Diff damage - MOT 2 wheel brake test

[nathanrobo]

Guys

I took my car to a local test station yesterday and as you'd expect with all the unnecessary jobs I've done on her, she passed with just an advisory on a windscreen chip (it's a big bit of glass and you can't notice everything .

Anyway, looking through the inspection window I saw the car having it's rear wheels tested on a two wheel system. When the tester came in with the certificate, I asked if it was okay to test a permanent 4 x 4 this way and to my surprise he told me that the car wasn't a permanent 4 x 4. Well as you can imagine this was news to me as it was when I bought it! After a bit of arguing I persuaded him that I was right and then his colleague chirped in that it had a 'hi-low' selector that they moved into neutral before testing, which I severely doubt if they didn't think it was full time 4 x 4 and can't see how that would prevent damage anyway.

I spoke to VOSA who told me that they should have used an decelerometer. Any views as to whether and where any harm could have been caused?

 

[sae70]

Wouldn't the centre diff save the day in this instance But maybe not after the brakes were applied I'd not be happy that's for sure

 

[Jon Wildsmith]

It won't have caused any harm Nathan provided the centre diff wasn't locked which they'd have noticed if it was It's more a problem on cars with viscous or clutch type couplings in place of or alongside a conventional centre diff. Putting the transfer box in N makes no difference BTW. Even so ours are always tested with a decelerometer but the place I go get a lot of 4x4's so would be more familiar with this than most.

 

[Chas]

Could someone explain why using a two wheel system would be a problem? I've heard before that it is but I don’t know why. I am thinking it puts unnecessary strain somewhere, is that right? My MOT man used a deccelarometer for my 80.

 

[Gav Peter]

My MOT guy uses a deccelarometer for Muxley's MOTs...

 

[Jon Wildsmith]

Could someone explain why using a two wheel system would be a problem? I’ve heard before that it is but I don’t know why. I am thinking it puts unnecessary strain somewhere, is that right?

Some permanent 4wd vehicles have a system that only allows a limited amount of slip between the front and the rear. If you only turn the wheels at one end as on a standard 2 roller set up it can cause damage in the coupling between front and rear and / or cause the car to jump off the rollers so might be seen as dangerous That's my understanding of why permanent 4x4 are usually tested with a decelerometer.

 

[Chas]

I think I see, does that mean that if the rear wheels are driven by the rollers then that drive would be transferred to the front and so pull the truck off the rollers?

 

[Toby]

Hi Nathan,

As Jon says this won't have caused any harm unless your car has a viscous coupling in the centre diff. AFAIK the 100 series didn't come with a viscous coupling whereas the post aug.'92 80 series did.

Even if you put a viscous equipped 80 on an MoT roller brake tester I doubt you'd damage it with the speeds and duration involved (unless it was repeatedly re-tested so the heat built up), however it would be impossible to get an accurate brake efficiency reading, so would be completely pointless.

All the best,
Toby
1990 HDJ80
1994 HDJ80

 

[Jon Wildsmith]

I think I see, does that mean that if the rear wheels are driven by the rollers then that drive would be transferred to the front and so pull the truck off the rollers?

Basicly yes but as the brakes are being applied to all wheels when testing the foot brake then the front wheels don't really want to move either so the viscous coupling or whatever arrangement the car has will have some fairly big forces applied. As Toby said the VC in an 80 is unlikely to be damaged by it but the VC will be upsetting the readings making them pointless anyway. Nathan's 100 doesn't have a VC, just an open diff so testing on the rollers should be none damaging and give valid results but they are still supposed to be tested with a decelerometer just because they fall into the permanent 4wd category.

 

[Chas]

I think I see, does that mean that if the rear wheels are driven by the rollers then that drive would be transferred to the front and so pull the truck off the rollers?

Basicly yes but as the brakes are being applied to all wheels when testing the foot brake then the front wheels don't really want to move either so the viscous coupling or whatever arrangement the car has will have some fairly big forces applied. As Toby said the VC in an 80 is unlikely to be damaged by it but the VC will be upsetting the readings making them pointless anyway.

I was thinking if it was just the handbrake being tested it might pull the truck forward.

 

[nathanrobo]

Guys thanks for the input! btw way my smoke test reading jump from 0.69 to 1.49 this year with only 9k miles difference. She smokes a little on start up. Any ideas? The smoke is blackish but not thick!

I had my injection pump stripped, cleaned and re-calibrated last year could it be anything to do with the adjustment?

Had my injectors cleaned at the same time.

 

[frank rabbets]

Just to confirm; I've had 13 test done on my 80; the first 10 with the VC and the last 3 without. No damage occurred . Compared with slipping wheels when driving and with the power of the engine, the MOT is fairly short and mild.

Frank

 

[nathanrobo]

Cheers Guys! Think I understand