Corsa EPAS Power Steering – Kit Car

Corsa C EPAS Wiring
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For all those fitting a Vauxhall Corsa C EPAS (Electronic Power Assisted Steering) to a Kit Car or Hot Rod the following should be helpful. It seems a very popular subject, as around a 30,000 people per month read my EPAS posts, yet the quality of information on other popular forums is quite surprisingly very poor and even potentially dangerous.

There are a lot of kits out there to run your Corsa EPAS and as I’ve found out some of them (the cheap ones on ebay) are somewhat unreliable and close to useless.

I therefore set about testing things for myself on a running Vauxhall Corsa C, doing sums and calculating what is really going on.

Conveniently, after years of service, we are sending our 2002 Corsa C off to the scrap-yard, so I snatched the opportunity, to put an oscilloscope onto the wires.

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Corsa C Power Steering Wiring

Power connector with 2 thick wires

Red – 6mm2 +12v ignition permanent positive (50amp fuse needed)
Brown – 6mm2 Earth

 
Signal connector with 5 thin wires

Blue / Red trace – 0.5mm2 Speed Sensor (ABS) Input
Red / White trace -0.5mm2 Engine Multi-Timer Input
Black – 0.5mm2 +12v Ignition switched positive
(5 amp fuse)
Green – 0.5mm2 Engine rpm Input minimum 55Hz maximum 30KHz
Brown / White trace – 0.5mm2 Diagnostic output

 

Speed Sensor ABS Input (Blue/Red trace)

I’d previously worked out, that a Corsa C fitted with 13″ wheels would generate an ABS signal of 321Hz at 45mph. I’d worked this out using the rolling radius of the tyres and 29 ABS pulses per rev.

I can now confirm that this is indeed correct.

As a rough guide, add 71.333Hz for every 1000rpm.

  • 10mph = 71.7Hz
  • 20mph = 142.7Hz
  • 30mph = 214Hz
  • 40mph = 285Hz (unconfirmed)
  • 45mph = 321Hz (unconfirmed)
  • 50mph = 357Hz (unconfirmed)
  • 60mph = 428Hz (unconfirmed)
  • 70mph = 499Hz (unconfirmed)

Corsa C EPAS Speed Input -10mph

The above is at roughly 10mph @ 71.7Hz (0-13.5v)

Sorry the pictures are blurred, they were grabbed from a dash cam video, taken off road down a gravel track used only by tractors! At 30mph my teeth were starting to loosen!

I’d tested one of those cheap boxes from eBay and at maximum assistance the box was simulating a vehicle travelling at 2mph (16.4Hz). At minimum assistance, the box was simulating a car travelling at 41mph (294Hz). However, I also found that over time and as the box became mildly warm these simulated signals changed dramatically, meaning you’d constantly be playing with the dial to keep constant assistance. The simulated signal was only 0-5Volts whereas on an actual Corsa they were 0-12Volts, but this didn’t appear to be a problem.

Using real signals from ABS sensors, if I wanted to reduce the amount of assistance, I could put a frequency multiplier onto the input. A cheap one would simply multiply the signal frequency by 2. Meaning 1000rpm = 143Hz, giving the same assistance at 10mph the Corsa has at 20mph (much less assistance). With a frequency multiplier, the assistance would be close to zero at 20mph. You can use a Speedo Tuner (HO-001) to do this job.

Alternatively, if your car doesn’t use ABS, but your hubs have ABS rings, you might be lucky and find a compatible ABS Sensor ring with more triggers per revolution. You could even get a trigger wheel laser cut and fit it to your differential or use (like me) a gearbox pulse output. There are lots of possibilities. Remember to pass any ABS or reluctance sensor trigger signals through a signal converter, before connection to the EPAS.

Siemens 09.185.826 Impulse Signal Converter

In1 ABS Sensor
In2 ABS Sensor
Out Speed Output (connect to EPAS Blue / Red trace)
15 +12v Ignition switched positive (connect to EPAS Black)
31 Earth (connect to EPAS Brown)

I’m using a MT75 gearbox that produces 8 pulses per rev. I haven’t  measured the output yet to see if it’s useable, but if it is that would give me very similar power assistance to a standard Corsa.

 

Engine Multi-Timer Input (Red/White trace)

Nobody, as far as I know, has worked out the exact reason for this input. Even without the key in the ignition, this input has the signal shown below, in fact as soon as the door is unlocked, the signal turns on.

