The Stovebolt Site Forums The Shop Area Driveline Two speed rear

This may have been asked and answered, but I don't know. I did a search and didn't find anything, but I'm not exactly sure how to word a search for this question, so here goes. 1948 Chevy 6400, has the two speed rear end. The high/low shifting is vacuum operated by a lever on the dash. My question is, can this system by converted to an electric switch, instead of vacuum? If so, would it be possible to take the switch off the dash and replace it with a more conventional switch clamped to the gearshift lever? Thanks for any help you can give me. Trucks of this vintage and size are kind of new to me.

Not sure why you would want to, not like you need to do much split shifting.

However, the answer is yes if you want to.
You would either replace the vacuum valve (on the frame under the cab) with an electric valve or rig a solonoid to switch the existing valve.
You would then install a small solonoid behind the dash to shift the 2 speed speedometer drive at the same time to keep the speedo in sync with the rear end.
Currently those functions are done by mechanical linkage from the under dash switch.

Are you aware the 'high/low' is actually low and lower? Unless you will be hauling some very heavy loads you will never need the 'low' setting.

I was not aware of that. Still learning about these trucks. Ill probably just leave it alone. Thanks for the replies

I started driving a 4 and 2 before I had a drivers license. And I drove a 5 and 2 in a semi for 400,000 miles. The vacuum shifts are nothing but trouble and they are not needed if you are not hauling a load. I just lock them in high range, which is really pretty low, and just shift the transmission. I figure I don't need any practice playing with a 2 speed. Eaton electric and air 2 speeds are easy to keep operational.

I put a lot of miles on an "Eaton 15 speed" which is actually a 5 speed main box coupled to a pair of 2-speed rear axles. By putting one axle in low range and the other one in high, an "intermediate" speed is created with the inter-axle differential making up the difference in ratio between the axles. Each gear in the main box is split into 3 ratios by manipulating the electric-shift rear axles with a 3-position switch on the shift knob. There's quite a learning curve involved in making that one shift smoothly.
Jerry

crenwelge (Kenneth),

"I just lock them in high range, . . "

That is what I intend to do with my 54/55 GMC 2-ton COE.

Am I correct in my understanding that the "default" range is "low", and I should mechanically lock the rear into "high" back at the rear-end (where the vacuum-switch unit makes the "move" selection)?

Thanks,

Yes, they default to low with a spring. Vacuum pulls them into high. I just stick something in the linkage at the axle to keep them in high. The factory recommendation is to park them in low range, but that is because the e-brakes were a joke and low range gives a little more engine braking power. Of course you are probably never going to park an over loaded truck on the side of a hill anyway. A chock block is the best parking brake you can get anyway. I just block off the entire vacuum circuit to the two speed. That eliminates a lot of vacuum leak problem and trying to fix stuff for which parts are no longer available.

Jerry, my Emeryville was a 5 and 3. The Eatons were air shift and the suspension was a Hendrickson rubber pad. I could out pull trucks in my class that had Roadrangers because I always had just the right gear. But using intermediate sure took its toll on power dividers. In fact I usually carried a spare. There were even times I had to make thrust washers for the planetary gears out of beer cans. The planetary gears would eat into the housing to where they would jump the sun gear on a hard pull. A naturally aspirated Cummins with a back drop manifold sure sounded pretty with a 5 and 3 behind it. The contacts on the shift know would carbon over from shifting so much. I could take the shift knob apart while I was driving and file the contacts with a match box. I with I would have had a recorder back in those days to record the sound and photograph the flame coming out of my stack. I drove that thing daily for about 10 years.

The truck I learned to drive on was a B-73 Mack with a 262 Cummins and a Tri-Plex 5X3 transmission. It carried a 1600 gallon water tank and we contracted it out for dust control on big construction sites in California. When I moved up to a 10 speed Roadranger in a 64 freightliner cabover with a 318 Detroit, I didn't know how to act with only one shifter to play with!
Jerry

I am searching for a higher ratio rear for my '54 GMC, since 2 speeds default to low, how do I select high, manually on a non-running truck to check the high ratio?

Thanks,

Ed

"Yes, they default to low with a spring. Vacuum pulls them into high. I just stick something in the linkage at the axle to keep them in high."

I believe the 48-52 Chevy two speed axles(as I recall made by Timken) do not default to low. On my '50 COE there are two vacuum hoses going to the rear axle, and through the action of the diaphragm, pull the axle into high or low depending on the switch selection. My '47 Diamond T has a single vacuum hose going to the rear axle (as I recall made by Eaton)and it does default to low.

Warning: If you have such a rear axle that defaults to low, AND a drum type transmission mounted parking brake, DO NOT leave it in high gear when parked and you are depending on the parking brake to keep the truck in position. As the vacuum bleeds down, the rear axle will find a "neutral position" and your truck may roll away!

