ABS warning lamp

[fixitflav]

I have an old, 2004 petrol Mondeo and for several weeks the ABS warning lamp comes on intermittently. Sometimes it doesn't happen for several days and I think it's gone away, but then there it is again! As it's intermittent I don't think it's a problem with any of the wheel sensors.
Thinking it might be the engine management computer I tried the trick of disconnecting the battery, headlamps on for 5 minutes, then reconnect. That cures the engine management lamp coming on due to the EGR valve, but it made no difference to the ABS lamp.

Any comments or suggestions gratefully received

 

[Rad1o]

Get a generic code reader. Guessing on modern - even 2004 - cars does not help.

 

[johnny2007]

On vw and audi there's an abs management unit that gives exactly that fault.
Go on a ford forum and see what they say.

 

[Mottie]

Any rough/grumbling noises? I think the abs ring is in the bearing on these cars. I’ve had that problem once but only with a faulty new bearing that I fitted.

 

[Harry Bloomfield]

Start, by understanding how the ABS works. Generally - An Hall effect sensor (switch) is located in a fixed location, around the hub has a series of magnets around its periphery, around 40 to 50. Each wheel as such a setup and the pulses from each wheel are fed via individual wires back to the ABS controller unit. The ABS controller compares the pulses coming from each wheel and if they are not similar frequencies, it makes an assumption that a wheel has locked and releases the braking effort from that wheel for a fraction of a second.

The ABS has two basic stages of diagnostics, static (when you turn the ignition on) and sub-10mph when additional checks are carried out. In the static check, the ABS controller ensures it has the Hall sensor connected at each wheel. Your ABS light should come on at switch on, then go off if it passes that first test.

The second stage checks it is seeing the correct number of regular pulses from each wheel, if not then it puts the ABS light back on and disables the ABS system.

Whilst on the road, the pulses continue to be monitored, and any failure puts the ABS light on and disables the system.

If the pulses from any sensor are delayed whilst braking, the ABS system assumes that wheel is locking up, so it releases hydraulic pressure on the brake.

Possible causes of the ABS light coming intermittently, are poor connections or damp where the sensor plugs into the loom. Sensor wiring loose and catching on the tyre. Metal or rust being caught up by the magnets in the hub or bearing. A worn hub bearing. One of the better diagnostic units or Fords specific unit, able to interrogate the ABS module, will be able to tell you the wheel with the issue. A normal cheap, general purpose ECU unit will not.

I made myself a cheap little gadget, to enable me to check my own system at the wheel sensor. It uses a very sensitive crystal earpiece, connected to a 'break-out' plug and socket. I simply unplug at the sensor and plug into the plug and socket. Wheel jacked up and rotated very slowly, as each magnet passes the sensor, I hear a click in the earphone. A brick placed alongside the tyre as a reference point, I make a chalk mark on the tyre at the edge of the brick. If all is well, I should end up with a series of chalk marks, the correct number and all perfectly evenly spaced around the tyre.

 

[fixitflav]

Get a generic code reader. Guessing on modern - even 2004 - cars does not help.

Will do, I've been meaning to get one for a while.

 

[fixitflav]

Any rough/grumbling noises? I think the abs ring is in the bearing on these cars. I’ve had that problem once but only with a faulty new bearing that I fitted.

Same here. I replaced a rear wheel bearing and afterwards the ABS lamp came on, intermittently if I remember right. I jacked up again and found the ABS sensor was wiggly, even though the screw was tight. Clearly the hole wasn't tapped properly. I put a flat washer on the screw and that cured it. Then in about 3 months failed MOT on that bearing, so a Friday afternoon in China! Bearing replaced free..

 

[Harry Bloomfield]

Will do, I've been meaning to get one for a while.

A generic one will only read the generic codes from the engine ECU, it will not help with fault codes from the ABS, SRS, climate system or anything else.

 

[fixitflav]

Start, by understanding how the ABS works. Generally - An Hall effect sensor (switch) is located in a fixed location, around the hub has a series of magnets around its periphery, around 40 to 50. Each wheel as such a setup and the pulses from each wheel are fed via individual wires back to the ABS controller unit. The ABS controller compares the pulses coming from each wheel and if they are not similar frequencies, it makes an assumption that a wheel has locked and releases the braking effort from that wheel for a fraction of a second.

