Technical Seicento Schumacher ABS issue - lots of info & what to check next

[Jamieboy]

Hi All, I have a very weird ABS problem with my Schumi that I would appreciate some advice on. After rebuilding all 4 corners of the car and replacing all ABS sensors with new items, I was very dismayed to see that the ABS warning light was still on. I hooked up an OBDII reader and got the following error:

The system is freely streaming data and live-updates the brake pedal status (not sure what the 1.75 km/h offset is about on all 4 wheels, the car was stationary).

I decided to also use FiatECUScan, which returned the following:

After some research, I found a Polish site which mentioned this error, so I put it through Google Translate:

I decided to get the cluster PCB out of the car so that I could test all diodes, including the light-emitting variety - using the diode test on my multimeter, everything seemed to check out.

The next port of call was to check out the signals getting to the cluster, so I referred to the technical manual for the ABS and cluster circuits.

The ABS circuit seems to feature a different representation of the cluster "E50", with 3 interface connections; my unit (part# 735290647) only has two. I used the wire colours and symbols to reconcile the correct connections.

I also decided to map pertinent tracks on the actual PCB and count the pins/parts and to work out which connector is which (note the colours I chose are just for high contrast - this is not the wiring colours):

Pin 4 of connector B is what I expect the ABS controller good/no good signal to arrive on. To give me some idea of what to expect, I probed Pin B10, the airbag good/no good signal (in which the warning circuit looks identical), I found that the voltage level started at ~3.5 V when the ignition key was initially turned, then rose to 12 V when the light went out. The ABS pin comes on at 3.5V and stays there.

I also decided to map the pins to colours, which I include here in the event it helps someone at some point in time, although there are a couple of omissions:

I then thought it logical to turn my attention to the signal source, at the ABS modulator, so I found these references and took some photos:

The ABS connector removed, everything looks pretty clean on the controller.

...and the connector side:

Here's where things get weird - with the connector off the ABS controller and the instrument cluster unplugged, pin 20 (verified pink wire) of the ABS connector is not only NOT connected to pin 4 of cluster A4, it has a 0 ohm short to earth. ABS connector pin 21 is also shorted to earth, neither or which I would expect.

Looking at how the signals route through the car, I followed the ABS loom from the ABS controller, but quickly found that it tightly tucks up under the bulkhead shelf and it is difficult to follow. I am looking for connection D5, which, from the ABS wiring diagram, seems to facilitate connection from the ABS look into the cabin. Does anyone know where this is?

If you're still reading and have not been put to sleep, thank you for your time! Any advice on what to look at next would be very much appreciated and if there are other tests I may perform/identification of things I may have got wrong so far.

 

[Jamieboy]

I have also read this ancient thread, which details replacement of "SMDs" on a Punto, which fixes an issue which has some similarities to my own.

I feel that this, like the Polish thread quoted in my initial post above are interesting for background and worth consideration, but the solutions are not applicable to the root cause of the issue(s) I am facing, since my fault is present without the cluster PCB attached.

 

[Jamieboy]

Yesterday, I made a bit of a discovery/realisation which reduces my issues to 1.

Inside the ABS controller receptacle is a little wedge of plastic:

I thought this was lock/alignment tongue for pins 20 & 21 in the loom plug:

But, I discovered that the plastic tongue actually serves to lift off the little metal bar, which otherwise shorts pins 20 & 21.

The bar itself is always connected to earth and shorts 20 & 21 similarly earth when the loom is unplugged. It seems logical that this is some kind of interlock that means should you drive without the loom attached to the ABS controller, it will flag up an ABS and brake warning light on the dash.

When the bar is carefully lifted off the pins (simulating it being connected) and temporarily held with an insulated wedge for testing....

....the handbrake and ABS warning feeds are isolated. I verified that the handbrake signal is influenced by application/release of the handbrake, which "only" leaves the ABS lamp feed as a problem, in that it is still not showing continuity between the controller end and where it arrives at the dash. I am hesitant to simply replace the feed wire as I want avoid cutting into the loom in the engine bay if I can help it and also have the hassle of routing a wire into the cabin, threading it up to and splicing it into the cluster, etc, when I would rather find the break and repair it (which could be at the elusive D5 junction).

 

[Jamieboy]

Update - I have found the discontinuity in the wiring between the ABS controller and the cluster plug. At (what I think is) junction D46 from diagram E7020 above, which runs inside the footwell outer panel on the driver's side (of my RHD car for non-UK members), the signal was not passing through the plug/socket:

In the closer view on the right, the ABS status signal does not pass over the connector pair. I have removed the pin and receptacle from the respective half and found both to be heavily oxidised. After cleaning the parts manually with a brass wire-wheel from a rotary tool, I reconnected the junction halves and now have continuity from the ABS controller to the cluster

I will update tomorrow once I put things back together to the point where I can test it.

 

[Jamieboy]

That fixed it. ABS light now extinguishes correctly. I had to take the cluster back to pieces as I hadn't tightened the LCD bezel clips tightly enough and the top halves of the characters were missing. Best check before building it all back up next time!