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Im working on a 938G s/n-4ys02571 and anytime I manually shift…

I'm working on a 938G...
I'm working on a 938G s/n-4ys02571 and anytime I manually shift between gears (or the machine does it in AUTO) there is a couple second delay between the previous gear disengaging and the new gear engaging. Each time I manually change the gear, P2 drops to zero and then comes back up to 300 psi a couple seconds later as the gear engages.

Pressure Tap readings:
P2 (high) 300 psi
P2 (low) 300 psi
P1 (high) 325 psi
P1 (low) 325 psi
P3 (high) 100

Torque Out (F) 50 psi at high idle neutral
Torque Out (F) 25 psi at high idle in any gear (under load)

Trans Pump (G) 400 psi

Trans oil and filter have been replaced with the proper replacements.

Any ideas? Thanks.
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Answered in 2 minutes by:
2/26/2014
Mechanic: catmastertech, Technician replied 8 years ago
catmastertech
catmastertech, Technician
Category: Construction and Road Equipment
Satisfied Customers: 4,042
Experience: Field service technician for CAT 19 years.
Verified

catmastertech :

Hello, let me research this a few minutes so I can see if something was listed about this problem.

catmastertech :

It seems I remember a related problem but, I need to check back on some history.

catmastertech :

Have you done any repairs to this machine? Does hot or cold oil change the problem?

catmastertech :

Any fault codes logged?

Customer:

no fault lights are coming up. doesn't have a display to show any codes. don't have access to CatET

Customer:

only repairs done was recent service of eng oil & filter, fuel filters, trans oil & filter.

Customer:

issue was present prior to trans oil/filter change

Customer:

oil temp doesn't seem to affect the issue

catmastertech :

Does the P1 pressure drop to zero each shift?

Customer:

p2 does. give me a minute and i'll check p1

Customer:

p1 drops to about 75 psi

catmastertech :

So both pressure drop and it takes three or four seconds before pressure returns to both?

catmastertech :

Do both build pressure at the same time?

catmastertech :

I think I found what I was looking for.

catmastertech :

Problem: Clutch engagement is slow. (This causes slow shifting in all gears.)


Probable causes:



  • Check valve (9) is not closing or check valve (9) is open to the drain. Screen orifice (10) for the load piston is dirty or plugged.



  • Oil pressure is low.

    Note: For the probable causes of low oil pressure, refer to Cause 5 in the problem "The transmission does not operate in any speed or the transmission slips in all speeds".





  • Load piston springs (19) are weak or damaged.



  • Modulating relief valve (11) is stuck.

Customer:

I've only been able to hook up to one pressure tap at a time. timing of the pressure returning is about the same

catmastertech :

Full Size Image

catmastertech :

The screen and check valve in the valve body is were I would start. #9

catmastertech :

Then check the load piston springs (19).

catmastertech :

And the relief valve (11)

Customer:

Alright

catmastertech :

You really need two gauges to watch P1 and P2 pressures at the same time.

catmastertech :

The pressures are usually 50 psi different

Customer:

my coworker just returned from the field and has a second cat gauge setup

catmastertech :

Problem: The pressure differential between P1 and P2 is not correct.


Probable causes:



  • Pressure differential valve (13) is not operating correctly. This may be caused by any of the following conditions:

     



  • Spring (14) is weak or damaged.

  • The wrong spring (14) was installed.

  • Valve spool (15) is faulty.



  • Differential relief valve (16) is not operating correctly. This may be caused by any of the following conditions:

     



  • Spring (17) is weak or damaged.

  • The wrong spring (17) was installed.

  • Poppet (18) is faulty.



  • Flow control valve (1) is stuck in the closed position.

catmastertech :

Sorry, spec is actually 42 ± 10

Customer:

p2 & p1 move together. p2 comes up to about 290psi and p1 comes up to about 330psi

catmastertech :

the screen and check valve is were to start.

Customer:

thanks. I will be starting there in the morning

catmastertech :

Ok. If you need my help, I will be back online tomorrow night. Thanks, Donnie

Customer:

the screen was broken and had a piece of plastic plugging it. replaced it and was good to go. thanks for your assistance.

catmastertech
catmastertech, Technician
Category: Construction and Road Equipment
Satisfied Customers: 4,042
Experience: Field service technician for CAT 19 years.
Verified
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catmastertech
catmastertech
catmastertech, Technician
Category: Construction and Road Equipment
Satisfied Customers: 4,042
4,042 Satisfied Customers
Experience: Field service technician for CAT 19 years.

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