ZF Suspension-RL-85/E and RL85/A Steerable Tag Diagram/Steering Gearbox 8098 - Printable Version

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ZF Suspension-RL-85/E and RL85/A Steerable Tag Diagram/Steering Gearbox 8098 - jdkskyking - 03-13-2022

UPDATE 10/05/2025 - Root Cause, know I know. Here’s the nuts and bolts.

Four years ago while on a Coach vacation, I started to experience a significant handling problem with my Newell.

Last week we completed a trip to Breckenridge Colorado from our home in Bentonville Arkansas.

The Coach performed well in the Rocky Mountains AND handled almost perfect. Almost…. More to come…

Here’s my recipe for success in a nutshell but it has been a steep and long learning curve involving many people and $$$$$.

1) ZF RAS Pressure Accumulator - That’s where my problems started. The diaphragm in the accumulator failed, the accumulator is only good for ten years if you’re lucky. When this happens, the TAG axle literally trails and is not held in the straight forward position for straight forward driving. The hydraulic pressure accumulator is responsible for holding the TAG axle straight forward when the steering wheel/steering axle is straight forward and within 5° left or right of center. I also set my ZF RAS system/accumulator to the ZF recommended pressure values of 10 bar nitrogen and 15 bar hydraulic. The system has been “bled” multiple times…. I have my opinion on that process and have probably posted that as well. It’s part of the secret recipe for success with the ZF RAS System. In the future I will accomplish additional modifications to my ZF RAS hydraulic lines and will post everything. Getting as much air out of the system is key to proper functionality.

2) TRAVEL HEIGHT ACCURACY- The Coach must be at the correct travel height front and rear, 11 1/4” on mine. The Steer Axle has one Height Control Valve (HCV) and the Drive Axle has two independent HCVs for each side. The Travel Height accuracy is critical for a proper ALL WHEEL ALIGNMENT on your Coach. If the Travel Height isn’t accurate and CONSISTENT, your bus WILL NOT driver/handle properly. I replaced all three HCVs in 2021 and didn’t realize the Steer Axle HCV was faulty until this year. I not only replaced the HCV but also reengineered the Steer Axle HCV actuating system. I’ve posted about it on this forum.

3) REPLACE ALL WORN ZF SUSPENSION COMPONENTS - I have replaced, or one could say, refurbished nearly all of my suspension. Some by necessity and some by desperation to make my Newell Great Again… Get to know your ZF Suspension. I’ve provided everything you need to know on this thread and probably in a few others. If you don’t know, you can’t grease any of the “ball joint” components.

4) ALL WHEEL ALIGNMENT - I could count the numerous times I’ve had my Newell aligned but it wasn’t until I took it to
Bill Rajewski in Columbia Missouri with Perry Legend Collision Center (573-442-7883) that the Coach drove properly. I’ve posted about this as well. Bill knows his business and knows how to make a vehicle roll straight down the highway. Trust me on this one because I guarantee you will not be disappointed.
Richard taught me the old fashioned way of aligning a Newell using string, plum bobs, math, and your noggin. He set me off on the right course but I simply didn’t have the right environment or time to complete the process. I’m sure Richard has posted his process.

5) Last but not least…. and this is my opinion… the Hydraulic System (not the ZF RAS system which is a stand alone closed loop system). My Coach has one little VICKERS V20 engine driven hydraulic pimp that “services” two critical components on the Newell, Steering Gearbox and Cooling Fan Motor. As I said, in my not so humble opinion, this setup is a loosing battle in the need for the ZF Gearbox to “function” properly and to have enough hydraulic pump to turn that Big Ass Fan that sucks air through your Radiator, Charge Air Cooler, and Hydraulic Fluid Cooler… not to mention force unneeded hot air out of the engine compartment.
In addition to the short list I’ve posted, I’m modifying my system to provide the ZF Gearbox it’s own private hydraulic pump the engine driven VICKERS V-20. I’m ADDING an Allison 4000 PTO driven large EATON/DANFOS hydraulic pump for the cooling fan hydraulic motor. Dedicated pumps for dedicated systems. Once I’m done, the ZF Gearbox will never be starved of hydraulic pressure or flow because it shared a system with one pump. Additionally… and this is YUGE…. The cooling fan can/will be properly pitched for proper airflow through the Radiator/CAC/Cooler stack. I’ll post about this as well.


