EAS controls Range Rover P38 Electronic Air Suspension

The kit OK. I admit it. After two and a half years of messing about with the Air Suspension on my 1996 Range Rover 4.6 HSE I had had a serious sense of humour failure. I can't complain about the main agent as they had long since ceased to charge me for the work on the EAS but even plaintive phone calls from them to the factory was just resulting in bits being changed and no fix that lasted.

So what was happening? I would drive for a while with all the abilities to change the ride height you would expect from a car that promises serious off-road abilities and was over £60K when new but after a while all the lights on the suspension controls would come on and adjustment would be no longer available. Then, some times a few weeks later, some times just a few miles, the computer would totally loose the plot, open the dump valves and drop me onto the bump stops and display the dreaded SLOW:35 MPH MAX message.

After a couple of limp homes like this, one from Derby to Brighton, and then discovering that when the main dealer plugged in his Test Book and reset things it all worked well again with no sensible reported error I started to get annoyed.

Scuba air I purchased a kit on EBay to fit T-connectors into the air suspension lines so you could blow them up manually. Now I suspect that when people are selling kits to fix a fault the problem is widespread so I am more surprised there wasn't a factory fix. Anyhow with the kit fitted, a handy scuba cylinder of compressed air in the boot and a scuba drysuit feed stuffed in a cheap garage air filler I could pull the power relay, manually blow the system back up and get home at 70mph like a civilised person. I had to do this from North Wales once and I did a trip to Falmouth and back towing a 6.5 meter dive club boat entirely on scuba power.

This, however, is not a fix but a bodge. For a start I still don't have control to run the height up and down to load and unload my elderly mother at the supermarket nor all my scuba gear at the Marina. Plus there is the annoyance factor of having a multi-feature trip computer that can't display anything because the space is taken by warning me the EAS is on the blink.

Once upon a time the Land Rover seemed to embody the very essence of get there and back come hell or high water so something that panics, puts up its hands in horror and bleats for a main dealer to pat it on the head seems wrong. It wasn't even trying to reset or report what went wrong. What was it running? Microsoft EAS? It threw a Blue Screen of Death and just wanted to be rebooted? I didn't feel I could even sell the mess with a clear conscience and, anyway when it worked it was just what I wanted, so I needed a fix.
This is the story of the fix.

ANR4499 Opening the case The works inside

Step one was the purchase of a spare ANR4499 Suspension Height Control Unit via EBay. It wasn't new and when I ran it in the car it worked for a while and then did just the same this the other one did so I felt assured it wasn't the computer directly, it was just the unhelpful software loaded into the thing. Well I'm an Electronic Engineer specialising in embedded computer systems. This is what I do for a living. I can fix this and I can fix it properly.

The case is pop-riveted together but that's hardly a problem. The first disappointment was a CPU with the code inside and a part number that implied it was a semi-custom system. OK so no quick reblow of an EPROM this time. This is going to be open heart surgery. Run up the hot-air solder gun, yes I do have full surface mount rework equipment at home, and out came the Central Processor.

An assessment of the wiring diagrams for the car give me a feel for the number and type of inputs and outputs in use so I selected a Arizona Microchip PIC 18F4550 CPU to replace things with. I like the Microchip kit and I have the compilers to work on it so for a one-off it that an easy choice. This particular chip has the added advantage that I can fit a USB interface and read and write data and even reprogram the whole core with new microcode via the USB line. This would mean that software tweaks could just be a matter of sitting in the car with the lap-top plugged into it and getting the micro-code just so.
The rest of the spec of the 18F4550 is 32K of program memory, 48MHz clock (the original ran at 4MHz), 2K of internal RAM and 256 bytes of EEROM for all those calibration constants and the all important error reports. A2D converters, pulse rate capture ports, serial ports, timers and lots of I/O pins. There is a 430 page datasheet downloadable from the Arizona website if you want it and it only costs about a fiver. I did wonder about using the surface mount version since I wanted to do a PCB for reliability but decided that as I had a programming jig in DIL for the Arizona range I might as well take the easy route rather than the pretty one.

