TRACS/ABS: 850: VOL-FCR ABS DTC 313 Sensor Signal/Fault in Circuit Diagnosis

[LordTodhead]

I would appreciate forum members' comments on the following ABS diagnostic attempts following two occasions, outlined below, of the TRACS and ABS console lamps remaining steady on after the ABS ECU module power on self-test.

(1) Following DTC 314 - "Right rear wheel sensor signal/fault in circuit" - I re-flowed solder on pins 1 and 17 on ABS module PCB, cleared DTC via VOL-FCR and all was well for circa 1,500 Autobahn miles.

(2) This week, following DTC 313 - "Left rear wheel sensor signal/fault in circuit" - I re-flowed (twice) pins 2 and 10 on ABS module PCB but still cannot clear this fault code.

All four wheel speed sensors (O/S R, N/S R, O/S F and N/S F) show resistance that I understand to be within specification at respectively 1044,1058,1068 and 1062Ohms.

With ABS ECU module power on there is 2.391Vdc and 2.371Vdc across the relevant pins (2-4 and 1-3) in the yellow connector (near fuel tank sensor housing) for left and right rear wheel speed sensors respectively. Is this the bias voltage for these sensors and is this value within specification?

I can't find technical data documentation specifying bias voltage for this Ate module (ID 0009140774 rev. 003 software code 09148591 rev. 003).

If, by using VOL-FCR, the DTC is cleared whilst vehicle is moving then no further DTCs are indicated and the TRACS and ABS lamps go off and ABS functions correctly.

After the DTC is cleared under vehicle moving conditions the TRACS and ABS lamps always remain off until vehicle speed reduces to less than circa 2mph whereby the lamps are switched on apart from one occasion when the TRACS and ABS lamps remained off when stationary and even went through the successful self test sequence of 2 seconds on and then off.

I intend to check for wiring shorts to ground with DVM (resistance) and oscilloscope (bias voltage) tomorrow.

The facts of a measured steady bias DC voltage and (whilst vehicle is moving) a sensor reluctor AC voltage the satisfies the ABS ECU module operating conditions lead me to believe that the solder re-work on pins 2 and 10 is sound - or am I missing something here as the problem of the failed power on self test whilst stationary remains?

Any ideas?

[LordTodhead]

Results of Philips PM 97 Scopemeter DC voltage measurements with respect to ground on rear ABS sensor connector 24/19 pins.

Where:
NS/R - nearside rear, OS/R - offside rear, Gr - Green, Wh - White, Rd - Red, Ye - Yellow.
Pn - pin number, Vdc u - sensor unplugged, Vdc p - sensor plugged in, Vdrop - voltage drop, %drop - percentage voltage drop.


Sens Wire Pn Vdc u Vdc p Vdrop %drop
NS/R Gr/Wh 2 2.416 2.319 0.097 4%
NS/R Gr/Bk 4 0.021 2.300 0.116 5%

OS/R Gr/Rd 1 2.397 1.847 0.550 23%
OS/R Gr/Ye 3 0.021 1.735 0.612 25%


DC voltage measurements across sensor pins (p-p) on rear ABS sensor connector 24/19:


Sens p-p Vdc u Vdc p
NS/R 2-4 2.392 0.020
OS/R 1-3 2.373 0.110


From the above we see there is a five-fold greater drop across the nearside sensor compared to offside sensor. The ABS module sees 1.7Vdc from nearside sensor whereas it sees 2.3Vdc from offside sensor. If, for example, the threshold for acceptable sensor Vdc signal is 2Vdc then the nearside sensor is below this level and is not seen by module which then flags DTC 313 fault.


I'll swap (crossover) the wires Gr/Wh to pin 1 and Gr/Rd to pin 2 tomorrow to determine if problem is located in sensor/sensor tail.

[LordTodhead]

Swapped ABS sensor tail pairs at connector 24/19 so that ABS module nearside channel is now connected to offside sensor and vice versa in order to test:

(1) If ABS module flux-reflowed pins 2 and 10 can detect known working offside ABS sensor.
(2) If nearside ABS DTC fault shifts to offside - implying that the problem is 'downstream' of connector 24/19 - ie. sensor related and/or sensor tails related.

A reliable connection for this swap was eventually achieved by removing the sensor tail pins from connector 24/19 and reinserting them in the required swapped positions. This can be achieved by opening the pin securing 'lid', depressing the pin's locating tang (located inboard on each spade-form pin) and pulling out with care. [see attached photos].

Running VOL-FCR ABS diagnostics now shows ABS 314 - a swapping of original results:

ABS-314 Right Rear wheel speed sensor signal, fault in circuit.
ABS-313 Left Rear wheel speed sensor signal, fault in circuit.

Voltage with respect to ground measurements now show essentially a swapping of original results:

Sens Wire Pn Vdc p Vdc p
OS/R Gr/Wh 2 1.848 2.319
OS/R Gr/Bk 4 1.716 2.300

NS/R Gr/Rd 1 2.302 1.847
NS/R Gr/Ye 3 2.282 1.735


As both points (1) and (2) above would seem to be satisfied I have returned connector 24/19 pinout to original and, sure enough, we're back to DTC ABS-313 as expected.

Weather has closed down now and, like many forum members, I am working hard core outside - so the checking of sensor and sensor tails will have to wait until (hopefully) tomorrow.

The keen-eyed amongst you will have noticed the change of label for pins 2 and 4 from incorrect "NS/R" in the previous post to the correct "OS/R" in this post. Also, pins 1 and 3 are now correctly labeled "NS/R" in this post. This is an unfortunate typographical error only - the testing was performed throughout on pins/wire colours where:

Offside Rear (OS/R) or Right Rear is pins 2 and 4 Green/White and Green/Black
Nearside Rear (NS/R) or Left Rear is pins 1 and 3 Green/Red and Green/Yellow

Sorry if this has caused unnecessary confusion.

[LordTodhead]

To recap:

From outset (and checked again today):

1. Disconnecting at 24/19 and measurement across pins 2-4 and 1-3 shows both wheel speed sensors resistance to be ca. 1020Ohm.
2. Resistance to ground measured at each OS/R pin 2 and 4 is OL (off scale, infinity) and measured at each NS/R pin 1 and 3 it is ca. 5MegOhm.

OK, weather is reasonable, so time for NS/R disk off and visual check on wheel speed sensor and sensor cable.

Cleaning off the usual brake dust on sensor and reluctor ring and freeing cable from a pitch point that I suspected was cause for voltage drop produced, sadly, no change from the original OS/R and NS/R voltage measurements of, respectively, 2.3Vdc and ca. 1.7Vdc wrt ground.

Perhaps voltage creep through the sensor cylindrical case to brake backplate/hub assembly explains the 0.6 Vdc voltage drop and 9MegOhm?

Looks like its replace ABS sensor time...

Anyone have another idea?