One of the other things that happens with cam belts (and frequently overlooked by people) is that it is possible to "yield" a cam belt. Just like over tightening a bolt and stretching it past the point of it's elastic limit (the ability to stretch a bolt and then have it return to it's original length.....common on head bolts, rod bolts, etc.), it is possible to over tighten a cam belt and damage the internal cords (which are the only thing that control stretching....the rubber certainly will not do this.)

The factory cam belts (Gates) are very well built with a high amount of internal cords of very high quality. "Aftermarket" belts....not so much.

It is possible to tighten a factory belt to very high tension (over "10" on the factory tool) and then loosen it without incurring any long term damage. The belt can be re-tightened to 5.0 units (the correct tension for a 32 valve engine) and it will maintain that tension, as if over tightening had never occurred.

No apparent damage to the belt.

The common "aftermarket" belt that is sold and used will reach 6.5 to 7.0 units on the factory tool and then just continue to stretch, without getting tighter, These belts, if over tightened by just a little percentage, "yield" and may be damaged.

In my shop, if we have any reason to remove an "aftermarket" belt, we dispose of it and install the correct belt.

I have a GT in my shop, as I write this, which suffers from this syndrome. This car belonged to a brand new owner, without much in the way of informative service records, however the new owner had been told that he cam belt was changed, very recently.

The vehicle came in for some service and we naturally checked the cam belt.....this is an almost "automatic" thing for us to do.

The aftermarket belt was at 2.5 belts units, with the belt light disconnected and grounded (really dumb thing to do.) We were going to tighten the belt, as it was relatively new, but saw that the tensioner had the adjusting bolt tightened extremely deep into the housing, without any more room to tighten the adjusting bolt.

We removed the tensioner and disassembled it, to find that the previous mechanic had assembled all the internal bi-metallic discs in the same direction. By doing this, the tensioner became a solid slug, without any ability to compensate for engine expansion. "The "spring" was gone. The stack of discs also became shorter by doing this, which caused the adjusting bolt to need to be very deep in the housing.

We repaired the tensioner, inspected all the other components, and re-assembled (customer did not want the belt changed, since it was virtually brand new.)

We adjusted the belt to 5.0, using the factory tool, repaired the belt light, and sent the car home.

A week later, the belt light came back on, after some "spirited driving" by the owner. I told him to shut the car off and restart the engine, too see if the belt light came back on. He called back and said the belt light came back on after a very short period of time (presumably 3 minutes.)

That was enough, for me. Time for the tow truck.

We inspected the cam belt, once the vehicle arrived at the shop, and found the belt to be back below 3.0 on the factory tool. Disassembly of the front of the engine found no problems. The cam belt was simply stretching at an alarming rate.

I've never been a traditional mechanic....one that just fixes things and moves on...I've ALWAYS asked "why" and figured out why things occur.

My theory is that this belt was initially tightened to somewhere near (or over) the factory specification. The first time the engine got hot, this belt (because the tensioner was a "solid slug") got stretched beyond its "elastic limit" and was damaged the internal cords.

The client got very lucky the belt did not break....the belt warning light certainly saved this engine from extreme damage.

Correct.

If you look at the second picture that Christopher took of his belt and the Kempf tool (random place), you will note that the rotor is not pointing in the direction of TDC #1.

His measurement, taken in that picture, is meaningless. Actual belt tension could be off by a huge amount.

Good to know. Hauling that engine around from the bottom, then popping up, then missing TDC on the cam, then having to go around again and again gets kind of boring. But it does make a demonstrated difference, as seen in the pics.

*nod* Agreed. I'm not sure how much difference there is on the Kempf tool (maybe I'll get a 90201 tool for Christmas since I'm going to have to do the 944S *again* soon and it would be cool to see if I can find a spot where the Kempf matches the Porsche tool which matches the auto-tensioner or something.

The 86.5 engines still had the mechanical fan on the back, so I was thinking I would have to pull the fan, idlers, belts, and all that jazz to get to the tensioner. What would be the impact of running it with low oil at this point for a few months before I take it in for a real servicing.

Note: Since I put it together I haven't seen a light, even after a trip to 5k RPM. In thinking about it I was moving that wire around when I was checking the oil, it's possible I loosened it on the contact or something. I'll keep an eagle eye on it and see if I can identify any failures.

Aside from calling the shop is there any way to tell if that was a genuine or aftermarket belt?

C
As I remember the tensioner oil level can be checked with minimal disassembly. I would pull my fan shroud and that takes about 2 minutes but then I have a Spal dual fan kit that facilitates such. The tensioner has two nipples on the front of its body- one for filling and the other to prove the level and they are reasonably accessible.[/QUOTE]

[QUOTE=GregBBRD;14435744

I've never been a traditional mechanic....one that just fixes things and moves on...I've ALWAYS asked "why" and figured out why things occur.

.[/QUOTE]

Greg,

A truly excellent post that encapsulates what I have been alluding to partly in this thread but also the other one.

A good technician can tell you what is wrong- a good engineer will tell you why! If winding in the tensioner results in no proportionate increase in tension then plastic deformation is taking place and failure is just around the corner.

It should be obvious that if a belt is in true distress then the alarm will not "go away". Most systems are tolerant to some degree of human error but disaster invariably happens when more than one parameter has been corrupted. In the case you mentioned over-tension and a faulty tensioner.

Hopefully the OP will take note and if he gets any further alarm he will call it a day with this belt. I have only used Porsche OEM or [more recently] the Gates belt from Roger- never had a problem with either [touch wood].

