|Mon, Nov-08-2010, 01:35:55 AM||#1|
The Frugal and Outspoken One
Join Date: Feb 2008
Location: Orlando (Metro West), FL
The definitive guide to self diagnosing your ABS/ASC light.
If you're like me, one of your biggest pet peeves is a Christmas tree dash. Some people don't mind trouble lights on; I am not one of those. I cant stand the feeling that something is wrong with my car.
A BMW dealer will quote you anywhere between 1-2 hours of diagnoses charges (@ ~$120.00/hr) to read your ABS/ASC faults for you. If you are all about saving money, and learning something at the same time, you may find this useful. I will show you how to find the faults in your ABS/ASC system using a simple multimeter without being at the mercy of the dealer.
To start off, download the proper ETM for the year of your vehicle from TIS online. In this you will find the proper pin out info for your year. Use this link:
Change the year in the above link to match your vehicle. Example: if you are working on a '99 change it to: "http://www.bmwtechinfo.com/etm/data/e36/e36_99.pdf"
You will need to remove your glovebox to access your ABS/ASC computer connector. Here they are:
Here is how the pin numbers are laid out in the connector:
I ignored the assignments off to the left, they're wrong as far as my '97 was concerned. Use the ETM you downloaded for the proper pin assignments for your vehicle.
For a '97 we refer to the following assignments for reference:
Some of the most common things that will cause an ABS fault in your system are amongst the following:
1. A faulty wheel speed sensor
2. A bad ABS relay
3. A bad pedal travel sensor
4. A stuck/clogged solenoid valve
5. A faulty ABS pump motor
6. A bad brake light switch
Testing your wheel speed sensors.
Perform all of these tests with the ignition in position 3.
Our cars use Variable Reluctance type wheel speed sensors. In the tip of the WSS is a magnet and a coil that produces an AC voltage proportionate to the rotational speed of the wheel. This signal is sent to your ABS computer where it uses it to monitor the speed of your wheel rotation to decide when to activate/deactivate the ABS solenoids.
Test the sensor for proper resistance:
The specification for resistance of the sensors per BMW is between .5 -2 Ohms. Test this by identifying the proper pins in your connector that apply to that sensor. Let's use the rear Right for example. Using the above pinouts we see that pins 10, 45 are the 2 pins for the sensor. To measure the resistance over the WSS, apply your black multimeter probe to pin 10 and the red on pin 45. With your multimeter set on ohms you will get a reading. Do this on all 4 sensors. If the sensor is out of spec for resistance, replace it.
Test the sensor for proper function:
With your multimeter set to read AC volts, remove the wheel, then unplug the sensor from the harness. Probe the + pin in the connector on the harness side with red, and the ground pin with the black and spin the wheel. You should see your voltage produced increase with speed of the rotating wheel. Do this for all 4 sensors. If the voltage is not consistent with wheel speed replace the sensor.
Verify proper power supply to the ABS computer.
The ABS computer gets numerous 12V inputs from different areas all pretty much through the ABS relays/fuses. Using your multimeter set to DC volts, connect your black probe to pin #1 on your connector and leave it there while you perform the following tests. Referring again to the above pinouts, we see that we should have a 12V supply to the ABS/ASC computer at pins: 3,16,33,35,51 with the ignition in pos. 3. Using your red multimeter probe, touch all of these (may be different for your year) pins and verify that there is voltage there. If there is not, you have a bad power protection relay (stuck open) and/or fuses, and your ABS computer is not getting power. Make sure that your ABS computer is getting power by checking all the necessary pins for 12V. Your actual voltage will vary but it should be close to the measured static voltage of your battery which is close to 12V but could be as low as 11 or more in some cases depending on your battery. A voltage below 7 volts will trigger a fault code and turn your ABS/ASC light(s) on.
Test your ABS power protection relay.
Your abs power protection relay is an internally fused relay that is designed to blow and open when subjected to over voltage protecting your ABS computer from damage. If the relay is bad you will not have any power at the pinouts outlined above in the 12V power tests. If you do not have power in the previous test at the specified pins, this relay has tripped. If this is the case, remove the relay, and test the pins again; you should have full power everywhere with the relay removed. If you have power again after removing the relay; replace it.
Here it is:
Some more in depth testing info:
The way this relay works is; when power is applied over pins 85, and 86 you energize the coil magnet and it closes the relay. The closing of the relay is what completes the circuit and allows current to flow through it.
Make sure the diode in the relay works. A diode is a one way valve for electricity; only letting it flow one direction. Put your multimeter on resistance. Put the red terminal on pin 30a, and the black on 30. You should see a resistance reading between 50-100 Ohms. Now reverse your probes, putting the black probe on pole 30a and red on 30, you should now see infinite resistance. If neither of the above cases is false, the diode is bad and warrants replacing the relay entirely.
Test the continuity of the inner circuitry. Before energizing the relay, put your multimeter on continuity test. Red on pin 30 and black on 87a, you should register contact and hear a beep and/or signal that your multimeter makes to signal continuity.
Power up your relay by adding 12v across pins 85/86. you should hear a click. If so, the circuits are closed. Verify that they work by testing again for continuity, this time red on 87, black on 30. 87a no longer applies when energized because it becomes disconnected when the relay closes.
Test your ABS pump inlet/outlet solenoids.
Do this with your ignition in pos. 3. Our ABS systems are 4 circuit systems. Each hydraulic circuit to each wheel has 2 solenoids. 1 to modulate inlet pressure and 1 to modulate outlet pressure. Referring to your proper pin out, select which valve you want to test. Example, lets pick the inlet valve from the rear right. According to the above pinout, the pin to control it is pin #39. Set your multimeter to measure resistance. Connect your black probe to ground (pin 1 or any other ground pin) and the red one to pin 39. Specs for resistance for the valves are:
Inlet: 2-9 Ohms
Outlet: 2-7 Ohms
anything out of spec is likely a stock/clogged solenoid and would warrant a replacement abs pump.
Test your brake light switch
With multimeter set on DC volts, probe pin 1 with black, and 32 with red. Push your brake pedal. With the brake pedal pushed, you should have 11+ volts displayed. With the pedal at rest with no pressure you should have 0 volts.
Test your hydro pump motor for operation.
Do this with accy pos. 3 on. Measure the resistance between the 2 ABS pump sensor signals; pins 49, 50 in this case. BMW spec is 10-40 Ohms for the connection. Out of spec means new ABS unit. Within spec = continue reading. If resistance checks out, go to your fuse box and pull your abs pump motor relay. Its the 5 pole relay, not 6 pole. Jump the always hot pole to your pump supply line. (ID this by looking at the diagram on the side of the relay.) You should hear the pump start up, if not, its nad.
More to come
Last edited by WRXEATR; Sat, Nov-13-2010 at 05:11:43 AM.
|Mon, Nov-08-2010, 03:02:56 AM||#2|
Join Date: Oct 2009
Posts: 6,850In the garage:
Location: Columbus, OH
I sense copious amounts of win to be dropped in this thread.........
Never run out of traction, real estate, and ideas at the same time.
|Mon, Nov-08-2010, 03:50:25 AM||#5|
Carlos is My Co-Pilot
Join Date: Aug 2007
Location: Near the Tannhäuser Gate
Right click and save as...
|Mon, Nov-08-2010, 05:46:34 PM||#9|
Carlos is My Co-Pilot
Join Date: Aug 2007
Location: Near the Tannhäuser Gate
|abs or asc, definitive, diagnosing, guide, light|