Ok here are the scans of the two sides of the PCB after I removed the metal frame that acts as a heat-sink for the power devices. Top and Bottom and here they are again with the component numbers I use in the following discussion: Top and Bottom. Please notice that these are my component numbers and contradict the few that are marked. Also all the semi-conductors are marked U something because in SMT it's hard to tell diodes from transistors etc. At least with these diagrams I could work my way round the PCB and try to understand what was going on.
Top Bottom Top with component numbers Bottom with component numbers

Ride Height Selector Switch PCB Ride Height Selector Switch PCB
OK snag number one. The little Ride Height Selector Switch with the lights on only has two wires for four lights so there is some encoding going on here. Back to EBay and buy a switch for a couple of quid and destroy it.
It has two ICs inside. One is a dual pulse generator and the other is an eight bit shift register. At first sight it clocks in four data bits on a rising edge and first bit in is the lowest LED and the final bit is the highest one. Interestingly the highest LED, denoting Boulder Strewn Riverbed mode has a wire back to the Body Electrical Control Module for some reason.
However, just because only 4 bits are used here does not force there to not be more bits as the two wires, data and clock, are also connected to the BECM directly. The BECM does not apparently drive the switch as with no power relay in the switch lights come up randomly and never change.
One half of the pulse generator is used to delay the clock 120μS so both data and clock can be set simultaneously and the data is given time to settle before being sampled and the other half is used to prevent the display being updated while the data is shifting so the lights don't flicker (a 37mS period). A quick look at the system as it starts with the EAS unpowered shows it running normally for about a second and then the error lights come on and the 35 mph message displays. I'd guess the new lamp code is sent every half a second or so and no message is interpreted as no EAS.

PCB with
take off points OK. I won't bore you with the internal details of the circuit of the EAS ECU mainly because I can't be bothered to redraw my scrappy sketches onto the cad system but here is the pin out for the connector on the ECU. I traced the circuitry and mapped the signals onto the PCB which gave me the connection points by name. All the logic signals are referenced down for a CPU running on 5 volts and the analogue inputs are buffered and amplified similarly. Nothing is muxed. Nothing seems to require special handling except the Ride Height Switch.

CPU Circuit
Diagram Right now this wants fitting to the 18F4550 CPU with the minimum of fuss.
The planned items were:
Make the In-Circuit programming work so I don't need the USB. This will save me pulling the CPU out if I send something really silly down the wire.
Connect the engine and road speed signals to the Capture/Compare module as they look like pulse trains.
Use the on chip analogue to digital converter as 10 bits gives us resolution to thousandths of max range. Nobody needs to set the ride height to one tenth of a millimetre so it must be enough.
Use the supplied USB reprogram module so keep the special switch for the reprogramming mode and do not put anything on the 'lights' it flashes.
Do a full high speed USB connection.
Put the 'datalink' stuff to the 'test book' plug on the built in serial port as, even if I'm not planning to connect anything to it, I might want too one day and all the stuff I hear implies it is a serial port.

Here's my plan for a 40 pin DIL cpu and the circuit diagram for the CPU board.

In car locationRight so the next problem is to reconstruct it so it can be reinstalled and maintained. In my book this does not involve pop-rivets.

The picture shows the installed position under the passenger seat where the EAS ECU is centre left underneath a gearbox ECU. In this view the Power Delay relay (the thing that keeps it switched on so you can work the suspension for a bit after turning off the ignition) would be in the socket with the blue blob on it but this picture is the old ECU in messed up head mode so the relay was out to keep some scuba air in the suspension.
The PCB is inverted when it is installed but putting a USB connector through the side of the case near the front on the outside seemed a natural test, diagnostic and reprogramming adaptation. Remember that there is still a fault with my air suspension, one that drives the CPU mad, so when this is all working I need sensible problem reports so I can fix that. Odds on a bad connection somewhere anybody?

Rebuilt ready for the new CPU to be fitted First I trimmed the metalwork to get room for the USB connector. Sadly I can't punch the USB socket into the PCB as I will never get the metalwork off again so it will have another mounting method. Then the rivet holes were drilled out and cinche nuts pressed in. M4 was perhaps a bit over generous here as in one hole the drill carried away all the metal so I'm a bolt short of ideal but it's not near anything that needs heat sinking an there were spare holes anyway so it doesn't need the screw to damp-proof it.

