Hello,

My XJ (98) has a brake warning light in the dash that's connected to a master cylinder proportioning valve switch and also to the e-brake switch under the floor console.

From what I understand the purpose of the proportioning valve is to equalized fluid between front and rear brakes. If pressure becomes too low (front or rear) or out of proportion then the warning light is activated through grounding the brake warning light.

If the e-brake is out of adjustment or doesn't work then the brake warning light is also activated.

My question is when only the e-brake switch is activated does this have any sort of an effect on the master cylinder proportioning valve, and behavior of the brakes?

I recently spilled some coffee on and inside the floor console that caused the brake warning light on the dash to illuminate. However at the same time the brakes started to behave weird, the same as when I once had a bad master cylinder with a warn proportioning valve.

The brake light remained on (key on, engine on or off) until I allowed the XJ to sit for a couple of hours (coffee must have dried up) and I kept pumping the brakes many times (key on and key off, engine on and off).

The warning light eventually went out and stayed off and the brakes and brake pressure began to work normal again. Uncertain what it was, I remember hearing a low "click" sound after pumping the brakes many time as if something adjusted itself. (Perhaps it was the MC proportioning valve?

Anyway it appears that the e-brake switch directly effected the master cylinder proportioning valve through the brake warning light and master cylinder switch which I assume is directly connected to the mc proportioning valve.

I haven't yet had the opportunity to remove the centre console to test this theory of operation and thought it was sort of strange that spilling coffee would directly effect the operation of the brakes.

The problem seems to have cleared up as the brake warning light hasn't come back on and the brakes are behaving normally.

Perhaps there's other issues with the brakes. In the past two years the entire braking system has had a complete overhaul with a new master cylinder, hard lines to the rear, new calipers, new discs and drums, new rear hardware. The only thing not replaced were the two front hoses which appear to not have any leaks. Brake fluid has been staying between Add and Full.

Possibly the mc proportioning valve in relation to the switch isn't working quite right?

This spring after the weather clears I think I'm going to go over the entire system including the brake lines again.

 

No effect on one or other. The e-brake light part of it only comes on when you pull the handle to apply the brake. The valve light is activated when pressure is lost on one side, ft or rear. When I was working on trucks you figured 70 o\o of the braking was done with the front and 30 for the rear.

 

I'm planning on eventually pulling the console and activating the e-brake to test what may have caused the problem with the brakes after spilling coffee on the e-brake switch which potentially caused the brake warning light to come on. I would need to test the warning light circuitry to both devices to determine if spilling fluid could on the e-brake switch potentially could effect mc proportioning valve and switch.

The wiring diagram from the service manual shows the two switches are connected.

C1 == C100 === Brake Warning Pressure Switch S211 === Park Brake Switch through a Grey/Black wire that connect to a Grey/White wire to the === Red Brake Warning Indicator




Why my brakes behaved as their was no brake fluid pressure after spilling the coffee and the brake warning light coming on is a mystery. Unless the new mc is defective or something.

Possibly there are other issues I need to address. When I have time I'll go through and inspect the entire braking system.

I started working on automobile when things were fairly simple back in the early 60s. The first car I worked on was a 52 Ford. I've owned many vehicles over the years however the 98 XJ is the latest year I've owned. After purchasing had to give myself a crash course to understand OBD2 from books checked out from the public library. Now OBD and OBD2 is fairly common with more information available from the Internet.