So there you go. That’s what I know as of today. “Learn as we Go… Grow as we Learn”… it’s what I tell my pilots that report to me…

I hope what I’ve learned will be a fast track to success for others. Learn from my experiences and mistakes so to speak…


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Read the Original Post Below ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

I’ve collected the following information using the ZF Data Plates on our 2005. The coach was built in 2004 as unit #744. I have no idea how long Newell used the ZF RL-85 suspension and drive train. The Drive Axle has a different designation (A-132/S-91/PP 476774).

Steer Axle Data Plates were located on the left and right steering hubs. 
Steer Tag Axle Data Plate was located on the aft side of the horizontal beam, left of centerline. 
Drive Axle Data Plate was located on the left driveshaft tube facing aft. The differential has its own data plate mounted on the housing.

Data plates may be difficult to see due road grime, they may have been covered by tape when the undercarriage was painted black. I used simple green HD to clean off the areas where the data plates are mounted. Note: Simple Green HD should not corroded metal parts. 

I worked with ZF technical customer service to identify the part numbers listed below. They provided me with diagrams and parts list which I’m also posting. ZF is a enormous German company that manufactures and supplies several lines of motor vehicle equipment. Maneuvering within their system is at first overwhelming and frustrating. I have requested from ZF availability and prices for the parts listed below. I will update information as it becomes available. 

Steer RL-85/E


Drive Axle A-132/S-91/PP


Steer Tag Axle RL-85/A


Steer Tag Axle Function, Maintenance, Inspection, Troubleshooting  


Steering Gearbox 8098



The following document covers several different types of ZF Servocom systems including the RAS, REAR AXLE STEERING. I urge every ZF owner to study the system so you have a solid understanding how your Active Steerable Tag system functions. It requires ANNUAL inspections and maintenance. There is a pressure accumulator that should be served annually and replaced after 10 years of service. 
Pressure accumulators in the ZF RAS look like a small metal soccer ball mounted in the frame between the drive and tag axle. The accumulator is pressurized with nitrogen and works in conjunction with the RAS closed loop hydraulic system pressurized to 250 psi. 
The nitrogen side of the accumulator can experience micro leaking of the valves used for testing and servicing. If the pressure in your tag system is not correct you may experience anomalies during operation. There is a pressure sensor installed in the system that is activated at or below 230 psi and turns a light on at the drivers position warning of low pressure situation. 
Low pressure in a tag system will manifest while backing up and the tag tires my not move in the correct direction. If the tag pressure is low enough the tires will turn in the wrong direction and backing up is impossible. Don’t force it! 

Steering Systems and Accessories for Commercial Vehicles


These are life limited parts steer axle and tag axle steer system. There may be additional parts such as steer axle hub bearings for the control arms. 

Steering Axle Tie rod Part # 0501.007.768 (one required)
Steering Axle Tie rod Part # 0501.007.769 (one required)
Steering Axle Tie rod Part # 0501.007.611 (sway bar link-two required)
Control arm bushing Part # 0501.314.963 (eight require)

Sway Bar Link Assembly - LEMFÖRDER 3079901
   

Steering Tag Axle Tie rod Part # 0501.005.797 (one required) 
   

I have not identified tie rod ends for the steer gearbox hydraulic assist system. 
I have not identified tie rod ends for the steer tag axle hydraulic actuator.

These documents provide a wealth of information about the ZF suspension. 
Lubrication and Maintenance 


Operating Manual


Repair Manual



Time in service is a limiting factor in these steering parts. Keep in mind the coach may have low mileage but time is an enemy to any of these parts. They live in a hostile environment far below the driver’s seat. The tie rod ends have rubber boots designed to protect the inner workings of these parts. If the boot is damaged in any way, the world of road grime and nature’s elements have an open door to enter and degrade the performance of these critical steering parts.

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RE: ZF Suspension-RL-85/E and RL85/A Steerable Tag Diagram/Steering Gearbox 8098 - jdkskyking - 09-07-2022

Greetings all.

Today I added ZF documents to my original post that include information about the steering gearbox, steerable tag axle diagrams, maintenance, and troubleshooting. I continue to gather data and information that will be valuable to owners of the early ZF suspensions and steerable tag axle. 

Take care, 
JK

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RE: ZF Suspension-RL-85/E and RL85/A Steerable Tag Diagram/Steering Gearbox 8098 - orbit1957 - 09-09-2022

TX for the info!

