School Me On 2Spd Rear Ends!

Bryce Frazier · Oct 5, 2018

First of all, and friendly hello to all my YT Friends!!!

I've got a 1970 Chevy C50 dump truck, Vacuum powered 2spd rear end.

I lost High the other day, we started messing with it today, I have air vacuum at one of the lines going into the little shifter on the axle, when the switch inside cab is flipped, vacuum changes to the other line, all of this appears to be working properly as far as I can tell?

After a little research, seems like the common problem is a hole in the diaphram in the actuator, but I have no idea how to identify what I have, or where to even begin to find parts for it?

I've also read that an electric conversion is a good upgrade, which makes sense to me! But how do I know what I need, and where do I get such a thing? Thanks!

DeltaRed · Oct 5, 2018

Go talk to a shop that does heavy trucks.Or a truck salvage yard.Do you have a truck dealer close.Good to hear from you again.

rustred · Oct 5, 2018

the ones i used to work on in the 80's were electric, so that must be an early style. they had a worm gear in there that moved the shifter.

DeltaRed · Oct 5, 2018

My '68 C50 has a vacume shifter. I have driven several other chevys from the '70s that also had vacume shifters.

Jim in Sask · Oct 5, 2018

My old parts book says it's part number 3847058 for the diaphragm only (1965-1974 trucks). Used with 2-speed axle / Bendix 374451. I've had to replace the diaphragm on my '73 C60 a couple times over the years, simple to do. I think they went to mostly electric in the mid-70's Chevys/GMCs, my '79 was electric.

farmer656 · Oct 5, 2018

it can be changed over but you will need the whole shift tower, hopefully you can find one at a truck salvage yard, ebay has the shift motors and internal parts , would be best if you found complete unit motor and all, 2 bolts hold them on , I am in the process now of installing one

TGP · Oct 5, 2018

Vacuum 2 speed were the standard Fifties, sixties,seventies Chev/GMC with corporate rears.
Anything ordered with Eaton or Timken rears got electric.

Now in my opinion they were very dependable.

Sounds like you have narrowed it down to the unit itself.
When you shift the axle sitting there idling does the engine rev up?
If it does it is leaking. If not there is something else in there going on.
As mentioned you most likely have a ruptured diaphragm.
Find a shop or parts house for heavy trucks, they can help you get parts.
If you pull the diaphragm and it is indeed ruptured depending were the tear is you can apply a tire patch for a temporary fix.
Remember it's a GM Corporate diff. in your 70 C50
Should help you narrow down parts.
Tom

rustred · Oct 5, 2018

i was just using the grain trucks today and one is 1972 and it has electric shifter. the other one is 86 so definitly electric was standard in the 80's.

ArleninOr · Oct 5, 2018

Bryce, I have the unit off of the rearend if that helps. Was working the last time I drove the truck.

MtMan · Oct 5, 2018

Bryce, good to hear from you as you have been missed for a while. Years ago I drove a few trucks with vacuum shift. One very important thing to remember is have a good parking brake or block wheels on a hill. Because if your vacuum leaks off your truck will roll off even if in gear.

mschwartz · Oct 5, 2018

Hello Bryce I will look in the morning and see what I might have down here, have a parts truck but I am not sure if it has gm or Eaton rear axle, Mike

used red MN · Oct 6, 2018

From my experience that rolling off thing is not true of the vacuum two speed in the vintage of Chevy truck that you have. As you have confirmed it takes vacuum to shift to hi or lo as noted by your test of the lines and the vacuum swapping from one to the other. I have been around a handful of Chevy trucks with vacuum two speed axles from the sixties and never heard of this being an issue. This may have been true of an axle with a different design where vacuum made the shift to hi or lo, but a spring returned it to the opposite speed. As far as having a good emergency brake if there was a disconnect in the rear axle it would not help anyway because the emergency brake on those trucks is on the back of the transmission and locks the driveshaft from turning. Wheel chocks would work in any case.

MarkB_MI · Oct 6, 2018

Bryce, those units have a spring to hold the shifter in low gear when parked, so when the diaphragm goes they won't shift into high. (NEVER park in high gear!)

The vacuum shift is simple and reliable, so I'd try to fix it rather than convert to electric. Of course if you can't get parts you can't fix it, so that might be your only alternative. I wouldn't waste my time trying to find a used shifter; replacing a fifty-year-old vacuum shifter with another fifty-year-old shifter probably won't turn out well.

A google search gives <a href="http://www.rebuildersenterprises.com/">http://www.rebuildersenterprises.com/</a> as a possible source for these parts.

MarkB_MI · Oct 6, 2018

> Because if your vacuum leaks off your truck will roll off even if in gear.

That should not happen if you park in low gear; there's a spring that holds the shifter in low. Park in high, though, and in the morning you'll find your truck at the bottom of the hill. This is also true of electric two-speed axles, BTW.

sms · Oct 6, 2018

It's been a while since I had a truck with an electric two speed but at that time it seemed the new parts were not as reliable as the older parts and I was fixing the unit regularly. As others have said the vacuum is simple, I would try to fix it.

caterpillar guy · Oct 6, 2018

Unless you are going to drive it long distances or lots of short distance mileage you can still use it with just the low side. It will use more fuel/gas but not hurt the truck unless you try shifting it and have it only half engage.

Bryce Frazier · Oct 6, 2018

You actually pointed out something interesting I forgot to mention!

When in low, truck idled normal, high, it would speed up the idle quite a bit.

This was happening for about a week, and I missed a low to high shift 4 or 5 times, like 4 lo to 4 high, I would be in a kind of netural, go back to 4 lo, speed back up, try 4 high again, and it would go.

I was leaving the gravel pit the other day, and it just refused to shift to 4 high from that point on.

Trkr · Oct 6, 2018

Used to drive an old CO 4000 IH in the 60's,had a Fuller 3 speed rear,air shifted.

MarkB_MI · Oct 6, 2018

> When in low, truck idled normal, high, it would speed up the idle quite a bit.

Yes, it pulls vacuum to shift into high, but relies on the spring to go into low. A ruptured diaphragm will act exactly as you describe. It's probably been leaking for some time but finally blew out completely.

TGP · Oct 6, 2018

(quoted from post at 10:30:46 10/06/18) > When in low, truck idled normal, high, it would speed up the idle quite a bit.

Yes, it pulls vacuum to shift into high, but relies on the spring to go into low. A ruptured diaphragm will act exactly as you describe. It's probably been leaking for some time but finally blew out completely.

Ditto!
What Mark said.
Easier to fix than convert to electric.
Tom

OregonAaron · Oct 6, 2018

I use a vacuum testing tool called mightyvac. It's a hand help vac pump with a gauge. It's a great tool for diagnosing vacuum problems such as leaks and torn diaphragm. They are not expensive. Give one a whirl!

hd6gtom · Oct 6, 2018

I have replaced those diaphragms before, always got them from Chevy. Make sure they don't give you a diaphragm for an air brake pot.

School Me On 2Spd Rear Ends!

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