ABS troubleshooting help

[WhoWhatNow]

I need some help troubleshooting the ABS system in my 1991 BMW 325i (e30). The symptom is a constant ABS light on. I have a late model car, so it does not have the fusible link in the relay under the dash which seems to be a common problem with these cars. I pulled both relays in the ABS pump and tested resistance between 85 and 86. Resistance was 57 and 49 ohms for the two relays. That is the extent of my abilities to diagnose this issue.

I would like to learn how to diagnose electrical issues on cars, however knowledge of electronics is limited. Can someone help with detailed steps I should go through for a proper diagnosis? I know my way around a multimeter, but I don’t always know where to pick up a ground or how to apply power to a circuit for testing. Basically can you explain it like I was a 12 year old?

 

[BillK]

Do you have a service manual ? It should have a troubleshooting procedure. That is always my first step. There should also be a description of how the system operates. From that you hopefully will be able find the problem.

By the way, you seem to think that this is an electrical problem ? It could very well be a mechanical problem that the ABS control module is sensing and then turning the light on. The only way to know for sure is to know how the system is supposed to work.

 

[gizardlizard]

Start by ohming the wheel speed sensors. Unhook them from the connector closest to each wheel. You need to isolate them before checking. Pull the sensors out and inspect the tone ring underneath where the sensor sits. Look for missing teeth and clean all the grooves really well. Being OBD1, not even sure if you can scan each sensor. That’s before my time. Basic multi meter tests will go a long way here.

 

[ToolsRCool]

In this case, paying the dealer an hour of diagnostic time @ $150 may save you a ton of time, headaches, and possibly $. If they find that just a $30 sensor needs to be replaced, then you could be back to 100% for less than $200 total.

Only suggesting this because the car is pre-1996, so the diagnostic codes and scan tool to read fault codes will likely be BMW specific (only).

 

[captaindiode]

There is some info here that might help.

 

[charbar]

Before we even start on this potential headache we need to know what you mean by 'constant abs light'. Is the light on even with the key off? Because that is a completely different route on the diagnostic path compared to light on with the key on.


Relays usually fail on the switching side before they do the control side so checking between pins 85 and 86 is only telling you part of the story in regard to the health of your relays.

 

[BurtEggley]

ABS is a system, at least on my Acura cars that were that era, had A LOT of pressure in the accumulator. I would read the manual on ABS front to back because a person could get killed with the pressures involved by opening something to see if it looked normal. The warning tags to leave "this" alone may be gone. My ABS had its own brake fluid reservoir that needed a special tool to capture the brake fluid that came out from the pressure. (Brake fluid eats car paint) I found this thread online. Good luck and be happy you got 34 years out of it. https://www.bimmerforums.com/forum/showthread.php?1789802-ABS-light-turns-while-driving

 

[CraigStu]

If you don't have a manual go here; https://www.alldata.com/diy-us/en

 

[andyvh1959]

The ABS light on is likely caused by some system issue that was not resolved by itself after the usual self test the system goes through at each start up. ABS of that era would create a "soft fail" issue that would clear if the ABS system self test confirmed the system voltage, speed sensors, the ABS pump, ABS solenoids. A soft fail may be a low voltage condition, enough voltage to start the car but voltage below the ABS computer expects. A hard fail would be recorded if a component or circuit didn't pass the start up tests. Then the ABS shuts down, sets the ABS fault light and the vehicle reverts to non-ABS assisted braking.

The speed sensors are simple coils, which output a low voltage range relative to the wheel speed. If the senor is out of adjustment the voltage will be out of range for low speeds, and record a hard fail. SInce the sensor is a coil the resistance is a fixed value easy to check with an ohmeter. Only way to check the actual sensor output on the car is with a wheel spining at a speed the system expects (above about five mph). Solenoids could likely be manipulated if you have the main connector pin-out to determine which pins to access, but even that would only tell you if the solenoid reacts, but not how much the solenoids cycle per the ABS controller signals.

All the testing at home still requires the car goes to the dealer/technician to get the fault codes and clear the codes in order for the system to reactivate. Dealers may not want to work on a system that old, so a specialist on ABS systems may be needed. Fortunately there are specialists who can even repair the ABS computer.

Back in the late 80s I worled directly with Bosch on heavy truck ABS systems. It was a great time, learned a lot. Even got to do a lot of test driving at Bosch's winter test facilites in northern Minnesota and Sweden.