Corsa C EPAS speed controller

Corsa C EPAS Wiring
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Have you ever wondered what the correct inputs to the Vauxhall Corsa EPAS column should be? The column requires 12v, an rpm signal and a speed signal. But what frequencies?

I’ve been trying to get my Vauxhall Corsa C EPAS steering column to work correctly.

From my previous blog article you can see the correct wiring diagrams….

corsa-electric-power-steering-epas

Corsa C EPAS wiring

The column has has two connectors that need inputs:

A power connector with 2 thick wires

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

A 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

When you apply just power alone to the Corsa EPAS column; via the 2 thick Red and Brown wires; it does absolutely nothing as it is expecting rpm and speed inputs.

The first thing I found, was that you have to apply a square wave signal to the thin green wire. This signal needs to be minimum 0 – 5V peak to peak and an absolute minimum of 55Hz (read on). Without it the column does nothing.

Corsa C EPAS Control boxes

I had one of the those cheap control boxes you buy off fleebay for testing purposes. They’re simply a little square wave oscillator with the frequency adjusted by a small dial or potentiometer (probably built around a 555 timer). These very simplistic units apply identical square waves to the green and blue wires. It doesn’t make any sense to me to put identical frequency signals onto the RPM and Speed inputs, surely that’s going to mess with the Corsa’s EPAS assistance maps.

Corsa Epas Controller

corsa-epas-controller-slow

In the above photo you can see the ‘maximum assistance’ output on the green wire.

The period of the square wave is approximately 61mS (6.1 divisions x 10mS ). The frequency is  16.4Hz (1/0.061).

  • 16Hz on a 13″ Wheel Corsa is roughly 2mph.
  • 16Hz x 60secs = 960rpm

At ‘minimum assistance’ setting the output is shown below:

corsa-epas-controller-fast

The period of the square wave is 6.8 x 0.5mS = 3.4mS. The frequency is 294Hz (1/0.0034).

Corsa C EPAS Speed Signal

A Corsa with 13″ wheels would generate an ABS signal of 321Hz at 45mph. – blue / red wire
At 45mph it will be doing roughly 2550rpm or 42Hz in top gear  (1 pulse per rev). – green wire
The speed input is roughly in the ball park (4mph out), but the rpm input is way off (it’s simulating doing 17,640 rpm!)

One thing I noticed about my control box, was the frequency and duty cycle weren’t that stable. Over the first 5 minutes, not only did duty cycle change, but the frequency doubled. Provided I kept the unit at roughly the same temperature it seem much better. Therefore, if you are using one, keep it away from heater pipes and mount it down to a decent sized lump of metal such as the chassis (as a heat sink). Also, I found it almost impossible to get a similar output by turning the dial and returning it to the starting point.

corsa-epas-controller-fast-period

Testing

After 60 minutes, the frequency had changed to 869Hz (1 / 1.15). Therefore, if you have one of the cheap non-speed sensitive units and the temperature inside your car varies by more than a few degrees, expect to keep adjusting that dial to keep the assistance constant.

I also had a pocket oscilloscope and a function generator (to generate a square wave). These allowed me to do much deeper investigations.

Whilst testing with the Function Generator, instead of the control box, I noticed the subtle click of a relay inside the columns’ built in control box at 55Hz. The amount of power assistance offered by the Corsa column isn’t overly noticeable on a car that currently weighs less than 600Kg’s, especially with the tyres turning easily on shiny plastic tiles, but when the relay clicks, there is definitely a slight ‘change’ of assistance (I think it increases – but it’s hard to tell). Once the relay clicks in, you can remove the square wave and the relay will remain ‘clicked’. It won’t ‘de-click’ until the main high current +12v supply is removed.  The ‘off the shelf’ control box I had went down to 16Hz, which seems logical (16Hz = 960rpm) – so what is the relay doing? – I’m confused, is the column tripping out?
When, you connect the ‘off the shelf’ controller, it clicks as you power on the unit.

Off Setting

I want an ‘off’ setting (these columns have been known to self-steer), so I’ll fit a kill switch on the main 12v high current feed. The reason being, if the relay is the assistance kicking in, the only way to stop ‘self steering’, would be to take the keys out.

I’m not saying all these switch or potentiometer, control boxes are the same, I’m just saying the one I bought off an auction site might be rather unpredictable to use. For this reason, I’ll not use it, instead I’ll be using a Speedo Tuner box and a big kill switch.

Speedo Tuner

Unlike the potentiometer controlled control units, I plan on generating separate RPM and Speed inputs – as close to the original Corsa as possible.

Corsa C EPAS Speed Input (Blue / Red trace)

Using the pulse calculator spreadsheet above, you’ll see that the 19x35x235 tyres I’m using will turn 792 times per mile.
I’m also using:

  • 3.92:1 ratio 2.0s 4×4 Sierra differentials
  • MT75 4×4 gearbox that puts out 8 pulses per rev.

792 x 3.92 = 3104.64 prop-shaft revolutions per mile, equating to roughly 24837 gearbox pulses per mile. Already, without any calibration, that’s not far off the 13″ wheeled Corsa’s ABS input of 25693 pulses per mile.

A Vauxhall Corsa uses the ABS sensor to generate the speed input. I’ll be using a gearbox feed.

Car rim width
(mm)
% Profile
(mm)
Profile
(in)
Tyre
Diam-

eter
(in)
Tyre
Circum-
ference
(in)
Revs
Per
Mile
ABS
Pulses
Per Rev
ABS Pulses
Per
Mile
Corsa 13 155 80 124 4.88 22.76 71.5 886 29 25693

25693 / 24837 = 1.0344 or approx 103%.

Depending on which forum you read or believe, the Corsa has between 3 and 9 assistance maps, but as a guide I’ll use the chart below to show that I might not even need a Speedo Tuner (a lucky coincidence).

Map Assistance Corsa Speed Range My non-calibrated Speed Range
1 Maximum 0-18mph 0-17mph
2 Medium 18-45mph 17-44mph
3 Minimum 45+mph 44+mph

Corsa C EPAS Engine Rpm Input (Green)

Assuming the Corsa uses a Crank-Shaft Position sensor (CKP) to generate the tachometer feed (1 pulse per revolution) and not a coil feed (2 pulses per rev on 4 cylinder engine), then I could fit a Crank position sensor on the Flywheel similar to the later P38 4.0 & 4.6 Rover V8’s.

What if the Corsa column has some kind of torque sensor built in?
I’m determined to get this sussed!

Let’s face it, a Haynes Roadster or Locost doesn’t need power steering, but it’s fun to play.

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