40 years. Here's what the tranny looked like when we popped it open. Have seen rear ends that everything above the lube is a ball of rust and everything submerged looks factory fresh. Don't have a clue.
The Answer : There has to be a confluence of ferrous metal , water and oxygen ~ the <magic> triangle ~ if the parts are submerged in oil , they won't rust . If they're submerged in water , they rust very slowly . If there's water half filling the tranny (see above) it all goes to heck fairly quickly . I've rebuilt a few 1940's English Moto gearboxes that were pretty bad , we got bearings and bushings but no gears were available so it was pretty noisy when finished .
