ABS warning light/sensors/ECU/wiring problems

[pete the bus]

I thought I'd start a new thread following on from my failed MOT post earlier, as this is now the only problem left.
The warning light on the dash is constantly on. I have searched for and read countless threads on this issue, but am still confused

I have measured resistance at the ECU plug and found the OSR sensor (or wiring) to be at fault. I have located the OSR sensor connector block and measured the resistance there. Open circuit, indicating sensor or wiring fault.

Before changing the sensor, I want to know that this is the only fault. So I ( as recommended in another thread), put a resistance across the connector (1100 Ohms). It now measures 1140Ohms at the ECU plug. In theory, this should fool the ECU into thinking there is a working sensor and put the light out. But it doesn't.

I have also read that when you turn the ignition on, the ECU warning light will go on and off a couple of times as it tests the circuit. It doesn't. I have checked the fuses relating to ABS function and they both check out OK.

Are there any other checks I can do? Will a Lexia test be of help? Or should I just change the sensor anyway and see what happens? I don't want to keep throwing money at it and finding it still isn't fixed.

Car is a 1999 (T reg, S2) Xantia 1.9TD LX Estate.

Any thoughts, ideas, help, much appreciated.
Peter

[Lighty]

The problem is that if the ABS warning light is on, ie fault apparent, it will not go off until you clear the light with a diagnostic tool, or the car see's a good signal from the sensor.
So if you replace the sensor then you must drive the car for about 30ft or so to clear the light. Obviously you resistor cant do this. I would say if you have open circuit on the plug, then change the sensor and all will be well.

[pete the bus]

Thanks for the input, Lighty.
I have read similar advice on here about driving the car (over 15kph), and resetting the ECU. Others, though, have remarked that once the ECU has done its testing procedure at start-up, it "sees" the sensor resistance and puts the light out. As I said, my ABS light does not appear to go on and off at startup so I'm not sure if the ECU is doing the test...

[VertVega]

... So I ( as recommended in another thread), put a resistance across the connector (1100 Ohms). It now measures 1140Ohms at the ECU plug. In theory, this should fool the ECU into thinking there is a working sensor and put the light out. But it doesn't. ...

It doesn't because it also must pass the test while driving.

When you replace the faulty sensor the dashboard ABS light will not turn off.
I don't know the equivalent value in miles but in kms the critical value is 20 km/h.
ABS sensor must also give healthy response just when the car passes 20 km/h speed
and then the ABS light disappears from the dashboard.

Edit: ABS sensor error will stay registered in ECU memory and can be resetted by Lexia
but this error marking doesn't cause ABS light ON continuously.

[HDI Dave]

Or should I just change the sensor anyway and see what happens? Car is a 1999 (T reg, S2)

Well that's what I'd do,did,

Yeah,like an open circuit. I was overthinking it,as I think you are

Had light on for years..mot guy getting miffed lol

Changed sensor..same

Went shop for some 'relaxants'..

..and the light went off! and has remained off

Got mine off the bay,around 15 quid iirc,altho some people don't recommend cheaper ones..

mine still seems fine though

[pete the bus]

Thanks for the replies Vertvega and HDI Dave.

I have ordered a new sensor off ebay, £13.00 inc. p&p.

I am now attempting to get the old one off. Have put the car on ramps, taken off the spare wheel, and managed to get the actual sensor off. I cannot though see how on earth the cable connector can be pulled through from underneath. I managed to pull the grommet upwards and out of its hole, and then had to cut it off. But when I try and feed the connector block down through the hole there is absolutely NO WAY it can fit through it.
Do I need to take the wheel off and try jacking the trailing arm up to give me more room. I'm at a total loss now. I could cut the cable, but I still would have to feed the new cable back up through the hole. HELP!

[Lighty]

Sometimes easier to take the rear arm bolt out and drop the arm a little, not a nice job on your drive.

[VertVega]

Sometimes easier to take the rear arm bolt out and drop the arm a little, not a nice job on your drive.

Exactly that's the correct way to do it but it's difficult as you have also said.

Try feeding the ABS cable from below to upwards with a metal wire guidance.
Difficult to explain but may be these pictures will help you to see what I mean

This one I have already sent earlier:



This pic shows the top of the rear subframe where the ABS sensor cable comes out and that location is
marked by the arrow. Normally you'll not be able to see this location and hope this pic will be helpful.



This shows the rear and right side (fuel filler cap side). Top of the pic pointing the front part of the car.

[xmexclusive]

