MOT fail on binding rear brake... but seems to me to be resistance in gbox/dif.... is this normal?

[Jezbraker]

Morning all,.

So my 4200 failed MOT on a rear calliper binding on one side. Whipped it off and its all very free and pistons retract at a gentle push... that said when I jack a rear corner up, and have her in neutral, both sides give quite a bit of resistance to being turned... feels/sounds like i'm turning stuff in the gbox...

so... is this normal and just an over zealous brake tester? or have i got something scary going on in the f1 stuff....

thoughts appreciated! I'm taking it back for retest at noon

 

[77GR]

That'll be the limited slip differential putting up resistance. You need to jack both sides up at the rear to allow the opposite side to spin a bit and reduce the resistance.

 

[Oneball]

You need to ask how they tested it. They shouldn’t be doing a single wheel test at the rear on the rollers with an LSD. It should be flagged by the computer when they put the car in but you never know.

 

[Jezbraker]

perfic - that makes proper sense. to be fair they dont come across stuff like this often had to call me to work out how to get it in neutral

 

[safrane]

If they did put it on the rollers you may now have a ruined LSD...you may want to consider how you ask them, as they may deny it and you could be left a few grand out off pocket.

It's why I always chaperone my cars on MOT day.

 

[TimR]

Im also not shy to tell the garage how to suck eggs when I deliver mine..It also gives me a chance to work out who the tech is doing the testing...
Not all testers are made equal...!

 

[Oneball]

Don’t worry. Your diff is fine. You’ve already proved that by turning the wheels.

 

[Jezbraker]

yup - reading the numerous PH posts its defo one of those that draws opposing views! some same never roller lsds and some same makes bugger all real impact...
I'm going to go with the get them to jack both sides and feel how clearly its not brake binding and pass me please approach. just jacked both up here and although not spinning like a top its clear the brake isn't touching it.

 

[midlifecrisis]

My MOT tester told me that they were going to use the Decelerometer box to test the GT...it might take a while as it was in Brighton, I live in Haywards Heath!

 

[TimR]

Its 'interesting' that the tester has claimed the caliper is binding whilst it isnt clear that the problem is the brakes...
I guess a fail is a fail, what do they care whether they diagnose it properly..? Not as helpful as they might be. Still, often not much financial incentive to do more than a minimum....

 

[philw696]

As an ex tester I don't know why they don't use the Tapley meter for this minimum standard test.
Pass and give an advisory is the way to go and cover yourself.
The customer can then investigate further if they feel the need.

 

[Zep]

yup - reading the numerous PH posts its defo one of those that draws opposing views! some same never roller lsds and some same makes bugger all real impact...
I'm going to go with the get them to jack both sides and feel how clearly its not brake binding and pass me please approach. just jacked both up here and although not spinning like a top its clear the brake isn't touching it.

The reason opinions vary is that there are different types of LSD. If you have a viscous LSD slip limited by hydralic action, or a torque sensing diff, then the ones that say it makes no difference are right. If you have a plate (mechanical) LSD - slip limited by clutch packs on the sides of the diff, then the ones that say it does damage are right.

Just so you know, you have a plate LSD and you need an MOT station the reads the manual / gives a **** about your car.

Further reading:

Thread 'MOT brake test on rolling road ?'
https://www.sportsmaserati.com/index.php?threads/mot-brake-test-on-rolling-road.25765/