P38 ABS - Help me please!!!!!

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2001 DHSE
I have 2001 DHSE with WABCO D. I had an ABS fault light on all the time (or would come on after a minute or two), checked everything, turned out it was the Control Unit in the footwell.

Unlike the WABCO C (which you can get for a tenner on Ebay), the WABCO D is expensive, even if second hand (and hopefully working).

Rachel
 
Along with Hub nut I've heard the wheel bearings can cause ABS faults if there's play. Our old 4.0 went through N/S Front sensors ( 3 in 2 years) no problem with the hubs, and I never got to the bottom of it.
 
Hi, in which case my swapping for a new ECU and getting the same fault should rule out the ECU (as it seems to be plug and play)??? I am still to check the wheel live data.
 
Resistance checking a wiring loom tells you very little apart from the fact that wire runs through from one end to the other, you can have 25 strands broken in a length of wire, and the remaining two strands that are connected will show zero resistance.

What you need to do is load the wiring loom with a voltage, and then check the volt drop.
To do this, disconnect both ends of the loom, supply 12v at one end of the cable you are checking,and put a load at the other end - a 60w headlamp bulb is ideal for this. Once this is connected up and the bulb is lit you check the volt drop. To do this you measure the volt at each end of the wire you are measuring, you will probably have to use a good length of wire to go from one end of the loom to the other to extend your volt meter leads.

What you are actually measuring is the amount of voltage that wont go down the wire, ideally you want less than 0.25 volts showing on your volt meter. Certainly any more than half a volt and you have a problem. As your reported problem is a possible short circuit, leave the volt meter in the wire you have just tested, and then check for voltage in all the other wires in the loom.

It may well be the ABS unit that is at fault, but check it is worth spending a bit more time checking the loom properly first.
 
N5Fav said:
Resistance checking a wiring loom tells you very little apart from the fact that wire runs through from one end to the other, you can have 25 strands broken in a length of wire, and the remaining two strands that are connected will show zero resistance.

What you need to do is load the wiring loom with a voltage, and then check the volt drop.
To do this, disconnect both ends of the loom, supply 12v at one end of the cable you are checking,and put a load at the other end - a 60w headlamp bulb is ideal for this. Once this is connected up and the bulb is lit you check the volt drop. To do this you measure the volt at each end of the wire you are measuring, you will probably have to use a good length of wire to go from one end of the loom to the other to extend your volt meter leads.

What you are actually measuring is the amount of voltage that wont go down the wire, ideally you want less than 0.25 volts showing on your volt meter. Certainly any more than half a volt and you have a problem. As your reported problem is a possible short circuit, leave the volt meter in the wire you have just tested, and then check for voltage in all the other wires in the loom.

It may well be the ABS unit that is at fault, but check it is worth spending a bit more time checking the loom properly first.
Click to expand...

Would think that is you stuck 12 volts down a cable with only two strands left and coupled a 60 watt bulb to it you pretty soon wouldn't have any strands left. Don't think the sensors use 12 volts more like 5, maybe wrong though. :D:D
 
Last edited:
The 1.6v held when i did the loom test
Everything seems corrosion free....
Does anyone know the rough voltage that should be read on the live data for the wheels when you spin them (slowly)?
 
Owen89VSE said:
The 1.6v held when i did the loom test
Everything seems corrosion free....
Does anyone know the rough voltage that should be read on the live data for the wheels when you spin them (slowly)?
Click to expand...

Wheel Speed Sensors (X137, X140, X158, X161)
A wheel speed sensor is located at each wheel. The
speed sensors generate an AC voltage signal as a
toothed ring rotates past the stationary sensor
pickup. The Anti–Lock Brake System ECU (Z108)
calculates the wheel speed by measuring the
frequency of the AC voltage signal generated by the
sensors.

What that voltage is don't have a clue. The ECU just counts pulses. So i suppose any pulse is good news.
 
I think I’m confusing myself, it gives an AC wave - which the ECU calculates in to a wheel speed. The live data gives wheel speed - that’s what i need to see..........
 
Thought the Sensors were Hall Effect sensors??

The ECU gives a reference voltage to the sensor and as the toothed wheel passes the sensor, the effect of the change in magnetic field (Hall Effect) changes the return voltage!!

This will give a alternating wave pattern of the return voltage!
 
If it's a Hall type sensor then there is no point in testing the sensor for resistance, in fact you could damage the sensor trying to test it.
I would expected though on a vehicle of this age it would have inductive type sensors fitted. (No doubt somebody with more specific RR knowledge can correct me on this).

If it is inductive type sensor, (which seems to be backed up by the fact that there is a specific resistance value available), then as the wheel rotates this produces an AC waveform. The ABS ecu translates that information into a road wheel speed and compares it with the other road wheel speeds.

