04 Disco SE7 Shuttle Valve Switch vs Modulator guts:

Valmon

New member
Hello! I own my 04 Disco w/83K original miles. Had it since — I drive it little obviously. I intend to keep it. Anytime it needs anything it gets new OEM. Lately, its dash started showing the three amigos erratically on and off. My mech scanned and read it pointing to the shuttle valve switch ($85): https://www.roverparts.com/brakes/calipers-cylinders-hydraulics-vacuum/SWO500030/

Contacts at dealer and two other reputable sources at supply parts concurred.

Another source #3 at Atlantic British suggested that in the last couple of years more reports and findings pointed to not just the shuttle valve switch but more than likely also the modulator guts that needed change ($730): https://www.roverparts.com/brakes/calipers-cylinders-hydraulics-vacuum/SWO500040/

I spoke with #3 and he stated, changing the shuttle valve switch alone might work. It may fail after a few months — if it does and the lights come on again then the modulator would have to be changed, which means changing the shuttle valve switch with it yet again for $730 + labor.

My contact at LR dealer, service foreman, that had the same vehicle, said the shuttle valve switch is all that he changed on his (same vehicle) and that was it.


Attached is a screenshot of the findings. How does one make an intelligent decision as to what to change at this point?
 

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Circuit is a serial of three 1kohm resistors between mass and a ECU input. 2 of these 3 resistors are shortcutted by 2 end-of-carrier switches depending how deep are You depressing brake pedal. Modulator will decide what to do between 3 different inputs: 1kohm, 2 kohm or 3kohm measured by ECU between input and mass.
But this serial resistor loop is used to get false contacts. Then ECU gets to read infinite ohm values. Knowing circuitry, you can arrive at following conclusion: you can change this plastic part with 2 switches and may be you will get third fault solved. If not, you will need to repair some solderings between internal modulators PCB's to get shuttle valve switchs terminals state in modulator main connector again.

Regarding first and second fault i didnt experienced them, but as you can read both messages are pointing to an electrovalbe that works together with a rear brake caliper, but both eleectrical faults are opposite. So i suspect ECU only is inferring these electrical faults as it detects some response error in the brake procedure, as if modulator valve is seized. Then, I would be much more suspicious that what is seized is the rear caliper itself. Have you changed the pads lately?
Scuse me for my eventual spelling mistakes (better for me is to write in spanish or german)
Regards
 

Valmon

New member
Circuit is a serial of three 1kohm resistors between mass and a ECU input. 2 of these 3 resistors are shortcutted by 2 end-of-carrier switches depending how deep are You depressing brake pedal. Modulator will decide what to do between 3 different inputs: 1kohm, 2 kohm or 3kohm measured by ECU between input and mass.
But this serial resistor loop is used to get false contacts. Then ECU gets to read infinite ohm values. Knowing circuitry, you can arrive at following conclusion: you can change this plastic part with 2 switches and may be you will get third fault solved. If not, you will need to repair some solderings between internal modulators PCB's to get shuttle valve switchs terminals state in modulator main connector again.

Regarding first and second fault i didnt experienced them, but as you can read both messages are pointing to an electrovalbe that works together with a rear brake caliper, but both eleectrical faults are opposite. So i suspect ECU only is inferring these electrical faults as it detects some response error in the brake procedure, as if modulator valve is seized. Then, I would be much more suspicious that what is seized is the rear caliper itself. Have you changed the pads lately?
Scuse me for my eventual spelling mistakes (better for me is to write in spanish or german)
Regards
I had the mechanic check brakes the same day last Friday, rear pads need be changed. Will replace w/genuine OEM pads and rotors with aftermarket All Makes NTC8781. Front ones still have 30%. I don’t drive fast. And I probably drive it 3,000 miles a year.

At times I do have to depress the brake pedal more than usual to get it to stop.

Thank you for a thorough explanation. I get the gist of it. I’ll share it with my mechanic Paul whom I’m sure will understand it whole.

Are you suggesting that maybe it won’t need a shuttle valve switch or Modulator?

If you were to write in Spanish or German, I would use Google translator into English. Not sure how well the translator would do with technical terminology.
 
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discostew

Well-known member
Sep 14, 2010
7,706
1,015
Northern Illinois
I had the mechanic check brakes the same day last Friday, rear pads need be changed. Will replace w/genuine OEM pads and rotors with aftermarket All Makes NTC8781. Front ones still have 30%. I don’t drive fast. And I probably drive it 3,000 miles a year.

