ABS Problem MG ZT-T

[larryr123]

Originally Posted by HarryM1BYT

Reading that, suggests you are misunderstanding how to carry out the test...

It is NOT just a matter of checking the voltages, but of marking the tyre where the voltage changes. A light coloured crayon, with a brick or similar as a fixed marker works well. That ensures that a full continuous set of changes of voltages/ pulses appear around the circumference of the wheel/tyre. You may need to repeat it several times to be sure the pulses appear without fail.

The judder happens when pulses signals fail to appear, when they are expected. Most likely they will be missed at lower speeds, hence the low speed judder you are experiencing.

T4 and other diagnostics will not see it as a fault, because it is doing more or less what it is supposed to be doing - generating mostly regular pulses.

Thanks Harry -- I relied on the guys at the garage to run the test. I don't recall any chalk marking. This was the first time I'd witnessed this kind of test, so wasn't aware of the full procedure, despite having read up the basics of how the ABS system works.

I have a low cost cable, and engine test software, but nothing that tests the ABS. I've run the test at the garage twice so far, so will avoid that since I doubt the guys understand what the effect of placing chalk marks on the tyres will do. They did rotate each wheel individually by hand, as well as trying a drive test with the test rig plugged in and monitoring the readings.
The only real way to measure this I suppose would be on an oscilloscope and look at the uniformity of signal sent back to the ABS control unit.

I'll have a go at a new hub swap, as I think the absence of a scope I'm not going to get anywhere -- and it's cheaper than another inconclusive ABS test.
I did notice on the rear nearside hub, that a lot of the green gel had leaked, so I cleaned it up before reinstalling it - perhaps this was a factor in the reluctor ring deteriorating. It still puzzles me why one has a visible brass perforated ring on the outside and the other doesn't.

Regards Larry -- I'd happily do without the ABS (it's a bit of a pain), as I don't drive like a lunatic and don't use the car to commute -- I won't be selling though as I've had this since new, even if I get another car.

[Mike Noc]

Best to get hold of a multimeter and carry out the checks before changing anything else.

It is easy to add faults to the ABS system when replacing parts without a correct initial diagnosis.

On my car the fault was a slightly too large air gap between one of the rear sensors and the magnetic reluctor - both aftermarket parts. Checked the mounting was rust free and clean and then filed 1.5mm off the back of the bracket to move the sensor a bit nearer and it worked perfectly.

[HarryM1BYT]

Originally Posted by Mike Noc

Best to get hold of a multimeter and carry out the checks before changing anything else.

It is easy to add faults to the ABS system when replacing parts without a correct initial diagnosis.

On my car the fault was a slightly too large air gap between one of the rear sensors and the magnetic reluctor - both aftermarket parts. Checked the mounting was rust free and clean and then filed 1.5mm off the back of the bracket to move the sensor a bit nearer and it worked perfectly.

I agree, any attempt to fix things by substitution has the risk of adding even more faults and can work out to be expensive. Do a proper diagnosis first and save yourself a lot of time and expense.

It is not easy to mark the tyre wall whilst watching a meter, so I devised a better method. The better method uses a piezo earphone connected to the output terminal of the sensor. That produces a click in the earphone at each segment. A piezo earphone has to be used, because they present a tiny load on the output - they are very high impedance, but they are not so common/easy to source as the low impedance earphones. They produce a sharp and very precise click at each segment.

I have not tried it, but I suspect the click might be able to be picked up if a bit of wire is connected to the sensor output pin and a medium wave radio tuned to a quiet point on the dial placed near the wire.

[larryr123]

Mike, Harry, -- I have a multimeter, will give it a go, bit it's nigh on impossible to connect to the tiny terminals pins inside the connector either on the sensor side or up to the ABS circuit. It's the output from the sensor we are aiming to look at here right?

Only option is to cut into the sheathing on the leads to get a contact and I'm a tad reluctant to do that.
I'm curious as to why changing a part would introduce an new ABS fault. At the moment I don't have any reported faults and a new part wouldn't do any damage to the system would it?

Mike -- how did the sensor move further away from the reluctor ring which is fixed in the hub? - The sensors are up tight against the hub backplate and held in with an 8mm bolt. Unless you had a bang on the backplate, buckling it away from the hub somehow? I'm going to check both these distances on both rear backplates to see if there is any variance, but both looked normal on the last inspection.

[Jakg]

Originally Posted by larryr123

Mike, Harry, -- I have a multimeter, will give it a go, bit it's nigh on impossible to connect to the tiny terminals pins inside the connector either on the sensor side or up to the ABS circuit. It's the output from the sensor we are aiming to look at here right?

Only option is to cut into the sheathing on the leads to get a contact and I'm a tad reluctant to do that.
I'm curious as to why changing a part would introduce an new ABS fault. At the moment I don't have any reported faults and a new part wouldn't do any damage to the system would it?

