Corsa C Electric Power steering (EPAS)

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People have been fitting Vauxhall Corsa B/C Electric Power Assisted (EPAS or EPS) columns to kit cars and hot rods for the best part of ten years, so you’d think Corsa EPAS information on the conversion would be easy to find on the web.
How very wrong I was. It has taken me the best part of 30 hours of research to get what I needed. Even getting the correct pin-out of the connectors was hard work. Interestingly, I found that the vast majority are happy in ignorance.
You may ask why I’m putting power steering on my car and I could answer with a load stuff about fuel economy, how electric power steering only draws current from the battery when the steering is used or that my Locost is rather heavy, but the answer is much simpler than that. I saw one and I wanted it.

I know you can get control boxes that have a potentiometer and a 555/556 timer chip inside them, but they are far from ideal and to be honest I’d be scared of using one. These systems run permanently, giving the steering column a fake signal to make it believe the car is doing say 1200rpm and 30mph. If you are not actually doing that speed then the level of assistance is obvious going to be wrong. With a light weight car, I have read horror stories about them self steering at high speed and being impossible to keep in a straight line without constantly playing with the adjustment dial. My car with it’s V8 and 4×4 system is heavy for a Locost but light compared with a Corsa, so ideally I need something automatic, stable and mappable.

I’m not fitting anything with a dial or a switch on it as I’d rather struggle parking. I want to do this properly, not bodge it and end in a hedge. When the torque sensor on a Corsa’s column wears out, people have been steered against there will with little warning and they zig zag you down the road when trying to go straight, so if you are buying a column from an auction site try to get proof of mileage.

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

Depending on how the torque is measured within the ECU, bad connection or long wires on these power cables could cause measurement errors. So make sure your wires are short and connections are good.

A signal connector with 5 thin wire

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

I shall try to explain some of these signals.

Corsa EPAS ABS Speed Sensor (Blue / Red trace)

The Vauxhall Corsa EPS column varies it’s steering assistance depending on steering wheel torque input and to a certain extent speed. Having maximum power assistance at 70mph would make the car very hard to hold in a straight line and a little too lively whilst changing lane. Therefore, the Corsa EPAS ECU contains 3 assistance maps – Max, Med and Min, which are switched between depending on the speed signal.

Map Assistance Speed Range
1 Maximum 0-18mph
2 Medium 18-45mph
3 Minimum 45+mph

As I mentioned, you can get horrid little control boxes, that supply the column with a fake signal. Their dials affectively allow you to manually switch between these 3 settings. The dial is essentially a crude 3 position switch that you would have to re-adjust every time you change speed. The reason they have a dial and not actually a three position switch, is so that they only have to make one ‘suits all cars’ unit out of cheap components. As these basic units warm up, the map points will change, which is another reason for the dial. The components inside these units cost only barely over £1, even at one off prices.
The speedo corrector mentioned later, has much more sophisticated frequency multiplying electronics that are going to be much more accurate and stable over temperature and time. Trust me, I used to design engine management for Renaults.

Speed Signal (0 – 12v Square wave)

Should the EPAS ECU lose the speed signal, it defaults to a single assistance map – This is how the non-speed sensitive power steering in Corsa Vans work.

This speed Sensor input can be taken from several places such as the differential, the gearbox, the speedo or in my case the ABS sensor.

A Vauxhall Corsa C has 29 ABS notches on its’ ABS hubs. It also has 175/65/14 or 155/80/13 tyres on the rear.

A Ford Sierra has 94 notches on its’ ABS hubs and 195/60/14 tyres.

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

eter
(in)
Total
Circum-
ference
(in)
Pulses
Per
Mile
ABS
Pulses
Per Rev
Pulses
Per
Mile
Corsa 13 155 80 124 4.88 22.76 71.5 886 29 25693
Sierra 14 195 60 117 4.61 23.21 72.9 869 94 81671
My Car 19 255 35 89.25 3.51 26.03 81.8 775 94 72838

You can see by the above chart, if I were to connect up my Ford ABS sensors to the Vauxhall Corsa Column, it would get a very inaccurate speed reading. I would have speed map changes at 6mph and 15mph.

However, by modifying the rate of pulses per revolution of tyre / wheel, either electronically using a speedo corrector or mechanically using a different ABS reluctor ring, the system can be fooled into thinking the vehicle is travelling faster (or slower) than it actually is, and hence change the points the assistance maps are switched.

Corsa EPAS Engine Multi-timer (Red / White trace)

A Vauxhall Corsa has a multi-timer unit that run all sorts of things on the vehicle like the variable speed wipers. It also has an output that goes into the column. This is a 12v signal. Some say that this signal has a 10 second delay before it is turned on to prevent the power steering motor from being activated whilst the car is being started. This theory makes some sense, but has a flaw because the EPAS control unit also receives a engine rpm signal, so surely it could sense that the engine is running from this?
I have my own theory and that is the EPAS ECU just needs time to initialise all of it systems and calibrate its’ highly sensitive torque sensors. For this purpose it is given a 10second ‘start-up’ delay or calibration period.
Ultimately, this pin is a little bit of a mystery.
All of the forum sites I’ve read seem to leave this wire floating or permanently connected to switched ignition +12v. Vauxhall have obviously done this for a reason so I will also build a circuit that, after 10 seconds, set the input to 12V. I will build a crude 555 timer circuit that simulates this delay. You can buy handfuls of these I.C.’s for £1.

