CVTs' and the MOT.

[mac]

Credit for starting this post goes to petefarrell and pettaw, their re-commisioning of Petes' vario has shown up a possible problem.

During the MOT test (now computer driven) there is a requirement to measure part load emissions which requires the engine to be held at a moderate rpm. (the actual figure is vehicle specific but can be as high as 3000 for 30secs.).

In the absence of a specific instruction to the contrary from the VOSA or a note in the testers manual, convention would dictate, that with an automatic, this test is carried out in 'park'. Result in a CVT - a fried clutch!!. This may have happened to Petes' car.

I have spoken to the VOSA (vehicle operator services agency) this morning about this potential problem. They confirm that there are no model specific requirements for CVT cars, and no instruction for emission testing to be carried out in neutral.
THey did state that they were not aware of a 'significally higher failure rate during testing of Volvo 300 models' (of course the final clutch failure may well appear some time after the test and not be associated with it). They did however fully understand the issue (I spoke to the technical standards dept.) and promised to 'take note'. If this will result in an 'advisory' I have no idea - but it has to be worthwhile for any member taking a CVT for MOT mentioning to the tester that engine speed must NOT be raised above clutch engagement speed if the selector is in 'park' !!

Mac.

[petefarrell360]

Well good work Mac! What can I say? Who'd have thought the work of one weekend could have shown up such an issue. After putting the pieces together and figuring out what could have happened Andy and myself felt pretty sure this is what happened. Hopefully in the future no one else will have their car damaged when it's handed over to have an MOT carried out.
Pete

[pettaw]

IIRC the part load test is normally only for cat cars to test the emissions are within a lambda range.

The only reason they seem to rev the b*ll*cks out of the engine for a period is to ensure its warmed up properly, I've not had any more than an idle test done for any of our cars.

However, good work Mac on the 'advisory' from VOSA, don't rev a CVT in P. One other thing though, does the vacuum servo release thingy kick in and raise the effective engagement point of the clutch when its in P? I know it does when its in N, and also when the button is pressed, but does it also do it in P?

[mac]

Yes pettaw, the clutch disengage servo does operate in 'P' as well as 'N', in fact the vacuum valve servo activates as soon as the ignition is switched on.
(it is earthed through the inhibitor switch). Depressing the interlock button on the gearlever 'takes over' the earth whilst 'D' or 'R' is selected, the earth being broken (and the servo de-activated) when the gear lever button is released.

However, with both clutch and clutch servo correctly adjusted the servo will only delay full clutch engagement until 2000rpm. Partial engagement and slip (if the transmission is locked) will begin much lower. In any event, as you say there should be no need for a part load test on a non cat. car - but as I have seen on many occasions a tester will often rev the bol..ks of a car to either warm it up or to 'clear it out'. especially pre '84' cars which tended to run richer at idle (I think it was 84 when MOT CO went down to 4%).
Treating a car this way would almost certainly exceed full engagement speed anyway - let alone the threshold of clutch slip!

Mac.

[pettaw]

Yeah, muppets. The number of times you pass garages, even main dealers, and you hear the sounds of engines straining holding revs. I'm convinced revving them is almost a hobby for some 'mechanics'

With the clutch still at about 1.3mm clearance it struggles to engage at all when that vacuum thingy is energized, but hopefully that'll all be sorted soonish....shims are on order. Thanks for the info.

[shimon340]

hi guys,

great work mac contacting VOSA

I'm also aware that CVTs must not go on the rolling road for the brake testing either

[foggyjames]

MOT time will be when I install the 3000rpm RPM limit pill in my new MSD box when it arrives

cheers

James

[shimon340]

MSD?

PS the rolling road advice is from the guy who I learnt mechanic with since I was 16. He's a CVT and Daf expert. same advice from the Daf club.

MOT stations were aware of this but since 340s arent so common and Dafs even less so, you need to ensure your car doesnt go on the rolling road.

