Permanent EPAS ?

[Stephen Hunter]

I’m a bit of an interloper here being a 93 RV8 owner but I have a particular problem where I am hoping your expertise and experience will help out.
I bought the car just over a year ago and have found that it has been fitted at some point with an MGF EPAS. The control box part number is YWC 103773 marked with an E which I believe makes it an early system. It is mounted under the passenger footwell and it looks to have been installed to a professional standard. Referring to the MGF Electrical Manual it appears to have different colour cables in places, does not have a warning lamp and I can find no diagnostics facility. But it works.

The MGRV8 is well known for its heavy steering and unfortunately it is still so even with the EPAS fitted. It cuts out at some predetermined road speed which makes roundabouts and long sweeping corners a lot of effort when above the cut out speed which I suspect is quite low. The signal for road speed is taken separately from the cable driven speedo transducer (a Reed switch type) which has been piggybacked with another separate sensor (type unknown). Unplugging the sensor to give the control box a zero speed works for a while but then after a short while the EPAS cuts out completely because I believe there is a safety feature which won’t allow revs above a set figure if the car appears to be stationary.
I would like the car to have permanent assistance which would make it easier and more pleasant to drive. I would be grateful if anyone has any suggestions or insights. Was permanent or variable assistance ever available for example? Does anyone have a schematic of the 16 pin plug to identify the connections in the control box? Or direct me to one.
The alternative is to generate the required 12v pulse electronically but being an analogue person, mostly, this would be my last resort unless someone has already done it.

[mgb281]

I will be fitting the MG TF EPAS to my MGB and have done some research into it. The EPAS conversion was very commonly fitted to MG RV8's in Japan, is yours an import by any chance? The EPAS ECU receives two pulses per engine revolution and about 4000 pulses (square wave) per mile from the speedo, I will dig out the accurate figures if you need them. When you switch on the ignition the EPAS ECU does an integrity check and after that it should function normally. As you have said the EPAS reverts to non assist if the engine is running above a certain speed (2500 or 3000 rpm) for more than thirty seconds if no speed is detected. Is your EPAS warning light on when you switch on then off after a few seconds? Does it come on when driving? To get it to give continual assistance you would need to reduce the number of pulses per mile, one pulse per wheel revolution would keep it working up to about 75mph. I wonder if the EPAS ECU is getting a speed pulse. If you fitted a sensor to the propshaft which created one pulse per propshaft revolution the EPAS would operate up to about 25mph
This should give you the EPAS ECU pinout; https://mgfcar.de/schedules/sb14.jpg

[mgb281]

I have re read your post and am still assuming that the EPAS ECU is functioning and the light on the dash is still working. To me this suggests that either the signal from the tachometer or the speedometer is not being received or the EPAS ECU is faulty. Without an oscilloscope diagnosing that might be difficult. Another possibility is that the ECU is faulty, the simplest way would be to try another ECU and see if that improves matters. The early MGF used a different ECU to the later MGF and TF, I believe the change came with the adjustable steering column. I think that I have an early MGF one that you can have foc if you would like it.
As I said in my previous post the EPAS ECU requires a square wave signal, from all the information that I have found the amount of assistance drops to zero at 15mph which equates to 1000 pulse per minute. I do know that some have tried using one of these; https://www.ebay.co.uk/itm/314159291900 ... media=COPY to get a pulse and others have used one of these to get an adjustable pulse; https://www.ebay.co.uk/itm/294569594520 ... media=COPY
However no one ever confirms if they did work. Another option is to call Paul (?) at Technozen who is the MG Rover electronics expert.

[Stephen Hunter]

Thanks for engaging with me on this problem and for your help. Just to confirm it is a UK car and I do not think the EPAS is faulty, just that as installed it doesn't give me enough assistance. It does not have a warning light but it manages well enough without it.

I found an RV8Note that examines the operation of the speedo transducer (part no. YBE 10007)
https://www.v8register.net/FilesRV8WN/1 ... OTE440.pdf
You may need to register to read.

I have since seen in an article from one of the Jaguar club magazines on fitting the MGF EPAS to the XK 120/140/150. In that it says that the assistance is speed related reducing as the speed rises. Its source information is stuff from the MGF Electrical Manual which I recognise. The author made up his own road speed transducer and reports that it works well.

From what I can gather from these and other sources my EPAS may be getting road speed signals that are too high. In addition being a V8 it may well be getting double the engine speed signal than the MGF 4 cylinder that it was designed for. That may well have some bearing on matters and explain why the assistance is below par

I am not into electronics but my pal is and has sorted out an adjustable pulse generator to suit. So we are going to give it a go.