The key trick is to make and mount the PCB so it will survive the kind of off road experience that was one of the reasons for buying a Range Rover in the first place. I located two points on the board that I feel I could safely drill a hole and put a pillar into and drilled it 3mm. To fit a 2x3 inch PCB over that are I only need 7mm of clearance. (Sorry I keep unit switching but PCBs are still in my mind as designed on the one tenth of an inch grid as that's how all our components come so we work in inches while our fastenings are all metric. We dive mechanical engineers nuts.) The PCB wants to be of ground-plane type construction as that makes for nice stable CPUs once you start clocking in the tens of MHz and above region.
The total height I have to play with is 21mm so with 8mm spacing, 1mm for the PCB thickness I still have 12mm before I bump the case. This means I can have test connectors and indicators on the prototype board - not that there will ever need to be another but, hey, I make production quantity product, I think that way.

New PCB Right now, here are the new PCBs with the new CPU and the USB socket.
Final version I must be smug and say that Combro do do a nice PCB. The surface mount LEDs are a nice touch. You can't see them? True. They are green and you don't see them until they light up.

The only snag is that although my brother is a wizz on the CAD system the other brother in the partnership missed one critical dimension on the small board and got a couple of net labels wrong on the other. Still once it was mounted the ghastly botch with a hacksaw barely showed.

It needed a lot of work before it was ready to boogie though but standing on a couple of M3 bolts it looks quite neat. I must confess it is less tidy underneath. There are 32 wires connecting the new CPU card to the PCB beneath and I started wiring in nice heat-proof hook-up wire but as the quantity of cable grew I got nervous about how the board would ever fit with all that underneath it so I shifted to some thinner stuff but with a slight tendency to shrivel away from the soldering iron. However it's all done and I'd rather have flexible cables in a car application.
Test 1 I can't finish this section without a shot of it hooked up to the remains of that ride-height switch I bought. OK the program I was running at the time was just "read the switches" "write the lights" but it was nice to see something working after all this time.

Well this was the point where I thought it would all be plain sailing and the software would be the easy bit. OK I'd had some problems with the programming jig but when I discovered that the problem was pin loading adding a secondary socket between the programmer and the chip with all the pins cut off except for the ones I needed sorted that. There is something very funny about writing to port A on these PICs but that is probably because I have it smeared over the Analogue to Digital converter pins. Also it was a bit of a head banger getting the USB going but there was more fun to come on that score.

Once I got over the elation of real lights changing to button presses I noticed that when I pulled out the USB cable everything stopped. This was worrying as permanently plugging in the lap top when I want to drive was not in the game plan. However I started to dig.

A quick poke with the 'scope showed the oscillator stopping as the cable came out and, since it picked up again from where it left off when you put the cable back, that had to be 'SLEEP' mode. A delve into the standard library from the chip maker showed it just happened to have helpful code to kill the CPU into power-saving mode when you pulled out the cable. A quick mod to the library and the oscillator kept running.... pity nothing else did. Now digging into the other parts of the library turned up code that loops to give the USB system the benefit of the whole CPU when it is in disconnected mode. Another library frig, and this one has to be a bit more wary as this code overlaps projects we are selling, so no bugs here. Finally the lights flash and move without the USB.

OK so it's 9.30 at night and wet as can be but this is going in the car with null software to see if it drives the switch there AND does it get rid of the SLOW:35 MPH MAX message? To cut a long story short it does. It did flash a 20MPH MAX message at me a couple of times but mostly everything was OK. Nothing turned on, so I have all the right initialisation values, and the dimly remembered RANGE and MPG displays showed up. Went to bed a happy bunny.

The next couple of days involved sorting out the Analogue to Digital converters, discovering that the 'Engine RPM' signal was, as suspected, spark rate so an easy conversion, checking the outputs really did output and improving the program that ran on the PC to keep track of all the data I could generate. With that done real numbers were being obtained so I was ready to move. forwards to actually controlling something.

