I've been trying to find a similar topic for an answer to this one, but can't see the same issue in other posts, so am looking for a bit of a steer (or a link!)

When I start my p38 in the morning the ABS pump runs as normal when I turn the ignition on, pressurises the system then stops. The three warning lights on the bottom left of the dash go out as normal, braking system seems to work fine. When starting it a second time if I've been out anywhere, no ABS pump on ignition just silence and the warning lights stay on. If it's driven then the brakes start to go spongey after a mile or so, I assume as the system loses pressure. Sometimes if I turn the engine off and turn the ignition back on then I can get the pump to run, system pressurises, the lights go out and it's ok again. The next morning on cold start it starts as normal, no issues. It's only started happening in the last week after a 500+ mile round trip. There's plenty fluid in the reservoir and I can't see any leaks and clearly the pump motor works, albeit intermittently.

I don't have a diagnostic tool, so am guessing a bit and my usual garage can't look at it for a couple of weeks. Has anyone had a similar experience of an intermittent fault or know what it's likely to be - relay, pressure switch, accumulator or pump motor starting to fail?

Many thanks in advance, if anyone's got any ideas.
Jim

Could be an intermittent pressure switch, failing relay or bad contacts in fusebox. The connectios from the pressure switch to BECM & ABS ECU are signals to them so they know the position of the switch. There's also some splices in the loom that can affect the pressure switch ground.

By the sound of it, diagnostics will not help as much as a good multi-meter & checking the wiring, etc.

RAVE Says:

ABS Hydraulic Pump (M102)
The hydraulic boost for the system is provided by the
ABS Hydraulic Pump (M102), which is controlled by
the ABS Pump Relay (K102) and the ABS Pressure
Switch Unit (Z104).
The Pressure Switch unit incorporates three
electro– mechanical switches. The first operates the
pump, two more illuminate the low pressure
condition and signal that ABS and ETC functions
should be curtailed. The Hydraulic Pump includes a
hydraulic accumulator and non–return valve, as well
as a pressure relief valve to protect the system.
When low pressure occurs in the brake system, a
switch in the pressure switch unit closes to ground
the coil of the pump relay. The pump relay now
energizes and applies battery voltage from the fuse
to the hydraulic pump through the closed relay
contacts. The hydraulic pump runs to increase
pressure in the hydraulic accumulator. When
sufficient pressure is developed in the system, the
pressure switch opens to de–energize the pump
relay and to turn off the hydraulic pump.

A couple of months ago I got out of main after parking on the driveway and could here a buzzing noise. Realised it was the ABS pump that was running even after the car had been switched off. Pulled the relay and it stopped. A subsequent check showed that the contact in the relay had burnt quite badly and at times would weld themselves together meaning the pump would run all the time. Swapped the relay with one from a spare fusebox I happen to have and ordered a new relay. The ABS pump relay is a high current, dual contact, one to handle the extra current drawn by the pump and not a standard on-off relay. It also has one pin that is smaller than the others so you can't fit a standard relay in its place. I would suspect that you have a dying pump relay.

If the pedal goes spongey once the stored pressure in the accumulator has been used up, you have air in the system. Normally with no stored pressure the pedal will be rock hard. It probably only needs bleeding at the first nipple on the modulator but a slight deviation from the process is opening the bleed nipple with the ignition on to ensure all air is out of the accumulator to modulator stage.

Thanks for feedback and suggestions Pete12345 & Gilbertd, - It started without any issues first thing this morning, pump ran then stopped, firm brake pedal and no warning lights then when I used it again later on, no pump, warning lights stayed on. I switched the ignition off then back on and the pump ran and everything worked, warning lights went out as normal, so am going to check relay and fuse box and probably look to replace the relay to start with and see if it keeps happening.
Jim

I had the same thing happen that Gilbert describes above, a few years ago. Also replaced the relay with no further issues.

+1 on dodgy relay. The hotter you get a relay coil the weaker it's magnetic field.
The weaker the field, the hotter the relay will get as you will have poor contact force.
Self destructive feedback loop!
Oh, then it cooks the base of the fuse box!

In addition to the above, I have one with a dodgy pressure switch. The pump would run after a light tap on the switch. Next time it happens, with the engine running tap the relay & pressure switch with a screwdriver handle & see if it runs ?

The switches are unobtainable, so I took a chance on a complete pump & switch assembly from eBay & swapped the switch over. Been working fine ever since.

Thanks to everyone who responded this one. Just to update - I started with the fuse box couldn't find anything wrong, checked the wiring, which looks in good condition, no signs of corrosion nothing untoward showing up with a multi-meter. I pulled the relay and relevant fuses, cleaned everything up with a bit of electrical contact cleaner and put it all back and haven't had any issues since. I've ordered a new relay which I'll fit when it arrives tomorrow just to be sure. Will update if the issue reoccurs.