Thanks also for the schematic which I have seen. It doesn't show which position on the 16 pin plug that each function is connected to. I have found a list showing functions for 1-16 but once again it doesn't say which pin on the plug is which or where no.1 is. I have worked it out the hard way from my own observations and the MGF Electrical Manual. It’s important to know given that cable colours vary and some pins are redundant.

Cheers

Steve

[mgb281]

Steve
You might like to look at post 27 on this thread on the MGExperience Forum; https://www.mgexp.com/forum/mgb-and-gt- ... 408/page-2
You will see that Phil Harry did post a pin out with the wire colours. You will also find some of the technical info regarding sensor pulse rates. If you follow Dieters wiring diagrams and look at the instrument panel diagrams you will see that a white/black wire goes from the tachometer to the EPAS ECU pin 15, a yellow/green wire is for the dash EPAS warning light although Phil Harry say's its a pink/blue going to pin 2. To confuse matters Dieter has the pink/blue as the speed sensor. I think that Phil may have looked at Dieters diagram ang got them the wrong way around.
If you search the forum you will find a few threads with the MGF EPAS fitted to a RV8.

[talkingcars]

<<<snip>>>
However no one ever confirms if they did work. Another option is to call Paul (?) at Technozen who is the MG Rover electronics expert.

www.technozen-electronics.co.uk

[Rob Bell]

Really interesting question - and the MGF EPAS has also been fitted to TR6s and other older British sports cars.

The solution may be two fold. One (likely the easiest), is to send the EPAS ECU artificial signals for road and engine speed. If you are happy with fixed, non-road speed related assistance, then you could just make up a box of tricks that sends a fixed road speed and engine speed signal - and so long as they are both appropriate, then the EPAS ECU won't really care, and send a fixed level of assistance. You could be even more clever and adjust the level of assistance by sending a different engine/road speed combination signal to increase or decrease steering loads.

If you want to keep road-speed dependent assistance, then the two options would be a separate ECU that received road speed and sends an appropriate road and engine speed signal to the EPAS ECU to give you the levels of assistance that you need. Or you find someone who can remap the original ECU. No idea if anyone can do that for you.

To get the ECU diagnostic data, then you will need to identify the ECU K-line from the EPAS ECU, and with an earth and a 12v supply, you could connect up a PSCAN to read the EPAS data in terms of the data it is receiving, the steering angle and the levels of assistance it is giving.

I have one final comment: EPAS tends to give very light steering. If it is always heavy, I would be a little suspicious that the assist motors are not quite giving the level of assistance you should be getting: EPAS gives assistance up to 50mph, so at roundabouts, you should be getting steering assistance.

Hope that helps Stephen, and do keep us in the loop!

[mgb281]

Rob that’s interesting, I had been led to believe the assistance cut out at around fifteen MPH but that would be an easy mistake for me to make. The info regarding pulse rates came from here; https://www.the-t-bar.com/forum/9-mgf-t ... ata#194037

[Rob Bell]

I had not seen that thread before - really interesting and valuable information. Big thanks to SundanceUK for that post 4 years ago

Note that the Mk1 MGF has an analogue speedometer, but there is a reed switch on the back of the instrument to generate the electrical signal for the EPAS ECU.

From the OP data on that thread, it looks as though you get 1.11 pulses per 1 mph. That seems a bit odd to me: the speedo ratio in the MGF PG1 gearbox is 1.15:1, so perhaps the OP's calculation a little off (or my data on gearbox speedo cable ratio is slightly out), but I imagine that one pulse per speedo cable rotation is about right.

From Dieter's ever-useful website (https://mgfcar.de/epas/index.htm):

Steering power assistance is provided by an electric motor mounted on the steering column which drives the column through a worm gear arrangement. The level of steering assistance is governed by the power output of the motor. The EPAS ECU controls the current to the motor and hence the level of steering assistance. The EPAS ECU measures the steering column torque input from the driver and the road speed of the vehicle, supplying the motor with current to achieve the required level of assistance.
The motor is connected to the column via an electromagnetic clutch which is controlled by the EPAS ECU. If the EPAS ECU detects a fault in the system, the motor will be disconnected from the column by disengaging the clutch. The steering reverts to a normal unassisted system with the clutch disengaged.
The driver steering torque input and direction of torque is monitored by a sensor mounted on the steering column. The sensor measures the angular displacement of a torque tube. Outputs from the sensor are processed by two independent systems, these readings being compared by the EPAS ECU for accuracy. A difference in the two calculated values indicates a fault. The road speed signal, from a speed transducer on the gearbox, is also processed by two independent systems.

