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Just purchased a few days ago, 1988 Jag XJ6, 90k original miles. I've searched around but cannot put my finger on the exact problem. The VCM flashes ABS failure, and sometimes brake fluid low (the fluid is full). I was assuming that the float in the sensor was broken but reading some other posts I guess it may not be. The main problem is the brake pedal. It takes more force than normal to push the brake pedal down, but has normal travel. It also releases back rather forcefully. If you push it down once it will have full travel, then if you push it again about half, but brakes still operate as usual. From the research I've done, the booster failure should have a firm pedal but limited brake travel. Any thoughts on the accumulator? Hydrolic fluid pump? I cannot hear a leak in the booster vacuum hose with the fan running so loud. It has only 800 miles of use in the last 6 years, so if that helps any...
Ok, key in the on position not running, after each try I started the car to reset everything: 6 times it took 7 pumps until light comes on, once it was 4 pumps, and once it was 8 pumps, does that help any?
Yes, I see that they only had this system in 88 and 89 I believe and the parts are pretty expensive, so I'd like to narrow it down before dumping all the money into it.
First of all, I want to make sure what you have on your car. I'm very familiar with the stock setup, but you earlier made reference to listening for a vacuum leak. Vacuum has nothing at all to do with the power hydraulic system: UNLESS your car hasbeen modified to use a more standard type of vacuum brake booster. I have not seen an XJ40 like this, but the guy in Cali who sells them saysconverted carsare all over the world.
Peliminary: Are you certain your Power hydraulic reservoir is full?
Are you getting a "hyraulic fluid low" warning on the VCM?
Are you getting a "low brake pressure" warning on your VCM?
In the last case, does the warning go out after the engine has been running above idle speed for after a minute or so?
Is your car still fitted with the original style ride leveling rear struts? If it has been converted, you will see a pipe disconnected from the bottom of the solenoid valve block, and a bleed screw in the housing to block off the port.
Sometimes on first statup I will get the "low brake pressure" on the VCM for a few seconds, and also a clicking from the passenger side engine compartment. When I just went back out it didn't do it. I took some pics of the rear system, not sure if it is original or not, the car only has 91k and was parked for the last 5 years. I tried getting the cap off of the green fluid resevoir on the passenger side engine compartment but it would not come off. I disconnected the hose closest to the front of the car on the green resevoir and when letting off the brake it did squirt fluid. No Hydrolic fluid low warning, please let me know if you need any more pics, info ,etc. Thanks!
I'll be back later, the cap doesn't come off the reservoir, you have to use the special tube that comes on the bottle of the correct Castrol Hydraulic Fluid
That car is absolutely bone stock, except for the factory accessory alarm system. The rear ride leveling struts are still fitted: they look dry, which probably means they're not getting fluid. I suspect the back end is rather bouncy...
Other asides, the rear tires are junk. After seeing that degree of corrosion in the back, I'd take a real good look at the front suspension crossmember, up top in the wheel opening where the front suspension upper wishbones attach. These cars rust out pretty good there, and the front suspension collapses. Because of the cost of the repair versus the value of the vehicle, this problem can be a car-killer.
Put the black plastic cover back on the ABS modulator (in front of the battery) and leave it alone. There is nothing wrong with this unit, there NEVER is. Most likely cause of a constant ABS warning on an '88-'89 XJ40 is the ABS over-voltage relay, open circuit,it's mounted on a relay panel behind the carpet adjacent to the fuel filler neck in the trunk. It's the tallest relay on that panel, original ones were white and some of the replacements are black. If you smack it, the warning may extinguish for a bit.
With the engine off, pump the pedal 15-20 times to exhaust any boost pressure you may be getting from the power brake accumulator. You should now have a very hard brake pedal, with only about an inch or two of travel. If the pedal return feels unusual when testing statically like this, the hydraulic power booster (that's the unit between the brake pedal and the master cylinder) has a mechanical fault. There is a snap ring that comes out, although this too is a relatively rare fault, and allows the pedal to come back higher than it should. Then you have lost motion on the next application.
Master cylinders do have internal (and external) fluid leaks on these cars, so if the pedal is creeping down you have a brake fluid hydraulic system fault, just like on any other car. The car is not running, this has nothing to do with the separate hydraulic boost pressure system.
The '88-'89 cars used sliding steel guide pins for the caliper mountings on all four calipers. These pins will become hopelessly siezed on some cars: even when all other brake and hydraulic assist components are functioning properly, you have a fairly firm pedal but lousy braking.The caliper cannot squeeze the disc with the outboard pad, it is stuck in a fixed position instead of sliding as designed.
The "brake fluid low" warning is a false message from a bad float unit on the master cylinder reservoir, so long as the brake fluid level is OK. Happens all the time.
The first thing you need to do is to buy the proper Castrol Hydraulic Fluid (mineral oil), which comes in a bottle with a tube that plugs into the top of the hydraulic fluid reservoir. That is the green tank in the RF corner of the engine bay, and the cap is not removable. There is a visual float to indicate whether the reservoir is full, do not trust it or an indication on the VCM for "hydraulic fluid low". Fill the reservoir until it vents fluid out of the base of the float unit: the float can be stuck either up or down, and the electrical fluid level sensor can be faulty, or both. Get it full of fluid and we'll go from there.
