antilock/wooden brakes
09-06-2005, 08:57 PM
#1
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antilock/wooden brakes
While driving on the highway this morning, cruise control set at 140kph, the antilock warning light comes on. Well I'm half way to nowhere at 5:30am so I keep going. Stop for gas, 
(OUCH!) restart the engine, the warning lights off.
OK, get into the city and now I'm using my brakes on a regular basis and they're very wooden in feel.........press the pedal, no dramatic braking but they do work.
Any historical input?
Brake fluid is new this spring, fully bled with the Motive power bleeder. Prior to this morning, brakes were very strong.
(OUCH!) restart the engine, the warning lights off. OK, get into the city and now I'm using my brakes on a regular basis and they're very wooden in feel.........press the pedal, no dramatic braking but they do work.
Any historical input?
Brake fluid is new this spring, fully bled with the Motive power bleeder. Prior to this morning, brakes were very strong.
09-06-2005, 10:54 PM
#3
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Sounds like a vacum leak. Check all hoses/conections to booster.
Is the engige reving higher when depressing pedal?
EDIT : Or you lost power to booster (for watever reason) wich make the pedal harder!
On the other hand, I know nothing of ABS....
Is the engige reving higher when depressing pedal?
EDIT : Or you lost power to booster (for watever reason) wich make the pedal harder!
On the other hand, I know nothing of ABS....
09-06-2005, 11:57 PM
#5
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Check the vacuum check valve by the brake booster.
Mine stuck open somehow and I had non-boosted brakes.
Symptom: Very hard brake pedal and weak braking power.
I was lucky and just tapped the check valve. That got it working again.
Some have recommended taking vacuum check valve off and cleaning it.
When my check valve got stuck open, I didn't get ABS warning.
ABS warning could be iron flakes (damn Magnetic sensor) stuck to sensor
and needs cleaning, or corrosion in the ABS wiring connectors.
The connectors look like plastic cans with wiring running in/out of them.
One each by the ABS sensor and 2 in the engine compartment.
High up by the top of the shock towers. Unplug the connectors and give them
a nice dose of contact cleaner. Recommend De-Oxit. Available at Radio Shack.
Ernest (NYC)
Mine stuck open somehow and I had non-boosted brakes.
Symptom: Very hard brake pedal and weak braking power.
I was lucky and just tapped the check valve. That got it working again.
Some have recommended taking vacuum check valve off and cleaning it.
When my check valve got stuck open, I didn't get ABS warning.
ABS warning could be iron flakes (damn Magnetic sensor) stuck to sensor
and needs cleaning, or corrosion in the ABS wiring connectors.
The connectors look like plastic cans with wiring running in/out of them.
One each by the ABS sensor and 2 in the engine compartment.
High up by the top of the shock towers. Unplug the connectors and give them
a nice dose of contact cleaner. Recommend De-Oxit. Available at Radio Shack.
Ernest (NYC)
09-07-2005, 02:55 AM
#6
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If the ABS "goes off' due to a system failure, braking reverts to "normal" non-ABS braking. The ABS valve system is very reliable. In "theory" the pedal feel should be the same regardless of ABS functionality, though you do feel a "tingle" in the brake pedal when the ABS is cycling.
The ABS valves enable the following:
- Increase pressure (but never greater than the pressure from the master cylinder)
- Maintain pressure
- Reduce pressure
Based upon the input from the wheel speed sensors, the ABS system controls three hydraulic brake circuits on the 928; left and right front wheels (independent) and rear wheels together, where the rear is "select low" or the braking is to the brake pressure of the wheel with the least amount of traction.
The fact that the ABS "increase pressure" is never more than the pressure from the master cylinder is the reason why when you drive an car with ABS you should always mash the pedals as hard as possible in a skid situation; this enables the ABS to do its best. If you let pressure off the brake pedal, you reduce the braking force to the wheels that have the most traction! I always suggest "learning the feel" of ABS so when you need it you don't panic.
There are several reasons for an ABS warning light. It can be anything
- corrosion at connection points
- a bad relay
- open or short circuits in the speed sensors
- speed sensors that have too much metalic crud in them or on the tone ring
- malfunction in the ABS controller
- malfunction in the ABS valve body
What ever the problem with your brake system, it should be checked out ASAP!
The ABS valves enable the following:
- Increase pressure (but never greater than the pressure from the master cylinder)
- Maintain pressure
- Reduce pressure
Based upon the input from the wheel speed sensors, the ABS system controls three hydraulic brake circuits on the 928; left and right front wheels (independent) and rear wheels together, where the rear is "select low" or the braking is to the brake pressure of the wheel with the least amount of traction.
