Hydramatic Heaven

(care & feeding of the GMC automatic transmission)

I have owned hundreds of GMC deuces, have rebuilt many transmissions and am currently using several trucks. Transmission failure issues seem to be a regular topic.
I have found that how you operate the transmission and diagnosis of symptoms are critical. Driving, the transmission prefers foot planted firmly to the floor until you
reach the speed you want. Typical transmission life seems to be right around 32000 miles. The transmission will actually wear out sooner if you typically accelerate at
partial throttle. The 2nd to 3rd shift will always be harsh as both the 1-2 and 3-4 clutch packs reverse on that shift. To get the most acceleration and power under load,
I put the shift lever in hilly, completely depress the accelerator, allow the engine to go to maximum rpms in first and second, release the gas pedal, push lever forward
to level, depress the gas pedal, the trans will shift into 3rd smoothly. After the shift has been completed, pull the lever back into hilly, run the engine to maximum rpms
and push the lever forward to level for the 4th gear shift.I have been doing this for over 30 years without damaging a transmission.

As far as the Memphis reb shifter, it allows you to accelerate the vehicle similar to what Ihave just described but you must use the reb shift to downshift.
There is no safety feature on that shifter to stop you from downshifting either intentionally or byaccident in too high an engine rpm. I have seen several engines
with thrown rods due to improper reb downshift. Our tractor is used primary to switch trailers aroundthe yard and we have equipped it with a reb shift.
It works very well for shuttling heavy loads at low speeds. Our fleet of dump trucks, cargos and other units that godown the road,
all have the original shift, as an automatic up and down shifting transmission is highly superior especially when hauling heavy loads.

As far as the variety of woes, improper shifting, slipping in reverse, etc., if you are seriously going to use one of these trucks, the TM 9-8024 Operation and
Organizational Maintenance Manual is mandatory(can be found at http://www.portrayal.com/manuals.html). It includes two sections that are a wealth of information.
Page 163 has a diagnosis guide. I have found this to bevery accurate as to the root of problems. Page 335 has a description of the function of the transmission.
Page 345 has the operation tests. On page 353 there is anillustration, item d, oil pressure plug, is where you install a gauge for test purposes. We use a gauge that
goes up to 300 pounds and has a flexible hose long enough toreach into the cab. The front pumps of these transmissions are a vein type and are very susceptible to
wear. This is the highest failure item and the root of most other failures. If the oil pressure is not up to spec, the transmission will wear out rapidly.

Here are the magic numbers. In any forward gear, you should never see less than 90 pounds pressure regardless of rpms or temperature. In reverse, never less than 180
pounds pressure. In other words, there is no justification for a transmission that does not hold 90 psi at idle when hot. When we are diagnosing a problem, the very first
thing we do is this oil pressure test. There is currently a discussion about a transmission that makes noise when engaging into reverse. Good transmissions will engage
instantly with no noise when you shift the lever into reverse. If the engine takes time to wind down and engage reverse, it is a good indication of low oil pressure.
Memphis has always told people not to use high reverse. There is no reason, whatsoever, to not use high reverse if the transmission has proper pressure and is in good
working order.

If your transmission has 1st, 2nd and reverse but will not shift into 3rd or acts like it is shifting into neutral, the 3-4 clutch pack is toast, usually due to low oil pressure.
If the transmission works properly in one range (high or low) but the truck will not move in the other range, the wavey release spring in the reduction unit has broken.
This is a very common failure. Transmission does not have to be removed to repair this problem. (By the way, the transmission works just fine without that part)
The most common reason for improper shifting, i.e. all shifts at too low rpms or too high rpms, is not transmission internal. By adjusting the throttle rod that goes from
the engine to the transmission you can change the shift point. Longer, shifts occur at higher rpms, shorter is lower rpms. The transmission will also shift at lower rpms if
it thinks that you do not have your foot all the way to the floor. I have seen many trucks, where the governor in the carb was not allowing the butterflies to fully open,
the vacuum lines from the distributor to the carburettor clog with rust from moisture and get holes worn in them from the cab rubbing on them and the distributor
fly weights get rusty. If your truck seems woefully slow, shifts happen at low rpms, or you've experienced exhaust manifold failures, check the ignition timing.
Make sure the distributor advances.

As to oil, for over 30 years we have used hy-tran which is the International Harvester hydraulic drive transmission oil. It will not deteriorate the early style clutches.
It has a higher sheer factor than Dexron and is more viscosity consistent than motor oil. This is by far the best choice for fluid.I have seen transmissions that have
failed for various reasons but the couple of items I just talked about seem to be where the highest failures come from. As I havesaid, the front pump is the weak item.
It was under-designed for the purpose. The rest of the transmission is actually pretty good. We have never babied our trucks andwe have grossly overloaded
them lots of times. Good fluid, proper adjustments, correctly working distributor and carb and correct driving technique can dramaticallyextend the life of the unit
to that magic 30000+ number. Of the hundreds of transmissions I've been in, I attribute the mass majority of failures to the few items I justtalked about.

The first thing anyone should do that owns a GMC is get the manual and read it.
Second is a transmission oil pressure test.
These are very good trucks, very durable transmissions but they are old. Transmission failures do occur.
I hope this dissertation helps some of you. The books may not agree with what I say, but this is what works for us.