The ABS has two basic stages of diagnostics, static (when you turn the ignition on) and sub-10mph when additional checks are carried out. In the static check, the ABS controller ensures it has the Hall sensor connected at each wheel. Your ABS light should come on at switch on, then go off if it passes that first test.

The second stage checks it is seeing the correct number of regular pulses from each wheel, if not then it puts the ABS light back on and disables the ABS system.

Whilst on the road, the pulses continue to be monitored, and any failure puts the ABS light on and disables the system.

If the pulses from any sensor are delayed whilst braking, the ABS system assumes that wheel is locking up, so it releases hydraulic pressure on the brake.

Possible causes of the ABS light coming intermittently, are poor connections or damp where the sensor plugs into the loom. Sensor wiring loose and catching on the tyre. Metal or rust being caught up by the magnets in the hub or bearing. A worn hub bearing. One of the better diagnostic units or Fords specific unit, able to interrogate the ABS module, will be able to tell you the wheel with the issue. A normal cheap, general purpose ECU unit will not.

I made myself a cheap little gadget, to enable me to check my own system at the wheel sensor. It uses a very sensitive crystal earpiece, connected to a 'break-out' plug and socket. I simply unplug at the sensor and plug into the plug and socket. Wheel jacked up and rotated very slowly, as each magnet passes the sensor, I hear a click in the earphone. A brick placed alongside the tyre as a reference point, I make a chalk mark on the tyre at the edge of the brick. If all is well, I should end up with a series of chalk marks, the correct number and all perfectly evenly spaced around the tyre.

Thanks for that. I'll see if I can find the wiring plug! Failing that it sounds like it might be one of the sensors, I'll have to go round them all.

Thanks for others' comments too.

 

[Harry Bloomfield]

Thanks for that. I'll see if I can find the wiring plug! Failing that it sounds like it might be one of the sensors, I'll have to go round them all.

Usually the plug are in the dry and colour coded. My front are mounted in the engine compartment, top of the inner wing, rear under back seat. Blue plugs for ABS, black for pad wear. Simply unplugging and reconnecting with a squirt of WD40 cured an annoying ABS issue on a Scorpio I used to own.

 

[johnny2007]

Usually the plug are in the dry and colour coded. My front are mounted in the engine compartment, top of the inner wing, rear under back seat. Blue plugs for ABS, black for pad wear. Simply unplugging and reconnecting with a squirt of WD40 cured an annoying ABS issue on a Scorpio I used to own.

Wow, a Scorpio!
One of the best cars ever built.
I wish I'd bought one when i could.

 

[Harry Bloomfield]

Wow, a Scorpio!
One of the best cars ever built.
I wish I'd bought one when i could.

My last one was the ugly frogeyed version, and metallic green and before it I had several Granada's. The Scorp was all Granada, but with a new front and rear body panels. Loved 'em. The Scorp Lotus engined version had cam chain wear issues. All the models rusted at the rear sill/mudguard, which was when I got rid.

 

[fixitflav]

A bit of a follow-up. I've ordered a code reader, but in the meantime I jacked it up to check the brakes for MOT, and while I was at it disconnected the ABS leads and gave a squirt of contact cleaner. Made no difference. Then I found the connectors under the back seat as Harry Bloomfield suggested, same treatment, same lack of cure!

There's something in the Haynes manual about removing the ABS hydraulic unit. I've no intention of doing that before getting the code reader, but Haynes says disconnect the wiring from the unit, so I thought I'd give that the contact cleaner treatment. Says remove the air cleaner to get at the unit. Did that, only a few minutes job, but there's nothing visible under it that looks anything like the ABS hydraulic unit. Haynes has a photo, but it's difficult to relate this to anything else, and I can only guess it's taken from under the car, but in that case why remove the air cleaner for access? I'll run the car up the ramps when it stops snowing and have a look under, but can anybody throw light on it?

 

[Harry Bloomfield]

I'll run the car up the ramps when it stops snowing and have a look under, but can anybody throw light on it?

I have had no need to investigate that part, so as much in the dark as you. You can find the ABS unit by following all the pipes, to where they all converge in the engine compartment. That unit may contain all of the electronics, or just part of them, with another ECU elsewhere.

I hope the unit you have ordered covers the ABS.

 

[fixitflav]

I have had no need to investigate that part, so as much in the dark as you. You can find the ABS unit by following all the pipes, to where they all converge in the engine compartment. That unit may contain all of the electronics, or just part of them, with another ECU elsewhere.

I hope the unit you have ordered covers the ABS.

OK, thanks for your help