The fact that the ABS "increase pressure" is never more than the pressure from the master cylinder is the reason why when you drive an car with ABS you should always mash the pedals as hard as possible in a skid situation; this enables the ABS to do its best. If you let pressure off the brake pedal, you reduce the braking force to the wheels that have the most traction! I always suggest "learning the feel" of ABS so when you need it you don't panic.
There are several reasons for an ABS warning light. It can be anything
- corrosion at connection points
- a bad relay
- open or short circuits in the speed sensors
- speed sensors that have too much metalic crud in them or on the tone ring
- malfunction in the ABS controller
- malfunction in the ABS valve body
What ever the problem with your brake system, it should be checked out ASAP!
09-07-2005, 03:08 PM
#7
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Thread Starter
Originally Posted by Rich9928p
If the ABS "goes off' due to a system failure, braking reverts to "normal" non-ABS braking. The ABS valve system is very reliable. In "theory" the pedal feel should be the same regardless of ABS functionality, though you do feel a "tingle" in the brake pedal when the ABS is cycling.
The ABS valves enable the following:
- Increase pressure (but never greater than the pressure from the master cylinder)
- Maintain pressure
- Reduce pressure
Based upon the input from the wheel speed sensors, the ABS system controls three hydraulic brake circuits on the 928; left and right front wheels (independent) and rear wheels together, where the rear is "select low" or the braking is to the brake pressure of the wheel with the least amount of traction.
The fact that the ABS "increase pressure" is never more than the pressure from the master cylinder is the reason why when you drive an car with ABS you should always mash the pedals as hard as possible in a skid situation; this enables the ABS to do its best. If you let pressure off the brake pedal, you reduce the braking force to the wheels that have the most traction! I always suggest "learning the feel" of ABS so when you need it you don't panic.
There are several reasons for an ABS warning light. It can be anything
- corrosion at connection points
- a bad relay
- open or short circuits in the speed sensors
- speed sensors that have too much metalic crud in them or on the tone ring
- malfunction in the ABS controller
- malfunction in the ABS valve body
What ever the problem with your brake system, it should be checked out ASAP!
The ABS valves enable the following:
- Increase pressure (but never greater than the pressure from the master cylinder)
- Maintain pressure
- Reduce pressure
Based upon the input from the wheel speed sensors, the ABS system controls three hydraulic brake circuits on the 928; left and right front wheels (independent) and rear wheels together, where the rear is "select low" or the braking is to the brake pressure of the wheel with the least amount of traction.
The fact that the ABS "increase pressure" is never more than the pressure from the master cylinder is the reason why when you drive an car with ABS you should always mash the pedals as hard as possible in a skid situation; this enables the ABS to do its best. If you let pressure off the brake pedal, you reduce the braking force to the wheels that have the most traction! I always suggest "learning the feel" of ABS so when you need it you don't panic.
There are several reasons for an ABS warning light. It can be anything
- corrosion at connection points
- a bad relay
- open or short circuits in the speed sensors
- speed sensors that have too much metalic crud in them or on the tone ring
- malfunction in the ABS controller
- malfunction in the ABS valve body
What ever the problem with your brake system, it should be checked out ASAP!
Thanks Rich........appreciate the comments. This morning there was no warning light but the same 'wooden' pedal feel. After work I'll go find some open highway where I can 'mash' the pedal and see if the ABS operates.
I'll be back.
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09-07-2005, 03:11 PM
#8
Rennlist Member
Thread Starter
Originally Posted by ew928
Check the vacuum check valve by the brake booster.
Mine stuck open somehow and I had non-boosted brakes.
Symptom: Very hard brake pedal and weak braking power.
I was lucky and just tapped the check valve. That got it working again.
Some have recommended taking vacuum check valve off and cleaning it.
When my check valve got stuck open, I didn't get ABS warning.
ABS warning could be iron flakes (damn Magnetic sensor) stuck to sensor
and needs cleaning, or corrosion in the ABS wiring connectors.
The connectors look like plastic cans with wiring running in/out of them.
One each by the ABS sensor and 2 in the engine compartment.
High up by the top of the shock towers. Unplug the connectors and give them
a nice dose of contact cleaner. Recommend De-Oxit. Available at Radio Shack.
Ernest (NYC)
Mine stuck open somehow and I had non-boosted brakes.
Symptom: Very hard brake pedal and weak braking power.
I was lucky and just tapped the check valve. That got it working again.
Some have recommended taking vacuum check valve off and cleaning it.
When my check valve got stuck open, I didn't get ABS warning.
ABS warning could be iron flakes (damn Magnetic sensor) stuck to sensor
and needs cleaning, or corrosion in the ABS wiring connectors.