There are two seperate ABS checks carried out by the ECU.
The dynamic ABS check is the one that needs 4 good sensors and a moving vehicle above a set speed.
A resistor replacing a sensor will not fool this dynamic part of the ABS test.
This test happens at regular intervals as the car is driven provided no brake application is being made.
When it sees a failure it records a temporary fault in the ABS ECU.
Get enough temporary faults and the ECU locks the ABS out permanently.
Get just the occasional fault and the ECU will clear itself by driving above the cut-off speed for that type of ABS.
Use a LEXIA and you can clear that fault record but not the original fault, you still need to correct that.
The Lexia cleared ECU might just get you through an MOT though.
This part of the ABS ECU check needs to be fairly fault tolerant as it is counting individual teeth on the ABS rings and comparing the results.
For example fractional wheel spin on a pot hole or drain cover will put up a temporary fault that quickly clears so no ABS light comes on.
A split ABS ring on the other hand will most likely show a fault nearly every wheel rotation.
It will quickly build up enough temporary ABS faults to lock out the ABS and put on the ABS light.
To get to see these dynamic ABS indications the ABS has to pass the static ABS check first.
The ABS Blink sequence on ignition switch on indictes only that the ABS has passed the static test.
If the light does not blink on ignition switch-on the fault is not just a defective sensor.
For the static test of the sensors the ABS ECU only checks the resistance of the sensors.
A resistor instead of a sensor will fool the ECU at this stage.
But sensors are also the final stage of the static test.
If the ABS light has not already blinked a couple of times it is failing the ABS static test before it gets to the sensor check.
So no amount of work on sensors alone will cure this type of fault.
From memory the earlier stages of the static check are:
Power supplies;
ABS saftey relay correct operation;
Microprocessor running test;
Second Microprocessor data agreement:
On a typical ABS system the first blink of the light is the correct completion of the power supply check.
Note that some ABS ECU's have up to 3 seperate fused feeds.
If any one is down for fuse or poor contact there will be no ABS light blink at all.
It then checks the ABS relay.
Pass this test and you should get a second blink of the ABS light.
Now the ABS relay also controls the power feed for the ABS light so in one fault condition it too can cause no ABS blink at all.
The third ABS light blink confirms that the Micro checks have completed ok.
The ECU now checks the resistance values of the 4 sensors.
It then starts again and repeats all the checks from power to sensors.
If these pass ok the ABS light will then remain off.
If it fails the light will stay on.
In most of the UK reaching this stage should get you a MOT pass (NI is the exception).
So for the initial ABS fault finding:
Ignore the sensor and forget about the Lexia just watch the ABS light blink on ignition switch -on.
Count the blinks if any and from that work out which bit of the circuit to check.
The process requires no test kits just a few minutes logical checking.
If you need to move onto sensors you will be there in less that 5 minutes.
Sensors can take you hours to work through and correct.
Thats the stage for the Lexia or meter.
Do not waste time on the sensors until you are certain they are at fault.

John

[ksanturion10]

Hi there,
sorry to ressurect this old thread, but I think this is the right place.
Had ABS light on for some time, found the FL sensor to be OL-ed. Changed the sensor, and after a short driving the ABS light went off. The strange thing is, that actually the light is not really off. I can see it during the night, but because it is kind of faint, it can't be seen during day time. Any suggestions? Maybe to check the ABS relay, as I read above, that it is responsible for powering the ABS light?

[admiral51]

What i think you are seeing is the bulbs that light up the display when the side/headlights are switched on.
Over time bulbs will have been replaced and may not have been fully secured and even the unit itself may have developed small areas where the interior plastic divides does not make a snug fit so light can pass through, a bit like not having the door shut properly.
The symbols are on a plastic type sheet, some red/yellow/green so any small amount of light behind will give you what you are seeing now.
Best way to test is to check in the dark with no exterior lights on, see if it shows up, then put the exterior lights on and see if you can see it then.
If it was an ABS fault you would see it all the time

[ksanturion10]

Hi and thanks for the reply,
I've checked your assumption, but regardless if the side/headlights are switched on or off, during night time, the abs light is still there. The intensity is not like during switching on the ignition, but rather more weak. I think it has something to do with the blocking diode into the abs relay according to these diagrams Wiring Diagram(s) for 1.9 TD Xantia Reqd. Will keep you posted if I find something.

P.S.: checked the ABS relay, it is OK. From the diagram the next suspect is unfortunately the ABS light connection to the ABS ECU. I will isolate the pin in question and will see if the ABS light goes off completely. Yes, I've read about the Teves system... nice one

[PJS956]

Hello All.

I appreciate this is an older thread but I found it very interesting while trying to solve an ABS and ECU warning on my 2009 Renualt Laguna 3 1.5dCi. Can anybody direct me to something Languna suitable and do the cheap OBD2 readers help in any way clear these warning if the causing fault is rectified?

[Doo]

I had weeks of hilarity (sarcastic face on ) with wife's car (07 C4) when an ABS situation arose.

Back in May, we returned from 2wks away and the car lit up like Christmas lights. Turned out to be the under bonnet fuse box (the BSM) had got contaminated with damp leaves (wife habitually parked under tree's at work....that's now been knocked on the head ) so I removed it, cleaned it and sanded each of the fuse holders.

Job done away she went.... Until July!

Wife came back from 18 days a way and the same old issue raised it's head. Figured it was the BSM again. Local Citroen dealer suggested they are guilty of going duff (not the description used ) so we replaced it with a known good BSM, but same issue.

Nothing would read the ECU and I wasn't prepared to pay hundreds for a "possible" outcome so I joined this forum and managed to order a Lexia.

Installed it to a laptop and plugged in. BOOM! It found the fault was simply the front left sensor.

I replaced it and the ABS fault disappeared as soon as I fired the engine, whereas before it would start and immediately find the fault and beep, bong and light up every warning under the sun.

Still going well. Also, it flagged up an issue with the glow plugs, but it has never failed to start and has never flagged up the emissions fault again since.

I have a heap of new parts for it, but don't plan to fit them until next year as I plan to have the engine out and do a full overhaul (glow plugs, timing chain, timing belt kit, clutch and gaskets, etc).

But I HAVE to endorse the Lexia as it saved me a heap of time, money and effort. Cracking bit of kit.

[GiveMeABreak]

That's good to hear - but just for the benefit of member: PJS956 - a Lexia / Diagbox won't help him as he has a Renault and it's not compatible. He'll need something like Renault Clip for a decent diagnostic tool, but depends on what he wants to do. A generic code reader will have varying capabilities depending on each one and of course quality, compatibility and price.