Which Computer Is Which?

As a longstanding member of the "if it ain't broke don't fix it" community, I've never had reason to delve into the electronics of my car - until today.

My cruise control recently stopped working so this morning I decided to do the computer checks listed by robertmnorton. Problem, which of these boxes of electrical trickery is the one I'm looking for and, for that matter, what do the others do?

On my car there is the main computer box nestling up against the offside wing. Inside there are three computers:
Item 1. The front unit. Has two multiplugs on top with the cables exiting out of the offside of the box. I am guessing that this is the suspension ECU.
Item 2. The centre unit. Has "Bosch" embossed on top. One multiplug with the cabling exiting through the rear of the box. I surmise that this is the engine ECU.
Item 3. The rear unit. Smaller, painted light grey, plug seems to be underneath. There is a paper sticker low down on its rear face with the word "Bosch" on it together with some indecipherable printing.

There is a black plastic cover clipped to the front of the main box. Inside this cover are:
Item 4. On the offside. Black and with "Bitron Type 240.104" on its top surface.
Item 5. In the central position is a mauve relay. Is this the cruise control relay?
Item 6. In the nearside position. A silver box with the following printed on its front face:
GR-STEUERGERET 12V
5GA 006 310-14
8 _ _ _ _ _ _ 1
12 _ _ 9

There is a further item attached to the front of this cover by a single large hollow rivet:
Item 7. Black, with "Euron 240107" and an electrical diagram printed on top.

Incidentally, in order to remove the covers I had to remove two relays from the panel behind the offside headlamp, there is a black socket with a green relay plugged into it and a green socket with a black relay. Now what's that all about???

Sorry for the long, rambling post. If any of you whizzkids can identify items 1 to 7 for me I'd be very grateful. Because I don't want to break anything I'd also appreciate any advice you can give on lifting any of these items out.

Cheers all.

Derek

OK, that didn't draw much interest.

I think the rearmost item in the main box is the gearbox computer. That would suggest the silver box under the front cover is probably - possibly the cruise control box, except that robertmnorton said it had 15 pins and this one is marked as having 12.

Also, where's the air conditioning computer?

Come on someone, I know it's Saturday night but you can't all be out on the town.

Derek

Hi Derek, cannot help much here as your car appears from your signature to be a 99 era one which is totally in the realms of the unknown for me in S1 land...

Just to clarify the 3 big ecu's though:

The first will be the hydractive computer - give away are the two large separate plugs.
The middle will likeky be the Bosch Fenix (well it is on a S1 24V) Fuel computer
The third, the other Bosch one, is your ABS computer!!

Not sure about the relays on the front of the box or the other computers you mention!! Didn't think the cruise was electronic - more cable/pressure controlled using linkages from the autobox. The AC, again shouldn't have a specific computer - it's simpler than that!! Hell, what do I know though, probably making a total ass of myself talking about things I've never seen!!

Sounds as though the two relays above the driver's headlight will be the lighting relays though so look after them...

Andrew

Hi Derek

Sorry to take so long to see your post. The cruise ECU should be the silver box nearest the engine and mounted on the front of the black ECU case. The one of mine I looked at after reading the CCC entry for cruise is marked HELLA GR-Steuergerat 12V 5GA 004 397-03 then underneath there is a diagram in an oblong box showing a top row of contacts marked 8 7 6 5 4 3 2 1 and next line on the right 11 10 9.
I have not worked out the function of Brown multi relay box that is at the other end by the wing but it has at least 15 contacts and mine is marked 96 196965 80 Type 240.104 and has a nice diagram in the plastic on top giving the circuit of 2 relays and 4 diodes. I should be able to sort this one out when I find my microfiches.
I think the green 12v 25A relay in the middle of mine is the cruise relay and is your purple one.
For reference the car I checked is a 96N V6 auto ORG 6910.
I agree with Andmcit about the 3 main box ECU functions and the headlight relay.
Aircon I think is part of the climate programming in the main (engine) ecu which does lots of other things as well such as immobiliser.

Regards

XMexc

This post has been edited by xmexclusive on January 21, 2007 06:25 pm

I'm not much help on these, however a 'Bitron' unit normally controls the heating / aircon (well, it does on my Xantia!).

Thank you Andrew, XMexclusive and Wirdy for your replies.

John, our two CC control units seem to be identical other than a slight difference in part number. On that basis I pulled the plug and carried out the tests listed by robertmnorton. There was no voltage on pin 9 so, with some trepidation, I followed his advice and bridged between terminals 9 and 10 with fusewire.