The first trials worked but weren't all that exciting but I now had suspension again.

The next CPU problem was the Microchip boot-loader. The problem was that it is designed to flash lights to tell the programmer what his program is doing but just to reassure the rest of us that things are working. However the two lines it wants to use are two of my solenoids and I don't want to flash them.
Originally I though I could just disconnect while program loading and I had arranged for the CPU to power from the USB to make this happen but this was beginning to get tiresome. The answer was to identify the program instructions that drive the lights from the supplied listing and hit them with a binary editor to NOPs...
So if anybody else needs a Microchip variant bootloader tested on a 18F4550 that does not even initialise the PORTE LED bits here it is nig-boot.hex.
It still obeys PORTB bit 5 low to go into boot mode but to change that would be a one location hit.

Now at least I just need to push one button while holding the other down and I'm in program load mode.
click for readable version More on the Ride Height Switch lights
I got the 'scope on the system and made a basic analysis of what was coming out and, since I had already dissected a switch, the protocol was simpler than I expected. The plot shows a normal display of the third light up lit. Click the thumbnail for a readable image.

The clock line, the blue trace, stays high (12V) most of the time but since the board inverts both signals that is a logic '0' out from my CPU. It goes low for 2mS and is then high for 8mS between pulses. The pattern repeats every 100mS. The data line (blue trace) is clocked into the switch's display shift register by the rising edge of the clock so if it is required to be low it sets low with the clock and sets high again 2mS after the clock. Between the 100mS frames the data line alternates between high and low and this probably supplies a simplistic I am alive signal to the BECM without the CPU there having to actually read the data. If you look at the plot you can see the rising edges of the blue line match up with the red being low, low, high, low in both cases. This sets the lights in ascending order. This is reasonably trivial to program. I have a 1mS heart-beat as my basic timer so I will just service the data lines every 2mS and cycle in 200mS to cater for the alternating state of data in the gap.

I started to cut the picture of my simulation but realised that was silly. You can't tell them apart. The only snag with testing indoors is that you need to add pull up resistors to the test jig, I used 5K6 to 12V, because the EAS ECU only pulls down.

Incidentally... If I set the line high in both gaps rather than alternating the BeCM displayed the 35MPH warning message and if I set it low it showed EAS Manual.

Version oneThe first version ran reasonably. The plot is a test run - time in seconds and height in mm. It was a bit slow but it followed the demanded height quite well.

It didn't do things like always lift the back first yet as it just trimmed round to taper into the required height and it was having trouble talking to the BeCM even though the ride-height switch on the same circuit was quite happy as this was before I got my ECU reset and 'scoped its outputs (see above). It did, however, allow me to dump out values and begin to see a pattern where it would drive high. Remember I am still hunting for the elusive fault that drives EAS ECUs mad and I begin to suspect a wiring problem in the sensors.

Revise that: a wiring problem to the rear-right solenoid. It was now pretty predictable. The rear-right wheel didn't respond. That is a killer to the standard box but not to mine. Rattle the cables on plug C152 and most times it comes back. Now the valve gear has been changed twice so the plug itself is worth looking at. So I put it into the shop for a new plug, specifically a nice new set of crimps. Watch this space - as they say...

Well that didn't seem to fix it but it's still intermittent and rattling the cables on C152 makes it come and go. I was frustrated and just put up with it for a while. What-ever the problem was it now failed first thing but once the whole system had warmed up it worked so it wasn't too bad now I had a box that didn't need resetting all the time.

So what next? I tried putting a bridge wire from the harness to bypass the whole C152 section but that worked for a few weeks then the fault came back. It does seem that disturbing the wiring improves things which, I suppose, is why the garage can always 'fix' it. It just doesn't stay fixed and I really want it to stay fixed.

