The Stovebolt Site Forums The Parking Lot Big Bolts Two Speed Diff Options

Has anyone tried replacing the vacuum shifter on the rear differentials with an electric solenoid? Is it worth attempting? How reliable are the vacuum shifters and the control valves?

What year/model are you asking about?

Mike B

I have no issues with the vacuum shift on my 58 2-ton. I have been driving it for 2 1/2 years.

Its a 48

I'm just worried that the lines will inevitably start to leak.

They’re steel lines and short rubber hose. You can replace either if necessary

Mines been great for over a year and a half. And that's with a temp repair in the middle. When I was attempting to diagnose it i found a rather large leaking area, fixed it with wrap and it's been good. A permanent repair is coming this winter I hope. Shouldn't be hard. 1/2" line from along frame from front to rear flex lines. It works so seamless and smooth I wouldn't change a thing. Like anything a little maintenance and it's good.

When I restored my truck I don't even think I had to replace the steel lines. I cleaned them up and just replaced the rubber hoses. I also replaced the diaphragm on the rear. I was lucky at the time to find one sitting on a shelf, NOS. Anyway, although the truck doesn't see lot and lots of miles every year, I have had no issues with my two speed rear since I got it on the road in 2003.

Yeah, the only issue with Marlow's truck is with the Steering Wheel Actuator .... Needs a complete rework ...



FWIW, as long as you are messing with it anyway, remove the rubber diaphragm and give it a good soak in some silicon (dot 5 brake fluid worked for me) to rejuvenate it a little.

DOT 5 (silicone) brake fluid has at least one serious disadvantage - paint contamination (be careful - be forewarned).

No denying that JM🤪

Having used both types of shifters pretty extensively, the vacuum shift is far superior in my opinion. The shifts are smoother and there's a lot less to go wrong. When I drove a Freightliner with electric-shift rear ends, I always carried at least one spare electric actuator to assure I wouldn't get stranded somewhere waiting for parts. The motors, and especially the shift switches were really bad about failing with no warning.
Jerry

Vacuum shifters exist in some yards.

Ed

Thanks for all the advice guys. I'll tear mine apart and try to fix it up. It will need all new lines and I don't know if the switch/valves are good yet.

If the truck doesn't have one, it would be a good idea to add a vacuum reservoir tank. With a one way check valve between the engine and the reserve tank you'll have a couple of brake applications available, and/or smoother rear axle shifts. I like to use a recycled Freon gas tank. Most refrigeration shops will give you one (or several) since empties are usually discarded, not refilled. With some flat steel strap mountings, one or two of them fit very nicely inside a big truck frame rail.
Jerry

Can you post some photos of this setup?

It's been 30-something years since I was involved in medium and heavy truck work. It's a pretty simple task to mount a reservoir tank or two inside a frame rail and run a vacuum line from the intake manifold to it. Then attach lines going to the brake booster and/or the axle shift system to the tank. Since the vacuum is trapped in the tank by the check valve, there's a reserve of vacuum available even if the engine stops. It's simple common sense that any plumber could figure out. On smaller trucks like a pickup or a 1 ton, a 3 foot length of 4" PVC drain pipe mounted under the cab with a couple of end caps glued on accomplishes the same sort of vacuum reserve.
Jerry

Hello everyone, I have the ashton 14 years and have had no problems with it's vac. system. because of where I live I have to use it right out of the stovebolt firehouse station 51. I agree with hotrod that the vacuum is smoother shifting. Shifting a vacuum shifter can be challenging at times. Like Billy I found a nos unit in the box some years ago and sucked it up just in case. Having a 51 GMC 350 and a 51 Chevy 6100 the gmc has the better setup in the vacuum valve and its mounted on the firewall. Chevy uses a linkage rod to go down to the inside of the frame rail. When I got the dump and once one humid summer since, it froze up and I couldn't actuate the shifter when I went to make a shift. Kroil and working the shifter slowly back and forth freed it up to the point of smooth shifts.

51 Ashton, your Chev's is mounted on the frame rail? Mine is one the firewall like you note for the GMC, and my 48 to 51 Canadian Maintenance Manual shows it that way for both. Maybe another US / Canadian difference?