option B install

**steve_a** · 25th September 2016, 11:34 AM

Originally Posted by sierrafery

I didnt say it doesnt work, what i mean is just that it has nothing to do with the tester bleeding cos it bleeds only the IV circuit between the modulator and calipers which is covered by pedal bleeding anyway

ABS is a fail safe system, pull the power to the block and you get normal brakes. That means that the paths from MC to each caliper is open.
Thinking on the fly, for the ABS function typically the valve for the locked wheel is closed (and vented?) to stop it from locking, carried out on a per wheel basis, the idea being is automated cadence braking on each wheel, with pressure supplied by you/pump. ABS mode.
ETC is the pump supplying braking to a wheel, so I guess if you want to brake FR you close FL,RR,RL valves and engage pump.
HDC is probably more like ABS with the pump replacing your foot.

I'm thinking from that to bleed the block you want to operate the pump to bleed the pump body and relief. If everything is open then I guess relief area may not get it, unless on over pressure.

FWIW, I've actually bled the block itself with a contraption to control all the valves and pump, but that was very specifically done to clean power steering out. Back on the car I used the relay bypass to bleed and didn't find any issues what so ever.
Other times I've used the relay bypass and nanocom to bleed, and again the pedal has been great afterwards.

**sierrafery** · 25th September 2016, 01:16 PM

Sorry, i disagree, IMO you misunderstood things a bit(and forgot about EBD which is the most complex of all

), there are 2 circuits in the modulator, the IV(inlet valves) and OV(outlet valves)...the IV circuit is N/O so that's the fail safe side of it for normal braking cos they close only in active braking mode(EBD, ABS, ETC, HDC) while the N/C OVs are opened by the ECU to achieve the desired action with pressure management according to the situation, better read again the system's description and operation mode.
I edited the modulator's diagram and what's with red is the pedal circuit on one side (same on the other), that circuit is opened all the time with the exception of situations during active braking when the ECU closes them to reduce pressure on that caliper/s and eventually opens OV's, there are many scenarios(each situation is described in the modulator's description in RAVE or in WABCO documents, i'm not gonna paste the whole thing here)

If you unplug the modulator or something fails only that red(IVs) circuit is open the OVs are closed, with tester the pump will be activated and the OVs opened too so then there will be full flow within the modulator otherwise the expansion chamber is islolated, if you bypass the relay this will not open the OVs only start the pump and the flow will be only between pump and calipers through the IVs so if you want a perfect job you'll use both methods pedal and tester bleeding(better at the same time which means activate that circuit with tester while you push the pedal) but if you want to use only one of them then pedal bleeding is more efficient than tester bleeding cos with tester the circuit between the modulator and master cylinder is not covered only between the pump and calipers... that trick with HDC can help but only if it's followed by another pedal bleeding otherwise the air which eventually was pushed out from the OVs circuit has no way out

this system is more complicated that it appears

**ozscott** · 25th September 2016, 02:10 PM

What do you think of Option B Sierra? I can't recall if you have commented before about it. Apologies if you have already. Cheers

**sierrafery** · 25th September 2016, 05:28 PM

I think is that option B is the perfect fix for the poor way in which the SVS signal side of the modulator was conceived and built... more modern modulators like they started to put on D3s dont even have SVSs they are taking the brakes on/off input from the brake switch circuit... i invented a way to bypass the whole SVS thing with a relay and two resistors and posted it here some time ago but i was thrown away with it and called idiot even though it was based on a prolonged study of the system and tested on my own car before(still working since then without any issues) but i'm not gonna speak about that again here... what's certain for me is that what ever happens with the system it's well enough conceived as any default mode for any fault will let the IVs open so normal braking should not be affected whatsoever, IMO fitting greater than 5% tyres is worst for the braking distance and system behaviour than my blamed mod and more dangerous in traffic...i explained that in some other thread a while ago.

**ozscott** · 25th September 2016, 06:40 PM

Thanks mate. I am interested in why say 10% increase in tyre size adversely affects the system. I suspected that the system is designed with 29.5 inch tyres in mind and any deviation might cause less than optimum performance but I don't pretend to know why.

Cheers

**sierrafery** · 25th September 2016, 07:30 PM

Welcome anytime

... the first part of the theory is here when i was just presuming based on study: https://www.landyzone.co.uk/land-rov.../#post-3290851 and some more after i made live tests too: https://www.landyzone.co.uk/land-rov.../#post-3620824

**ozscott** · 26th September 2016, 05:57 AM

Thanks again. It makes sense. Here is a Wabco tech bulletin that for a bus/truck air brake ABS system that actually refers to design parameters in wheel revolutions per mile - 437 to 591.

https://www.google.com.au/url'sa=t&s...uvS7CxZBsachsw

Cheers

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