Dynatest FWD - 17.6 General Functional Checking of the Hydraulics

2017.06.19 FwdMan2.8.17.docx 17-139
E2) and/or E3) can be caused by an external leakage which will cause oil spilling, usually
making it rather easy to locate.
One or more of the above errors can also be caused by UUinternalUU leakages, such as:
L1) A leaking or loose centre cylinder piston.
L2) A leaking or loose piston in any of the side cylinders.
L3) A leak in the (one of the) A/B valve(s), e.g. due to a stuck plunger.
UURe. E1:UU E1 can be caused by L1 and/or L2.
UURe. E2:UU E2 can be caused by L3.
UURe. E3:UU E3 can be caused by any of the three errors L1, L2 and L3.
Revealing/correcting L1, L2, L3:
L3 can in some cases be cured by alternately activating manually the A and B solenoids, as
this may remove particles that caused the plunger to stick. The same applies for optional AL
and BL solenoids.
L1 and/or L2 can be revealed by cutting off the cylinders one by one until the error(s)
disappear(s), as follows:
 Lower plate to the ground and DO NOT raise the weight.
 Release all hyd. pressure by pressing a rod alternately into the A and B solenoids a couple
times.
 Disconnect centre cylinder flexible hoses from the hyd. block and plug the hoses as well as
the block connections using the hyd. plugs from the Std. Spare Parts/Tools Kit, to prevent
oil spilling.
 Raise the plate off ground. If the plate can stay raised without sliding down more than max.
2 mm (1/10”) per minute or so, then the side cylinders and/or the A/B valve(s) are OK, and
the centre cylinder must be leaky and should be repaired/replaced.
If centre cylinder cut-off did not help, then the A/B valve MAY be leaking OR, more likely,
at least one of the two side cylinder pistons must be leaking.
17.6 General Functional Checking of the Hydraulics
17.6.1 a) Brief Functional Description of the Hydraulics
The different operational modes of the Trailer Hydraulics are activated by powering of one or
more of the four hydraulic solenoids (coils), denoted A, B, C and D, plus the motor starting
relay solenoid denoted M, refer also to 11.3, “FWD/HWD Hydraulics”, from which the truth
table is repeated here for convenience:
Mode
A
B
C
D
M
AL
BL
Raise Weight (Lower Plate)
OFF
ON
OFF
OFF
ON
OFF
ON
Drop (Lower Plate)
OFF
ON
ON
OFF
ON
OFF
ON
Lower Catch
ON
OFF
ON
OFF
ON
OFF
ON
Raise Plate
ON
OFF
ON
ON
ON
ON
OFF
From the hydraulics diagram it can be seen that A/B valve(s) is physically one unit with three
possible positions, also called a directional control valve or a 4-way valve. Therefore A and B