All of the verbiage about belts and tensioners (and water pumps, for that matter*) is extremely amplified by people trying to save a couple of dollars or by people with their own personal agenda.

In the end, people are not saving any money by constantly redoing their belt job because they used a cheaper timing belt (or water pump) or because they used other terribly engineered parts that have been changed multiple times in an attempt to make them work better. (Called "engineering and development on the consumer's dollar.")

The fact of the matter is very simple:

Use the stock pieces, replace the worn ones, assemble them correctly, and your belt will last 60,000 miles, minimum. (I even think the "Internet Buzz" of replacing the belt every 5-7 years is a crock.) Yes, it will need to be adjusted every 15,000 miles, which is an extremely minor operation.

I have no idea of how many stock timing belts with stock tensioners I've installed, over the years....literally thousands! I've NEVER had one of these belts fail. I've NEVER had one of these tensioners fail. I've NEVER had any engine failure with the stock parts. NOTHING bad has ever occurred.

There's millions and millions of absolutely trouble free miles on the stock pieces.....use them, replace the worn pieces at every belt change, correctly tension them, and forget about it.

Throw all that aftermarket crap into the trash can, like I do. Use the stock pieces and rest assured that the belt is going to be the least of your problems with these vintage cars.

(*) I maintain that if aftermarket/rebuilt water pumps for the 928 had never been offered or sold that the 928 community, as a whole, would be hundreds of thousands of dollars ahead. It's my firm belief this is also true for "aftermarket" belts and tensioning devices.

Fair enough, is it possible to tell if that is an aftermarket or OE belt. Remember I was burned with the -00 belt on the 944S; the 02 belts have never let me down.

As for a root cause, the possibility is either a loose connector (maybe, but odd it only happens at higher RPMs), oil loss in the tensioner, or some magical condition where the belt stretches while running but not when being tested over and over statically.

If tensioner oil, how much of a problem is this if the belt is still tensioned properly? If a stretching belt then why doesn't it show up on the tensioner tool? If a loose connection then I feel silly but I could rig a Raspberry Pi and some analysis software on the line to see if this is some weird transient or consistent error.

I had two different failures of the warning circuit in my car. The first occurred after a shop did the timing belt post initial purchase and somehow didn't orient the spade connector at the idler pulley correctly, causing it to rub against the center cover, eventually causing an intermittent loss of continuity to ground which would then trigger the warning. Bill Ball and I chased this over a few weeks before we finally tracked it down as everything tested fine at rest and failures only occurred while in motion. The second failure was a fracture of the copper strap from the tensioner to the idler pulley (fairly common failure) this one also started out as an occasional error and eventually became permanent. When I did the timing belt I replaced the copper strap with the updated wire from the S4 setup and have had no issues since.

Chris, after reading this thread I would humbly suggest adding another tool to your toolbox: Occam's Razor.

Indeed. What's the most likely cause of the problem? It's not the tension, we checked that and I can't sit here and troubleshoot possible metallurgical issues with the Kempf tool. :-)

Christopher: I don't see any acknowledgement that you corrected this error when taking a reading.

THE ENGINE AS PICTURED IS NOT AT TDC.

The notices on back of cam (and the plate on engine wall) will align as well as the indicator on the crank at TDC.

Hi Steve! Which picture? The captions are below each pic. It's possible I got it upside down, but unless the marks in the housing and the marks on the camshaft gear are not indicative of TDC then I believe I had it right. But it's always good to have multiple sets of eyes on these things...

If you're really sure none of those pictures are at TDC, then I can take it apart again next weekend and shoot a video of all this; I can also pull the #1 spark plug and stick a measuring tool into the barrel to verify it's at the top....

If Kempf tool is showing adequate tension at TDC then you need to troubleshoot the warning circuit. Check for continuity to ground at the center cover plug while manipulating the cover and examine the copper strap that runs from the end of the tensioner to the idler pulley. One other potential issue with the warning circuit is that a washer at the end of the tensioner frequently doesn't get reinstalled and has been identified as the cause intermittent warnings. If either the copper strap or the washer is the cause you'll need to remove the belt to fix.

The obvious visual cue for being at TDC No1 is when the rotor arms point to the 3 0'clock position when viewed from the front of the motor but the check should be done against the scale on the engine damper. The second photo in your sequence suggests this was the case for the 1/4 cam. Later photos show you checking the tension but as I understand that was not meant to be on TDC No1 as you were checking tension elsewhere in the cycle- correct?

Did you actually correct the over tension when you checked No 1 TDC? Assuming you have run the motor since then have you seen any further alarms?

Right. The other points were just random checks simply to get used to the tool and all. I was more concerned I wasn't using the tool right, I was expecting a loose belt.

No, I didn't correct the over-tension yet because I'm thinking it's still within a margin of error and I don't want to introduce too many variables. Haven't seen any further alarms yet, but have only driven the car a bit (focusing now on the transmission, was low on fluid, and the stupid brake/ABS lights).

I'll pop it off again next weekend and take another look. But I'm thinking the alert might be low oil in the tensioner. If I can get the second shroud off without too much difficulty/removing other belts I'll see what the tensioner looks like. Simple enough to top off with gear oil, (I assume I can pull a vacuum on the top nipple while having the bottom one connected to an oil resevoir, then pull till no bubbles with the engine slightly elevated nose up).

Lack of oil in the tensioner has nothing to do with the tensioner warning circuit being tripped. The oil in the tensioner is meant to transfer heat to the bimetallic washers in the tensioner assembly which DE-tensions the assembly to compensate for expansion of the block. Not to say that lack of oil shouldn't be addressed, but adding oil will not cause the warnings to disappear.