The Stovebolt Site Forums The Parking Lot Big Bolts Two speed Rear ends shifting

My owners manual says declutch to shift the diff.,but it also says the truck has manual linkage,which it doesn't. My new decal that shows my vacum shifting switch says don't declutch, turn switch lift on the gas petal count one two and depress gas pedal. It shifts a lot smoother since I have the right info. Are COE's the only one that have vacum shifting? What's the deal?

General Motors used vacuum 2 speeds into the 60's on their trucks around 2 ton. Larger ones used electric shift. Most other manufacturers used electric on everything until air shift became popular. However, the technique for shifting vacuum, electric or air is all the same.

It takes a little practise but you can shift beter without the clutch. There is a strong spring in there. When you move the selector it winds up the spring but it doesn't shift till you let up on the gas to release the pressure.

In the 46 era they must have had manual or vacum shifting. From the owners manual the manual linkage said you use the clutch but on the decal showing the vacum switch it says don't declutch. Are the vacum shifting valves only on COE's?

I don't know about 46 specific, but vacuum was not COE specific. Vacuum was used in conventional cabs into the 60's. I have never driven one of the old manual linkage 2 speeds, but I would suppose you might need to use the clutch. However, with the vacuum, electric, and air shifting without the clutch is necessary to keep from grinding the gears and actually damaging the rear end.

My 2-Speed Electric Shift Trucks always said to use the Clutch???

I got in the habit of using the Clutch when I was young and always shifted them that way. But I would always shift First and then hit the button down just before I let the clutch back out. It shifted so smooth and in rythem that some thought the truck had a 10-Speed Road Ranger.

My 42 (now owned by JLucas) had a mechanical linkage, which had a cable that went from the linkage rod to the rear end. The cable was VERY stiff and hard to shift. One day I could not get the cable to move at all so I rigged up a solid linkage, which worked great. I used the clutch to shift the two speed on that truck. BTW it has a Timken double reduction axle.

Most later trucks I have driven had either electric or vacuum two speeds. I would use the clutch when shifting the axle at low speeds, for instance, split shifting when hauling a heavy load of wheat out of a soft field. I would not use the clutch when shifting the axle at road speed.

Let's hear from someone who actually used/uses the 2-speed vacuum-actuated rear in this vintage of trucks (40s through mid-50s, GM trucks 2-tons or less).

51 ashton, where are you (he tried to teach me how to shift his 51 Chevy dump truck - I was not a good student)?

in my two 51's, a gmc and chevy both are vacuum. shift mechanisms are different and located in seperate locations. the shift chambers and the rears are different but they both work the same way. i live in mountanious country in eastern pa and i use my trucks, so knowing how to shift up and down is a must if your ever going to get somewhere. what i have found is that by tapping the clutch while upshifting the rear will make it a smoother shift. when downshifting the rear while going up a hill i find that just before hitting the lug stage is to shift the lever and take my foot off the pedal and press it back to the floor. during that brief period it shifts. timing has a lot to do with it. for instance my fire truck weighs the same all the time 11,000 lbs. the shift point are always the same. with the dump they are not depending how much weight im carrying at the time. maybe the difference is that one has a 270 and the other one has a 235 but thats how i work them and how they work for me with the least amount of blown shifts while downshifting.

for splitshifting then you need to use the clutch because your changing the transmission and the rear takes care of itself.

i dont advocate trying to downshift the rear while your going down a hill either like you would the transmission. that is unless you want a new experience in how to make strange noises from you truck

liar liar your pants and your hair is on fire

let the teacher be the judge of that. if i had to grade you i would have given you a grade of lets say a "B" which where i come from (even though i never got one) is a passing grade. the next lesson will be more difficult. nest time we wont be on the flat. more like at my house instead

You're an easy grader, Mike.

I've been too lazy to lift my left leg since I started driving over 50 years ago. In an upshift I let off of the gas slowly until I feel the shift has been completed. On a downshift I let off the gas a little and then slowly throttle up easily until I feel the shift has been completed. As underpowered as the trucks were 50 years ago and as bad as the roads were, I got lots of practice. When I first started, I tapped the clutch a little right about the time the shift was completing, but I soon found that unnecessary. I'm still lazy. I always specify manual transmissions when I go to Europe. I only use the clutch when I stop and I probably scrape less gears than people who use the clutch.

My 46 owners manual that decribes mechanical linkage says declutch , but Mongos Garage's factory reproduction decal that shows the exact vacum switch for shifting like I have, said (no need to declutch) which does make it shift a lot smoother on my 46.

Back in the 60s I worked in a Truck leasing shop. We had mostly IHC with the electric 2 speed drive. You could split shift them through the gears & keep the RPM in the torque range. I alwaus thought the electric worked smoother then the vacuum.