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optional tranny for M915 series?

maddawg308

maddawg308

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The M915/916/920 has a Cummins 400 for an engine, and a Cat 7155 semi-automatic transmission, which based on feedback from others, is quirky and isn't very reliable. Is there another drop-in replacement for the CAT 7155 tranny that is more robust, preferably something in a full automatic for road use?
 
jaxsof

jaxsof

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Allison makes some of the best automatic transmissions in the world for OTR use. That said, there are some companies making "Allison" transmissions umder license, and have had questionable service life.

The Cummins has an SAE Standard bellhousing, wherein any Industrial/heavy duty (big truck) reansmission can be attached, provided you can find the appropriate clutch housing.

(Bell housing, in HD engine speak is the part that is from the engine manufacturer, and is attached to the block. The clutch housing bolts to the transmission, and to the bell housing. We dont usually change these unles we really have to. Usually an expensive part.)

On our trucks, it is usually an SAE 2 or 3. Mine is a 2, guess the rest should be as well
SAE Bellhousing
 
wreckerman893

wreckerman893

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When I drove M916/915's in the Army I loved the CAT transmission....once you get the hang of "slap shifting" it you can move on out.

The issue in the civilian world is that most CAT dealers have little or no experience working on them because there were few civilian applications.

SS member Soni put an 18 speed behind a 600 HP Cummins in his highly modified M920.

The Allison is a good transmission but they are gearbound and you have to run wide open to get 60 MPH out of them.

I just bought another M915 with the Allison in it so I guess I'll have to get used to it.
 
R

R Racing

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I have heard of people trading out for a 13 speed manual. I love the cat 16 speed. its easy to use once you get use to the shift points and with the lower rears you can pull some serious weight. the older m915s had the cat trans. then in the early 80's they went to the allison. I would like to try 1 out someday and feel the difference.
 
eldgenb

eldgenb

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The allison in the 915 has a final drive ratio of 1:1 the cat is a .87:1 if memory serves, maybe .83, anyways if you swap a allison from a 915 in you will be a 50mph truck tops due to the much deeper rear end gearing. I love my Cat tranny when it loves me back, when if fights me I want to drive it off a cliff. my .02. I will say however that Jim (r racing) told me about the turbo charged air compressor mod and even though my truck is down right now I look forward to seeing how that affects the trucks ability to maintain full air tanks going down the road.
 
maddawg308

maddawg308

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Is there perhaps a commercial replacement for the 7155 that would give it the same speed, but in a 100% automatic package like the Allison?

Another question: with the 6x6 and 8x6 capability in the M916/M920, is the transfer case in each of these trucks divorced or non-divorced from the tranny? That might affect the replacement options of the transmission...
 
R

R Racing

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The transfer case is indeed divorced. I found a fully auto trans a while back that was the modern day replacement for the Cat 7155 It was also a cat and had the PTO output . It was fully automatic but I would have been scared to ask what there going for . I believed it may have also used a computer.
 
R

R Racing

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I may be wrong but If I remember correctly his came that way as a airforce truck ( they were special oredered that way) They go really high on GL . Think the last 1 I saw went for over 12K
 
eldgenb

eldgenb

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R Racing said:
I may be wrong but If I remember correctly his came that way as a airforce truck ( they were special oredered that way) They go really high on GL . Think the last 1 I saw went for over 12K
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Marine corp trucks, the conversion was done as a retrofit and also included an air ride cab which was ridiculous by design, when half the cab is rigid mount and the other is air, not real effective. They are an allison 7050 hd I believe, the model # is military specific and is not available for public purchase outside of surplus. Allison makes a ton of auto's that would work for an m920 application.

Transmission Specifications ? United Transmission Exchange is a Full Service Independent Facility with over thirty years of experience rebuilding Allison Transmissions
 
Nonotagain

Nonotagain

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maddawg308 said:
What model Allison is your tranny? Did it come stock in your truck or did you swap it in?
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Mike, I believe Justin's truck has the MD3070PT just like the new 5 ton trucks.

I purchased one via GL a while back with plans of running it in a M35 A3 or possibly a 923/925 series truck.

I purchased the trans computer for a grand and still need the shifter, another grand or so.
 
Ferroequinologist

Ferroequinologist

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Anyone put a manual in a M915? I've got a lead on one missing a transmission that I am thinking of getting and putting a 13speed in.

I think with the SAE bellhousing it would just be a simple parts scrounging exercise, right?
 
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