[foggyjames]

Multiple spark discharge. It's a very high performance and clean burning ignition system - yanky stuff derived from drag racers. The box pulls 20A off the battery....that should tell you how hungry it is!
www.msdignition.com

I just bought an MSD6AL, which has an adjustable rev limiter via slot-in 'chips' or 'pills'.

cheers

James

[shimon340]

ooh sounds good foggy - is this ignition hardware or dya need to programme the ignition map? how come you're upgrading?

cheers

Shimon

[foggyjames]

It goes between the trigger (be that a mapped system, or a mechanical system) and the coil, rather like the 'power stage' that many cars have. It's rather like an ignition signal processor.

I'm getting the upgrade because it means I can get better combustion, and better tolerance of dubious fuelling (i.e. big carbs). I'm switching to B23E ignition at the same time, so the advance can be manually set up for 95 octane rather than 92, and so the timing matches the cam. The MSD add-on is just an upgrade to that system - the real advantage is the B23E system.

cheers

James

[wjp01908]

Hi Guys

Cheers for the heads up on testing - I usually put a card on the dash asking politely that the car is not started or revved in P and have not had any problems with this. ( There is an original windscreen sticker in the car giving this advice but it`s not that obvious )

Can anybody explain why the rolling road should not be used though - all it does is turn the wheels against the applied brakes. How would this be harmful?

Surely this is the same in effect as braking while on overun.

I`m not saying the advice is wrong - I`d just like to understand the reasoning behind it.

There is no mention of it in any of the handbooks I have.

Will

[shimon340]

hi there,

Ive wondered myself why the CVT cant go on the rolling road but I was told by quite a few people ( Daf club etc ) that they cant. Also, MOT stations used to know this but as the cars age, the info isnt around.

Im guessing the reason is the same as why CVTs can be towed with the front wheels off the ground ie running on the rears. I guess the CVT box doesnt like being turned from the lowest ratio.

Mac - hello! what dya know about this from Volvo?

cheers

[mac]

Afternoon all,

This issue of towing or roller brake testing CVTs - hummm - I myself would be very carefull before making any definite statement about this. I have heard the stories but suspect most are apocryphal.

The only recommendation I can recall from the factory regarding towing CVTs is to remember to select 'N' - in fact the handbook states the car can, if necessary, be towed any distance. (just checked an '81', '84' and '89'). From an engineering point of view I can't see any reason not to tow - the only downside I can see at present is the 'big wheel turning little wheel' senario - the tow car would probably like it less than the CVT (try pushing a manual car in top gear and you'll get the idea!).

I suspect that if belt adjustment were very slack you might get the onset of belt slip and if you tried towing at speed you might run into rotational speed problems with the primary unit (no vac. assisted change up - though the transmission would bo its best via the centrifugal weights in the primary pulleys).

As for roller brake testing, again, I personally can't see why not - the test is very low speed (equivalent to 5mph or less in most cases - certainly below ABS trigger speed) - you do much the same to the transmission mecanically every time you roll up to a stop with the throttle closed!

I think the 'problem' may have arisen with earlier Daf vehicles with separate (twin) reduction gear secondary units (no differential) - I can see an imbalance throwing the car off the rollers sideways. I can see a case for not roller brake testing there, but for the 300 - ??? I certainly cannot remember any problems (we used our own rolling road/brake tester at the import centre and don't recall any violent explosions!!).

If there is a sound technical reason for not towing/brake testing I, like wjp would quite like to know what it is.

My brain hurts! - Mac.

ps. Shimon - I see (on the VOC forum) you're after the 'CVT booklet' by George Dickinson. - the article I recently wrote has gone to Jack Cluer and may appear before Georges does! Aymat also has a copy and says it will appear in V3Ms technical (under construction) section very soon - in the meanwhile if you're desperate I can always send you a hard copy or a CD of scanned jpegs.

[antiekeradio]

the early CVT's without differential usually show a higher brake difference than normal on the rolling road. if there is some (permissible) difference, this might put it over the limit. MOT failure would result, unnecessarily.

The CVT's with differential only have problems with the 'plate type' brake testers. the inertia of the secondary pulleys throws the system totally off balance. A car that brakes fine in real road conditions shows up totally awkward on such a brake tester.

Our APK(mot) station tests the brakes like I do myself; on the road.
If it decellerates firmly and securely without pulling to either side, its OK what more do you need to know