OPERATION
Power supplies
Power for the EPAS motor is supplied from the in-line EPAS fuse to the EPAS ECU on a brown wire.
Fusible link 4 in the under bonnet fuse box supplies battery power to the ignition switch on a brown-purple wire. When the ignition switch is in position II, the power is fed to fuse 1 in the passenger compartment fuse box on a black-yellow wire. Fuse 1 supplies the feed to the EPAS ECU on a green wire. A system earth is provided by a black wire connected between the EPAS ECU and an earth point.

General
When the ignition switch is first turned to position II, the EPAS ECU performs an integrity check to ensure that the system is operating correctly. During the integrity check, the EPAS ECU earths the pink-blue wire from the instrument pack to illuminate the EPAS warning lamp. The warning lamp remains illuminated until the EPAS ECU receives an engine speed signal from the ECM on a white-black wire. The road speed signal is transmitted on a white.orange wire from the speed transducer to the EPAS ECU . The EPAS ECU outputs a 5 volts supply to the torque sensor on an orange wire and connects the orange-yellow wire to earth to complete the circuit. Two independent torque signals are input from the torque sensor to the EPAS ECU on the orange-blue and white wires.

NOTE: Although installed in the harness, the blue-green and orange-white wires are not used.
When, from the torque sensor signals, the EPAS ECU detects a steering input, it switches the supply from the in-line EPAS fuse to the EPAS motor. For a right turn, the supply is switched to the brown-blue wire between the EPAS ECU and the EPAS motor , and the brown-red wire is connected to earth. For a left turn the polarity is reversed. The EPAS ECU regulates the supply to the EPAS motor between approximately 0.5 and 12 volts, depending on the level of assistance required.

The clutch in the EPAS motor has a power feed from the EPAS ECU on a blue-slate wire. The earth side of the clutch is connected to the EPAS ECU by a blue-pink wire. The EPAS ECU continually monitors the performance of the system. If a fault is detected, the EPAS ECU energises the clutch, to disengage the motor drive from the steering column, and illuminates the EPAS warning lamp in the instrument pack; a fault code is then stored in the memory of the EPAS ECU and the steering operates without power assistance. Fault codes can be read using TestBook, which communicates with the system on the pink wire connected between the EPAS ECU and the diagnostic socket . For certain faults, the EPAS ECU may temporarily reduce power to the motor instead of disengaging the clutch.

All that data fits with SundanceUK's observations - but it looks like a 2.5v square pulse signal for both engine speed and road speed based on his oscilloscope data? Or perhaps that was the minimum voltage amplitude that was recognised by the instrument pack?

Steve, for your RV8, you may be best to get hold of an oscilloscope to work out the rpm:pulse ratio (2:1 on an MGF from this information) and the roadspeed:pulse ratio (1.11:1). If all you want is fixed assistance irrespective of road speed, then just sending 50Hz on the rpm signal input (=1,500rpm) and 2Hz on the roadspeed channel (2mph) should give you manoeuvring speed assistance at all times. If you want roadspeed dependent assistance, and the engine speed and road speed signal is different to the MGF, then you'll need an intercept box that will step down or step up the signal appropriately.

You probably already know this, but engine speed is largely there to ensure that the roadspeed signal is appropriate - if the ECU does not get an appropriate road speed for a particular engine speed, it will automatically go into a fail-safe mode, as over assistance can be dangerous, particularly if it were to suddenly change mid-corner!

[Rob Bell]

I am no electronic guru, but looks as though the 4017 counter chip might be helpful in a signal step down application...

[Stephen Hunter]

Gentlemen thank you for your knowledge and willingness to share it. I now have a better idea of what is involved and I think I will sort out the road speed pulse first as suggested to get a fixed rate of pulse via a variable generator which my pal has in hand. See how that goes and if not satisfactory pursue variable engine speed which I suspect will be harder to achieve but I have seen a gizmo that can halve the pulse rate that looks promising. The Pscan looks useful. Not sure how it connects when there is only one diagnostic lead coming from the EPAS ECU but I can cross that bridge when I get to it. And Technozen Electronics is the fall back if all else fails. What can go wrong!
I will keep you in the loop as requested as I think my findings could useful.