Hi all. My 1992 XJS V12 Convertible lost the electric power brake boost. It was working fine, and I was cleaning the car, opened the roof to clean back seat etc, and after returning the roof to the closed position, I had lost the power brake boost. Checked the relays, all fine. Temporarily wired from brake switch and electric motor boosted brakes as normal.
Any suggestions would be welcome. Thanks.
Richard

 

Richard,

Can you just explain a bit more what you mean by "lost the electric power brake boost"? Do you mean that when you start the car the motor doesn't run? Does the brake light extinguish (indicating that pressure has been reached in the accumulator)? When driving, which of the brake lights are illuminated?

Cheers

Paul

 

Hi Paul. Thanks for trying to help me out with this. This model has the electric motor which activates when the brake pedal is depressed to increase brake pressure. When the car is being driven the pump is idle, it only comes on when the brake pedal is depressed. When driving the " brake " light on the dash now remains illuminated.
I had assumed that since with my "jerry rigged" wiring, the pump and brakes are operating normally, that the accumulator was reaching pressure.
I know the ABS ECU is over the rear drivers side wheel arch, and originally thought that perhaps a wire had been hanging and was damaged by the operation of the roof, but I could not find anything damaged.
Hope that was the extra information you were looking for.
Richard

 

Richard,

The brake system is a bit more complex than that. It's really worth a read through the manual to understand it. The rear brakes are driven completely by the accumulated pressure, not by the brake pedal pressure. In simple terms, the rear circuit is always "dynamic". The front circuit is "static" and driven by foot pressure under normal operation and then boosted by dynamic pressure under ABS conditions.

The pump should not run every time you press the brakes. There should be enough pressure in the accumulator to last a number of applications and power the rear brakes before the pump runs.

If your orange light is on, your ABS has been disabled. However, if the red light is on, with either fluid loss or system pressure loss, you have a much more fundamental problem. Don't drive the car!

As mentioned, have a good read through the manual first. Run the 32 diagnostic tests and then report back.

Good luck

Paul

 

If you had done something to the ABS controller the ABS light would be on.

The brake light will come on if the brake fluid reservoir is low on fluid or the system pressure falls to 105bar. All other failures will turn on the ABS MIL.

Check the fluid level first.

 

Thanks Paul. Now you taught me something I honestly did not know about the car. True what they say, never to old to learn. To be honest I don't know if the motor runs every time I use the brake, just that since my jerry wiring, the car brakes normally, ie, no excessive foot pressure required to stop. I did have a motor fail on my sovereign and had to almost stand on the pedal to stop from 30 mph.
In actuality I have not driven the car in a while as I have to replace the water pump, and wasn't wanting to risk overheating the engine with the dire consequences which could have followed, and this year had a fair amount of personal trauma to deal with. That being over along with the heat and humidity of a Florida summer, I decided it was time to get my baby back on the road.
Memory is a strange thing, but I am pretty sure the light which was on was orange. However, I will install a battery at the weekend and check that, and follow through with your advice.
Very many thanks for your help and replying.
Richard

 

Thanks for reply Warrjon. Fluid level is ok. But between you and Paul, I think I can hopefully see some light at the end of the tunnel.
I used to have a Jag mechanic locally, bit of a hole in the wall affair, but knew his stuff, but he disappeared. Tried another place and didn't feel at all comfortable.
eg, I mentioned to them that I had just replaced water pump and made mistake using old one off my 85 XJS and they wanted to rip my cylinder head off and have it planed as first option. I tried to explain to them that the pump was the problem as the car I took it out of ran hot, and with the original pump the car ran at normal temp. They still wanted to pull the head. You see my point. I think they should have called their shop "shark central"
Thanks again, will be back with update, Richard

 

The ABS pump does not run every time you put your foot on the brake. It only runs to keep pressure at a pre-determined level.

The pump should run for 30-60 seconds when you turn the ignition on, and then it runs occasionally as needed to maintain system pressure.

The pump should not run every time you put your foot on the brake, this symptom could indicate a leak in the system, or valve body back to the reservoir.

 

Hi Warrjon, thanks for your reply. Since I Jerry rigged the wiring, the brakes work as normal. So I am assuming no leaks. As I said in my original post this happened immediately after I raised and lowered the ragtop. That was why I was thinking an electrical problem. I know the ABS equipment is located over the wheel arch on the drivers side, and was thinking that something from the ragtop mechanism did something to the wiring or the processor.
Anyway, I am going to hook up a battery this weekend, have not used the car for several months. Been too much going on, moving house, lawsuit etc plus too hot in Florida to be working on the car.
Now house move, lawsuit over and so is summer.
Will update with anything more specific once battery connected.
Richard