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Thread: 1978 931 1P-0571 Valve Body help needed

  1. #11
    Join Date
    Apr 2019
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    Mineola, TX
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    45

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    Quote Originally Posted by Rome K/G View Post
    Your positive the springs were good in the control valve and modulating relief valve?
    Yes, as positive as I can be in this perplexing situation. Replaced all three springs, replaced both check valves with OEM parts. Local Holt Cat mechanic of 30 years looked at the valve body parts, inspected the spool and sliding valve and said all looks as it should. He also inspected the transmission pump and said didn't see any wear or slack in it either. Said the parts looked fine.

    The new transmission pump I installed last week gave the same performance as the one the Cat mechanic said looked good to him.

    As of right now, I think it is in the valve body - but I have no way of determining that since Cat doesn't make the part any longer and the rebuilt ones on the interweb are in the $5,000 range. The Cat mechanic could not find anything wrong with the valve body, so I don't plan on spending that type of money on a part that probably isn't bad.

    I am stymied, but I appreciate all of your help and comments. It is nuts and bolts, so there is a solution - just have to keep searching.

  2. #12
    Join Date
    Apr 2019
    Location
    Mineola, TX
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    45

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    Quote Originally Posted by edb View Post
    Hi Team,
    the plug in the valve body you refer to would be removed to test what Cat calls "Initial Pressure" of the Direction Clutch Modulating Relief Valve (MRV)

    Initial pressure is when the Modulation Relief Valve load piston stays back in the relaxed position.

    This low pressure, usually around 40 PSI, is so the clutch engages softly and then as oil is fed in behind the MRV Load Piston the MRV spring is compressed at a fixed rate of pressure rise to its set maximum pressure.

    The MRV load piston oil is supplied thru an orifice in a floating check valve so it can dump the oil trapped behind the load piston quickly ready for the next clutch engagement where the process of modulated clutch pressure rise starts again.
    If the load piston or its check valve are sticky or there is crud floating in the system that can periodically block/restrict oil flow thru the check valve orifice or the load piston leaks more oil than can flow thru the orifice then the low pressure you see may occur--I stand to be corrected as it is some 35 + years since working on these things.

    From memory at least one erosion wear point to check is the valve body land that aligns to the rear face of the MRV load piston when it is in the compressed position as dirty oil will scour this land away and I guess if enough is scoured away then the load piston will not full travel to achieve full valve set pressure--a similar erosion takes place on the valve body relief land for the working end of the MRV. really need a Parts Book to be able to describe the position of these lands clearly, I do not have literature for these later machines.
    The grey sand blasted wear lines/tracks to the sharp edges of lands would be evident as you look in thru the ports on a dismantled valve group.
    We often had to discard such erosion worn bodies due to excessive leakage issues.

    Cheers,
    Eddie B.
    Eddie,

    Thank you for the explanation of how that floating check valve (slug) works and why. I have been looking at the schematic and trying to figure out why they designed it using that floating slug, now I know. I have looked pretty closely at the spool valve when I had it out multiple times, but I wasn't looking for what you described. I will look for that type of wear next time I have it out, which will probably be real soon. I have gotten real quick at disconnecting the fuel lines, taking out the screen, disconnecting the steering clutch lines and removing the valve body - I need to start challenging myself and doing it blindfolded to make it more fun!

    The one step in this process that always gives me the most pause is inserting and taking out the clip that holds the tube in place from the valve body to the steering clutch lines. I am worried about dropping it into the transmission each time, but so far I am batting 1000.

    Thanks for your help.

    Newt

  3. #13
    Join Date
    Nov 2006
    Location
    Corralitos, Ca.
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    14,958

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    Have you run a full set of pressure tests? and what did you get for results?

  4. #14
    Join Date
    Dec 2006
    Location
    victoria australia
    Posts
    3,165

    Default Rag and Vice Grip Pliers

    Hi NDW,
    maybe stuff rags around the clip area. I think I used to snap on a pair of small vice grip pliers onto the clip ring to save it shooting off too.
    I recall in at least one trans. valve group from which unit I do not recall now that if the check valve was incorrectly fitted (reversed) it caused issues with leakage and maybe it getting stuck on the end of a projecting stop plug--thinking aloud here.
    Cheers,
    Eddie B.

  5. #15
    Join Date
    Apr 2019
    Location
    Mineola, TX
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    45

    Default pressures

    Quote Originally Posted by Old Magnet View Post
    Have you run a full set of pressure tests? and what did you get for results?
    OM,

    I've only tested the steering clutch and transmission pump pressures, I have not tested the torque converter inlet pressure. I can test the TC pressure, and report back.

    Eddie, good idea about the rags, I'll start doing that.

    Newt

  6. #16
    Join Date
    Nov 2006
    Location
    Corralitos, Ca.
    Posts
    14,958

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    Check it all....no short cuts!!
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  7. #17
    Join Date
    Apr 2019
    Location
    Mineola, TX
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    45

    Default pressure test

    Quote Originally Posted by Old Magnet View Post
    Check it all....no short cuts!!
    OM,

    I will check it all out, I need some help thinking through the process and arriving at a plan of attack. Since the tractor loses pressure with time and I believe oil temperature, should I start the pressure tests on the different ports when the oil pressure has dropped? Or, should I do pressure tests on the ports when the oil pressure is high in the first 15 minutes of operation?

    What are your thoughts to get the best data to arrive at a decision on the culprit of the pressure drop?

    Thank you,

    Newt

  8. #18
    Join Date
    Nov 2006
    Location
    Faunsdale, AL USA
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    4,161

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    Initial cold pressure and then again when hot will give the most information.
    D2-5J's, D6-9U's, D318 and D333 power units, 12E-99E grader, 922B & 944A wheel loaders, D330C generator set, DW20 water tanker and a bunch of Jersey cows to take care of in my spare time

  9. #19
    Join Date
    Nov 2006
    Location
    Corralitos, Ca.
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    14,958

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    Well,we already know your losing high pressure so the lower pressure checks would be more meaningful, particularly at the torque converter and lube pressure which is the final flow item of the oil circuit. Looking at a suspect torque converter leak seeing as how there doesn't seem to be a problem at the transmission clutches. This based on your description that your not getting below 200 psi or getting close to other pressure requirements.
    Also do you suspect there is a problem with the steering clutch controls? That's another source for robbing system pressure. If this is suspect a test would be to block off this circuit.
    In general be looking for any pressures that are below specification.

  10. #20
    Join Date
    Apr 2019
    Location
    Mineola, TX
    Posts
    45

    Biggrin1 test port identification help needed

    I was able to find some time to get the belly pan off of the 931 today and prepare to do the full pressure test on Saturday. After looking through my service manual I could not find a diagram to identify the two test ports. One is the torque converter outlet and the other a lubrication port. Any help is greatly appreciated.

    The pictures are taken from under the tractor looking up at the left side of the transmission by the oil cooler lines (inlet and outlet).

    I also snapped a picture of my knowledgeable helper inspecting my work.
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