Well, I have been having fun today...
I may have got somewhere with the Bendix ABS problems on the XM
Paul (Citroenxm), bless him, gave me ABS two valve blocks at CXM, one good one and one bad one that I was free to play around with. And have I ever played today...
The relay board is sealed but with the use of a sharp craft knife around the seam it can be opened. There's not much in there, just a diode and a relay along with some potentially dodgy soldering. All aspects of the board can be tested without needing to open it and if anyone wants, I can go through the tests.
The one I opened tested fine so I turned to the valve block itself. I noted when I tested the resistance of the five electrovalve coils on the one presently on my car that four read 3.5 ohms but one was much, much higher. I found the same on Paul's duff block, one being sometimes 85 ohms, sometimes 22 ohms and at other times nearly open-circuit or nearly right
I checked the good valve block and found all five electrovalves were all 3.5 ohms.
I then pulled the valve block apart cautiously by undoing the four tight Loctited Allen bolts and splitting the block in half. The electrovales can then be removed. They're a lovely piece of engineering..
The valves themselves can be further disassembled by gently unscrewing their caps nearest the coil wires and the coil bobbin lifted out.
Here's a shot of the block in bits:
By looking at the resistance of the duff coil I could make it vary by wiggling the wires from normal 3.5 ohms to open-circuit...
With a craft knife I have scraped away enough insulation to almost get to the coil terminations. Work has stopped at this point...
This is the partially excavated bobbin:
So, it looks like that could be the problem. The ECU must check the electrovalve coil resistances on self-test just as it does the sensors. It looks like the electrovalves may be amenable to repair and if not, a good valve of the right type could be swapped from another duff block or the coil bobbins swapped over or re-wound.
To aid the identification of the duff coil, measure the resistance between the following pins on the 7-way valve block connector and pin 5 of the 5 way valve block connector with the middle plug on the relay board plugged in. A good coil will read 3.5 ohms
Each electrovalve has different coloured wires going to them to aid identification as follows:
7 way connector Pin 1 - Yellow
7 way connector Pin 2 - Green
7 way connector Pin 3 - Red
7 way connector Pin 4 - White
7 way connector Pin 5 - Brown
Although it's a 7 way connector there are only 5 pins and pin 1 is the one nearest the edge.
So, don't throw away duff Bendix valve blocks - they can yield valuable spares
A huge thanks to Paul for making this possible
Also today Xac and Danielle popped around for a very nice visit whilst Cassy was having new tyres fitted. I guess they thought my place was a far nicer waiting room than that at the fitters
It made a very pleasant afternoon indeed