Result, magic, the cruise control works. Selects OK, increases and decreases cruise speed when demanded, cuts out when brake or accelerator are touched.

Only question remaining is, is it safe to run with it permanently cross-connected like this? Robert wrote about the gearbox reverse relay and brake pedal swich. The brake switch is ok, wonder what the gearbox reverse relay does and is it likely to muck up my autobox. Alright, I know that's actually three questions but you know what I mean.

Time for me to delve into the circuit diagrams.

Derek

Right, with all the skills of someone with no electronics qualifications and minimal electrical knowledge, I can report my findings. Reference is page 1472 in the electrical (gearbox) section of the XM manual.

Terminals 1, 6 and 7 of the CC control unit supply the vacuum pump motor, terminal 1 also supplies the CC electrovalve - I assume this opens the vacuum line in order to reduce the selected speed? Terminal 8 connects to earth. Terminals 2, 3, 4, and 10 are inputs from the steering column selector switch, terminal 10 also has a feed to the holding coil in the CC relay.

A 12v supply to terminal 9 seems to be crucial to the operation of the cruise control. The circuit diagram shows it normally gets its supply from two sources, via the clutch contact (but my car with auto box doesn't have a clutch so this doesn't apply) and through the cruise control relay - in fact, this seems to be the only thing that the relayed supply goes to. By cross-connecting terminals 9 and 10, I am cutting out the relay, probably overloading the supply line to terminal 10. So if terminal 9 isn't getting its 12 volts, it looks as if the relay is faulty. I intend to pull it tomorrow to see what happens.

Is my diagnosis faulty?

Derek

Hi Derek

Have you seen the James Noble cruise control article in the CCC magazine for Sept last year. He gives a circuit diagram and shows pin 10 to be fed direct from the dash on/off switch. This feed also energises the column stalk and the cruise control relay. Pin 9 is fed via the brake light switch back contact then through the switched part of the cruise control relay.
So on that basis either your cruise control relay or your brake light switch is defective and one remaining open when it should close or there is a break in the connecting wires.
I think that pin 9 loosing volts tells the cruise control to switch out on braking so giving it a permanent feed is not such a good idea I suspect. This does not tie up with you getting cruise cut out on braking. You should not get cruise cut out on accelerating as the car should respond to the accelerator then settle back to previous speed when you take your foot off with cruise remaining fully operational.
The CCC correspondent says the auto box relay is on the opposite side of the engine bay and his problem turned out to be corrosion on the contacts of the connecting wires that had lost him feed to pin 9.
I have not seen the robertmnorton cruise information so a reference would be useful.

Regards

XMexc

Hi XMexc,

robertmnorton posted a very useful series of checks on the CC control unit on 14 January in this forum, XM issues, "Cruise control gone again".

No, I havn't seen the article in CCC magazine although I'd heard about it. Perhaps I should join?

I agree that the switched supply to the relay comes from the brake switch, the diagram in the manual also shows an additional supply to pin 9 that bypasses the relay. This also comes from the brake switch and goes through the "clutch contact" - whatever that is.
So your diagnosis is correct, if I have bypassed the relay supply and taken the brake switch out of the loop then operating the brakes should not cut out the cruise control.

And yet........ I did the road test before I looked at the diagram so I didn't know what to expect. I entered the main road, accelerated to 45mph and engaged CC. I then operated the lever to increase speed to 50, held for a couple of seconds then touched the brake pedal and watched the speed decay to 40. I then turned off into the village, slowed to 30 and clicked the lever up to re-engage cruise. the car immediately accelerated so I again touched the brakes and cruise again disconnected.

Looking at the diagram I can't explain it, it shouldn't happen but it did.

Of course my comment about it switching out when I accelerated was nonsense.

Getting back to the failed supply to pin 9, I had assumed that the fault was in the relay energising coil, your comment about the fault originating in the brake switch will have to be investigated tomorrow.

I think that pin 9 supplies the vacuum pump motor through the control unit, that would explain why it's supplied through the relay. For this reason it's probably not a good idea to persist with a feed from pin 10 direct to pin 9.

So it looks as if I'm in for either a new brake switch or relay.

Regards

Derek

Hi Derek

The brake switch contact we have been talking about to pin 9 comes off the third extra cruise contact that all Mk2 XM's seem to have on the brake pedal. According to the CCC article there is a further Cruise ECU connection to pin 5 from the brake light side of the switch. Perhaps this switches off cruise as well as putting the brake lights on.

Regards

XMexc

Hi XMexc,

I'm sure you're right about the supply from the brake light switch to pin 5 switching off cruise, that would explain the anomaly. Sounds like belt and braces from Citroen.

