ABS Problem MG ZT-T
 

Hi -- I have an MG ZT-T CDti Diesel. Developed an ABS brake pedal judder problem recently and struggling to clear it.
ABS light came on 2 weeks ago and speedo failed. Unplugged ABS fuse and cleaned it and reinstalled it - worked OK, then did it again. Same procedure cleared it.
Since then the ABS light has stayed off but experienced brake pedal judder on braking, as the ABS is coming in erroneously. I confirmed it was an ABS problem by removing fuse again and braking went back to normal, no judder.

Replaced sensor on front right, thinking this would be the cause of an intermittent fault causing the speedo to go off in the first place.
Still had judder at various braking speeds, so took it to garage to be tested. £40 lighter, there was no definite conclusion. Static test showed fault on front right and one on rear right. Cleared faults and took it for a test. Retested it and faults didn't show. Also tried a wheel by wheel test rotating the wheels and recording the variation in voltage - no abnormalities found, they all showed voltages that rose and fell pretty much the same.

Next step - took off rear hubs (that have integrated ABS reluctor rings), cleaned up the rusty back plates (told these could cause the problem), cleaned stub axles and the hubs. Reassembled and it was much improved, ABS not kicking in on general driving, however, it still kicks in at very low speed, almost a few miles per hour.

I noticed on the rear Hubs, one hub had what looked like a brass ring on the outside with oval holes cut out at intervals - I assumed this was the reluctor ring, but there wasn't one on the rear nearside hub.

Does this make a difference, i.e. is this brass ring the actual magnetic reluctor ring or is it actually embedded into the hub? As it's been working fine since hubs were last changed in October 2014, I'm at a loss to know what to do next and what could be causing this to judder at low speeds, and yet no ABS light is coming on and no faults recorded.

I guess the ABS is working OK but the signals going back to the ABS control unit are such that it thinks the wheel is locking, but only at low speeds.

Any help welcome.

Hi Heddy, is the mag ring visible on the outside of the back of the hub when you take it off, or is it embedded inside the hub and you can't see it?

I'm still confused as to why the rear right hub has a brass ring visible on the outside with regular oval cut outs that coincide with position of the sensor on the back plate, yet the hub on the rear near side doesn't have this ring at all. The system has been working for 2 years with both these hubs on until now, so completely confused as to what's what.

LR

Depending on the manufacturer , some hubs have the cover ring you describe and some don't . Not a problem ; I'm running one of each kind :}

Thanks Chris, that's one off the list -- any idea what could still be causing the ABS to kick in at low speed. I also took the battery and casing out to check the ABS computer plug was dry. I still couldn't get to it but it was as dry as a bone around that area, and the plenum's are dry. Also I guess if there was an electrical fault, this should trigger the ABS warning light. Since mine is not lighting up, I'm still looking toward a reluctor ring problem. I'm hoping it's not the front bearings as they are not fun to take off and replace -- the rear hubs are a doddle in comparison.

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.

When I had a similar situation I bought a cheapish rear hub to use as a test piece . If I had to bet I would say that one of your rear hub reluctors is duff :shrug: Try it on both rears in turn and see if it fixes the problem , and if it does then leave it on :} aapoldham on ebay do a decent one for a good price .http://stores.ebay.co.uk/aapoldham?_...p2047675.l2563

This sounds very familiar to me as it follows my own experiences , and is good advice , methinks:}

Follow Harry's advice below and make sure you rotate the wheel slowly to check each magnetic reluctor segment.

Have done this a few times now and it is a very good way to test all the ABS sensors and magnetic reluctor rings. :D

Chris, Thanks -- sounds like good advice, I'm going to go ahead and order one today and try swapping each of the rears out see what happens.

Regards Larry