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Discussion Starter #1
Hello

I must be going for a record. two posts on different subjects in the same night (got to get out more).

I have a failed electronic cruise control unit. I have pulled the unit and fiddled with it (which make it not repairable by that nice company who do repairs (price on application) but lay down conditions [like untouched or no way]).
Not that I am concerned. The item itself seems pretty straight forward and I am building a test box to see if I can fault trace it. As long as the micro-controller is OK, it must be repairable.

However

I have purchased off Ebay a VDO unit from a BMW E34 of the same era. Since these can be picked up for bobbins (I now have 2 for under £20 for the pair), I have opened up this unit too (force of habit). They are remarkably similar and so is the actuator in the engine bay and the controls also. I will post some pics for you to see (later). The only difference could be the speed pulses from the tranny, which may be slightly slower (or faster) than the RR. I doubt this will be a problem. It will be no great hardship to find that I can't use my cruise control slightly above or below the nominal 30mph as the original unit is set to.
I have done my research (those nice BMW people put a lot of info on the web) and as I see it, it is very straight forward to use the BMW control box on the Bentley (remember mines an 89 Turbo R). The connector is different and the pins many, but only the same number as the RR version are required to get the unit to work (it seems).
So

I am planning to do a tempo graft of the BMW unit and see if it works. I'll also build my test box and see if I can fix my original unit.

So what is the thrust of this post?

Well I guess I'm after other opinions if you have them as to the pros and cons of doing this.

Cheers... Rob.
 

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BMW control unit

Hi Rob!

I'm afraid there aren't many people as deep in electronics as you are.
So we're all standing around with open mouths waiting for your results!

Should I get myself some e34 ecus? Munich should be a good place... :)

Regards, Lutz
 

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The cruise ECU is a GM issue. Some GM ECUs can be used, but the ECU from a Holden of that era is a "plug 'N play" unit.

UPDATE: The ECU is a GM unit made by VDO. VDO makes ECUs for many cars.
 

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Discussion Starter #4
As promised, a couple of pictures to show you how close these two units are. In all these the Bentley unit is on the left. The BMW unit is the same width (and the box the same width and depth), but the BMW unit is a little longer (~13mm). The mounting arrangements are different, but that is easily sorted... If it works.




If we open up the cans and slide out the PCBs, we can again see similarities. You should note that the LM2901N is just an upgraded (higher temperature rating) LM339N (which I had on the shelf) so I swapped that in already to see if that cured it. I guess the fact that I'm continuing this thread here will tell you it didn't. That's where my test box for the Bentley unit will come in, but that's not built yet....

The extra couple of wires you see were for me to power the Bentley board up quickly on the bench. I know the Bently microprocessor is running as the oscillator starts up on the chip, driven by the crystal (metal can). You will note the BMW connector is radically different, but that is no great hardship.



After looking at the Bentley unit, and following around the circuit, I moved onto the BMW unit and effectively traced around the same circuits on the BMW board. You will notice that the BMW is a dual layer board having tracks as well as a ground plane on the component side of the PCB. I then compiled a list of the cross connections to convert the BMW pinout to the Bentley pinout. I then looked at the BMW circuit diagrams as I found for the E34 car on this most useful site www.e38.org/e34/ and in particular this wiring diagram file www.e38.org/e34/e34_88_etm.pdf and I back checked on what I had worked out. Hey I was almost perfect. The only slight confusion was over a switch setting. BMW drivers have the luxury of a Decelerate/Set switch as well as our accelerate/set switch. Otherwise I got full marks. This little reverse engineering excercise has given me high confidence that this should work.

I have now built up my temporary cross adaptor cable so in the very near future (if it ever stops raining in Blitey at the moment), I'll have a trial fitting and test it out.
More soon.....



Cheers... Rob.
 

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Discussion Starter #5
OK

Hey it stopped raining for half an hour.
I got the temporary installation ready to go and have installed it. However since its now 22:30, it would be after curfew and wifey doesn't allow me out on the streets after dark.
So
Just thought I'd post a couple of photos of what I've done.

First one below shows the adaptor cable I made to test the unit in the car and the BMW board temporarily assembled into the Bently housing. This was done just beacuse it was easier than modifying the BMW housing as with its mounting bracket at present, it is much wider (i.e. did I it for speed).



I then installed it by zippy clipping (cable ties) the unit onto a captive nut lug above the accelerator pedal and plugged in my carefully labled wires into the Bentley cable connector.



Final neatened up temporary installation.



So all I have to do now is drive the car and test it....

.....An update when I have it.

Cheers... Rob.
 

