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Hi guys, I took my P38 for its annual test, this morning, and it failed on an ABS issue.

I had new front brake pads fitted, and a new caliper on the front passenger side, last week. The brakes, under normal road conditions, appear to be working perfectly. However, the guy in the test centre failed the test because he said that the drivers side front brake is "locking" when under braking. I don't quite understand what he is on about, but he said that it is an ABS fault.

Would anyone here on the forum have any idea what could be happening ? The brake caliper on the drivers side is the original item, but my garage guy says that it is working fine and doesn't need replacing.

I am wondering, have a memory of seeing ABS issues, whether the ABS unit could be faulty, and would it then produce the test failure fault, i.e. "locking" up the right hand wheel ?

Pierre3.

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If he did the test on two-wheel rollers, it will do strange things.

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As Pete says, if tested on two wheel rollers, the Traction Control will kick in and brake the wheel that is turning fastest. So the rear wheels won't be turning at all and it will brake one or other than is turning faster than that. So it isn't a fault but by design. A permanent 4 wheel drive vehicle must NEVER be brake tested on a 2 wheel roller. I know you are in ROI but they should know that as just the same as in every other country that has a test.

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I know that the test centre do the tests on a two wheel roller, I have watched the test being done.

If, and I ask in deference, if the guy did the test right [but it looks like he didn't] could the ABS unit be faulty ? Could it cause this type of issue ?

Pierre3.

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The ABS unit isn't faulty, it is doing exactly as it was designed to do, slowing down the fastest turning wheel. If they tested the parking brake on a roller too, the usual method is to check one wheel at a time. If that is done it will show no brake effort as the other wheel will be rotating in the opposite direction as the parking brake is on the propshaft. So I would expect that it to be a failure too. The other problem is that they can destroy the viscous coupling in the transfer case by having one axle turning with the other stationery.

The UK MoT testers manual clearly states:
Using a roller brake tester
Ensure that the vehicle, or system, under test is suitable for testing using a roller brake tester. If the vehicle or system is unsuitable, it should be tested with a decelerometer.

I've taken a couple of Audi saloons and a Bentley Continental with permanent 4 wheel drive in for MoT and IVA tests and all have been tested with a decelerometer and not on the rollers. I suggest you go back and ask them to check their instructions on brake testing something with permanent 4 wheel drive.

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And do a couple of tight circles on tarmac to test your Viscous. If it has failed, you'll be buying a new front diff in a few months time.

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Hi Morat, just out of interest what should happen if I do tight circles on the road ? How will I know if there is a problem with the front diff ?

I was reading another article regarding the failure of the ABS unit, and it mentioned that one of the symptoms of a faulty unit is a wheel locking up. Perhaps my unit is dodgy and I am reading too much into this issue. Perhaps I just need to replace the unit ?

I had the ABS/TC issue earlier this year but I fitted a new sensor, which cured that problem, and this current issue is different, I have no fault lights.

Just out of interest, will my Nanocom tell me anything ?

Pierre3.

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If you do full lock turns and you find the front wheels are 'skipping', the viscous unit in the transfer case has seized. That will subsequently kill the front diff. Two ways to cause the viscous to seize is to drive the car with one propshaft disconnected or run it on a 2 wheel brake test roller.

If you have no ABS light on, you won't have any faults stored and no fault with the ABS system. You can replace anything you like and it won't make the slightest difference. Your problem is a tester that doesn't know his job.

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Wot 'e sed!

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Hi guys - well, I have got to the bottom of the problem. As Richard correctly says - the tester didn't know his job !

I went back to the test centre in my car [not my P38] and looked for the supervisor and we had a chat. He looked through the test results and agreed that there is nothing wrong with the brakes, that all the brake test results were a "Pass".

Where the problem escalated was that the knobhead who did the test firstly wrote [typed - whatever] that the ABS was in fault. He then proceeded to explain to me that the front right wheel was "locking up", and I couldn't figure out what he was on about. I asked him whether it was an issue with the ABS light being on but he said it wasn't on !!! But he hadn't driven the car after doing the brake test, although he must have seen that there was no light on when he drove the car into the test bay, about 100 yards. His last comment was that the ABS wasn't working and it needed to be repaired.

The supervisor that I spoke to this morning went back and rechecked the test results stored on their computer and he said there wasn't a fault, and the tester should have driven the car, or at least asked me whether the ABS light goes out when driven.

