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Mapping Power Steering Weight

Using the brilliant Digital Pulse Adjuster kit to map the steering weight from 0-200+ km/h...

by Julian Edgar

Click on pics to view larger images

At a glance...

  • Fully re-mapped power steering assistance
  • Adjustment of power steering weight over all speeds
  • Uses the Digital Pulse Adjuster kit
  • Incredibly fine control
  • Vastly improved steering feel and stability
  • Suitable for all electronically-controlled variable weight power steering systems
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Back in February 2002 we showed you how to modify the weight of a speed-sensitive power steering system (Modifying Speed-Sensitive Power Steering). In summary, we used a heavy duty pot to reduce the average current flowing through the solenoid valve that controls power steering weight. With reduced current flowing through it, the valve opened less which in turn made the steering heavier. The guinea pig car was a 1998 Lexus LS400.

And what were the benefits of changing steering weight? Better feedback, more ‘meat’ in the steering inputs, better stability on freeway sweepers. In short, if you haven’t experienced the ability to alter the weight of the steering, you honestly don’t know what you’re missing out on.

The driving difference can be HUGE.

However the ‘pot’ technique had some disadvantages. The change to the steering weight couldn’t be varied at different speeds – the curve provided by the factory was retained, it was just moved downwards a bit. This resulted in the fact that when the pot was set to give good steering weight at speed, the steering was noticeably heavier than normal when parking.

Using the pot was an effective but blunt instrument.

The pot technique could also only be used in cars where a reduction in duty cycle resulted in an increase in steering weight – in a car with a system that worked the other way around, the pot couldn’t be used.

Click for larger image

But as you may be guessing from this preface, we now have available a more sophisticated technique to altering power steering weight that results in vastly better adjustability. It uses the Silicon Chip Digital Pulse Adjuster interceptor kit - electronics engineer John Clarke did the design and development. We’ve covered the kit in two detailed articles – Part 1 and Part 2. The kit – and the book in which it is featured – are available from the AutoSpeed Shop and Jaycar stores. Kit cost is just AUD$79.95, with the Digital Hand Controller another AUD$59.95.

Variable Steering Weight

So what’s this steering stuff all about, anyway? Most recent cars have power steering that varies in assistance with road speed.

(Older cars use a system that vary in assistance with engine speed, where as revs rise, the power steering pump output alters to reduce assistance. These systems are usually purely hydraulic, with no electronics able to be modified.)

In the case of the Lexus, a solenoid valve is used to vary the flow of hydraulic fluid to a reaction chamber - a fluid force that actually resists the power assistance. If a lot of fluid is allowed to flow to the reaction chamber, the steering effort is higher. If little fluid flows to the reaction chamber, then the steering effort is lower.

The current to the solenoid is varied by means of pulse width modulation - the current is pulsed on and off quickly. If it is on for only half of the time (ie it has a duty cycle of 50 per cent) the coil will ‘see’ only half battery voltage, and so will not close fully. If the duty cycle is reduced to, say, 30 per cent, then the valve will open a little more.

The pulses being sent to the solenoid valve look very much like an injector duty cycle signal – they’re a square wave where the ’on’ time varies. But note that unlike an injector, the frequency of the pulsing is so quick that the valve doesn’t open and shut to the individual pulses - instead the plunger hovers at mid-points.

Variable duty cycle pulses are exactly what the Pulse Width Adjuster is designed to intercept, so the device can be used in this application.

Digital Pulse Adjuster

As we’ve already covered in detail in the earlier articles, the Digital Pulse Adjuster takes in the signal from the ECU, alters it as the user wants, then spits the revised signal out to the solenoid. In fact, it completely takes over the driving of the solenoid.

In this application the variable duty cycle signal that normally controls the power steer flow control solenoid is diverted into the Pulse Width Adjuster (DPA), and the power steer solenoid is wired to the DPA’s output. Without making changes on the DPA’s hand controller, the power steer solenoid behaves exactly as it did in its unmodified state.

But the beauty of the DPA is that it can makes changes to the output, based on the duty cycle coming in. So for example, all 50 per cent duty cycle signals can be altered so that they are now 40 per cent. All 60 per cent signals can become 55 per cent. And so on. But you really don’t need to understand all of that – with the DPA working, all that you need to do is to press the up or down buttons on the hand controller until the steering weight at different speeds is as you want it.

Wiring-in the DPA

The DPA controls the solenoid by switching it to ground. In other words, it expects that one side of the solenoid is always connected to 12V, and the other side is earthed through the action of the DPA whenever the solenoid is turned on. The DPA is set up like this because that’s the way most solenoids (including injectors) are organised in a car. Unfortunately, in the case of the Lexus, the solenoid wasn’t wired like this. However, a simple wiring change got around this problem.

Click for larger image

This is how the electronics of the Lexus power steering system are arranged. As you can see, it’s pretty simple with the ECU having only one input (road speed) and one output (solenoid duty cycle).

