BimmerFest BMW Forum banner

1 - 2 of 2 Posts

·
Seek to understand,^Value
Joined
·
25,199 Posts
Discussion Starter #1 (Edited)
I'm not sure if this is a new wheel speed sensor diagnostic test, or if it's just a badly written repeat of the OLD test #5 for the wheel speed sensors when diagnosing the trifecta.

Can someone (who knows better how mechanics think & write) help me figure out what this test is supposed to be?

The test showed up in post #35 of this thread just now:
> E39 (1997 - 2003) > Trifecta lights + no speedo, again

... took it to the mechanic today and this is what they came across:

So my module is the culprit it appears.
I find their test VERY INTERESTING if it purports to test a wheel speed sensor.

Is this an accurate rendition of the mechanic-speak of that paper?

"With wheel speed sensor disconnected (from where?) and key on (in which position, 1 or 2?), harness side should read 8V (presumably across the relevant wheel speed sensor pins in the blue female ABS control module). harness connector)".
 

·
Seek to understand,^Value
Joined
·
25,199 Posts
Discussion Starter #2
Is this new WSS test just the same old test #5 repeated in a different way?

This 8V disconnected-sensor power test seems similar to the "TEST #5" we already know about which is in the canonical ABS diagnotistic post, but, it has the extra confusing (to me) step of "disconnecting the wheel speed sensor".

Here's the best I can make of the procedure documented by the mechanic above:

  1. Disconnect the wheel speed sensor at the wheel well.
    • But, why bother disconnecting the wheel speed sensor?
    • It will be disconnected from the ABS control module when we pull the ABS control module harness off anyway, right?
  2. Turn the ignition key on (I presume to position #2?)
  3. Test the static voltage across the relevant WSS pins in the ABS control module harness.
    • Presumably it should read 8v across the two leads
Seems to me that test is only checking the power going to the wheel speed sensor (since it's presumably disconnected). I'm not sure that will tell us anything useful. Can someone tell me what I'm missing here?

Note: We already have a similar #5 WSS test explained in the canonical ABS control module diagnostic thread which consists of checking for the voltage to the sensor:

UNDERSTAND WHEEL SPEED SENSORS:
Note: The wheel speed sensors are two-wire hall effect transducers which send a digital square wave signal with a low of .75 volts and a high of 2.5 volts to the DSC control unit. Each sensor receives a well-regulated 8 volt power supply from the control module through one wire. The ground path for the sensor is through the second wire back to the control module. The signal is generated by a pulse wheel affecting the voltage flow through the hall element in the sensor. The pulse wheel is integrated into the wheel bearing assembly, behind the seal. This protects the trigger wheel from foreign substances which may affect the wheel speed signal.

TEST WHEEL SENSOR CIRCUIT FROM THE ABS CONNECTOR
(also checks wiring circuit):
OPTIONAL: Jack car up (so that all four wheels can be spun to test voltage & resistance fluctuations of the hall-effect sensors)
- Turn the car off and remove the key from the ignition.
- TEST 1: Switch the DMM into the diode test position
- Wrap a stiff 20AWG wire onto the ends of your DMM probe for sticking into ABS-connector pins
- Label the positive 20AWG wire with white tape so that you won't get confused as you switch back and forth
- Stick the ends of the wire into the appropriate female holes of the ABS connector (13-29, 30-31, 28-12, 15-16)
- In one direction, you should see 1.7 to 1.8 volts (note the pinouts mentioned are in order, positive to negative)
- In the other direction, you should see OL or some other infinite reading (open circuit)
- TEST 2: Switch the DMM into resistance checking mode (optional)
- You should see around 3.3 Mega ohms in one direction & approximately twice that in the other direction (but some say more)
- TEST 3: If desired spin the wheel at about 1 revolution per second, by hand (the resistance should fluctuate as the wheel spins)
- TEST 4: Switch the DMM into millivolt mode (optional) & again spin the tire & wheel assembly by hand (test-lead polarity won't matter)
- You should read between 1 and 5 mV when you spin the hub (no voltage implicate the sensor or circuit)
- OPTIONAL TESTS BELOW REQUIRE FLYING LEADS WITH THE IGNITION SYSTEM ABS SYSTEM CONNECTED & POWERED UP:
- TEST 5: Swith the DMM into the 10v and attach flying leads to the sensors with the power on
- You should see the voltage going to the sensor and the return signal
- Expect a baseline voltage of about +5 to +12 volts depending on the ABS system (does anyone know this value?)
- Expect that baseline voltage to the sensor to change (by how much?) as you spin the wheels

- TEST 6: Hook an oscilloscope with "flying leads" to the ABS sensors (notice that the ABS system must be powered)
- You should see nice clean square waves generated as you hand spin the wheels at about 1 revolution per second.
Note: The oscilliscope can detect problems that can't easily be found with a DMM (A scope pattern for a wheel speed sensor should show a classic sine wave alternating current pattern that changes both in frequency and amplitude with wheel speed. As the wheel is turned faster, signal frequency and amplitude should both increase. Damaged or missing teeth on the sensor ring will show up as flat spots or gaps in the sine wave pattern. A bent axle or hub will produce an undulating pattern that changes as the strength of the sensor signal changes with every revolution. If the scope pattern produced by the sensor is flattened (diminished amplitude) or is erratic, it usually indicates a weak signal caused by an excessively wide air gap between the tip of the sensor and its ring, or a buildup of metallic debris on the end of the sensor. A weak signal can also be caused by internal resistance in the sensor or its wiring circuit, or loose or corroded wiring connectors.)
So, can someone explain if the old #5 test is the same as this new-to-me test, and, if so, what's this about disconnecting the wheel speed sensor as a first step?
 
1 - 2 of 2 Posts
Top