ABS Pump & TC light

Hi all,
Got a problem with my ABS pump. It does not start on its own with ign ON, specially after parked for a while. At that moment virtually no brakes. The 3 amigos will stay ON & when the pump starts the TC light goes out and everything is normal thereafter. (hand brake light goes out with HB down & ABS light after moving )
Most of the time I have to tap the ABS relay 17 ( new relay & new fuse box, about two months old).
Nanocom says ' Internal ECU failure'. The problem started with the new fuse box and vanished after few days.
Now when I start the car, if the TC light stays on, I know the ABS pump is not working
Seek your advise.

As the problem started when you replaced the fuse box and relay then either the fuse box and/ or relay are defective, or there's a defective connection to/ from the fuse box.

Many mis-construe the 'ABS Pump' ...... it is not an ABS Pump.....the entire braking system on the P38 is reliant on the Hydraulic Booster Pump....

Most other cars use a Servo Unit to boost hydraulic pressure based on pedal pressure - the P38 doesn't have a traditional servo assisted system and instead hydraulic pressure is generated by the pump alone.

In most other cars, failure of the ABS system, the ABS Pump or the Modulator will still give you full braking pressure as it uses a Servo for the assistance.....in the case of the P38, no pump = no brakes.

So with your knowledge that the pump isn't working - this is why you have no brakes.....

Find the reason for the pump not running - could be blown fuse, dodgy wiring or the motor is fecked...try running it with a direct 12V feed to the appropriate connections and see if it will run and go from there.

The pump works but only thing is it won't start , randomly, after parked for 4 - 5 hrs. Otherwise, it runs happily.
What is the TC light got to do with the brake pump?
Why no pump no brakes, isn't the accumulator suppose to keep the pressure for 20-30 brake applications? (accumulator was replaced about 5 years ago with genuine WABCO).

Checked the wiring and reinserted the under fuse box wiring connectors. Can't get to the brake pump's wiring connector at the pressure switch end, it's too tight & no space too.

The accumulator will hold enough pressure for efficient braking for only between 6-10 pedal applications at the very best, 5 or below it is getting weak, 3-4 is it tired, below 3 it needs replacing.

the 20-30 pedal pushes is to fully depressurise the system to allow the changing of the accumulator, after about 10 pushes, there is not sufficient hydraulic pressure to stop the car efficiently, but enough to cause havoc if you unscrew the accumulator, so RAVE stipulates 20-30 pushes to ensure all pressure is spent.

The TC system uses the ABS system for operation - no brake hydraulic pressure - No Brakes, No ABS and No Traction Control

Saint.V8 said:

The accumulator will hold enough pressure for efficient braking for only between 6-10 pedal applications at the very best, 5 or below it is getting weak, 3-4 is it tired, below 3 it needs replacing.

the 20-30 pedal pushes is to fully depressurise the system to allow the changing of the accumulator, after about 10 pushes, there is not sufficient hydraulic pressure to stop the car efficiently, but enough to cause havoc if you unscrew the accumulator, so RAVE stipulates 20-30 pushes to ensure all pressure is spent.

The TC system uses the ABS system for operation - no brake hydraulic pressure - No Brakes, No ABS and No Traction Control

Click to expand...

There should always be braking on the fronts regardless of hydraulic pressure. But stopping power will be next to nothing. The rears will not work at all without hydraulic pressure.

Saint.V8 said:

The accumulator will hold enough pressure for efficient braking for only between 6-10 pedal applications at the very best, 5 or below it is getting weak, 3-4 is it tired, below 3 it needs replacing.

the 20-30 pedal pushes is to fully depressurise the system to allow the changing of the accumulator, after about 10 pushes, there is not sufficient hydraulic pressure to stop the car efficiently, but enough to cause havoc if you unscrew the accumulator, so RAVE stipulates 20-30 pushes to ensure all pressure is spent.

The TC system uses the ABS system for operation - no brake hydraulic pressure - No Brakes, No ABS and No Traction Control

Click to expand...

wammers said:

There should always be braking on the fronts regardless of hydraulic pressure. But stopping power will be next to nothing. The rears will not work at all without hydraulic pressure.

Click to expand...

Morning Tony,

I did say '...not sufficient hydraulic pressure to stop the car efficiently....' and whilst I said '...no brake hydraulic pressure - No Brakes, No ABS and No Traction Control...' What I meant to mean was 'in relative terms there are no brakes....'....

Saint.V8 said:

Morning Tony,

I did say '...not sufficient hydraulic pressure to stop the car efficiently....' and whilst I said '...no brake hydraulic pressure - No Brakes, No ABS and No Traction Control...' What I meant to mean was 'in relative terms there are no brakes....'....

Click to expand...

Mornin Ant.This is true relatively.

It behaved as nothing wrong, so far. Diags not much help. Could it be the accumulator?
To remove the wiring connector from the pressure switch, do I have to remove the whole pump first? or is there any easy method. I want to clean that connector & see.

wammers said:

There should always be braking on the fronts regardless of hydraulic pressure. But stopping power will be next to nothing. The rears will not work at all without hydraulic pressure.

Click to expand...

forgive me, I thought having 'brakes' & 'stopping power ' , relatively has the same effect.

kapilamuni said:

forgive me, I thought having 'brakes' & 'stopping power ' , relatively has the same effect.

Click to expand...

Are you serious? Stopping power is relative to the amount of pressure being applied. There will always be some braking effort on the front brakes even if there is no stored hydraulic pressure in the system. But that would of course be relative to the power of your leg muscles. Why do you think the front brakes are bled without any stored hydraulic pressure, but the rears need stored pressure to be bled. The fronts are hydrostatic the rears are not. If you have to tap the relay to start the pump it must be pretty obvious where the problem is.

If the TC light stays on it means there is less than 110 bar pressure in the system. It also indicates that there is a made circuit to pull the pump relay and run the pump. If the pump is not running and starts when the pump relay is tapped it means the relay is faulty. When the pressure rises to 110 bar the TC lamp will go out and the car can be driven, as minimum working pressure has been achieved. The pressure switch has three functions. Look at RAVE for descriptions of what lamps on or off mean at various stages.

wammers said:

Are you serious? Stopping power is relative to the amount of pressure being applied. There will always be some braking effort on the front brakes even if there is no stored hydraulic pressure in the system. But that would of course be relative to the power of your leg muscles. Why do you think the front brakes are bled without any stored hydraulic pressure, but the rears need stored pressure to be bled. The fronts are hydrostatic the rears are not. If you have to tap the relay to start the pump it must be pretty obvious where the problem is.

Click to expand...

You are correct.
But the relay is new.

kapilamuni said:

You are correct.
But the relay is new.

Click to expand...

See post #2 from OrangeBean!

A new relay can be faulty straight out of the box same as any other electrical component.

will change the relay n see.

The issue came back again even with a new relay already in place. While hovering through the fuse box, noted that the issue is with the relay-15, IGN relay (and not the ABS relay 17.) It too is a new relay but wriggling it, the pump came back on. Changed that too with another new relay which put the pump into action immediately.( Fingers crossed.).
I'm going to look into the ABS ECU connector too, tomorrow.

You should not be wriggling things to get them to work on a new fusebox!

Yes and mostly new fuses too.