Click for larger image

Here you can see how the duty cycle signal from the ECU is now fed to the DPA. The DPA then drives the solenoid which has both of its wires completely disconnected from the ECU. Because the DPA drives the solenoid by earthing it, 12V now needs to get fed to the other side of the solenoid.

Note that a change to a link on the PCB needs to be made when adopting this wiring configuration – see our stories on the DPA for more on this.

A Fake Load?

In many cases a load will need to be provided to the ECU to replace the solenoid, otherwise the ECU will register a fault code or may not even have an output. This is achieved by placing a resistor across the wires that once went to the solenoid. The resistor will normally be a 10-watt design of the same resistance as the solenoid coil (which is easily found by measuring it with a multimeter). Check that the resistor doesn’t become hot in normal use – if it does, double the resistance value and wire two resistors in parallel. In some cases the resistor(s) will need to be mounted on a heatsink in airflow eg under the bonnet in a cool spot.

A single resistor (red) is shown in place here. However, in this case it wasn’t needed as the power steer ECU in the Lexus was happy without it.

Click for larger image

Tuning

Unlike say an interceptor being used to tune mixtures, the tuning of the power steering weight is nearly entirely based just on personal preference. But what’s the actual process used?

The DPA’s LCD has two lines, one showing the input value (called ‘load points’) and the other showing by how much the output has been altered at each point. The load points correspond to the different duty cycles coming in, but again that’s not something that you really need to worry about.

In the case of the Lexus, the input load number when the car was stationary was 40, and when the output at this number was increased, the steering became lighter. Driving the car showed that the input numbers covered the range from 40 to about 90, and so changes could be made over this range, with most being reductions so as to make the steering heavier. (In cars with systems that work the other way, the duty cycles of the valve could have been increased. However, if increasing duty cycles, feel the temp of the solenoid to make sure that it isn’t becoming too hot with the extra current passing through it. Normally, there won’t be a problem.)

Click for larger image

This graph shows the changes that were made on the Lexus. Low load numbers (left-hand side of the graph) relate to low speeds and high load numbers (right-hand side of the graph) to higher speeds. (See the breakout box below for more on why this is the case.)

Load sites around 33-44 are for when the car is stopped or only just moving. As you can see, at these points we increased the valve duty cycle, making the steering lighter than factory. However, once the car is moving (around 45 - 53) we add in steering weight by reducing duty cycle. Load sites 68-73 correspond to cornering at about 70 - 90 km/h and you can see at across these speeds we made the adjustments similar. Load sites 73-78 are at about 100-110 km/h, while from load site 80 onwards we made the steering much firmer. In this land of speed limits, I am rarely driving for long periods above 110 km/h, so the steering could be made heavier at speeds higher than this to give better stability during high-speed passing manoeuvres and so on.

Note that the highest load number seen was about 92 but the rest of the map (up to load site 113) was completed in case a circumstance ever develops where these numbers are reached.

Load Number vs Duty Cycle?

The DPA assigns input load numbers from 0-128 to the various duty cycles covering the range from 0-100 per cent.

Therefore, 0 on the input on the hand controller = 0 per cent duty cycle, 32 on the input = 25 per cent duty cycle, 64 on the input = 50 per cent duty cycle, 96 on the input = 75 per cent duty cycle, and 128 on the input = 100 per cent duty cycle.

Since in the case of the Lexus, input load numbers covered the range from about 40 to 92, the valve duty cycle was actually varying from 31 to 72 per cent.

As discussed in the main text, in the Lexus the load number shown on the hand controller (ie valve duty cycle) rose almost in proportion with road speed. This indicates that the factory set-up has the steering being lightened as speed increases!

No wonder the standard car always felt nervous at high speed....

With the map of tuning values stored by the DPA, the hand controller could be disconnected, Link 3 on the board removed (this locks in place the frequency of the solenoid valve operation) and the box mounted under the dash.

The Results

It is difficult to describe the magnitude of the difference that has been made to the car. Using the pot technique, I’d already improved steering weight in medium speed corners – say around 80 km/h. But until I’d used the DPA and realised how light the factory control system was making the steering at higher speeds, I simply hadn’t realised the potential gains in this area.

Now the steering weight is perfectly-weighted everywhere. Of course it is – I have modified it to exactly suit my personal preferences!

For example, I need to do a three-point turn to get into my garage. Now it’s a doddle – the steering is super light when the car is moving at slower than a walking pace.

For example, most of the corners on my country road home are taken at around 80-90 km/h – the steering weight now gives a firm feedback that allows the placing of the car more accurately, without the steering being overly heavy.

For example, I have always been unhappy with the stability of the car in crosswinds at 100-110 km/h on the freeway. You could get into a little juggling act with the wheel as you inadvertently over-corrected when the car moved around in gusty crosswinds. In comparison, it now feels rock-solid.

For example, at very high speeds the road always seemed to narrow more than it should. Now it doesn’t.

I could go on and on. If you think that a change in wheel alignment can make a difference to how a car steers, try modifying the weight of the power steering. In this car at least, the difference is quite remarkable.

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