[AT] Vacuum controls and other rolling abortions

[Alan Nadeau]

I've got a '71 GMC 5500 that has a vacuum rear end.   Works just like an 
electric one until the diaphragm gets a hole in it.  When that happens it 
will hang between gears and when the hole gets too big it defaults to low 
range.  Never gave a thought to parking it in low range but I can see where 
it could pop out if high as the vacuum bled off.  This one is geared perfect 
for our hilly country.  Low range is somewhere in the 8.8-1 range.  Power is 
a 350 V-8 with a newer Holley 600 carb which is plenty with those gears. 
The  truck GVW is 19,700 and I've had it grossing 24,000 over the quarry 
scales.  At that weight you can start the load without touching the 
throttle.

I saw a much earlier truck where the 2-spd was controlled by a rotary vale 
on the dash.  Can't imagine trying to split shift that one.

Even more odd was a truck we had on the farm.  Cabover 1600 IHC with a 5 
speed column shift and electric 2 spd rear.  The linkage was worn ad you 
learned quickly not to wrap your hand around the knob when going for 4th as 
the knob would ram into the dash, it had actually beaten a bit if a dent 
there.  To top it off you had to split your gears as the truck was terribly 
underpowered and the 2 spd switch was in the dash.  Made for some wild 
gyrations of the right hand driving that.

Vacuum heater controls can be just about as much fun as the wipers were. 
Co. I used to work for had a Jeep pickup, 1979,  I think it was, straight 
six/3 speed.  Not terribly underpowered but it took all the throttle there 
was to maintain speed up any sort of hill.  I got caught in freezing rain 
100 miles from home on the other side of the Green Mountains.  The grades 
took all the throttle there was, which left no vacuum to hold the defroster 
duct open.  I crawled around under the dash and could find the diaphragm and 
linkage and figured out what the problem was.  Finally stopped at a hardware 
store and bought a small pair of Vise-Grips.  I was able to pull the linkage 
out to move the control flap the right way, then clamp the pliers on so it 
couldn't retract.  Made it home with a clear windshield after that.



Chuck Bealke wrote:

This reminds me of another vacuum operated feature that might never pass
safety rules today.  Starting at about 14, I occasionally drove a long
'53 GMC 1 1/2 ton truck with a grain bed, sometimes topped by a cattle
rack.  It had a two-speed axle, which I think was vacuum actuated.  The
farmer I worked for, who owned the truck, gave me a serious instruction.
He warned me that if I left the axle in high speed and parked it in gear
facing up a slope without remembering to put the brake on, it would
eventually lose connection to the engine and start rolling backwards.
He had done it once to some kind of ill effect.  My rule was to ALWAYS
set the axle to low speed before parking and leaving it.  That way, if I
forgot to put the brake on, nothing would get bent.  Vacuum operated
windshield wipers were not so comforting either - 'specially when you
were climbing a long grade on long winding road with oncoming traffic.

Chuck Bealke