The Stovebolt Site Forums The Parking Lot Big Bolts 1946 2 ton vacuum shift

Took my 46 for a ride today and developed a problem with the vacuum shift. When I go into high, it shifts fine, just sounds like air gushing through the valve, when I go back into low it’s seems fine. It did loose vacuum on the gage, and did at one point start idling rough. Any idea what it is, and are the valves able to be cleaned or rebuilt? Oh yes it is the one that is on the dash!Thanks

My '57 2 ton now has a vacuum shift 2 speed rear differential. The truck was originally a single speed but through the grace of one of our big bolt members I was able to purchase from him the parts needed to convert. The shift valve on mine was in pretty good shape internally, I disassembled it, cleaned and lightly lubed with silicone based lube so as not to destroy the rubber. I did replace the shift diaphragm in the differential actuator. When shifting on the road, I can't hear the air movement through the vacuum system, if the truck is sitting still with the engine not running, I can shift it and hear a little air through the exhaust filter. I added a vacuum reservoir so I do have the ability to shift it a couple of times with the engine not running.

I saw a post from one of the old topics on the vacuum shift saying you can use a unlit propane torch to check for vacuum leaks. While the engine is running, spray some propane around the lines and listen for the engine speed to increase.

I kind of visually check the lines while the truck was running, didn't really hear anything. It is really present inside the cab, maybe the valve on the dash, or check valve under the hood? It has a vacuum tank mounted on the side, but if the truck is not running, you are not able to shift or hear anything. Just curios if anyone experienced this problem, so I kind of know what I'm looking for.

The valve, check valve, and actuator diaphragm can all be disassembled and inspected, some cleaning and light lubrication goes a long way.

I will give it a try, Thanks!

I guess my previous post didn't go through. But letting the vacuum pull WD-40 into the shifting valve fixed mine, it's been working fine for years. Also that factory reproduction sticker showing the shifting speeds and which way is high or low is cool.

ok, i will try that too, thanks!

My valve is on the dash and a sealed unit. Mongo's Garage has the shifting stickers if you interested.