Bill Ball

The stock tensioner warning trips when the belt tension is less than that of a coil spring on the end of the tensioner arm, so it's related to belt tension. The coil spring is insulated from the rest of the tensioner by a plastic bushing. The ground circuit brakes when the coil spring manages to separate the tensioner pin from the arm by overcoming the reduced belt tension. I've tested it on a few cars and found that it trips pretty accurately right at the lower end of the belt tension spec.

Your proposed warning would trip when the maximum extension of the tensioner piston is exceeded. That would be great to know, but the belt tension could be dramatically low before that happens. Still, a good idea. You'd have to come up with an insulated spring like the stock system to be more responsive to belt tension.

PorKen

Theory
Belt tension is more of a concern with the stock loose-belt-control-system.

Actual belt tension on the working/tight side is controlled by how strongly the crank gear is pulling against the friction of the cams/pumps.
More or less belt is deposited on the take-up/loose side when it is stretched between the gears while running based on the size=temp of the engine and engine speed.

Because the stock system does not react to actual belt length, in order to keep the belt wrapped on the crank gear, and not flap too much, from hot<->cold and at different rpm, the stock system requires the belt to be pre-stretched, hence the attention given to the static tension reading.
Compare to a V-belt driven accessory like a power steering pump. Without a belt tensioner, the belt has to be stretched to near its working elastic limit to stop it from flapping while running. (Yes, a v-belt is different than a toothed belt as it relies on friction to transfer torque.)

With a self-adjusting system (like the PKT), which reacts in real time to belt length, belt tension only needs to be high enough to keep the belt tightly wrapped around the crank gear.


Application
A warning system for a self-adjusting system needs to show if the tensioner/damper has failed, allowing uncontrolled belt movement, IE retracted, OR there is a situation where belt length has grown out of range for some reason, IE fully extended.

The proposed grounding system should break contact in either case.

Avar928

Resurrect for point of reference to another thread.

Livio928

Out of curiosity, how many T-belts have failed with the PKtensioner? After being on the market for the last few years, there should be some type of failure rate number out there.

As per my understanding, the biggest issue with this system as it currently stands is that there is no warning light given if the belt lets go or the tensioner stops working. Am I missing something here?

69gaugeman

Go look at the how many miles do drive a year thread..... better not be any failures.....

Livio928

If it is based on the last three years of my miles, my tensioner will last for ever :-o

Tom. M

Colin runs the Pktensioner....if it can survive with his driving...then it'll survive under any conditions...

I won't run the stock system on any 928 I have anymore.. When my GT engine was rebuilt years ago under the PO...the 911 qualified mechanics totally ignored the tensioner..when I got to it to fix bent valves after the crank sheared the teeth off ...I found broken belleville washers and it would never hold oil... I suspect it just got put back on (after the po crashed all the valves )...

After the tensioner bolts started backing out on the block (on the way to a track event).. I switched to the PKtensioner..and it has been on there since 2006ish... lots of miles..no belt dust to be found and gears still look great..
I bet the uniform tension provided by the Audi tensioner probably extends the life of all the rollers, bearings, waterpumps pulley etc etc..

It's just better...

polecat702

FWIW, before he passed, Greg Nettles went back in to his 928 and replaced the belt just for his own personal info after running Ken's tensioner for 50K miles. He told me that every thing still looked new. And Greg didn't baby his car. Greg's recommendation, was the deciding factor for me to use the PKtensioner, when I did my TB/WP along with all the seals and gears last year. I also opted to replace my Porsche water pump with Ed's Guardian pump. I believe it's prudent to replace the old with new and better parts as they become available. Porsche doesn't really support these cars any more. It's our vendors, and some really talented owners that keep the 928 on the road.

BTW I just put over 4000 miles on my car in November, with out a single mechanical problem, in 7 days. For me it was money well spent to purchase Ed's pump, along with the PKtensioner.

Imo000

Since Gregs' belt looked good at 50K, why did he replace it? Did he explain it?

Livio928

I wouldn't take a chance to leave the old belt on even if it looks good. The resins still age and, IMHO, it is still too much of a risk to leave an old belt in unless you are a gambling man. Why do we replace WP even if they look good? Not worth the actuarial science kicking for failure rates.

Lizard928

Service interval is supposed to be 60k miles, so to leave a $56 belt on to only go back in in 10k seems stupid.

I've put harder miles on this system than most and it's held up wonderfully.

Schocki

OK how many PorKensioners have failed? The question was asked before...
I never heard of any issues in Germany (and I installed many of them and sold many too) and nothingnegative from Roger.

Installed mine in 2009 and I never had any issue with the TB. I actually refuse to service the TB if the owner wants to keep the original tensioner system. PorKensioner or nothing, sold my Kempf tool in 2010 and never missed it since... :-)

Imo000

Yes the service interval is 60K with the stock system. Since the Audi system is gentler on the belt, why not increase the belt interval too? Why not use the Audi belt interval? Would be nice to have some research data on this. The cost of the belt is not the issue.

Lizard928

The service interval on the audis and vws is around 100k miles. But most still change them at 60k miles.
I've only seen a couple of people hit the 60k miles in the 5 years. All others get replaced due to age instead of mileage.

69gaugeman

Just because most replace them doesn't mean that they should be replaced. Or that they need to be replaced. I am not sure what most means anyway. More than 50% of the people I know wait till the 150 or more.....

5 Years is one of those urban myth things that have no basis in fact. Is there a life limit in age on a belt? Sure. But it is not 5 years. or 10 years for that matter.....

I never replaced one before 150,000km on an Audi. Audi also does not have a year expiry on the belts. Mileage only. The only thing they have a time interval on is brake fluid every 2 years.