The signal does not vary with speed or with rpm. It’s a 2mS Pulse every 10mS (100Hz 0-12V). 0-13.5v when the engine starts.

The signal goes off (0 volts DC) after the 4 seconds of starting the car. It turns back on (100Hz 0-12v) 3 seconds after turning the engine off.

For the car to start and return to idle rpm takes around 4 seconds; almost identical to the timer input. Meaning when the timer signal turns off (0v), the engine is idling around 700rpm. Maybe the timer input is there to signal that the car ‘should’ have finished starting and assistance ‘should’ have kicked in? It maybe something to do with the in-built diagnostics?

As a test, I turned the ignition off at 3000rpm. It took just under 3 seconds for the engine to stop turning – coincidence?

On the cheap eBay boxes, this signal is permanently set at 0volts (grounded).

Corsa C EPAS Timer Input
 

Engine Rpm Input (Green)

From previous testing on the column in my Haynes Roadster, I knew that the power assistance didn’t start below 55Hz and above 30KHz turned off again. But until now, I didn’t know how this related to rpm.

To get power assistance to start you need a signal of 55Hz on the green rpm wire. Once assistance has turned on, to get it to turn off, you’ll need to remove the main 12V feed.

As far as I could sense, power assistance doesn’t vary with rpm, therefore this rpm signal is simply there to prove to the power steering EPAS that the engine is running. Indeed, why would you want different assistance in different gears?

Corsa C- PM Input 1000rpm

The above is at roughly 1000rpm – pulse width = 28.6mS (35Hz 0-13.8V)

  • 1000rpm = 35Hz
  • 1428rpm = 55Hz
  • 2000rpm = 70Hz
  • 3000rpm = 105Hz
  • 4000rpm = 140Hz
  • 5000rpm = 175Hz

During starting, with no throttle applied, the Corsa appeared to hit around 1600-1700rpm. At cold it was closer to 1700rpm and when warm was closer to 1600rpm. This bust of start-up revs would, in theory, be enough to trigger the power steering.

On the cheap eBay boxes, at maximum assistance, the box only produced 16.4Hz. Therefore, to get assistance to kick in, you’d always need to start the car on minimum assistance, then turn the dial back to maximum, to manoeuvre the car. I imagine remembering to do this could get rather annoying, but it could explain why so many people on the forums publish posts similar to ‘my EPAS only works 50% of the time’. Also, each time you turn the dial from min to max, getting the cheap potentiometer to produce similar outputs will be somewhat of a trial. As an ex electronics engineer, I know this type of potentiometer isn’t designed for accuracy and doesn’t last too long when in constant use.
 

Conclusions

The EPAS power steering on a Vauxhall Corsa makes the steering extremely light. For those that prefer good feedback from the steering, it may be far too light. I know from moving my Haynes Roadster around the workshop, even with a V8, 4wd, plenty of caster and 255/35/19 front tyres, the assistance makes the steering incredibly light. On my car, over roughly 10mph, there would be absolutely zero need for any assistance.

From my experience, I can now say, under normal circumstances, I wouldn’t put EPAS power steering on anything as light as Haynes Roadster. I’d leave it for something much more ‘substantial’, meaning EPAS is probably over-kill for a large proportion of Kit Cars. Also, with EPAS, you’ll loose much of that self-centring action generated by the caster angle. So ignoring the extra weight, EPAS would be a non-starter for track cars.

That opinion is coming from an old bloke with a hand, arm and shoulder pinned together with nuts and bolts.

Personally, if I was using Corsa C EPAS I’d do things properly and simulate all signals as accurately as I could. I’d take inputs from rev counters and ABS sensors. Maybe, for peace of mind, for use in emergency situations only, I’ll put in a secret kill switch.

If you are simply building a road only car, for occasional use on short runs, then you could argue a cheap simulator is an option. You’d need to keep the simulator box at a constant temperature and you’ll spend lots of time tweaking that dial. Simply setting the simulator at maximum assistance and turning it on ‘when you need it’ isn’t going to work, as the simulated rpm signal wouldn’t be fast enough. I’d also keep all wires very short, as close to the battery and steering column as possible. Keep the loom away from all high voltage, high frequency signals such as ignition and injection. Keep in mind, if you had a real Corsa, disconnected the sensor connector and plugged in a cheap simulator, the resultant assistance isn’t going to be anything like what Vauxhall intended and would no doubt cause the EPS warning light to come on.

Hope that helps

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