Question about Caterpillar American Construction Equipment 1:87 Scale

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Accumulator charging valve

Dear sir
I have 988B caterpillar wheel loader 50W06041 powered by 3408 engine. And I have a problem with the hydraulic system the problem is the bucket does not raise when the engine is on low idle until I push on the brake padle then it raises. But it raises normaly when the engine is on high idle. I have an other wheel loader the same kind of it and I disassmbled the accumulator charging valve of it and installed it on the loader that has the problem and it worked out perfectly. So I knew the problem with the accumulator chargeing valve. So I brought a new charging valve and installed it on the the loader but it did not work out it gave the same problem that the loader has. and I tryed other four valves second hand but they did not work as well and the loader has the same problem. So my equestion is there possible problem with the valves them selves?
thank you

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This sound weird but the small pump on the back of your steering pump is the brake and pilot system supply. It is the possible source of your problem. Check for flow from the pump. At low idle there may be low flow.

Posted on Jul 30, 2009


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  • Servos/Accumulators The servos are hydraulic pistons and cylinders. They resemble the hydraulic actuators used on many other machines, such as bulldozers. Hydraulic fluid enters the cylinder, under pressure, and forces the piston to move to engage the band or clutches.
    Servo operation tccs7015.gif

    The accumulators are used to cushion the engagement of the servos. The transmission fluid must pass through the accumulator on the way to the servo. The accumulator housing contains a thin piston, which is sprung away from the discharge passage of the accumulator. When fluid passes through the accumulator on the way to the servo, it must move the piston against spring pressure, and this action smoothes out the action of the servo.
    Hydraulic Control System The hydraulic pressure used to operate the servos comes from the main transmission oil pump. This fluid is channeled to the various servos through the shift valves. There is generally a manual shift valve, which is operated by the transmission selector lever, and a shift valve for each up shift the transmission provides.
    Most automatic transmissions are electronically controlled; electrical solenoids are used to control the hydraulic fluid. The shift solenoids are regulated by an electronic control module. Shift timing is regulated through sensor feedback information provided to the electronic controller.
    On older transmissions there are two pressures that control the shift valves. One is the governor pressure which is affected by vehicle speed. The other is the modulator pressure which is affected by intake manifold vacuum or throttle position. Governor pressure rises with an increase in vehicle speed, and modulator pressure rises as the throttle is opened wider. By responding to these two pressures, the shift valves cause the up shift points to be delayed with increased throttle opening to make the best use of the engine's power output.
    Older transmissions also make use of an auxiliary circuit for downshifting. This circuit may be actuated by the throttle linkage, vacuum that actuates the modulator, or by a cable or solenoid. It applies pressure to the downshift surface on the shift valve or valves.
    The transmission modulator also governs the line pressure, used to actuate the servos. In this way, the clutches and bands will be actuated with a force matching the torque output of the engine.
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