1989 XJS. All new brakes, calipers, lines, and a used ABS with new accumulator purchased off eBay.

How often does ABS pump cycle? I hear my ABS pump cycle on when the key is on and then it turns off. And then it comes on every few minutes after that (as best I can tell-hard to hear when driving).

I'm trying to trouble shoot the red "brake" warning light staying on in my car and maybe this is related? I keep reading about low pressure bleeding which I *think* I did when I completed brakes.

Should I just re-bleed brakes and see what happens? Pedal is firm, but I did have the entire system apart so....

 

Hi Andy

Near enough I've also had the very same problem as you and if you have bled the Low Pressure side, then I'm sure you would have remembered, as after you have de-pressurised the system (20 pumps on the pedal, as I'm sure you know)

Then you have to pull the Pin by the side of the Brake Actuator, so that you can pull the Pipe out of the side, which is made of Plastic (so don't break it!)

It can be a PIA to pull out, as there is an 'O' ring on the end, so having protected that pipe with some rag, I put some 'mole wrenches' on it (not to tight) so I had something to twist and jiggle about till it came out.

Put a Towel right over the Top, just in case there's any pressure left, as you do not want break fluid, squirting every where.

As soon as you pull the Pipe out, Brake Fluid will flow under gravity and you should see lots of bubbles, as the Air Bleeds out. So you need to keep the Reservoir Topped up while you are doing it.

Then while it's Still dripping Clear Fluid, push the pipe back and replace the pin.
It's going to make a hell of a mess, with Brake Fluid running all over the place, so stuff the Engine Bay with Rag, to keep it off the Paintwork.

As a bit of belt and braces, I also covered the engine bay in Spray Grease, but don't rely on this alone, just stuff rag everywhere, and when your done, pull all the Rag that will be soaked in Brake fluid, out from under the Car.

As if Brake Fluid drips onto the Paintwork, then you are Screwed.

This should be the only time, that you will need to Bleed the Low Pressure Side, unless you pull it all apart again.

Then when you've done that, then Bleed all the Brakes, in the Normal way, using the Key to bleed the rears, with the Key Turned for 15 (ish) Seconds Max to prevent the Pump Burning Out, and then do the Fronts on the Pedal.

I also made a Tin Foil Shute, to divert the dripping Brake Fluid, to a Washing up Bowl, that I put under the Car.

Then The Red Light should go out!

If the ABS light goes out, then it shows that your Accumulator (Black Ball) is working fine.

This should be a 'Walk in the Park' for someone of your 'Calibre' who can build a New Car out of Scap!

 

I followed those steps and bled the brakes again.

They feel normal, but red brake light is staying on, (ABS light turns OFF after self test when key is first turned). Red brake light on supposedly indicates that the fluid is low or that pressure is low.

Fluid is not low. I'm not sure what the symptom of low pressure would be, I'm assuming a hard brake pedal. My pedal feels "normal" (Don't have a comparison to another XJS, it might be a little hard).

My ABS pump cycles every few minutes while driving. Is that normal? Seems excessive.

 

The hydraulic pressure accumulator is pressurised by motor pump unit, controlled by
Pressure switch.

The accumulator is maintained in an operating pressure range between 140 to 180 bar (when system functioning correctly)

When ignition switched on power is applied to abs ecm (electronic control module) which performs self test routines and switches abs warning lamp (and brake warning lamp) on and motor pump on to build pressure in the accumulator.
The time the lamp remains on depends on how long it takes for accumulator pressure to be increased to level.
Brake warning light and abs warning light should extinguish in a maximum of 40 seconds after ignition switch on, with about 1 second between the two lights.

When pressure reaches 130 bar the combined pressure warning switch is actuated , the brake warning light goes off, followed a short time later by abs warning light going off.

Motor pump will continue to build accumulator pressure until 180 bar when pressure switch will turn motor pump off.

I believe maximum time to pump switch off should be 60 seconds.

If pressure switch fails to switch motor pump off at 180 bar a pressure control valve will operate to release pressure at 210 bar to protect against system damage.

When pressure drops below 140 bar motor pump is switched on again.
Brake warning lamp is switched on if pressure is below 105 bar.

 

I think that my pump is building pressure adequately, it turns off well below 30 seconds after starting the car and the accumulator appears sounds as more than 15 pumps of pedal (car off) was required to harden pedal.

How does pressure switch "know" that there is fluid, or more precisely, low fluid?

My car sat with no fluid in system for some time and I'm wondering if there is a corroded electrical contact that doesn't register the fluid in reservoir?

 

Low fluid level is monitored by two Reed switches operated by floats inside the reservoir, which has separate chambers for 1)front brakes,master cylinder via main valve
2) motor pump unit for booster and rear brakes
3) return flow from booster, rear brakes and valve block.