kmascotto

When I replaced the last belt, I was about 1000-1500km into the replacement. As I was driving home on the highway and just having passed another car. I came off the throttle and it triggered the belt warning. At that point I had two choices 1) I could pull over and shut the car off in the middle of nowhere and call a tow truck or 2) Keep driving at a nice steady pace and drive directly home. Knowing I just needed to have the belt re-tensioned I took the chance and drove home. The belt never displayed the warning on the rest of the drive home and when I checked the belt a few days later, it was a little out of adjustment as expected.

PorKen

Always 3 min delay after engine start. After that, any break of ground/earth, however brief, triggers a BELT TEN.o<O warning.

The delay originates in the Central Informer brain which only receives a separate switched high temperature (small spade term) ground from the coolant temp sensor, not the variable resistance (large spade) which goes to the dash gauge.

GregBBRD

Porsche says (in their service information) that a tensioner that has no/low oil will trigger the light. And I've fixed a few cars with good belt tension that have lit the light, only to find out that someone forgot to fill the tensioner with oil. Filling the tensioner did fix these particular cars.

It's always bothered me not knowing how the oil would affect the light...perhaps the oil dampens the movement or the net movement increases without the oil?

I always assume that Porsche knows more about their designs and pieces than I do....

mj1pate

Greg; it's actually left up to you and the "Old Guard" on the board to propose to the rest of us how this works. .... several thousand miles before I changed my belt/tensioner/roller-bearings/WP, I managed to over stress the tensioner rear gasket while filling it with oil and lost most of the oil. But it *never* resulted in a belt-ten warning; the tension electric circuit worked and at cold TDC the Kemf pointer sat in the middle. When you and the Emeritus have the cycles, reverse engineering these details would be most excellent.

RFJ

Assume nothing.

Alan

The Belleville washers in the stock tensioner are basically just springs.

They can be configured to have more spring tension or more net deflection under the same tension based on their directional configuration (stacked or opposed washers) so follow the prescribed config. The oil just likely serves to avoid binding-up that would cause the whole stack not to move smoothly as the loading from the belt varies e.g. belt flutter/temp variations of the belt and the temperature related growth of the block & head dimensions causing the belt to tighten. It think it only changes the dynamic behavior to be more smooth Vs load. Such a change in dynamic behavior (e.g. as belt tension drops - not immediately extending) would be enough to trigger the warning - since it senses instantaneously (after 3 mins after start anyway).

Alan

GregBBRD

Technical Service Bulletin #9010. (Book "G", Page 33)

jeff spahn

Greg, at this point I think you know more about the 928 than Porsche does. They haven't touched them for how long..... Thank God for you.

GregBBRD

Considering I can't remember what I had for dinner last night....that's a scary thought!

Rob Edwards

PorKen

Editors Note: the following is part of a continuing series of stuff-PorKen-has-had-to-figure-out-because-the-so-called-expert(s)-never-bothered-to-verify-or-truly-understand-the-stock-system. They spent many hours measuring, trying to find fault in the PKT, but are/were content to 'assume' the stock system works as advertised.

Today's thought: the Belleville Washer Pack is not a spring at all. ALL of the 'spring' force is provided by belt stretch = tension. {edit: not well thought out}

The BWP is for warmup only. The washers are just too firm¹, compared to the (stock) belt to be a spring, when the engine is cold. When hot, the washers flatten out and the BWP is transparent, the assembly becomes a spacer for the adjusting bolt and nut.

Before it all leaks out, the oil lubricates the BWP and helps transfer engine heat to the BWP. Post '82, it may provide some damping², cold and hot.


¹ Turning the engine backwards may overload/crack the stiff washers
² In order to provide damping, one set of washers were removed, reducing BWP force and distance

PorKen

The valve in the '83- tensioner...it opens if the Belleville Washer Pack expands when belt tension is reduced (length is increased) and closes when tension is increased (length is reduced), but only when the engine/BWP is cold and the bi-metal-thermostatic washers are cupped.

So what I said in the quote above is wrong. There is no damping when hot. When hot, the whole BWP assembly is always a fixed length. Regardless of the amount of oil remaining in the tensioner, the belt has to pre-stretched (tensioned) to a high enough level statically. At high rpm, when the belt is further stretched to its maximum between the crank and gears, creating extra belt and lower tension at the tensioner, oil can't help prevent the tension light from being tripped.

phendriks

As the OP, I'd like to thank everyone for chiming in. Although as a relative newbie, I'm damn near lost on what we are now discussing.

Anyway, the belt was 1/16-1/8" out of window on the loose side (at TDC of course , thank you Rob). Adjusted to tight side of window (per Sean). What a PITA that was.

Question - How often do you need to test the belt tension? Mine was fine two years ago, and that was after a suitable break in period(albeit done by a Volkswagen dealership in Northern Ontario)

If nothing else, the system seems to work. It's interesting that you can get this far out of spec before the alarm triggers. I've seen threads on which side of the window to adjust to. And I'm well outside it (I think). Back to Freds' original point, how critical is this warning. Drive home or flatbed? Do you feel lucky, punk?

Cheers, Phendriks

OTR18WHEELER

, you guys are going too far..., he has no porken, light came on at shutdown. Belt tension seems to be ok, but timing is not a concern here?..

Adamant1971

Glad you found the source of your warning. If your around this weekend it's Sharks In Ontario, Holiday Inn Burlington.

How many miles have you done in the last two years? I think that is pretty far out of spec for 2 years in service, unless you have driven a ton.