ABS switch on an ABY & bias valve

[sam80quattro]

put relays back in

[re2500]

Touche!

[Rusty]

So, having read the guide above, I decided that I wanted to know why things happened and so got cracking. Initially, things weren't too clear and I've tried a few things to get the ABS to turn off...

I started with the wiring diagrams for 3B and ABY but it wasn't clear what controlled what, so I went out the car and started to play. A few things later and I still hadn't worked out what was what, so back inside and as ever, the wiring diagrams made a load more sense after seeing it all in the flesh so... here's what I found:-

The ABS light is controlled by the two yellow wires that go to pin 7 on the ABS unit. When the ABS relay is energised, the lamp negative is switched to 12V, thus there's no voltage across the bulb (12V-12V). When it is not energised, the negative is switched to earth, hence the lamp turns on (12V-0V) (A possible MOT solution???

From studying the diagrams, pin 1 on the ABS ECU takes a direct feed from a blue/black wire from Fuse 31 and feeds 2 other blue/black wires for the diff lock.

In the 3B, the switch latches the ABS combi relay and I think puts a 12V pulse through the black / blue wire to pin 1 on the ABS ECU (which is connected to F114 - acceleration switch).

The green/white wire that goes from pin 17 (ABS ECU) to pin 3 (ABS unit) provides the positive feed to the ABS motor and solenoid valve relays.

The brown/yellow wire that goes from pin 28 (ABS ECU) to pin 5 (ABS unit) provides the earth for the ABS motor.

The green/purple wire that goes from pin 27 (ABS ECU) to pin 1 (ABS unit) provides the earth for the solenoid valves.

I have tested disconnecting the green/purple wire and this turns off the ABS. I'll try the other ones at a later date.

I'm still making sense of it all, so will post again when I've worked out which bit the diff switch interrupts. I'm guessing that it gives a pulse or switched signal to a pin but I'm at a loss as to which one...

[Rusty]

Got it

As the difflock confirm switch (on the diff) is normally open, so the circuit is incomplete and thus the brown/black wires feeding the difflock confirm light and pin 13 on the ABS ECU are held to +12V. The light does not illuinate, as the live on the other side of the bulb is also at +12V.

When the difflock confirm switch is activated by the diff locking action being complete, the circuit is completed and the brown/black wire is grounded; completing the circuit and so turning on the difflock confirm switch and also grounding pin 13 on the ABS ECU. This explains why rj_scotty had an illuminated difflock confirm light when he did his version, as he was both turning off the ABS by grounding pin 13 and at the same time, completing the circuit of the adjoined difflock confirm light! I stumbled on this after the difflock confirm light stopped working as the same times as the ABS light, due to no more vacuum. Start the engine, get vacuum, all works... I then confirmed this with a multimeter.

There is a 2 pin white connector under the rear seat which has 2 brown/black wires in one terminal. This terminal feeds pin 13 of the ABS ECU and the difflock confirm light from the difflock confirm switch circuit. This is where I will be interrupting the circuit to do my install and I am planning on using the aux relay holder to make it all neat.

I will look in to getting a latching relay, which I can use to interrupt the difflock confirm circuit and thus toggle pin 13 to disable the ABS. This will be separate to the difflock confirm light, so when active (ABS off), it will break the ABS ECU to difflock confirm light circuit and so will not illuminate the difflock confirm light. When inactive, the relay will complete the circuit and operate as normal, meaning that pressing the difflock button will still diable the ABS. The ABS off light is a separate circuit attached to the ABS unit and so is not affected in any way and neither is the safety aspect of the light.

Obviously, the much easier way is to just remove the fuse... but where's the fun in that

One last thing I noticed relates to the speed control wire. I had thought that this was one and the same with the similarly coloured wire on the difflock switch but it is not the same (difflock switch = grey/blue; difflock control unit = white/blue). This explains why cutting the wire at the difflock switch doesn't work No idea what that grey/blue wire actually does...

[Rusty]

So, turns out that all you need to do to cancel the ABS on an ABY is to bridge the difflock sensor (white plug under the rear seats) As a belt and braces, I've also plcoked both vacuum pipes to the controller, so that it doesn't accidentally lock the diff...

I'll be getting a bias valve in the brake circuit, initially under the bonnet by the ABS unit, as I don't know where it'll do inside the car with all the interior in there just now.

Obviously, the addition of a relay to a bridge circuit would enable the switch to operate the ABS cancel using a standard relay, so that's a lot simpler than retrofitting a 3B ABS relay, like the one I have next to the keyboard!

Just needs the appropriate connections to the switch and then a wire running to the rear to switch the relay.

[Rusty]

So, having bought a 3B relay over a year ago, I've not done anything with it, so I decided to use today to see if I could do anything.

I studied the wiring diagrams for a while and got waht I thought were the right pins, so went and did a test but no joy.

So, after some more effort, I gave up with plan A and moved on the plan 2.

Plan B was to swap the ABS switch face from the puch button ABS body to a latching body, like those used for fog lights

The latching switch simply grounds out the black and brown wire that goes to the diff lock switch and a quick test indicates that it's sorted it

Very simple - only requires an earthing point and a couple of metres of wire. I've yet to make a permanent connection at the diff end, so for now it's either the diff lock or ABS switch but that will be sorted when I have access to a soldering iron that I can use in the carpark.

Just the early style bias valve to install now and then I'll go and test the balance - if that doen't make things a little less rear biased, then I'll buy an adjustable valve and mount it under the bonnet.

[Rusty]

Fitted the early spec bias valve today. Noticed the early spec was marked 28 and the late was 37. This seems to sort of tie in with the S2central quoted output pressures for 50 Bar input but who knows...

Testing tomorrow with the ABS cancel switch, to see if it now locks the fronts... and not the rears

Alignment Tuesday, so see what front camber I can muster...

[Rusty]

Did some testing with the car today and the ABS switch works a treat. Unfortunately, although it offers less rear bias, the old style bias valve is not quite enough for my setup, meaning that the rears do start to drag a little under heavy braking. This may be a different story on track - which I'll find out - but I think I'm going to have to go custom...