The connectors look like plastic cans with wiring running in/out of them.
One each by the ABS sensor and 2 in the engine compartment.
High up by the top of the shock towers. Unplug the connectors and give them
a nice dose of contact cleaner. Recommend De-Oxit. Available at Radio Shack.
Ernest (NYC)
09-07-2005, 10:49 PM
#9
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Thread Starter
Went or a ride tonight and the ABS does not appear to be working. I've checked all conections that Ernest has suggested and all are intact and clean. Rich has suggested the relay.........how do I check it?..........its a 4 pin device 928 615 124 01 date stamped 14/09/87.
I assume having the relay removed will not be of concern as the ABS is not working?
I assume having the relay removed will not be of concern as the ABS is not working?
09-08-2005, 09:11 PM
#10
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Malcolm,
If ABS warning comes on at startup before car moves, then it's very likely
failed relay.
If ABS warning comes on after car's in motion, then there's a differential/non signal
sent to the ABS brain so the sensors/connectors may well be the problem.
Think removing ABS relay will just disable ABS circuit. Been driving like that with
no brake loss.
Ernest (NYC)
If ABS warning comes on at startup before car moves, then it's very likely
failed relay.
If ABS warning comes on after car's in motion, then there's a differential/non signal
sent to the ABS brain so the sensors/connectors may well be the problem.
Think removing ABS relay will just disable ABS circuit. Been driving like that with
no brake loss.
Ernest (NYC)
09-09-2005, 11:36 AM
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Which sensors/connectors need to be checked, and where are they? I have the odd problem that my ABS warning light comes on (sometimes, but not always) after hitting a large bump or pothole. That's got to be a loose/corroded connector somewhere, right?
Mark
-86 928
Mark
-86 928
09-09-2005, 11:54 AM
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There are quite a few components in an ABS (928-style) system that can fail or be damaged. I have two cardboard boxes full of that stuff taken off my car, and personally, I am amazed it EVER worked at all, given how much complexity there seems to be. Look into your wires at each wheel and the sensors and connections. Vunerable to road debris and chafing. Look at your pads, could be a worn pad or two. Check the ABS Brain...that can fail and of course, the relays, wherever they are. Your "wooden pedal" description sounds like problems with the power assist system.
Don Hanson
Don Hanson
09-10-2005, 01:02 PM
#13
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Thread Starter
OK............drove the car in the city all week with no further ABS warnings. Get on the highway yesterday to drive the 3 hours to home and an hour into the trip on comes the ABS light again. I'm in the middle of nowhere cruising at 150kph and all of a sudden!!!
One thing I forgot to mention earlier, I'm now running the car with the turbo twist rims and big fat Michelin PS2s...........could be that one or more of the wheels/tyres is rubbing the ABS wire?
One thing I forgot to mention earlier, I'm now running the car with the turbo twist rims and big fat Michelin PS2s...........could be that one or more of the wheels/tyres is rubbing the ABS wire?
09-10-2005, 02:08 PM
#14
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Originally Posted by the flyin' scotsman
OK............drove the car in the city all week with no further ABS warnings. Get on the highway yesterday to drive the 3 hours to home and an hour into the trip on comes the ABS light again. I'm in the middle of nowhere cruising at 150kph and all of a sudden!!!
One thing I forgot to mention earlier, I'm now running the car with the turbo twist rims and big fat Michelin PS2s...........could be that one or more of the wheels/tyres is rubbing the ABS wire?
One thing I forgot to mention earlier, I'm now running the car with the turbo twist rims and big fat Michelin PS2s...........could be that one or more of the wheels/tyres is rubbing the ABS wire?
a. ABS light. An intermittent problem can be caused by any of the problems that cause a full time failure - I suspect an issue at any of the connection points OR the relay.
b. Hard pedal, as suggested, look into the brake booster.
c. The wheels and tyres should have no effect on your problem - unless you have mismatched sizes where one wheel would roll at a very different rate than the other, and this would cause an ABS fault early on.
09-10-2005, 04:09 PM
#15
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Thread Starter
Rich...........is there a schematic for the relay so that I can do some testing? I did remove it during the week, cleaned the relay contacts ( they weren't really dirty) and cleaned the respective holes on the fuse panel. I also checked the connection points that Ernest mentioned under the hood.
Its pouring with rain here so I'll do an oil change and pull the front wheels.
To reiterate, there's no warning on start up, only after the car has been driven for a considerable distance does the light come on.......apprx 100kms.
Its pouring with rain here so I'll do an oil change and pull the front wheels.
To reiterate, there's no warning on start up, only after the car has been driven for a considerable distance does the light come on.......apprx 100kms.