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Discussion Starter #7
Well

I'm not pretty sure about it, I'm certain.

It works virtually perfectly. So what does that mean?.

Well. Basically all functions worked exactly as expected. The only difference of note is that the speed at which I could get it to start working was ~25 mph. The manual says this should be 30 mph, but I don't know how accuate that is. I am of course judging the actual speed by what I read on the speedo in front of me. I didn't get the sat nav out and check it that way (and I won't be doing so). Life is too short. The main news is that its fine by me. Note that I also won't be checking the top end shutoff!.
I include below the details of my cross connections to the BMW unit pins.



I now have to decide how to proceed with my unit. I think the neatest solution would be to remove the plastic bottom connector end of the device from the Bentley unit and transfer it onto the BMW unit (having removed the plastic bottom connector from that), and then connecting up the appropriate pins direct to the Bentley connector. This would allow the Bentley loom to plug straight in.

I'll also have to modify the BMW mounting bracket (or swap the Bentley bracket to the BMW housing). This is easy enough.

More photos of my final installation ready version when I have it.

Cheers... Rob.
 

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Rob,

You've gone way past my abilities. If you can adapt a BMW ECU, just think how easy it would be for you to adapt an old "El Cheapo" Chevy ECU.
 

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Discussion Starter #9
Hello Lew

These BMW units are easily sourced second hand from ebay. I think I paid £8.00 + £3.00 postage (which was a little high considering the unit weighs almost nothing). Maybe £10 for the 2nd one I won. The best bit is that they made a whole lot more BMWs than Bentley/RR, so supplies should be available 2nd hand. One thing is for sure, at this sort of price, these items are also 'el-cheapo'.

What would be really neat would be to see if the microcontroller chip from the BMW unit could just be trasferred onto the Bentley board. I'll have to do a rather more detailed investigation of that at the pcb board level, but a tertiary glance over it makes it look feasible. This all assumes that its the chip that has failed. I'll have to complete my test box to know that with any certainty.
This swap of BMW to Bentley works only for the electric motor type type of Bentley/RR cruise control. From what I can see, pre 87 (non EFI UK spec) cars had a vacuum powered actuator. I have not seen the control unit for that device so cannot comment. If I remember correctly, all non UK SZ series cars had EFI (and maybe the electric actuator?). EFI only became standard fitment in the UK after 1986 I believe

There are a number of UK vehicles that seem to have a Hella manufactured unit. Landrover being popular (and available). What I don't know is if they use a vacuum powered actuator. I may get one of these off Ebay just for a look see. Maybe someone has posted wiring diagrams for those cars also on the net. Hmmm........

Cheers... Rob.
 

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Vacuum actuated speed control

This is what I have in my 82 SZ. Until now I haven't noticed any electronic failure yet. :) Keep on looking and testing, Rob, I'm so glad someone does. Your research is of so much help to all of us! Thanks!

Greets, JimKnopf
 

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If I'm not mistaken the electric actuator started with the 87 1/2, 20,000 series cars.
 

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Discussion Starter #12
Just to really do this to death, I've gone and spent another £20 on a Merc W126 cruise control unit and the actuator, which looks suspiciously like the one installed on the Bentley (well at least on the photos on ebay). No doubt I'll get that next week and we'll be can opening that one too. I'll post photos. I also already have the car wiring diagrams for this installation having traded up from my old w126 300SE to this Bentley.

More shortly and hopefully another potential candidate for conversion.

Cheers... Rob.
 

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Discussion Starter #13
Ok
Finally received the Merc unit from Ebay and there is good news and bad news on this one. The control unit is as below.



However, when we pry it open, its rather old fashioned in the electronics department as opposed to our nice RR microcontroller version.



I have no doubt that this can be grafted onto the RR/Bentley and I have the car wiring diagrams to do this, but for now I am going to leave that alone. Maybe a job for a rainy afternoon (and we have plenty of those at the moment here in Blitey).

However

On a more good news front, I did also get the actuator from the same 85 S class. It might look familiar...







I believe this is the same unit used on Bentley/RR (and I've looked at the installation on my car from many angles). Except for the Merc standard plug connector, it is the same.

Cheers... Rob.
 

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Discussion Starter #14
Well, I modded the BMW mounting to more easily fit onto the mounts on the Berntly/RR behind the dash. A little careful cutting with a junior hacksaw and using the original Bentley unit to mark out the drilling details, I arrived at the following.





This installed a treat behind the dash. I still used my converter cables shown in the earlier post whilst I am still in a testing stage.

Cheers.... Rob.
 