I didn't do much on the car today, too busy hauling apple trees vertical and propping them up after the recent storms.

I did disconnect the cruise relay and check the supplies at the plug. I expected to see two 12V supplies, one to the coil from pin 10 and one from the brake light switch. There was only one showing, which I presume was from pin 10 as its a direct connection. Its opposing connection showed 4.5V which I assume is backfeed from the auto gearbox relay - or is it a backfeed from the innards of the control unit, in which case the 12V is coming from the brake swich! Hang on a minute, there's a 12V feed from the bridged connection between pins 9 and 10. Ow, my head!

In other words - no nearer a solution. As far as my worry about overloading the supply to pin 10 is concerned, all the cables look to be similar thicknesses so there should be no problem. But in that case, why bother with a relay at all? Except that the coil is earthed through the gearbox relay and must therefore de-activate the supply to pin 9 when the gearbox is in - what? reverse?

Regarding my original post concerning the"Euron" plug rivetted to the outside of the cover, now I've cleaned it off it doesn't read "Euron", it reads "Bitron". So there are two of the b**** things. From what I've read elsewhere I think they are to do with engine temp and fan operation.

The clarity of this posting accurately reflects the state of my brain.

Thanks for your help so far,

Regards,

Derek

Hi Derek

Forgive me for being a little confused here. I thought that the CC on our cars (late 24v) was wholey electronic with no vaccum aspect? Having driven the CC on my old 12v and the 24v they seem to operate very much differently. The CC on the 24v being much smoother and less prone to rev up and down. Hence my assumption that it is electronic?

Cheers

Tony

PS. The CC isn't working on my 24v but since I only use it maybe 3-4 times per year I haven't got around to getting it fixed. Although I'm thinking about having a look in the spring.

Hi All

With the 2.5TD the electronic cruise control is operated by the main (engine) ECU and there is no separate cruise control ECU fitted to the car. With this introduced for the 1995 model year I had always assumed that Citroen would have progressively updated the other models. It seems that they did for 2.1TD's bound for Scandinavia but not the UK ones. If Derek has a Vac operated cruise on his V6 then it looks as though Citroen never updated to electronic the V6 cruise design throughout the entire XM production run.
Interesting how we keep getting surprised by the little differences that keep cropping up as we learn more about ours and other peoples XM's.

Regards

XMexc

Hi Tony and John,

As I understand it, the 2.5TD has fly-by-wire throttle control with the pedal merely operating a rheostat (hope that's the right term). So it's easy to insert a control signal from the engine ECU for cruise control.

The late model V6 (ES9J4 engine) has its throttle butterfly operated directly by cable from the pedal. As a result, the only way that cruise control can influence throttle opening is by mechanical means.

For anyone reading this who doesn't know how this is achieved, there is an electrically driven vacuum pump that is connected by rubber tube to a vacuum operated actuator, this in turn operates a rod connected to the throttle butterfly. The vacuum pump only operates when the throttle needs to be open and if you rest your foot on the pedal you can feel it moving under the influence of the actuator. [That's OK for opening the throttle, closing (for example to hold the set speed when changing from uphill to downhill running) is achieved by a solenoid valve which admits air to control the vacuum.]*

The operation of the vacuum pump and the solenoid valve is electronically controlled by the cruise control control box - don't know whether it justifies the term "computer" - whose operation is modified by the lever to the left of the steering wheel.

Two days ago I knew nothing about the electronic side but I'm learning fast! What I really want is a schematic of the inside of the control box.

Regards,

Derek

Ps. *This is wrong, information just received shows that the solenoid valve simply dumps the vacuum whenever the cruise control is deactivated- for example by touching the brake pedal or switching it off. I believe that throttle closing is obtained by running the vacuum pump in reverse.

Derek


This post has been edited by DerekW on January 23, 2007 02:56 pm

Further to the Bitron story.

When fitting my new H.I.D. kit I had to follow Wirdy's advice and site one of them on the separate cover in front of the computer box. In order to do this I had to move the Bitron (item 4 in my initial posting here) and that involved disconnecting its multiplug.

To cut a long story short, when I came to test the headlights the next day I forgot to reconnect it. I got in, switched on the ignition and all hell broke out! The gear indication flashed snow and sport continuously whilst it recycled through all the gear positions, the left hand digital readout cycled through all the warning messages at two second intervals, whilst the key pad flashed both red and green lights together and refused to accept any imput.

I thought for a moment that by converting my headlamps I had succeeded in reducing my car to scrap!

So it looks as if the outside bitron takes information from the warning systems around the car, collates them and sends the result to the relevant warning indication.

Derek