[kmckmc]

I do not know if my 1992 968 ROW-spec (6speed) 's ABS is faulty or not.



I have installed new rotors/brake pads (Pagid sport) for about 1000 miles.



First, ABS light at the instrument panel comes on (when key is inserted and turned 1 notch) and then off (after the engine got started). I suppose this means that ABS unit is not faulty.



BUT, while cruising straight line, when I slammed the brake, then both rear wheels got locked, and I would see tire smoking. I did not feel the ABS nor hear any noises from the ABS pump. Without hard braking, the brake system seems normal.



What had happened? Is this normal?





Please help.

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[Bulti]

Did the brake proportioning valve got changed? It is responsible for how much braking pressure is divided between front and rear. When braking hard, a lot of the weight goes to the front of the car making the back tires lock up if the pressure gets too high.



Just a thought...

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[flash]

also, are the outside diameters of the front and rear tires within 3% of each other?



when is the last time the system was flushed?

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[Spyderman]

Do you know if the original rubber brake hoses are still on the car? Over time, the inside of the rubber hoses can swell, constricting the inner diameter, causing issues with braking.

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[flash]

good point. at this point in time, anybody running on the original brake hoses is likely to start having problems. flushing the fluid regularly, as per the instructions in the manual, is key to longevity, but i'm willing to bet that almost nobody does it.

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[Spyderman]

I've cut open many an old, hardened rubber brake hose to find that the insides have collapsed from the effects of age and dirty brake fluid, to the point where you wonder how any brake fluid was able to flow back and forth through the hose. Akin to plaque building up on the inside of blood vessels and arteries and raising the blood pressure, or becoming blocked off entirely...

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[williamoss]

Regarding flushing the brake lines, in our 968 can it be done with 2 people one pushing the pedal all the way to the floor while the other opens and closes the bleeder screw with a clear plastic tube connected and submerged in a glass jar so one can see the black fluid change to clear. Or does one need to use a tool to do this job as in a pressurized jar or a vacuum brake jar?

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[flash]

use a motive power bleeder - period - you'll be glad you did - nuff said

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[kmckmc]

The brake fluid (ATE Blue color) was flushed about 6 months ago. I will check the condition of those brake hoses.

Thank you all.



Happy New Year.

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[MB968]

Don't see how the brake lines would have any effect on your symptoms. If you are getting lockup, that means that pressure is getting through to the rear brakes. So, the ABS should see the fact that the wheels are not rotating (no signal from the sensor) and should activate to keep this from happening.



You might try the same test on a large wet parking lot where you can keep the speed a lot lower. See if you get the same results. It would get you another data point. And, you could also try the parking lot test with the fuse pulled for the ABS system, again to see what results you get.

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[MCL968]

Anyone have any suggestions for sources for stainless lines?

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[Ernie]

Got mine from automotion but I am pretty sure pelican has them too.

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[kmckmc]

My mechanic replaced a used ABS pump.  Now, my 968 ABS pump is back in action.

So, it was a case that the ABS pump was seized in the valve block.

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[Waylander]

Been there done that, you can rebuild the ABS pump with parts from same era Mercedes, the merc pumps were in the engine bay so no corrosion


Mine was in the same state as yours rusted motor and pistons, swapped the parts out of the merc one all works sweet as, cost me the total of about $50 to fix including brake Fluid


If you get one out of a 190E even the relays are the same


The valves and motor are the same the Bosch part numbers change due to the location of the pipes.


Simple check is turn the stereo off and move off real slow you should hear the pump run a test at about 3-4 mph

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[MLB]

Aha, Waylander! You mention ABS pump test at a slow roll;I have been puzzled for over two years since I got my car, trying to figure out why I hear that ABS sound for about one second almost every time I back out of my garage!

My mechanic friend thinks I'm imagining things.

So that's normal?

Thanks,

Michael

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[Waylander]

yes its part of the ABS testing itself.

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[MLB]

My mechanic still calls bunk on the abs self test idea. Is that documented somewhere? It would be great to show him I found out something about this car that he didn't know, and that the information was first mentioned by a forum member (he is also very skeptical of these sorts of forums due to the misinformation and questionable info found on some of them)

--michael

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[Waylander]

Well he is wrong, the 928 does it so does the 944 and the 968 they all share the same abs system


The ignition on does the ecu and sensor test, the move off tests the pump and relays, it's in the extra abs test sheet from Porsche the move of test also calibrates the ecu for the sine wave it gets from the wheel sensors.


this is my post after fixing my ABS on the UK PCGB site




Hi All I have been trying to build a post to help anyone with ABS issues so if anyone has any thing to add to this it would be cool


So far....


When the ignition is switched on the ABS light on the dash should illuminate.

When the engine is started the light should extinguish

When the car reaches 3-5 MPH the system should carry out a sensor and valve check sometimes a noise can be heard from the front OS wheel arch.


If the ABS light stays on after engine start there could be a few reasons for this.


1. Brake light Switch Fault

2. ABS pump Relays

3. ABS relay G20 (944) G22 (968)

4. The ABS ECU Grounds

5. The ABS Wheel Sensor Grounds

6. The ABS Pump Ground

7. The ABS ECU


If the light goes out with engine start, then comes on when reaching 3-5 MPH this is caused by a malfunctioning wheel sensor, ground or the sensor.


Wheels sensors will only provide output to an oscilloscope for full diagnosis but it is possible to test them with a DVM.

When the hub is spun as fast as you can by hand (direction does not matter) the sensor should output a very small voltage AC, they should all be about the same output, for the same spin speed.


Checking the resistance will only show they are not open circuit.


Checks

1. Make sure the toothed section on the rear of the hubs is free from rust and metallic particles and dirt.

2. Make sure the head of the sensor is also clean and free of dirt.

3. Check cable continuity for the ABS sensor cables.


If one sensor puts out a significantly lower voltage than the rest replace it


The ABS sensors can be tested at the ECU cable connection.


Right Front ECU Plug Pins 21 and 11

Front Left ECU Plug Pins 6 and 4

Rear Right ECU Plug Pins 24 and 26

Rear Left ECU Plug Pins 8 and 9


It is also possible to bridge out the ABS relay G20 / G22 for testing

Bridge terminals 87 and 30 in the fuse box.


It is also possible to test the ABS pump Motor by applying 12v to one pin in the ECU plug and one pin in the fuse box With the ECU unplugged.


Apply 12v across pins 28 (ECU Plug) and terminal 87 (ABS Relay fusebox) the ABS pump should fire into life.

If you have no warning light after moving off and no ABS, I would suggest that the ABS pump is seized in the valve block and needs replacing.


if anyone has any more information or tests that would help please add them in


Pins for the ABS Relay

Image

Pin 15 Is the voltage sensor from the Altenator


ABS ECU Information


Two ABS ECU's can be used in our cars but don't be fooled by the part numbers, I have had this checked out by OPC in Polaris which is their parts history system.


The ECU that appears on eBay most often

928 618 119 05

This ECU was used between 1984 and 1991 for 928,944,944S2


928 618 118 00 is the later ECU fitted to 968, later 928

This is the correct one for our cars but both will work


The later ECU is now only available as an exchange part at the paltry sum of £744.00 plus £200 parts deposit plus VAT



Abs Pumps Modulators that can be cannibalised for parts.


Porsche 944S2, 968 Standard Abs Modulator

Bosch Pump 0265 200 038


Donor Pumps

Bosch Pump 0265 200 043 Mercedes W124 Chassis from 190 to 500SL

Bosch Pump 0265 200 042 BMW All sorts

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