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Discussion Starter #15
Fine tuning and becoming fussy in my old age

Although the installation 'works', I felt it was not ideally matched to the Bently. Remember that this control unit came from a BMW. A classy, but not the same car. So I set about improving on the one main issue I had with this conversion. This was that the unit gave a small surge when the set button was activated and I found that a little annoying. Also I noted that if the set/accelerate button was depressed again, there was a distinct urge about the acceleration that was not quite in keeping with the cars character (my opinion and this is for my car).
So
I set about a tedious investigation of the original circuit board and the BMW unit, especially around the speed sensor circuit from the cars drivetrain mounted pickup. I traced my the most suspiciously different resistor on the circuit (between the two PCB boards) and installed a temporary variabble potentiometer into the circuit that would allow me to vary it whilst driving. My solution is shown below.



I installed this into the car and mounted the potentiometer next to the wiper control switch on the dashboard using one of the dash mounting screws to hold the plastic box, and routing the wires appropriately to the cruise control unit.



The end result is that I decided on a value for this resistor of 100k Ohm. This gave a reasonably soft takeup of cruise control, and also on pressing the set/accel button again to increase speed, I did not get a sharp kick in the back and have the transmission kickdown to accelerate hard.



I hope that this is going to be my final modification to this system and hope to install this permanently behind the dash by bolting it to the original mounting locations.

I will then move onto my next job, the cars 54,000 mile major service and curing my small LHM leak from the front pump body.

Oh

I am still building my test box for the original Bentley/RR Cruise control unit.

Cheers... Rob.
 

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Discussion Starter #16
Thought I'd add just one final post on this. I stated that I was building a test box, but I decided to try something a little more radical beforehand.
I decided to attempt a heart transplant.
To this end I carefully unsoldered the 40 pin microcontroller from both the BMW and the Bentley units (having applied a label to the chip first).
A couple of points to note on this job is that the BMW PCB uses all plated through holes for the components. This makes de-soldering the chip a rather more difficult and time consuming job. They also bonded (glued) the chip down at one end. This needs some careful cutting (carefull insertion of a very sharp modelling knife) under the chip to cut the glue.
Once removed, I installed a 40 pin chip socket on the Bentley PCB to allow me to easily install/remove the chip next time. These sockets are relatively inexpensive and make things so much easier.
Here are a few photos of my work.









And... How does it work?
Well, in the absence of another car to test with an original unit for comparison, I'd say it works just great and the whole unit can now be re-installed as originally done.
To conclude on this, it seems that the BMW unit offers a full set of replacement parts to fix the Bentley unit. Although the layout is somewhat different, almost without exception, with a little detective work and tracing, all the components can be swapped to repair a faulty Bentley unit to get your unit back into service.

Finally
This has been an unashamedly personal journey to repair this unit. All the work I did was for my own benefit. I have documented it in this public arena for your interest only and so do not recommend or instruct other owners in any way as to how to get your unit fixed. You takes your own chances as they say.
If you found anything on this subject useful then that is great. If you just found it interesting that's fine also.

Good luck!
Cheers... Rob.
 

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I am envious that you can work on the electronics as you do. About the closest I have come is replacing the memory ECU back-up battery that is soldered in the circuit board. :?
 

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Rob,

Have you thought about doing repairs on these ECUs for profit? I'll bet there are many owners that would like to get them repaired instead of forking over mega money for a new one.
 

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Discussion Starter #19
Hello Lew

Nice to hear from you.
In reponse to your last comment.

Well, I have a full time job and not enough hours in the day for all those other things in life.
I would have a go at repairing anothers unit, on the understanding that it was just as a favour done on a best efforts basis. I'm not made of money, but I don't think I want the extra pressure of having to produce. I'm afraid I don't fulfill the American vision of a striving to succeed individual. I'm an enthusiastic amateur at home, though I am an electronic engineer at work.

Doing this sort of thing as a business venture would be hard work for me.
Sadly, having a go can be a problem. Those nice people that offer the mega bucks repair service on this particular unit (and I'm not talking about the earlier econocruise based item) are basically getting money for old rope, especially if you can procure a cheap 2nd hand (but serviceable) BMW unitand you can use a soldering iron and a desoldering pump.
The professional outfit do however offer full backup but won't touch a unit that has been tampered with. A nice touch to put the frighteners on the customer to keep them away from the likes o' me. I take my hat off to them though. They must make money at it.
BTW I have another BMW unit in addition to the one I used for my repair. Make someone a good spare......

Cheers... Rob.
 

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spare creuise control unit

Rob, I come back to you when I i have the appropriate car AND a defective ECU. :) Keep the stuff on stock!


Regards, JimKnopf
 
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