STEERING IN THE WRONG DIRECTION

 

Welcome

Well this is my first blog post, where I can go more in-depth into the content posted on my YouTube channel.Here I can explain the how's and why's, where on my YouTube channel it needs to be short and engaging.

The How And Why

As I always say to my apprentices, understanding the how and why of a system is critical to diagnosing a fault.

It’s no good changing part after part without evidence of a failure or at least a bloody good educated guess on the component you’re about to replace.

Which brings us onto how an EMAS system functions hydraulically

Electronically Controlled Multi Axis Steering

Truck Running - No Steering Input

The first critical component in the system is the pump.This is a high capacity geared motor and takes oil from its own power-steering reservoir. The pump self regulates the oil pressure and supplies fluid to the control valve.

From the motor oil gets pumped into the control valve (labelled SP201 in the diagram). This literally does what it’s called, limits pressure to the steering circuit for the trailing axle .

This valve is an essential safety component of any hydraulic system to prevent catastrophic damage caused by the enormous oil pressures generated.

The relief valve ensures that if the pressure rise is too great, the system will leak off to the return-line and back to the reservoir. When the valve gets energised, pressure flows through to the next valve.

SP051 valve, when energised, causes the pressure port to be blocked off and won’t allow The C1 and C2 connectors on steering cylinder (10) to be linked. This causes pressure in the steering circuit to rise. If we do not energise this valve, all ports are connected, we equalise pressure in the trailing axle.

With fluid not getting through SP051, it flows round to SP011 valve. This has 4 positions, the position now (3) is the centre position, where no fluid flow is possible. This keeps the trailing axle in a specific position.

Finally, the steering pressure heads to the accumulator relief/loading valve, also known as SP041. This is yet another 2/2 spring return valve, which, as you’ve guessed, lets fluid through or not and in this case stops the high pressure entering the ram.

This valve controls the pressure in the centring circuit and works to keep it at a pressure of 28 BAR throughout. It can open the valve to lower or raise the pressure if it deviates too far from this.

The Centring Circuit

With SP041 as the gatekeeper of valves which “tops-up” our centring circuit, we have a few important components in the centring circuit I need to talk about.

SP071 is yet another 2/2 valve (I’m sure TRW who made this system did a bulk buy on these things). 

This has a one-way valve fitted which allows oil to flow FROM the accumulator when SP071 isn’t energised. This also allows the centering circuit to maintain system pressure. 

When the valve is energised, it allows oil to flow from the centring circuit freely to the accumulator BUT, what is an accumulator?

This component is a glorified capacitor in the centring circuit. It consists of two sections separated with a diaphragm. One half is filled with oil from the centring circuit and the other half is charged with nitrogen. If the oil pressure in the centring circuit drops, the diaphragm forces oil back out into the centring circuit to maintain system pressure. The reverse is also true in that if the pressure becomes too great the accumulator can absorb and store it.

To The Left, To The Left

To carry out a left turn, this is how the trailing axle operates;

The EMAS ECU energises the fluid flow control valve block SP201 and the release valve SP051. This action disconnects the C1 and C2 connections on the steering cylinder (10). This causes pressure in the steering circuit (black) to increase. The proportional steering valve SP011 is energised to move to the left by the ECU. As a result, connections P and A are linked together. Oil (black) is routed to connection C1 on the steering cylinder. Connection C2 is linked to connection T on the control valve, allowing oil (light grey) to flow back to the reservoir (1) from C2.The piston rod is pushed into the steering cylinder (10). Oil (dark grey) must be able to flow from the steering cylinder connection Y2 to the accumulator (12) with this steering action. The ECU also activates the accumulator valve SP071 to allow this.

When the times RIGHT

To carry out a right turn, the trailing axle moves due to the following process;

The EMAS ECU energises the fluid flow control valve block SP201 and the release valve SP051 yet again. As a result, the C1 and C2 connections on the steering cylinder (10) are no longer linked, causing pressure in the steering circuit (black) to rise. The SP011 proportional steering valve is energised to the right. As a result, connections P and B are crossed over in the valve and linked. Oil (black) is routed to the steering cylinder connection C2 (10). Connection C1 on the control valve is linked to connection T on SP011, allowing oil (light grey) to flow back to the reservoir (1). The steering cylinder's piston rod slides out. Because oil (dark grey) must be able to move with this steering movement, the ECU also activates the accumulator valve SP071.

All's Well Ends Well?

With model year 17 coming in, DAF replaced this system with EHS (electro-hydraulic steering) which from my personal experience has been fault free compared to EMAS. Im glad they simplified the system as trying to understand EMAS from just reading the description above is rather difficult but hopefully someone will find it useful one day and it supports the video which you can find here : emas steering angle replacement