Hydramatic Heaven

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majortom

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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 I have 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 by accident 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 around the 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 go down the road, all have the original shift as an automatic up & 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 & Organizational Maintenance Manual is mandatory. It includes two sections that are a wealth of information. Page 163 has a diagnosis guide. I have found this to be very 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 an illustration, 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 to reach 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 & 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 have said, 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 and we have grossly overloaded them lots of times. Good fluid, proper adjustments, correctly working distributor & carb and correct driving technique can dramatically extend 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 just talked 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.
 

emr

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Awesome info, i for one have absoulutly NO experience with the GMC Auto's, BUT have followed there use thru the Hobby for years, one of the best looking trucks ever built,ALL of these vehicles have best ways to drive them, most are really common sense, mixed with experience, I truly believe the easy way out for the uninformed is the typical, "design flaw" But have to say in "my opinion" 90% of the time its driver error, lack of maintanence , lack of ability, and not an intentional lack, ive lived it too, I dont believe by a long shot a first time MV owner should get one. I do think those who do and have em running, are showing the type of hobbier they are. "experienced" and deserve respect, these trucks need to be used and refurbished, they are important to the hobby," in my eyes",. I recently met a guy who bought just a pristine beuty of a GMC Mseries, and after one season sold it and dropped the hobby, sad, he was very missinformed, oh well, I think this site is really the place to be, great post!!... Randy
 

hippiedude

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Thankyou so much for your imput on this subjeck....It seems that you here more Con`s about thes tranies than pro`s,,,,I would think these trucks would have to be cappable trucks for the militay to have used them.......Be sides I love my 1952XM211.........hehe thanks Tim
 

gwalker

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I was always under the impression M-211's were 2 speed tranny's , although I couldnt figure how that was possible.
I was also under the impression these were light duty truck trannys in size and capacity? What is the orign of that tranny model,was it unique to the military?
 

papabear

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Thanks for that post major tom. My 211 shifts just fine when she wants to. Recently, she's started shifting fine and suddenly she'll shift down to about 3rd and even though I've got my foot to the floor - she won't upshift.
If I stop and take off again she shifts fine again - until she acts up again.
 

M215

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Hydramatic questions

Majortom, great information on these trucks,
Maybe you can help with a problem that I have been trying to work out on my GMC. My tranny is set up with a REB shifter, when cold the tranny will slip in 1st and 2nd in high range, does not slip in low range. Once the tranny is warm, no slipping and will shift fine all day. Even if I let the truck sit for hours and cool down, wont slip. Usually I'll start the truck, put the transfer in neutral and shift the tranny in gear, I'll let it run at a fast idle for approx 10 minutes. After that it shifts fine without slipping.
Changed the fluid from dexron to 30 SAE oil, this reduced the cold tranny slipping some what. Pressure tested fine, same hot or cold, adjusted the bands and shift linkage, no throttle linkage because of the REB shifter.

Any ideas?
Thanks Karl
 

stephenfeldmeier

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I’m glad you informed us of your experience with all the trucks you owned so I have a question or two.

Since you have so many miles on these transmissions, you stated you can get 32,000 miles out of a transmission. What in the transmission wears out (seals, clutch plates or pumps) and can you rebuild them to new after words? Or is the transmission just shot after that many miles?

All my experience with rebuilding these only have 3,000 to 5,000 mile on them. All the parts I have encountered were broken parts. I have never seen any wear in them. So if you can give a summery of items you had to change due to wear.

Also I like the info you gave about the oil. Nothing is better then a long running history like you have with all these trucks. Your info and experience probably rewrote the manual for the proper oil to use.


Thanks for sharing all this information with use.

Steve
 

majortom

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karl the tm p164 lists your problem and cause read k your rear band was most likly misadjusted classic high range cold slip drop the pan replace rear servo adjust band flush trany band or clutch filings will kill front pump if this is a me rebuild they very greatly in quality
 

majortom

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steve a proper trany that is tired from high miles will need a front pump bands clutches and seals ihave redone one trany three times 116k total miles then it was all junk low mile broken tranys are usualy from abuse they dont like shifting ranges or into rev when moving bad things happen
 

DDoyle

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Re: RE: Hydramatic Heaven

gwalker said:
I was always under the impression M-211's were 2 speed tranny's , although I couldnt figure how that was possible.
I was also under the impression these were light duty truck trannys in size and capacity? What is the orign of that tranny model,was it unique to the military?
The 302 and 303 transmissions are military, and in addition to the G-749 trucks were used in a number of armored vehicles.

Regards,
David Doyle
 

JasonS

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I had always heard that the internals of the military hydramatic (rear reduction unit excluded) were the same as the standard gmc civilian truck. I had also heard that Cadillac internals could and have been used for rebuilding. Not true?

My wife's uncle tells me that the hydramatic was used in dragsters and was quite strong.
 

M215

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Hydramatic

JasonS,
One of my parts tranny came out of an old Cadillac (40s vintage?) and I have used some parts for my GMC. The Caddy tranny does not have the two speed reduction unit of couse. I don't know how much of the interal parts are interchangeable.
Karl
 

stephenfeldmeier

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hippiedude said:
Thankyou so much for your imput on this subjeck....It seems that you here more Con`s about thes tranies than pro`s,,,,I would think these trucks would have to be cappable trucks for the militay to have used them.......Be sides I love my 1952XM211.........hehe thanks Tim
Actually, I get the same information when I read all these responses to these threads. But for 8 years with the fire department and for all the maintenance we did on various types vehicles from the multi fuels to the gas Reo's, the M211 needed no maintenance. Just routine oil changes, all other vehicles, required engine changes, thrown rods, transmission changes, and gear box changes. The M211's were abused but kept on going. The auction truck I bought came from the same fire department I was with but when it was time to send it to auction, the people who were driving it to the GL yard, put the REB shifter in 1st gear while going 35 mph and blew out the front clutch pack. (They thought it was funny)

This is the same transmission I'm rebuilding now, and it still looks like brand new inside as you will see when I finish the videos on it. A new drum and slip ring is all it needed.

These truck are tougher then what we read.

Me for one, when I first got my M211, I was afraid to read anything about transmission failures because I did not know how how they worked or how to fix them. At one time I was ready to drive the truck back where I found it and let it rot with the rest of them because my confidence level was a negative 10.

Once I learned a thing or two about them, the fear is gone and my confidence in these transmissions is that they are indestructible if driven with any common sense. In other words you almost have to try to break them.

So anyone who owns one or thinking of getting one, enjoy your truck and don't let the fear factor get to you while reading all these responses.
90% of the problems people write about is curable by a proper adjustment of the transmission linkage, front band, or transfer case.

I'm with you Tim, I also love my truck, by the way I have a 1952XM211, mine was built in Dec.
 

badgmc56

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My thoughts are with you guys.I wouldn't trade my GMC for anything.Not to say I wouldn't want a wrecker or a diesel deuce but my truck is a keeper.In the coming years I feel these GMCs will go up in value quite a bit because of the rarity of well preserved trucks. So keep them if you have them. I went to a MV rally last year and there wasn't one GMC there.Hopefully mine will be there this year. The transmission deal doesn't bother me at all. I have seen this kind of thing in the repair business. Rumors start and then multiply.Most of the time it's the loose nut behind the wheel that is the problem. Take the advise of Steve and Majortom and these trucks will last a long time and run with the best of them.
 

butch atkins

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dont know how i missed this thread :oops: ,extremely good info here,just reading this today,maybe AM GENERAL will see this also,been on since 04 ,still dont know how i missed this,thanks majortom,good luck to all on your projects
 
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