MCI SERVICE INFORMATION
Rear Axle Steering Troubleshooting - Renaissance® and E4500 Coaches
In Section 2, Rear Axle Steering Troubleshooting - Renaissance® and E4500 Coaches, the operation of the rear axle steering (RAS) system is explained. The purpose of this tip is to concentrate on troubleshooting this system. The attached troubleshooting guide, taken from Renaissance and E4500 maintenance manuals, does a very thorough job of covering various scenarios that could be encountered. Two additional possibilities are:
Repeated failures of the oil pressure switch mounted on the manifold assembly.
- Unusual or abnormal tire wear on the tag axle and possibly the front axle; the coach has had the wheels re-aligned, possibly multiple times, and the problem still exists.
The cause: the steering gearbox was not properly centered prior to wheel alignment.
- It was not as important to center the steering gearbox on trucks and earlier coaches that didn't have hydraulically controlled steerable tag axles. Instead, the technician could simply align the wheels straight ahead, and if the steering wheel was not properly aligned, it could be removed and replaced until straight. This procedure is not acceptable for the Renaissance or E4500 models.
- Renaissance and E4500 models require that the gear box is centered to ensure that the master cylinder is centered in the middle of the dead band. If this is not done, the symptoms are sluggish steering in one direction, aggressive steering in the other direction, and excessive tire wear on both the front and tag axles. See Figure 1 to view the alignment marks on the steering gear box.
When the gearbox is centered, a tab on the shaft dust cover aligns with a raised boss on the gear box case. The marks are aligned by turning the steering wheel, which rotates the input shaft and dust cover. See Figure 2. The two marks are slightly misaligned for clarity.
Centering the gearbox guarantees that the master cylinder is positioned properly in the middle of its "dead band".
- The dead band is an area in the center of the master cylinder stroke which allows slight movement of the cylinder and no pressurization of either steering line (L1 or L2).
- This allows the coach to make minor steering maneuvers such as a lane change while maintaining the tag axle in the straight ahead position.
The Cause: The RAS accumulator has failed, and can no longer suppress any fluctuations in system pressure while the tag axle is steering. The high-pressure spikes damage the pressure switch so that it is no longer calibrated to activate the alarm at 100 psi.
- Tag steering fault tell tale light is on.
- No visible leaks in system.
- When a pressure gauge is connected prior to re-pressurizing system, system pressure is well above pressure setting of switch.
- The RAS pressure switch is pre-set to activate the alarm at approximately 100 psi (7 bar). The alarm should not activate at pressures above that setting.
- Failure repeats itself again in a short period of time.
- The accumulator is designed with an internal bladder (similar to a basketball) that is pre-charged with nitrogen to 150 psi (10 bar).
- As the system is pressurized, this bladder is compressed until the accumulator internal pressure equals the oil pressure.
- When the steering wheel is turned, pressure in the L1 or L2 line (depending on the direction steered) can reach as high as 1470 psi (100 bar). At that point, a pressure relief valve located in the oil manifold opens, allowing oil to bleed off into the L3 line.
- At this point, the pressure on the L3 line increases due to the additional oil added to the circuit. The function of the accumulator is to allow space or volume within the system to make room for the additional oil with minimal increase in pressure
- If the nitrogen charge has escaped into the atmosphere and the RAS system was pressurized without this accumulator functioning properly, no additional space or volume is allowed in the L3 circuit. When the pressure relief valve opens adding additional oil to that circuit, the pressure in the L3 circuit will increase dramatically.
- Normal pressure fluctuation on the L3 circuit while steering is approximately 100 - 200 psi above system pressure.
- If the accumulator has failed, the L3 circuit pressure could spike to well over 1000 psi, damaging the pressure switch.
Further information is available on the procedure for checking RAS system pressure and accumulator in section 11 of the Renaissance or E4500 maintenance manuals, or click to view the troubleshooting guide.
For further assistance, please contact the MCI Customer Support Center at (800) 241-2947.
Motor Coach Industries - A New Flyer company: Corporate Address 200 East Oakton Street, Des Plaines, Illinois 60018 | Phone: 866-MCICOACH
Copyright 2004-2016 Motor Coach Industries Int'l, Inc. and its subsidiaries. All Rights Reserved.