Mike -- how did the sensor move further away from the reluctor ring which is fixed in the hub? - The sensors are up tight against the hub backplate and held in with an 8mm bolt. Unless you had a bang on the backplate, buckling it away from the hub somehow? I'm going to check both these distances on both rear backplates to see if there is any variance, but both looked normal on the last inspection.

You can use a (drawing) pin to go into the wire and connect the meter to that (french mikes trick).

[chris75]

Originally Posted by larryr123

Mike, Harry, -- I have a multimeter, will give it a go, bit it's nigh on impossible to connect to the tiny terminals pins inside the connector either on the sensor side or up to the ABS circuit. It's the output from the sensor we are aiming to look at here right?

Only option is to cut into the sheathing on the leads to get a contact and I'm a tad reluctant to do that.
.

With the help of folk on here I was able to get hold of two spare blue plugs and make up a loom which I could plug into the sensor circuit . It had two extra wires to provide an output to my multimeter , so no wires were harmed during the test
For the sake of £28 , I still support getting a new rear hub for a substitution test ! The world would be boring if we all agreed on everything ......

[larryr123]

You guys are great - well ahead of the garage. I ordered the hub this morning, for the sake of £25 I'll change the back rear one anyhow, given that it had green lubricant leaking when I took it off.

Jakg, thanks for the drawing pin, it made me laugh, but it ain't such a bad idea. I'm going to make up a rig with spare plug -- I have the old one from when I took it off the from right wheel. Chris I'll make this up and give it a go on the wheel test.

That way I'll have it covered from both angles.

I'll post back when I've done -- cheers -- have a great weekend.

Games -- I couldn't possibly sell the old girl now after all this hassle, who knows it might be a future classic -- well you can dream can't ya!!

[Mike Noc]

Originally Posted by larryr123

Mike, Harry, -- I have a multimeter, will give it a go, bit it's nigh on impossible to connect to the tiny terminals pins inside the connector either on the sensor side or up to the ABS circuit. It's the output from the sensor we are aiming to look at here right?

I also made up a cable a few years ago from an old ABS plug and socket, some speaker wire and a bit of chocblock. The original idea was to use it as an extension cable so that any ABS sensor could be plugged into the OSF and the speedometer used as a diagnostic.

Here it is being used to check a hub before fitting to the car:

Originally Posted by larryr123

I'm curious as to why changing a part would introduce an new ABS fault. At the moment I don't have any reported faults and a new part wouldn't do any damage to the system would it?

Mike -- how did the sensor move further away from the reluctor ring which is fixed in the hub? - The sensors are up tight against the hub backplate and held in with an 8mm bolt. Unless you had a bang on the backplate, buckling it away from the hub somehow? I'm going to check both these distances on both rear backplates to see if there is any variance, but both looked normal on the last inspection.

Very easy to damage the magnetic reluctor rings if drilling out old ABS sensors, or get a fault as I did when I replaced first the bearing due to it wearing out, and then a couple of years later, the sensor when it packed up, with non original parts.

There was no damage to the hub or backplate and no rust build up around the mounting, so likely the combination of the two aftermarket items needed a slightly smaller air gap to function correctly together.

As French Mike says: Measuring is knowing.

[larryr123]

OK - quick update.
Ran the tests on the back two wheels, by adopting the drawing pin in the leads to the sensors. Steadily made simple marks at each progressive change in the signal voltage with my multimeter by placing bluetack on the wheel. Also monitored the changes in the voltages whilst spinning the wheel slowly and a little quicker and the changes +ve and -ve of the voltage were pretty uniform. Also tested that there was a voltage present on the other lead to the sensor from the ABS control unit.
So sadly it doesn't look like a back wheel problem. my new hub still sits in it's brand new box.

I then tested the front right wheel that I had already replaced the sensor in. This was a different story -- again used drawing pins in the leads to get a connection for my meter.
This was the wheel I suspected originally as the speedo went a couple of times.
On this wheel there were large areas (about 1/6 of the wheel circumference) where the voltage did not change at all, which leads me to believe there may be a reluctor damage in this wheel bearing. They are supposed to be fitted for the life of the car according to the Haynes Manual.

The problem still only resides at very low speeds, a few miles per hour, and not on general braking and I still have no warning lights.

I tried the rock test to see if there was play in the bearing (I think there is a little but I may be imagining it) and there is no rumbling grinding sound on the bearing when I rotate the wheel freely.

Before I think about the nasty job of replacing the front wheel bearing, can anyone offer a view as to whether I might be barking up the wrong tree here?

[HarryM1BYT]

Originally Posted by larryr123

the changes +ve and -ve of the voltage were pretty uniform.

Before I think about the nasty job of replacing the front wheel bearing, can anyone offer a view as to whether I might be barking up the wrong tree here?

I will assume you meant - absolutely uniform, rather than pretty uniform?

There should be no absent pulses at all..

It is obvious your problem is with the front off side hub or sensor, but before swapping the hub, I would try moving the sensor a bit deeper/ nearer the reluctor to be sure it is not just the distance causing the issue....

File the lug, so the sensor is a fraction nearer.