Through intense use of Google, I found a book and various websites that had all of the electrical specifications for the inputs and outputs of the Teves Mk2 system.  Armed with these and my £4.99 multimeter, I set to work.

I managed to establish that unlike 99% of Sierra ABS faults, mine was not down to a faulty wheel sensor.  The entire system checked out perfectly with the exception of the Fluid level sensor.  The fluid level sensor is connected to Pins 9 and 10 on the ABS ECU.  When the system is properly pressurised and the correct amount of brake fluid is in the reservoir, you should be able to meter continuity across pins 9 and 10.  If the circuit is made, the ECU enables the ABS, if the circuit is broken, the ABS is disabled.

The penny dropped after I spent hours deciphering various circuit diagrams of the system - and when i remember reading how to disable the ABS in a Group A rally preparation manual.  I came across the sensor on the Anti-lock Brake Diagram in the Sierra Haynes comic.  In real life, the cap on the top of the brake fluid reservoir has two connectors - one has three pins and the other has two pins.  The one with two pins is the ABS system’s fluid level sensor - it’s connected through the pressure sensor on the hydraulic unit, and to the ECU.  The one with three pins is the normal low fluid level switch which illuminates the red brake warning light on the instrument cluster.I spent two hours today barking up the wrong tree - I thought my problem was in the three pin connector, so I set about cleaning the contacts and checking it for continuity.  After I’d worked out how it worked, I realised it has nothing to do with the anti-lock part of the braking system.  I’ll explain why:

During Normal operation:

• Current passes from the battery to the instrument cluster.
• If the handbrake is up, the current can return to the battery via the bulb and the handbrake switch, and the red handbrake light illuminatesIf the handbrake is down, the current has no route back to the battery, and the red handbrake light can not illuminate.

During Failure:

• Current passes from the battery to the instrument cluster.
• If there is insufficient fluid in the reservoir, the float on the cap’s shaft closes the circuit to the red handbrake light - the red light illuminates, warning you of a low fluid level, regardless of the handbrake’s position.

It’s basically a circuit that can be closed by either of two switches.  It took a while to work it out but I’m glad I did because now I know how it works, and why there are two connectors on the top of the cap.  Other variations of the Teves Mk 2 system have slightly different arrangements - the one on the Peugeot 505 Turbo has a single 5 pin connector. Whilst trawling through the circuit diagrams I also discovered another difference between the 4x4 and 2wd versions of the system.  You’ll read more of this later.

I’d ruled out the 3 pin Fluid Level sensor.  Next was a simple test - the two pin connector was now the target of my assault.

I’d also worked out how it’s connected to everything, and that the ECU is looking for continuity.  I disconnected the plug, bridged the two pins, and disconnected the ECU.  I metered 9 and 10, and there was continuity!  Just to see if my theory was correct, I reconnected the ECU and turned on the ignition.

The pump started, and seconds later, the yellow ABS light was off!  Off for the first time in ages!  I’d finally found my problem.  To prove it, I pulled the bridge out of the connector - the yellow ABS warning lamp illuminated again; reapplying the bridge re-enabled the ABS system.  Eureka!  I bet Archimedes couldn’t have worked that one out…
I don’t mind failing the first time - knowing what doesn’t work, and why, is just as important as knowing what does work.

Anyway, now I’m up there with Einstein and Newton, I set about cleaning up the contacts on the cap’s connector.  I reconnected the plug, switched on, and hey presto!!  I was back in a loin cloth with the cavemen.  The amber light was staring at me.  Again, I’d been outwitted by the Teves Mk 2.  Grrrr…

I tested the two pin connector on the cap, and there was no continuity, regardless of the float‘s position.  It appears that the cap is damaged internally, and needs to be replaced.  No big deal really - at least I KNOW where the problem is now, and I don’t have to plan for further diagnostic sessions.  Let’s see if I can find a cap!

I mentioned earlier that the 4x4 and 2wd ABS ECUs are different.  The first difference that springs to mind is that for one, the 4x4s use a different caliper/disc setup to the 2wd Cosworths.  The other difference is a little less obvious.

Sierras with ABS that were built up to May 1989 had an orange relay in socket VII in the fuse box.  This relay is an interrupt that cuts power to the ABS ECU when the engine is started. After the engine is started, relay VII returns power to the unit.  This means that the ABS ECU has no connection to pin 19.  From June 1989, relay VII is absent, instead, the power is interrupted by the ABS ECU itself when it detects a voltage on pin 19.   You'll find this in the Anti Lock Brake Wiring Diagrams in the Haynes Sierra comic.