The rhythmic knock from the right front of the car is the solenoid valve block pressure relief valve, it is probably faulty and the picture of the valve block shows it to be the original one and has not been updated. Don't worry about this until the system is full of fluid, if faulty it will cause a knock back through the brake pedal but not affect the brake system operation otherwise.
We can start to trouble-shoot everything once the fluid is full. In all likelyhood, you are also looking at a solenoid valve block pressure relief valve, some stuck caliper guide pins, and a rear strut conversion to retrofit the ride leveling if you want good handling and reliability. That will already be alot of money.
Wow, thanks for all of the help and time spent researching this. It's really too bad, the car was parked for the last 5 years and only has 91k on it, I'd hate to see it go to parts. I was unaware of all the other potential problems (especially the ride control). I'll try to get the Castrol Hydraulic Fluid and go from there. The brake pressure low is only on for a few seconds then goes away. The car stops normally, just takes a little extra pressure on the pedal. No pressure is lost on the pedal either. Also, when driving last night the "knock" was coming on at random times and could be felt in the brake pedal. Looks like I'm in over my head, so it might be showing up in the classifieds soon [sm=smiley24.gif]
That car is absolutely bone stock, except for the factory accessory alarm system. The rear ride leveling struts are still fitted: they look dry, which probably means they're not getting fluid. I suspect the back end is rather bouncy...
Other asides, the rear tires are junk. After seeing that degree of corrosion in the back, I'd take a real good look at the front suspension crossmember, up top in the wheel opening where the front suspension upper wishbones attach. These cars rust out pretty good there, and the front suspension collapses. Because of the cost of the repair versus the value of the vehicle, this problem can be a car-killer.
Put the black plastic cover back on the ABS modulator (in front of the battery) and leave it alone. There is nothing wrong with this unit, there NEVER is. Most likely cause of a constant ABS warning on an '88-'89 XJ40 is the ABS over-voltage relay, open circuit,it's mounted on a relay panel behind the carpet adjacent to the fuel filler neck in the trunk. It's the tallest relay on that panel, original ones were white and some of the replacements are black. If you smack it, the warning may extinguish for a bit.
With the engine off, pump the pedal 15-20 times to exhaust any boost pressure you may be getting from the power brake accumulator. You should now have a very hard brake pedal, with only about an inch or two of travel. If the pedal return feels unusual when testing statically like this, the hydraulic power booster (that's the unit between the brake pedal and the master cylinder) has a mechanical fault. There is a snap ring that comes out, although this too is a relatively rare fault, and allows the pedal to come back higher than it should. Then you have lost motion on the next application.
Master cylinders do have internal (and external) fluid leaks on these cars, so if the pedal is creeping down you have a brake fluid hydraulic system fault, just like on any other car. The car is not running, this has nothing to do with the separate hydraulic boost pressure system.
The '88-'89 cars used sliding steel guide pins for the caliper mountings on all four calipers. These pins will become hopelessly siezed on some cars: even when all other brake and hydraulic assist components are functioning properly, you have a fairly firm pedal but lousy braking.The caliper cannot squeeze the disc with the outboard pad, it is stuck in a fixed position instead of sliding as designed.
The "brake fluid low" warning is a false message from a bad float unit on the master cylinder reservoir, so long as the brake fluid level is OK. Happens all the time.
The first thing you need to do is to buy the proper Castrol Hydraulic Fluid (mineral oil), which comes in a bottle with a tube that plugs into the top of the hydraulic fluid reservoir. That is the green tank in the RF corner of the engine bay, and the cap is not removable. There is a visual float to indicate whether the reservoir is full, do not trust it or an indication on the VCM for "hydraulic fluid low". Fill the reservoir until it vents fluid out of the base of the float unit: the float can be stuck either up or down, and the electrical fluid level sensor can be faulty, or both. Get it full of fluid and we'll go from there.
The rhythmic knock from the right front of the car is the solenoid valve block pressure relief valve, it is probably faulty and the picture of the valve block shows it to be the original one and has not been updated. Don't worry about this until the system is full of fluid, if faulty it will cause a knock back through the brake pedal but not affect the brake system operation otherwise.
We can start to trouble-shoot everything once the fluid is full. In all likelyhood, you are also looking at a solenoid valve block pressure relief valve, some stuck caliper guide pins, and a rear strut conversion to retrofit the ride leveling if you want good handling and reliability. That will already be alot of money.
Hello JagtechOhio, I have a noticeable varying thump which matches a pulse in the hose from the brake power hydraulic system valve block to the pump mounted on the front of the engine (1988 XJ6, 110,000 miles). While driving, the thump noise varies down to zero at idle and stop. Is the valve block serviceable and if not, is a replacement still sold? Thanks.
Please help. Of the two accumulator switches, my leaking switch is the bottom one, part number 74661549, one black and one red wire, on my 88 XJ6(40). Jag Bits parts vendor tells as current switch ends in 9, I need replacement JLM 1562 = charge switch. Manual shows bottom switch is low pressure warning switch, and shows the two tubing connections on accumulator base at 12 o'clock and 3 o'clock which seems to match my photo. Also, my two switches only have two wires each and I don't find the ABS modulator so I presume no ABS although parts vendor tells me last 6 of VIN 527360 has ABS? New part number JLM 1563 low pressure warning switch has a red and black wire in part description, JLM 1562 has yellow and black (both have two other wire colors which I presume is for ABS). Looking up.
04-07-2008, 09:42 AM
1988 Jag XJ6 Brake problem/ABS/Hard pedal
