Switch Pitch with a 3.21

[bc...@juno.com]

Thanks for describing your experience. It might be concluded Switch
Pitch is not best for hilly country. I found no specific weaknesses in
more than a half million miles, but that was in 4100 lb vehicles. With
a 2.41:1 axle, the Switch Pitch took a second off my 0-60 and slightly
improved my mileage. Did even better behind a 5.7L diesel.

I would point out that the gear shifting components, which failed for you,
are exactly the same for the SP or non SP trans. There were some
small upgrades to these over the years, possibly all by 1970. But all
of these may be had by using the SP pump and chain set with a later
transmission, which is my usual method. The bands used for engine
braking are the same.

CASPRO and others stated that the SP in a GMC must be used with
a trans temp gauge. Otherwise on a long pull in high stall, the trans
can be overheated, a disasterous state. Watching this and often
switching to low stall might be a trial in hilly country. There are gauges
with electrical outputs at set points, that could do this automatically.
Bruce Roe
+++++++++++++++++++++++++++++++++++++++++++++++++++++
TH425 Trans
From: "gbec...@graestone.org" <gbec...@graestone.org>
To:EFI GoogleList <gmcm...@googlegroups.com> 16 Jun 24

I had a switch pitch with a 3.21. Nice setup, but we are in hilly country
and after 3rd gear clutch problems 3 times, I put a guage on it. Each
one made about 4K miles.

Driving 65mph, let off the gas, pressure went 55lbs. (Edge of low.) The
failures were always going down hill. I would hear a change at say 65,
look at the tach and it was at idle. Of course, that burnt things. Then on
flat land it would slip out of third gear into a 2nd gear ratio. However if I
let off the gas, the engine would go to idle. Freewheeling sorta like the
old overdrives.

After new internals, new torque converter, and a new case, I abandoned
the Switch Pitch. My current transmission presseure never goes below
82 psi when idling or deacclerating.

Manny uses switch pitch in his diesels, but they have no vacuum
braking so there is no pressure on the transmission when the pressure
drops. Dont know what is happening internally, but the SP transmissions
were not designed to slow down a 11K vehicle.

[gbec...@graestone.org]

Correct, the internals are the same. My theory is that when the pressure was down to 55lbs at 60 under full vacuum engine braking, the clutches and bands weren’t holding. It didn’t happen the first long downhill but I rarely made more than 4K on each transmission. I never shifted down when going down hill. 

Manny: You have disk brakes don’t you?

Me: Yes

Manny: Then Use Them!

I’m pretty sure Manny isn’t putting SP in gas GMCs. 

I’m a pretty easy driver, and never held in switch for long. It was handy occasionally when almost cresting a hill ad had to decide to change to 2nd or switch. I also liked the low stall when not in switch.  

On Jun 17, 2024, at 6:58 AM, bc...@juno.com wrote:

I would point out that the gear shifting components, which failed for you, 
are exactly the same for the SP or non SP trans.  There were some 
small upgrades to these over the years, possibly all by 1970.  But all 
of these may be had by using the SP pump and chain set with a later 
transmission, which is my usual method.  The bands used for engine 
braking are the same.  

---------

Best Wishes,

George
gbec...@graestone.org