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.
 
Last edited:

discostew

Well-known member
Sep 14, 2010
7,141
803
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
46
8
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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