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How do we adjust the transmission Charge pressure? Caterpillar…

How do we adjust the...
How do we adjust the transmission Charge pressure?
Caterpillar 938G Front end Loader Ser No. 6WS04119

What should the pressure be? Current Pressure 126 Psi
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Answered in 9 hours by:
8/13/2012
Mechanic: catmastertech, Technician replied 10 years ago
catmastertech
catmastertech, Technician
Category: Construction and Road Equipment
Satisfied Customers: 4,043
Experience: Field service technician for CAT 19 years.
Verified
Test pump pressure at the filter test port.
graphic

graphic

Illustration 1 g00281968

Schematic for the Transmission Hydraulic System

(1) Flow control valve

(2) Transmission hydraulic control valve

(3) Selector spool and slug (Forward High)

(4) No. 1 solenoid (Forward High)

(5) Selector spool and slug (Forward Low)

(6) No. 2 solenoid (Reverse)

(7) Selector spool and slug (Reverse)

(8) No. 3 solenoid (Forward Low)

(9) Check valve

(10) Screen orifice

(11) Modulating relief valve

(12) Slug

(13) Differential valve

(14) Spring

(15) Valve spool

(16) Differential relief valve

(17) Spring

(18) Poppet

(19) Load piston springs

(20) Converter backflow check valve

(21) Load piston

(22) Transmission oil filter

(23) Selector spool and slug (First Speed)

(24) Transmission dump valve

(25) Transmission oil pump

(26) No. 4 solenoid (First Speed)

(27) Selector spool and slug (Second Speed)

(28) Selector spool and slug (Third Speed)

(29) No. 6 solenoid (Third Speed)

(30) No. 5 solenoid (Second Speed)

(31) Torque converter inlet relief valve

(32) Suction screen and magnet

(33) Torque converter

(34) Oil cooler

(35) Transmission lubrication

Oil pressure is low.
This may be caused by any of the following conditions:

Oil level is low.

An oil strainer is dirty or there is a restriction in the oil flow circuit.

An oil pump failed.

Air is leaking on the inlet side of the pump.

There is leakage inside the transmission (worn seals around the clutch shaft or around the clutch piston).

Modulating relief valve (11) is stuck open.


No real relief valve to adjust but, other valves work together to regulate flow and pressures.

Components and Functions
Component Function
Solenoid Valves (2), (4), (6), (24), (29), and (30) These valves direct P1 oil or P2 oil to the selector spools in order to keep the selector spool in the engaged position or in the disengaged position.
Speed Clutch Selector Spools (23), (27), and (28) These spools route P1 oil to the first speed clutch, to the second speed clutch, or to the third speed clutch.
Directional Clutch Selector Spools (1), (3), and (5) These spools route P2 oil to the forward high clutch, to the forward low clutch, or to the reverse clutch.
Differential Valve (11) This valve maintains P1 pressure at a level that is
275 kPa (40 psi) higher than the P2 pressure.
Differential Relief Valve (12) This valve prevents excess P1 pressure after a shift.
Converter Backflow Check Valve (22) This valve prevents excessive flow in the converter circuit which is caused from dumping into the load piston spring cavity during a shift.
Modulating Relief Valve (10) This valve works with load piston (17) in order to provide a controlled pressure rise in the clutches and in order to control P2 pressure. This valve sends excess oil to the torque converter.
Load Piston (17) This piston works with modulating relief valve (10) in order to provide a controlled pressure rise in the clutches and in order to control P2 pressure.
Check Valve (7) This valve senses low P2 pressure during a shift and the valve moves to the right. This opens load piston cavity (25) in order to drain. After the clutch is full, the valve closes. The valve then allows modulating relief valve (10) to complete a controlled pressure rise.
Dump Valve (21) This valve rapidly removes the pressure oil in load piston cavity (25) during a shift. This is done in order to allow load piston (17) to fully reset.

For serious pressure control problems, having the manual is a must to find all the possible problems.

I can help you with some of this but, it will take several post to narrow down the problem.
If you would like more help on this, please provide all the pressures of the transmission. This will help identify what is working or not working. Below is the normal pressures for this transmission.
Table 2
Table for Transmission Pressures
Pressure Specification Actual Pressure
(A) P2 pressure (high idle) 1896 ± 170 kPa (275 ± 25 psi)
(A) P2 pressure (low idle)
1896 ± 170 kPa (275 ± 25 psi)
(B) P1 pressure (high idle)
290 ± 70 kPa (42 ± 10 psi) higher than P2 pressure
(B) P1 pressure (low idle)
290 ± 70 kPa (42 ± 10 psi) higher than P2 pressure
(C) P3 pressure (high idle)
1000 ± 140 kPa (145 ± 20 psi)
(D) Transmission lubrication pressure (high idle)
190 ± 70 kPa (28 ± 10 psi)
(F) Torque converter outlet pressure
380 kPa (55 psi) min

515 kPa (75 psi) max
(G) Transmission oil pump pressure
2750 ± 170 kPa (400 ± 25 psi)

Here is the location of all taps. Transmission Pressures - Test and Adjust

Let me know what you find and we can get deeper into this problem.
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Customer reply replied 10 years ago
Hi Donnie

thanks for the information. we have removed and inspected the pump and it looks fine, no score marks on the housings and gears. see pictures.


Pressures taken as follows.


1. Pump pressure at filter - 950 KPA at Idle - 1500 Kpa at high idle.

2. P1 = 950 @ idle and 1500 kpa @ high idle

2. P2 = 950 @ idle and 1500 kpa @ high idle

3. P3 = 0 kpa

We suspect one of three: 1. Torque Convertor inlet relive valve. 2. differential valve. 3. Flow control valve.


What do you think, we will visit the machine tomorrow.


Mechanic: catmastertech, Technician replied 10 years ago

Great pictures! Pump looks good, no deep scratches. Gears do appear to have passed some debris. I would go with possible trash in the modulating valve(11) causing it to stick open. The next issue could be the flow valves to the convertor dumping oil. Either way the valve needs to be removed and inspected. Here are details on the valve.

Transmission Hydraulic Control Valve - Disassemble

Be sure to torque valve body bolts to torque of 30 ± 4 N·m (22 ± 3 lb ft).

catmastertech
catmastertech, Technician
Category: Construction and Road Equipment
Satisfied Customers: 4,043
Experience: Field service technician for CAT 19 years.
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