So I have to spend my time going back and forward to the test centre to sort the issue out, and with no sort of compensation for their inability to do the test properly. I have to take the car back tomorrow so that the supervisor can check that the ABS light does, in fact, go out - which it does, every single time.

I suppose the only upside is that I don't have to buy a replacement ABS unit, fit it, and go through the pain-in-the-bum process of bleeding the brakes.

So thanks for all the advice, as usual.

Pierre3.

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I'm glad you're sorted out!

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enter image description here

Pierre3.

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Just a quick question - I believe that, in RAVE, there is mention of how, and how not, to test the ABS system. I have looked through the manual, in the section about brakes but I can't find anything that explains how to test the ABS braking, and how not to.

Maybe someone might have a page number, section number, where I can find this information ?

A further update to my annual test - I went back with the car, this morning, and I was expecting the guy to just road test the car to ensure that the ABS light goes out, but instead he took the car back in to do another brake test. I watched three guys try to figure out how to do this, until eventually the supervisor called me to tell me the car had passed the test. But he then said that they couldn't test the system fully because the drivers side front wheel would lock up on their rolling road thing, but not when the car was driven on the road [inside the test warehouse, so not very far]. So they spent about half an hour going backwards and forwards before eventually agreeing that there is nothing wrong ------- BUT I would be advised to have the problem looked at !!!!

The supervisor said that their system can test permanent 4 wheel drives as it tests one wheel at a time, but the car is still sitting on a two wheel "rack" [or whatever it is called]. I have checked that the four wheel drive is working OK by following Richards instructions about doing a couple of tight circles, and there is nothing untowards happening.
enter image description here

What problem ???? There is no problem as far as I can see. I even went to the extent of ringing a guy in Brislington, an ex-Landrover guy called Steve Hill who runs his own LR garage and is considered an all-round good guy, and he agreed with me, from my description, that there isn't anything wrong and it is the way that the tester is trying to test the car.

The big problem, in Ireland, you can't just go somewhere else because the test centres are few and far between. The next nearest centre is around 3/4 drive away and about 35 miles each way. And, and, you can't just ring these test centres up and ask what method do they use for testing different vehicles as the bookings are done through an automated, on-line system.

Pierre3.

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They are two wheel rollers and you CANNOT test a permanent 4 wheel drive vehicle on a 2 wheel roller. He's talking out of his arse if he says it can test them as it tests one wheel at a time. If he watches what is happening, one wheel will be turning in one direction while the other one on the same axle will be rotating in the opposite direction while the two wheels not on the rollers will be trying to push the car off them. There is no problem with your brakes at all, one wheel is locking due to the Traction Control. If they were sat in the car while doing it, they would have seen the TC lamp flashing on to tell them that. It may be further away but I would suggest going elsewhere next time the test is due.

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Richard, the more information that you suggest the more I think that they are getting wrong. I was watching this morning, and there were two fella's in the front of the car, and the guy in the passenger seat was leaning across looking at the dashboard I suspect. I bet that, as you suggest, the ABS light was probably flashing on and off.

I have to say, I couldn't figure out how you can use a two wheel test rig if the vehicle is permanent four wheel drive, because the description alone, "four wheel drive" indicates that all the wheels are going to drive if the car is in gear.

I wonder, Richard, whether you might know where I can find the information in the RAVE manual, about the method of testing the ABS four wheel drive system, specifically, what to do and what not to do. The guy I spoke to earlier today, Steve Hill, said that the information is in the RAVE manual, but I didn't ask him where.

Pierre3.

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Brakes - ABS, Description and Operation. However, it doesn't tell you how to test it but it does show the following:

enter image description here

A dynamometer is a bit different to a brake tester but I suspect if you were to turn off the Traction Control then it wouldn't do what they said it was doing. It would have been the TC light flashing on, not the ABS light. ABS light only comes on to show a fault.

If you want to test the ABS and Traction Control, you need to find a piece of road with a verge of mud. Stick two wheels on the road and the other two on the mud and floor the throttle. The ones on the mud will try to spin and the TC will kick in to slow them down. Then stomp on the brake pedal and the ones on the mud will try to lock and you will feel and hear the ABS modulator doing its thing as it pulses the brakes on and off on the wheels slippy side.

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Richard, very many thanks for your guidance.

I will actually be able to try the methods you suggest as my brother-in-law owns a farm up country. I will give him a patently false pretence of expressing interest in chickens, so that I can drive across his fields - and then head home again !

Thanks for including the piece from the manual about testing, I am going to print this off and call into the test centre when I am passing, just to see what they say.

Pierre3.