At times I do have to depress the brake pedal more than usual to get it to stop.

Thank you for a thorough explanation. I get the gist of it. I’ll share it with my mechanic Paul whom I’m sure will understand it whole.

Are you suggesting that maybe it won’t need a shuttle valve switch or Modulator?

If you were to write in Spanish or German, I would use Google translator into English. Not sure how well the translator would do with technical terminology.
Just the shuttle valve switch probably won’t fix your problem. German posted a great write up of the modifications your going to have to make. Don’t forget to clean the grounds near the left headlamp.
 

special ed

Well-known member
Apr 11, 2012
182
115
Elsinore
Shortcut all of that. get a new pair of shuttle valves. cut both wires at the connector and solder extensions on them. A foot is more than enough. drill a hole in the bottom of the shuttle valve plastic plate and run the wires out. reinstall the new shuttle valves in the modulator. then cut the yellow green stripe wire in the harness and connect one side of the shuttle valve switch to that on the car side. The side going to the connector at the modulator is left alone and no longer used. Then ground the other wire out of the shuttle valve. Right next to the front of the modulator is a screw holding an ac line. you can shorten the wires before connection to make it clean.

The computer is just looking for ohms to ground and the circuit does not need to me inside the modulator. It can be run entirely external to the unit. Taking the modulator apart is completly a waste of time and a lot of unneccesay work.
 
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nashvegas

Member
Feb 12, 2009
23
11
I agree. I'd do this:

1 - Replace the shuttle valves. Note if they're wet w/ brake fluid you technically ned to buy a seal kit and replace the little seals up at the top of the cylinders (looking at it from below when shuttle valve is removed). Lots of folks don't do this and it's fine... will go for a few years but can foul the new shuttle valve I guess

2 - Do the option B ground fix.

and I'd bet 99.999% that your prob is fixed.

I've never - other than that Atlantic British video - heard of anyone fixing the 3 amigos by replacing that other $700 part.
 

mlnnc

Well-known member
Mar 23, 2008
266
31
Charlotte
I agree. I'd do this:

1 - Replace the shuttle valves. Note if they're wet w/ brake fluid you technically ned to buy a seal kit and replace the little seals up at the top of the cylinders (looking at it from below when shuttle valve is removed). Lots of folks don't do this and it's fine... will go for a few years but can foul the new shuttle valve I guess

2 - Do the option B ground fix.

and I'd bet 99.999% that your prob is fixed.

I've never - other than that Atlantic British video - heard of anyone fixing the 3 amigos by replacing that other $700 part.
No one replaces the shuttle valves. The code is for the shuttle valve switches.

I respect the OP's desire to keep everything OEM, but in this case that would be crazy expensive and would not be any better than doing Option B to bypass the failed solder joint connection buried deep in the ABS module.

But hey, it's your money!
 

discostew

Well-known member
Sep 14, 2010
7,706
1,015
Northern Illinois
No one replaces the shuttle valves. The code is for the shuttle valve switches.

I respect the OP's desire to keep everything OEM, but in this case that would be crazy expensive and would not be any better than doing Option B to bypass the failed solder joint connection buried deep in the ABS module.

But hey, it's your money!
I’m with minc on this. Do the shuttle valve modification and clean the grounds.
Keeping shit original is good for radios, wheels and that type stuff. But this will fix your truck, and nobody will know it’s not original. I would suspect it wasn’t original but just cause it works and the lights aren’t on.
 

best4x4

Well-known member
Mar 1, 2015
595
72
Beaumont, TX
Option B is a simple way to bypass the weak link inside the Wabco Unit itself. No point in buying another Wabco Unit to only have the flaw show up again down the line. Once Option B is done you really just have to deal with ABS sensor faults. I've saved I dunno how many Shuttle Valve Switch assemblies by simply using some 3/32 heat shrink over the bare wiring (brown insulation was LONG gone) and then performing Option B.
 
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jastutte

Well-known member
Nov 10, 2009
461
71
Option B is a simple way to bypass the weak link inside the Wabco Unit itself. No point in buying another Wabco Unit to only have the flaw show up again down the line. Once Option B is done you really just have to deal with ABS sensor faults. I've saved I dunno how many Shuttle Valve Switch assemblies by simply using some 3/32 heat shrink over the bare wiring (brown insulation was LONG gone) and then performing Option B.
this is the best long term fix for the Shuttle Valve Switch issues.
 
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