XJ6 & XJ12 Series I, II & III 1968-1992

1988 Jag XJ6 Brake problem/ABS/Hard pedal

 
  #2  
Old 04-08-2008, 12:37 PM
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Default RE: 1988 Jag XJ6 Brake problem/ABS/Hard pedal

Try pumping the brakes and see how long before the warning light comes on. My knee jerk reaction is the accumulator ball.
 
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Old 04-10-2008, 09:07 AM
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Default RE: 1988 Jag XJ6 Brake problem/ABS/Hard pedal

Start with the accumulator ball. That is weak.
 
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Old 04-13-2008, 01:05 AM
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Default RE: 1988 Jag XJ6 Brake problem/ABS/Hard pedal

Are you looking for some help with this?
 
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Old 04-13-2008, 01:39 PM
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Default RE: 1988 Jag XJ6 Brake problem/ABS/Hard pedal

ESTS,

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.

 
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Old 04-13-2008, 05:45 PM
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Default RE: 1988 Jag XJ6 Brake problem/ABS/Hard pedal

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
 
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Old 04-13-2008, 10:19 PM
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Default RE: 1988 Jag XJ6 Brake problem/ABS/Hard pedal

Nice pics, here's what I see so far:

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.
 
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Old 07-27-2015, 02:55 PM
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Hello JagTechOhio,


Possible to query you on this topic?


Thanks,


Neil
 
 
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