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RE: ZF Suspension-RL-85/E and RL85/A Steerable Tag Diagram/Steering Gearbox 8098 - kaptain - 10-12-2022

@jdkskyking, thanks for sharing.
Could you elaborate a bit more info on the steering tag pressure accumulator? The location on the coach and perhaps some photos?

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RE: ZF Suspension-RL-85/E and RL85/A Steerable Tag Diagram/Steering Gearbox 8098 - jdkskyking - 10-12-2022

The steering tag system is a closed loop system pressurized to 250 psi in conjunction with a pressure accumulator.  The accumulator has a ten year life span, the pressure should be checked annually. There is also a 230 psi pressure sensor mounted in a hydraulic block that turns on a light at the drivers warning panel.
The accumulator is mounted up high between the drive and tag axels. 
 
The system incorporates two actuators, the rear is the slave and can easily be seen by looking between the drive and tag tires. The forward actuator is a combination unit that also serves as the steering assist actuator but the two hydraulic systems are separate. The steering tag system is stand alone and does not use the engine driven hydraulic system. 

                   

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RE: ZF Suspension-RL-85/E and RL85/A Steerable Tag Diagram/Steering Gearbox 8098 - jdkskyking - 11-24-2022

Hello again,

Here’s what I do on Thanksgiving…

Carve stuff up to see how it works. This is the ZF Servocom RAS “Master Cylinder”.

This cylinder and its center section is responsible for controlling the Steering Tag Axle. 

   

   


   

   

   

   

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RE: ZF Suspension-RL-85/E and RL85/A Steerable Tag Diagram/Steering Gearbox 8098 - Richard - 11-24-2022

I see you cut the cylinder. Did you then see what was preventing the rod from coming out?

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RE: ZF Suspension-RL-85/E and RL85/A Steerable Tag Diagram/Steering Gearbox 8098 - jdkskyking - 11-24-2022

I haven’t had time to study it. I’m sure there’s a assembly trick that gets reversed with special tools. Look at pic 2 & 3, there are collars that fit in the channel in the rod held in place by a O ring.
I‘ve requested the ZF manual for Master and Centering Cylinder.
When I have time I’ll see what makes the center section tick.
More pics to come.

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RE: ZF Suspension-RL-85/E and RL85/A Steerable Tag Diagram/Steering Gearbox 8098 - jdkskyking - 09-07-2023

My journey continues. I’ve learned more about the steering and suspension on 744 than I care to admit. Subject matter expert??? Possibly or very close. One day when I’m done I’ll summarize everything I know so others can benefit. 

Most recently the subject is suspension bearings or better yet, drive axle and tax axle control arms which contain the rubber bearings. 

I ordered every suspension bearing under the bus. Eight for the steering axle control/A arms and eight for the rear. 

These are the steer axle bearing. They press into the upper and lower control arms then bolt to the coach frame. 
       


These pics are a small selection of my installation of the TAG Axle control arm replacement and Drive Axle. Each axle has four arms each. My bone pile has grown, I’m down to two control arms, drive axle uppers. 
                                                                   

ZF Part numbers for rubber bearings and complete control arm assemblies for the drive axle and tag axle.

The steering axle control arm bearings are all the same, eight total. A press is required to remove and reinstall the bearings. 

The drive axle control arms are different lengths upper and lower. The upper are the longer. 

The four tag axle control arms are the same length. 

0501.314.963 Rubber Bearing-Steer Axle Upper/Lower Control Arm (eight total).

0501.222.516 Tag Axle Control Arm Assembly 

0501.222.518 Drive Axle Control Arm Assembly  
0501.222.517 Drive Axle Control Arm Assembly

This is a sampling. Some day I’ll write up the details and provide all the ZF part numbers. 

Essentially I’m zero timing the suspension. 

Take care,
JK

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RE: ZF Suspension-RL-85/E and RL85/A Steerable Tag Diagram/Steering Gearbox 8098 - hypoxia - 09-07-2023

Jeff, you got my attention with the statement that he accumulator has a 10 year life, that's where 1482 is since it was manufactured in 2012.  I see another project for this winter figuring out what is needed.  Do you know offhand if there have been any/many changes in the system from yours to mine?  Are you finding the "O" rings in good condition when you remove them?  Also, is your maintenance due to leaks, mileage or preventative?  Where are you acquiring parts from?

Your data is outstanding and will be an immense help for all of us that will go down that rabbit hole.

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