I lost my brakes and nearly crashed...!

[GingerMagic]

Just wondering if anyone on the forum has an in depth knowledge of how ABS works?
I replaced my comm unit with a ' similar ' one as my auto lights are random and indicator was floppy - but this led to my ESP warning light coming on. I ignored this as figured it was the new comm unit not being right for the car but at least auto lights and indicator were fixed. My joy was cut short when upon braking at low speed, I nearly slammed into the back of a vectra!!!!
I have since put the floppy but correct comm unit back on the car to try and fix the fault but no luck.
It seems that brakes are hunky dory at all speeds, exept when going less than 10mph, when there is a grinding/buzzing noise and no brakes at all, no matter how hard I push the pedal. The brakes do cut in when at 2mph so my car will stop but by then I have either hit the car in front or soiled myself..
Now, I also have the ABS warning light coming on too. My heart sank at this point but as it turns out, the brakes are working normally dispite the ABS defective message..?????? Took it to a local garage to clear fault codes then had no brakes again until ABS defective popped back up, now alls good.
So, no messages are bad ( no brakes ) but 2 brake defective messages - ESP and ABS - is good ( brakes work )
I am perplexed by this, fault reader said "fault at rear corner but seems correct"
I do have pp2000 but waiting for laptop charger to set it up.
If anyone can help I would be much appreciated - I am no mechanic but try my best to do what I can. This is a family car so really need brakes.
Thanks, Kelv.

[V6Exec]

ABS is a simple concept = check that all wheels are turning at the same speed. If one locks release it until it rolls then increase the braking force until it stops again.

It fails to safety in that if there is a problem normal braking will work (as you have found out)

Best option for now is to pull the ABS fuse and see if that gives you normal braking.

It will need some code reading to work out what is wrong and how to fix it. Could be a fried ABS ECU or could be wheel sensors.

[madmadmax]

abs should not cut the brakes, i would check the brakes and pipes for bulges and leaks but pull the abs fuse and see what happens.

you can scan it with pp200 and look at the abs unit.

the abs light being on all the time meas it is disabled and will not work.

how abs works is there is a computer looking at wheel rotation speed and if it sees one or more of the wheels slowing down more than the rest it will put more or less presser to the wheels depending on what its doing, the abs pump may be doing brake bias

if you had air in the abs pump and the pump was activated then that air would go in to the system so bled the brakes using pp200 to activate the abs pump and have a good look at it all

edit: where are you as i would be willing to help

[V6Exec]

abs should not cut the brakes

The thing to remember about ABS and the symptoms reported is that it is actually doing its job.

Assume no fault and a difference in wheel speed gives and indication that there is a loss of grip. At this point the ABS will back off the pressure, over-riding the driver's command. If there are multiple sensor failures showing stopped wheels when there is rotation, there will be no brakes. The ABS will back off the pressure all the way and still see a "lock up" on one or more wheels.

[steve_earwig]

The first car I ever had with ABS was a Granada Scrapio, the first time I took it on snow I decided to see what happened when I stood on the brakes - with the ABS releasing the pressure repeatedly the pedal dropped worryingly towards the bulkhead with a grating noise...

The trouble is I didn't think the ABS on 406s worked the same way Definitely could be a knackered wheel sensor/damaged reluctor if it does.

[V6Exec]

This sounds quite similar to the fault above

http://www.thepeugeotforums.com/peugeot ... oblem.html

[madmadmax]

Now, I also have the ABS warning light coming on too. My heart sank at this point but as it turns out, the brakes are working normally dispite the ABS defective message..?????? Took it to a local garage to clear fault codes then had no brakes again until ABS defective popped back up, now alls good.
So, no messages are bad ( no brakes ) but 2 brake defective messages - ESP and ABS - is good ( brakes work )
I am perplexed by this, fault reader said "fault at rear corner but seems correct"

check the wheel bearing, reluctor ring and check the sensor with a multimeter at the side it reg the fault on

the abs systems fail safe so puts all the control back to the drive this is so you can take it to a mechanic and get it repaired asap.

where are you as i can have a look and check it over

[grasmere59]

It might have no bearing on the probem but i was told when fitting a comms unit with ESP it has to be calibrated to the car as the comms unit has to know when the wheels are in the straight ahead position when it is fitted.Probably nothing to do with the fault but perhaps something to remember in the future.

[GingerMagic]

Thank you everybody for all your input. I kinda knew the basics of what ABS did as far as controlling the wheels during a skid but didn't know how it ' thinks '. I did think that if the ABS was faulty then it couldn't give me any grief so pulling the fuse was what I would have done. I am glad ABS is a fail safe system, much better and peace of mind. I was just concerned when ABS was somehow telling brakes not to work at low speed, leaving me helpless as couldn't rely on ' trusty ' 406 handbrake to slow me down...!!!

I drove from Bournemouth to High Wycome for a family party with both warning lights blazing and brakes worked fine, confidence was high. When we left High Wycombe at midnight to drive home in the rain, no warning lights were on and I was shi**ing myself with the thought of driving a car that was trying to kill me, turned out it has sorted itself out.
When I first replaced the comm unit, it took about a week of grinding and buzzing under braking until it finally decided to not brake anymore. Maybe it has taken a week of grinding and buzzing to finally realise the old comm unit is back on the car. Do ECUs take time to adjust ??

I will still run the car with PP2000, figure out how to use it to bleed brakes while I'm at it just to make sure, but touch wood all is good.
Thanks again for all your help, I feel very supported by people willing to come and help out and maybe someone will have a similar issue in the future and start looking for answers.

[V6Exec]

Do ECUs take time to adjust ??

Long answers short - yes

Short answer long - when a fault is sensed this is stored, but degraded after every cycle until it is effectively ignored by the system and stored for historical significance. In a sense they are self healing and tolerant to minor issues.