So I delved again with a voltmeter on the wire and jiggling things about. The connection seemed good and yet the valve was not pulling in so it was time to take the whole box apart, something I'd rather been putting of until then. I got out the camera again so I had a record of how it was to go back together again (they are big files so don't bother unless you really need them).
PA120979.JPG PA120980.JPG PA120981.JPG PA120982.JPG PA120983.JPG PA120984.JPG PA120985.JPG PA120987.JPG PA120989.JPG PA140990.JPG
Sadly, by the time I've got it on the bench the intermittent fault is not showing but I am somewhat surprised to discover that the valve block may be dated 2004 but the electronics package is dated 1984 and, frankly, looks it. I wiggle the wires and I think I got a momentary break on the right one but it's impossible to be sure but I decided to change it anyway. I Googled the part number and www.p38spares.co.uk came up and they had one in stock at £170 so I ordered it and fitted it.

Refitted So what happened? I wasn't sure at first but it seemed to be working and after most of a holiday away I swapped back onto the original computer to see if it would stay going. Well so far it has.

If it does turn out to be this I will be annoyed. The garage have removed this lump twice to fit a new valve block. If you're going to do an expensive job and then redo under warrantee it some checks might have been in order. I don't know. I expect there are lots of good excuses but it doesn't do much for my opinion of garage men. Like the twerp who has been cutting back and refitting the perished end of my cruse control vacuum pipe for it to only last a fortnight before it goes again. It may yet fail but it's still going a month or more later and lots of miles. I'm taking it down to Geneva and back soon so it it does that without a tremble I'm going to count it as a done job.

After another four weeks I refitted the cover on the boxes under the passenger seat. How's that for confidence?

So what else?

Well things all trundled along for a while with the usual EAS grumbles but then it started developing a 'leaky bag' symptom. So I sigh a big sigh and bung it in the shop for a fix.

Well they fixed the worn front disks. They fixed the corroded rear brake pipes. They fixed the worn anti-roll bar rubbers but, despite having it for three days all they could say was that they couldn't get their computer to talk to the EAS computer. By now the bill was getting into 4 digits so I brought it home and promised myself I'd fix it.

Jig assembly The problem was that I didn't really have 'nice' control of the system with my current system. The on-board program had only been developed as far as it needed to go to keep things running. I wanted a proper 'car simulation' to plug it into at my desk so I could do the job simply.

XEAS executing So, out came the soldering iron and this is what I made.
I used a real up/down controller from Ebay to 'prove' it worked but the rest of the switched inputs are simple switches, the outputs are LEDs, the strut variable resistors are normal 'volume control' style components with knobs and the MPH and RPM pulse trains are generated by small oscilators. The engine speed is a simple on/off while the road speed has an off state and two speeds so I can see the ride hight change. The whole thing is powered by a 12 volt power supply out of the junk box that used to power some gadget years ago.

Jig panel Since my EAS box is USB controlled I also took the opportunity to upgrade the original simplistic software so something that would give me more intuitive control. It is written in WPF and is seen here running on Windows 7. I managed to catch it with the screen grab when it reports the 'exhaust' and 'rear right' valves open. If the 'Inhibit' switch is closed (ie the push button on the Rangie dashboard next to the Up/Down selector the controller shuts all the valves and waits for instructions. Clicking a valve toggles it between open and closed while click-and-hold on the big up/down arrows next to the blue strut height graphics open both the strut valve and the appropriate exhaust or inlet valve and releasing the mouse closes them both again.

Jig open Jig scematic 1 Jig scematic 2


People keep asking for the source code and I usually warn them that the device was never finished beyond the point where it allowed me to diagnose the problem with the 'real' system so it won't be much use to them. However the the requests keep coming so so I release it.

Usual GNU terms apply. This is UNSUPPORTED code that I wrote quite a long time ago now and I am NOT answering questions on it because I don't remember any details. It doesn't work well because it was only ever just a test framework.

CPU code using the FED compiler (which I would NOT recomend)
XEAS Windows display module for VS2008

One last note: in 2017 the poor thing threw its MOT test like a ton of bricks with a list of advisories to make you weep. It also had a bunch of other faults I was 'saving' for after the MOT and when several required parts came up 'no longer available' it just had to go the way of all things that have become more expensive than their value. It went to a farm and probably was cannibalised to keep two of its fellows working but I like to think of them sitting in a barn, chatting about the good times. It was frustrating at times but fun. Rust in peace.

Hit counter