Now here it can get interesting, effectively the ECU is counting how many times the AC waveform is passing 0v - (ie going from +v to -v as its AC current remember). The ECU is not interested in what the voltage is, as when the car goes faster, a greater voltage is produced by the sensor (a bit like a bicycle dynamo light gets brighter the faster you cycle). This is why there is no data for voltage to/from the sensor available. These voltages will certainly exceed 12v when you are at motorway speeds.


Wammers, I agree that two strands of wire would burn out quickly when it has to light a headlamp bulb. But now your testing has located and proved a wiring loom fault. :)
I was trying to show and explain that just doing a resistance check on the loom wiring is not a complete test.

HTH and that I've made sense
 
Saint.V8 said:
Thought the Sensors were Hall Effect sensors??

The ECU gives a reference voltage to the sensor and as the toothed wheel passes the sensor, the effect of the change in magnetic field (Hall Effect) changes the return voltage!!

This will give a alternating wave pattern of the return voltage!
Click to expand...

Saint I hope you don't mean that a Hall sensor gives a AC wave form, but that the voltage changes in square wave pattern. ie on / off
 
N5Fav said:
If it's a Hall type sensor then there is no point in testing the sensor for resistance, in fact you could damage the sensor trying to test it.
I would expected though on a vehicle of this age it would have inductive type sensors fitted. (No doubt somebody with more specific RR knowledge can correct me on this).

If it is inductive type sensor, (which seems to be backed up by the fact that there is a specific resistance value available), then as the wheel rotates this produces an AC waveform. The ABS ecu translates that information into a road wheel speed and compares it with the other road wheel speeds.

Now here it can get interesting, effectively the ECU is counting how many times the AC waveform is passing 0v - (ie going from +v to -v as its AC current remember). The ECU is not interested in what the voltage is, as when the car goes faster, a greater voltage is produced by the sensor (a bit like a bicycle dynamo light gets brighter the faster you cycle). This is why there is no data for voltage to/from the sensor available. These voltages will certainly exceed 12v when you are at motorway speeds.


Wammers, I agree that two strands of wire would burn out quickly when it has to light a headlamp bulb. But now your testing has located and proved a wiring loom fault. :)
I was trying to show and explain that just doing a resistance check on the loom wiring is not a complete test.

HTH and that I've made sense
Click to expand...
Just read RAVE.....you are quite right, they are Inductive Sensors.....

A sensor is mounted at each wheel sensing a 60 tooth exciter ring. When the vehicle is in motion the inductive sensors send signals to the ECU.

The front exciter ring is fitted adjacent to the constant velocity joint in each front hub. The rear exciter ring is inside the axle adjacent to the wheel bearing assembly.
Click to expand...
 
N5Fav said:
If it's a Hall type sensor then there is no point in testing the sensor for resistance, in fact you could damage the sensor trying to test it.
I would expected though on a vehicle of this age it would have inductive type sensors fitted. (No doubt somebody with more specific RR knowledge can correct me on this).

If it is inductive type sensor, (which seems to be backed up by the fact that there is a specific resistance value available), then as the wheel rotates this produces an AC waveform. The ABS ecu translates that information into a road wheel speed and compares it with the other road wheel speeds.

Now here it can get interesting, effectively the ECU is counting how many times the AC waveform is passing 0v - (ie going from +v to -v as its AC current remember). The ECU is not interested in what the voltage is, as when the car goes faster, a greater voltage is produced by the sensor (a bit like a bicycle dynamo light gets brighter the faster you cycle). This is why there is no data for voltage to/from the sensor available. These voltages will certainly exceed 12v when you are at motorway speeds.


Wammers, I agree that two strands of wire would burn out quickly when it has to light a headlamp bulb. But now your testing has located and proved a wiring loom fault. :)
I was trying to show and explain that just doing a resistance check on the loom wiring is not a complete test.

HTH and that I've made sense
Click to expand...
Usually the ECU input circuit from an inductive sensor is configured to produce a voltage limited near square wave, so it will only be AC at the connector. Therfore, diagnostics cannot show voltage, the signal is either there or not there as far as the ECU is concerned.
 
Hawkeye defo gives a wheel speed on each corner. I am yet to plug her in a check all four corners. I read that the ECU drops the data link over a predetermined speed. So it’s slow driving to see what’s going on. If that comes back ok, I really will be stumped!
 
Well, I can add to the list that is not the brake pads, hub nut loose/bearings and Hawkeye says each corner is giving a good wheel speed and working as it should.
I am totally lost now.
 
Owen89VSE said:
Well, I can add to the list that is not the brake pads, hub nut loose/bearings and Hawkeye says each corner is giving a good wheel speed and working as it should.
I am totally lost now.
Click to expand...
Maybe a rubbed wire shorting somewhere, otherwise it's usually the ECU, I know you have changed it, but did you carefully check the connector for corrosion?
 
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