P38A Random braking problem-sometimes extremely hard brake pedal

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Zaku

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I have a problem with the brakes in my P38 4.6. 2000.
When driving, once in a few brakings, the brake pedal is very, very hard to press, so it's hard to brake. And a moment later it works normally.
When driving, sometimes (quite often) you can hear something like "trtrtrtrtr" near the brake pedal.

There are no errors on the dashboard. The lights are not flashing. There are no errors in the Nanocom. There are no leaks (at least I didn't notice anything with the brakes or the brake pressure pump).

For the test, I took out the black ABS relay. Of course, the errors appeared, but the brake worked normally for a long time (a dozen or so brakings), so I think the high-pressure tank is tight.

I don't have a RR repair shop far away, so I have to fix it myself. Looks like some kind of sensor? Any ideas where to start?

I would be very grateful for any advice!
 
sounds like the abs is activating when perhaps it shouldnt, i think this will give a hard pedal as it assumes you already have you foot on the pedal.
check the live data from the wheel sensors, could be a dodgy sensor or broken reluctor ring
how many pedal presses between the pump running?
 
Thank you for your answer.

Can I find live data from the wheel sensors in Nanocom? If a damaged ABS sensor at the wheel can cause such symptoms, maybe it would be worth disconnecting them one by one?

I don't know yet how many presses there are before the pump starts, I'll try to check (when I was driving without a relay, i.e. without a pump, it braked normally and effectively, I think about 20 times).


EDIT:
ABS sensor damage did not show an error on the dashboard?
 
Zaku said:
Thank you for your answer.

Can I find live data from the wheel sensors in Nanocom? If a damaged ABS sensor at the wheel can cause such symptoms, maybe it would be worth disconnecting them one by one?

I don't know yet how many presses there are before the pump starts, I'll try to check (when I was driving without a relay, i.e. without a pump, it braked normally and effectively, I think about 20 times).
Click to expand...
Live data is available on Nanocom. Disconnecting a sensor will throw a fault as the ECU expects pulses from the sensors.
 
kermit_rr said:
Nanocom only collects liver wheel data during the diagnosis period, when the abs light is on. It goes out at about 5mph
Click to expand...
That's definitely the case on Wabco-C, but Nanocom Wabco-D (1999 onwards) might be different ? The later guide does not mention it, but the earlier guide does ?
 
Zaku said:
I have a problem with the brakes in my P38 4.6. 2000.
When driving, once in a few brakings, the brake pedal is very, very hard to press, so it's hard to brake. And a moment later it works normally.
When driving, sometimes (quite often) you can hear something like "trtrtrtrtr" near the brake pedal.

There are no errors on the dashboard. The lights are not flashing. There are no errors in the Nanocom. There are no leaks (at least I didn't notice anything with the brakes or the brake pressure pump).

For the test, I took out the black ABS relay. Of course, the errors appeared, but the brake worked normally for a long time (a dozen or so brakings), so I think the high-pressure tank is tight.

I don't have a RR repair shop far away, so I have to fix it myself. Looks like some kind of sensor? Any ideas where to start?

I would be very grateful for any advice!
Click to expand...

As others have said, it could be a sensor. Check they're all banged in all the way on the hubs.

There are 2 switches in the brake switch unit. Might be worth checking as they should be mutually exclusive, that is one goes on as the other goes off and vice-versa. You reset the switch by popping it out, pulling the plunger out and then refitting it. It will adjust itself to the right length when you use the pedal.

If the brake fluid hasn't been flushed in a year or so then do a full flush as per RAVE.

Finally, wiring connectors can be problematic. I cannot recall if the ABS one is behind the A-post but that's a favourite place for connectors to corrrode.
 
Thank you all for your answers!

I did a test with nanocom wabco D - sensors voltage and wheel speed.
I drove long distances many times, very slowly (over 6 km/h disconnects them from communication). All the time "Wheel speed" were practically identical on all wheels (max differences 0.01- 0.02).
Sensor voltage was also almost identical. A bit higher on both front wheels.
A few times right after the test, as soon as I accelerated, I had a hard brake pedal, but not always. During the tests it was always suitably soft.

Does this mean that the sensors and reluctor ring are rather efficient?
What should/can I check next?
 
As Mark said above, count the brake pedal presses before the pump cuts in.

1) Open bonnet, start the car, and listen for the pump.
2) Wait until the pump stops & dash lights go out.
3) Press brake pedal several times until the pump cust in again.

It should take 4-5 presses before the pump runs. Three presses means accumulator is getting low. 1-2 presses means it needs replacing.
 
pwood999 said:
As Mark said above, count the brake pedal presses before the pump cuts in.

1) Open bonnet, start the car, and listen for the pump.
2) Wait until the pump stops & dash lights go out.
3) Press brake pedal several times until the pump cust in again.

It should take 4-5 presses before the pump runs. Three presses means accumulator is getting low. 1-2 presses means it needs replacing.
Click to expand...
Should it be cold or after a long ride?

Press and hold or just click the pedal?
 
Thank you all!

I did 20 tests or more ;).

The pump turned on every time (maximum the second time) I press. So now we know it's the pump's fault.

Should the pump and the sphere be replaced (regenerated) together or can I do it separately? Does it make sense or not?
 
Zaku said:
Thank you all!

I did 20 tests or more ;).

The pump turned on every time (maximum the second time) I press. So now we know it's the pump's fault.

Should the pump and the sphere be replaced (regenerated) together or can I do it separately? Does it make sense or not?
Click to expand...

Probably just the accumulator (sphere). Try that first. Pumps tend to be very reliable. The pressure switch would be the next port of call.

Sphere first. I think Mark found some cheaper OEM ones somewhere.
 
Zaku said:
Thank you all!

I did 20 tests or more ;).

The pump turned on every time (maximum the second time) I press. So now we know it's the pump's fault.

Should the pump and the sphere be replaced (regenerated) together or can I do it separately? Does it make sense or not?
Click to expand...
Not the pump, just replace the sphere. 30 presses of the brake pedal before you try to remove it then it should unscrew fairly easily. No bleeding of the brakes needed but if the fluid has not been changed it would be a good idea to do a change.
 
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