I drive my two larger trucks at 6250 feet elevation and up and down low mountain hills. My engineer friends and family tell me that you lose 3% of your horsepower for every 1000 feet elevation. My COE weighs 7500 lbs and is powered by a later 235 (1958) and my Diamond T weighs 6500 lbs and is powered by the stock Hercules JXC 282 cubic inch that is supposed to get 100 horses. Needless to say, I need all the gears available and enjoy having the two speed rear axles, even though they are truly low and lower. Especially the night I was the designated driver and went at 2:30 am to Whisky Row with the COE to pick up my two sons and 25 of their friends. I had a thousand foot climb over 5 miles back home. So I would try to maintain the function of your two speed axles if possible.
Kent

Kent,

I only know about the 2-speed mechanism in my 54/55 GMC 2-ton truck.

It defaults to low speed when there is no vacuum. I was told this was logical, in case you lost vacuum with a heavy load - you could still get a heavily loaded truck rolling?

I do not see a clear statement of the "default" speed (low or high ratio) for 48-53 Chevrolet trucks, but the Chevrolet 1948-1951 Shop Manual (which applies to 52/53) seems to indicate in the diagnostics that loss of vacuum results in low gear/range? ( bottom left of this "Troubles and Remedies" diagnostics page )

I don't know why GM used the vacuum 2 speed in their 2 tons. The electric 2 speed used by 450 and bigger GMC and Ford and International was so much better. Vacuum leaks were always a problem. Either you would get stuck in low with not enough vacuum to get to high or if you were in high and lost vacuum, it would try to go to low the first time you let off the gas. If you didn't give it gas to complete the shift it will grind and grab and sound like the whole rear is torn up.

Down shifts are completed by the drive shaft speeding up. A dead truck can be shifted into low by putting transmission in neutral and turning the drive shaft until she ratchets into low. Up shifts are completed by the drive shaft slowing down. This can be completed in a dead truck by jacking up one rear wheel and turning it until it ratchets into high range. There were several ratios available for the the 1350 axles. The ratios were pretty good but the vacuum shift was crap. 5.14/7.15,5.83/810, 6.33/8.81. 1954 calls it a 13600 with either 6.33/8.81 or 5.83/8.10

Tim, you know I am of the generation that tries to fix it and then reads the manual if we can't fix it. I think it is saying that a leaky diaphragm leaves you stuck in low. Back when these things were pretty new, we usually checked to see if we were getting vacuum to the chamber first because chambers usually weren't old enough to fail yet. Now after 60 years, very few chambers still work.

Tim,
Does your 2 speed axle have one or two vacuum lines going to it? I have disassembled the vacuum shifter on my COE with 2 lines and I don't believe it defaults to low, at least I have never experienced that while working on the shift mechanism or driving it. I assumed that the two lines going to separate chambers on each side of the diaphragm were to use vacuum to pull it into the position you want with no spring needed.

As I stated above, my Diamond T has only a single vacuum line to the shifter on the axle and I have experienced it defaulting to low (or neutral) several times when leaving it in high range and then shutting down the engine.

Maybe someone with more specific knowledge about these particular axles can chime in to clarify this for me. I guess I could test the COE by leaving it parked in high range and jacking up the rear and count drive shaft/wheel revolutions to see what gear it is in.
Kent

Kent,

My vacuum unit (54/55 GMC) has one vacuum line going to it. It is not like yours - if yours looks like this.

Sorry for adding confusion to this thread.

Tim,
Yes, that is what I have on the COE.

My only experience with two speed rear axles is my two trucks, neither of which are ridden hard now. My COE had 90K+ miles on it when I got it and the vacuum control still worked. The Diamond T only has 23K on it and never had a problem. For a professional, like Crenwelge, I suspect reliability was a problem and I respect his opinion. For a hobbiest or light use, my previous comments are to encourage the owner to maintain the system, if possible, as it is useful to have available the gears the engineers originally designed.
Kent

Kent,

The most I will carry on my COE bed is my Suburban or pickup - less than 50% of the rated capacity of the 2-ton. I think I'd get by with just the high ratio gears.

I also have a five-speed with over-drive fifth-gear transmission. I bet (I hope) it will not require the lower-ratios of the 2-speed rear.

Trade journals such as Heavy Duty Trucking, which has been around longer than I have, has always had articles about operating trucks efficiently. Back in the 50's and 60's, when 2 speeds were common, there were many articles written suggesting not to split shift a 2 speed on an empty truck to save fuel and to save wear and tear on the drive line. Two speeds are much harder to shift smoothly on an empty truck than a loaded one. We had 2000 four compartment steel tanks with a pump and meter on our bob tails in those days. They probably weighed empty what Tim's vehicle will weigh with a pickup or suburban. We never split shifted them when they were empty. It was a sign of a kid just learning to drive by splitting the gears. I was guilty of that my self when I was 15 or so. By the time I was 16 and got my commercial license on my birthday, I was driving like the experienced drivers. My first semi was a 550 GMC with a 5 and 2. After I put a Neway drag axle, I weighed about 24,000 with an empty trailer. I usually only split 4th and 5th gear when I was empty.