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ABS Out of Control?

I have a 2005 Subaru Forester which has developed an annoying and scary problem. It generally surfaces on the highway at about 75 mph. The whole car starts vibrating as though I were driving over a finely ribbed surface. Of course, I’m actually driving over the nicely paved surface of the highway.

Taking it out of gear and coasting, clutch in, clutch out – it doesn’t matter. The vibration persists until I pull over to a full stop. Then, upon resuming the drive, the vibration is gone – completely.

One time when this occurred, upon pulling over I noticed that hot-brake smell that we all know and don’t like. Sure enough, the rotors were so hot that I singed my finger by simply brushing by the rotor lightly: VERY HOT! Even the rear drums were a little toasty.

I think that the ABS is deciding to ABS me out of the blue. It happens, as I said, at high speeds. I am not braking, and the roads are warm and dry.

Does anyone have experience with this symptom? If so, how can I correct it?


- - Herb

ABS usually works by releasing pressure on a skidding wheel, not applying it. Maybe you have air in your system, and the high-speed rubbing results in expansion? Regardless, I’d take it to a good brake shop, have the brakes fully bled and the fluid replaced. If that doesn’t help, I’m stumped.

I stumped,too. The only reason I thought of ABS is that the vibration suggests that the brakes are being “pumped” many times per second. All four brakes are hot when this happens. The car seems to free-wheel okay. When I release the hand brake, the car will start to roll, so it does not appear to have seized calipers or wheel cylinders.

Even if there were a seized caliper, why would that make the whole car start vibrating?

I could bleed the system, but the reservoir is 100% full and there are no leaks. How would there be air in the sealed system?

    • Herb

Stop the car sometime when this is NOT happening and you’ll find that the brakes are too hot to touch.

ABS does not apply the brakes.

Check the owner’s manual and the maintenance schedule for information about periodic replacement of the brake fluid. I’m guessing it’s recommended more often than every five years.

Brake fluid absorbs moisture. The system is not “sealed.” It’s closed, but not airtight. Moisture causes internal rust, and can also boil under the right conditions, leading to all sorts of problems.

This is why periodic brake fluid replacement is recommended. Try it. What have you got to lose?

It is possibly you have a sticking caliper. Vibration occurs when the rotor heats up enough and warps a bit.

ABS does not apply brakes, it only makes sure the wheel does not lock up during braking. So that theory is not really valid.

"ABS does not apply brakes"
This is true, but traction control and stability control systems can and do, via the ABS.

I wonder if all of the wheels have matching diameters?

I have stopped the car when this problem is not occurring, and the disks and drums are only slightly warm.

I will try bleeding the system, but I don’t have high hopes that this will cure the problem.

A sticking caliper would not explain all four wheels getting hot. Also, the car rolls to a gentle stop. With a seized caliper, the car would roll to a braked stop.

All four wheels are matching Nokian tires: matching diameters.

There is one component in a car that can lightly and constantly apply all four brakes as you drive…the vacuum booster.

Normally as you drive vacuum is applied to both the front and the back of the booster’s diaphragm. When you brake, valves operated by the rod open and close to close the front chamber off and vent the back chamber, such that vacuum is applied to the front of the diaphragm only.

If the booster is misadjusted or malfunctioning in a way such that engine vacuum is always applied to the front of the diaphragm with the back of the diaphragm always vented to ambient, the booster will constantly apply forward pressure to the brake system.

This should be simple to test. Disconnect and plug the booster vacuum line and drive the vehicle…with care, and as a test only, as you’ll need more force to brake. The reconnect the line and drive it again. If the problem only occurs with the vacuum line connected, it’s a good bet that that’s the source of the problem.

You could also probably test this by putting the car on stands and doing the “line on/line off” testing by spinning the wheels by hand and feeling for drag.

This sounds plausible.

It feels like there is some kind of “loop” happening: apply-release-apply-release, but very rapid. This is why I first suspected the ABS.

I wonder if some sort of cycle like that is happening at the vacuum booster.

I’m thinking that the rotors are overheating at highway speeds from the slight drag and causing the vibration.

There’s no permanent warp, since the brakes are smooth as glass normally – no “pumping” that no associates with warped rotors and such.

The odd thing is that I can pull off the road for, literally, one or two minutes and the problem disappears (until the next time).

Do you have traction and/or stability control?
I still suspect that it is instructing the ABS to apply the brakes. An electronic problem could clear as soon as you turn the key off and back on, a mechanical one probably wouldn’t.

The next time it does it (if you are in a clear area), turn the key off briefly (while still moving) and then back on. See if that stops the problem for a bit.

Good idea! I’ll try cutting the engine. If that clears it, then it may be electronic.
I could also try cutting the engine and taking it out of gear. That way, there would be no vacuum, thus testing out the notion presented by “mountain bike”.

“turn the key off and back on”

I’m wondering if it isn’t possible and just safer to find the appropriate fuse to disable the system temporarily. I don’t know how this system is set up or what else would be on the same fuses, but I’d look into that before cutting the ignition on & off @ 75mph. It wouldn’t tell you as definitively since you’d just be looking for the continued absence of the problem.