EPAS multi-timer 555 monostable circuit

Pin 3 would connect to the Multi-timer input. The two wires on the left of the image labelled + & – would connect to switched +12V. Using common component values of Ra = 910KΩ and C = 10µF gives a 10 second delay. Or use the 555 Monostable Calculator for other timings.

Corsa EPAS Engine Rpm (Green)

The Corsa EPAS Engine Rpm signal is used to ensure that the system is not drawing power without the engine running, as it would quickly flatten the battery. Without an engine rpm input, the column will not offer any assistance.

To get power assistance this wire needs a  square wave input  of at least 55Hz (55 per pulses per second) 12v  20-50% duty cycle. The maximum frequency is 30KHz (but that’s not important).
square-wave

The easiest way to get this input would be to take a feed from the rev counter, cam sensor or (in some cases) crank sensor. Check the voltage of these signals, if they are not 0-12v then use one of the orange amplifiers mentioned below. You need a minimum of 3.7 pulses per engine revolution.

55Hz = 3300 pulses per minute
3300 ÷ 900rpm = 3.7 pulses per engine revolution minimum!

A car typically idles around 900-1200rpm.

NB. A Corsa comes with either 3 or 4 cylinder engines.

Warning, If you are using a coil feed you’ll need 8 cylinders minimum.
If you are using a feed from the coil, then the signal is going to be 1 pulse per spark.
On a 4 stroke, 4 cycle engine, each spark plug fires once every two revolutions of the crankshaft, so at 1000 rpm, each plug fires 500 times per minute.
500 sparks x 4 cylinders means the coil fires 2000 times per minute.
2000 / 60seconds = 33 sparks per second (Hz) – Not fast enough! You’ll need 55Hz for the unit to activate

8 cylinders
900rpm, each plug fires 450 times per minute.
450 sparks x 8 cylinders means the coil fires 3600 times per minute.
3600 / 60seconds = 60 sparks per second (Hz) – At 55Hz, you’ll hear a relay click inside the control box and power assistance will kick in.

If the Coil is used, connect to the low voltage side of the coil!

Corsa EPAS ABS Sensors & Sensor Input

Without an ABS input, the column will offer maximum assistance. The higher the frequency input, the less assistance the rack will offer.

You can’t just connect an ABS sensor to the EPAS; the sensor first needs to be put through a little amplifier. You can get one off a non-abs Astra G/Corsa C/Zafira A. An orange relay shaped box that is found in the fuse box. It’s takes two feeds from an ABS sensor, an ignition live and ground whilst its’ speed output could be fed directly to the EPAS ECU. If you are fitting your EPAS column to something unusual, then its’ output can be fed through an intermediate speedo corrector to the ECU. My orange amp cost £1 but they are plenty on the auction sites for £4+. The speedo corrector cost £20.

These little boxes take the low voltage ABS signal, clean it and amplify it to a 12v square shaped waveform perfect for the EPAS ECU.

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)

By modifying the pulse rate per revolution of tyre/wheel, the system can be fooled into thinking the vehicle is travelling faster (or slower) than it actually is, and hence the speeds at which assistance is changed can be restored to the Corsa defaults or modified.

To modify the pulse rate, you could change the number of ABS notches at the stub axles reluctance ring (believe it or not these are often pressed on) or you can use a frequency modifier / Speedo Corrector see here. The Speedo corrector solution is much less work and allows the column to be re-mapped at a later date.

Corsa EPAS Speedo Corrector

The standard Corsa setup had 25693 pulses per mile but my setup has 72838. Ideally I need 35-36% of this pulse count . Using a speedo corrector, I would simply set the blue switches to 35.

Pulses per Second @ 18mph Pulses per Second @ 45mph
Corsa 25693 x 18 / 3600 = 128.5 25693 x 45 / 3600 = 321.2
Sierra 81671 x 18 / 3600 = 408.4 81671 x 45 / 3600 = 1020.9
My Car 72838 x 18 / 3600 = 364.2 72838 x 45 / 3600 = 910.5

Using a speedo corrector you can get 1 % steps in speed correction

Percentage
change needed
Modified
Pulses Per Mile
Speed Maps
Corsa – Standard
(13/155/80 tyres & 29 ABS pulses)
0% 25693 18mph & 45mph
Sierra
(14/195/60 tyres & 94 ABS pulses)
31% 25318 18.2mph & 45.6mph
My Car
(19/255/35 tyres)
35% 25493 18mph & 45mph

I found a very handy MsExcel spreadsheet on the web. I’ve done a couple mods and here’s my version:

Within the spreadsheet, you can enter your wheel / tyre ABS combo and it’ll tell you what percentage correction you need to restore the Corsa default map settings.

If your rack plays up, check this link out.

Update… YOU MUST READ THIS!

Conclusion

By using a Siemens sensor amplifier and a speedo corrector, you’ll have spent less money than buying one of those nasty auction site controller boxes and you have something that actually works like Vauxhall intended. You won’t have any dials to dash mount and you can fit and forget. Alternatively, in future, should you decide to change the mapping, turn those two blue switches.

It’s automatic, it’s stable, and it’s mappable. Perfect. Once you’ve got your head around it, it actually very simple. Don’t let the circuit boards scare you, but if you get stuck, many auto-electricians would enjoy a job like this.

Just, don’t forget to buy a low mileage column!

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