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Tension light on, tension ok, huh??? (86.5 928S)

 
Old 08-24-2017, 09:20 PM
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Christopher Zach
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Default Tension light on, tension ok, huh??? (86.5 928S)

EDIT: The problem turned out to be a slightly loose belt. You have to use care when using the porken tensioner, make sure it's at true TDC#1, make sure the tensioner is on the belt right at the point the belt goes into the lower cam housing cover, and make sure you're not pushing on the tensioner when making the test. Also for 32v cars the pointer should be at the top end of the window, not in the center.

Ultimately I turned the adjustment bolt 1/2 turn, and problem has gone away.

Original post follows:

So tooling around in my 86.5 US 928S a week ago I saw the dreaded BELT_TEN light come on at about 5200 RPM. Drove home at slow RPMs, checked car later, was fine. Few days later it came on again at >5k RPM. Figured the tension light was coming on with a cold engine and might have been loose I parked the car in the driveway, ordered the Keffer tool, and waited.

Arrived over the weekend, took off the passenger cam cover and took a look at the belt. Looks ok. Cranked the engine with a 28mm socket (toooough) clockwise from the front of the car, stopped, checked tension.

According to the tool, fully on the belt, and twisted so the top of the tool touches the back of the cam tower it looks like the indicator is just outside the window to the *front* of the engine, which would indicate a belt too tight. Does that make sense?

I'll try it again tomorrow morning with the engine rock cold from sitting overnight (was in the 80's today, haven't run it since last week) turning the engine through two complete cycles to TDC and mark on the passenger cam sprocket and see how it looks, but from what I can see the belt doesn't look loose. Would this be normal, anything else I should do to check what's going on?

C

Last edited by Christopher Zach; 10-12-2017 at 03:54 PM. Reason: Final resolution.
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Old 08-25-2017, 01:09 AM
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I had a problem wit the light coming on, belt tension seemed OK, but I was still concerned so I kept checking the belt tension and confirming it was OK. It turned out to be a connection problem where the wire connects to the terminal in the cover. I believe others have had issues with the ground strap inside the cover. Once you are convinced the tension is correct you need to confirm that the indicator circuit is not compromised.

In you case it seems rpm related which points to the belt or tensioner rather than the indicator circuit but don't forget about the 3 minute delay.
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Old 08-25-2017, 04:48 AM
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The consensus amongst those who have access to both the Kempf tool and the officially sancioned one is that the Kempf tool should be at the full extent of the window.

I have run mine in the middle of the window and had no issues but others have noted alarms at that setting. That alone tells me we are in the right window as it were.

On the other hand in the current thread of the same subject the good Doctor advises that a loss of oil in the tensioner can induce alarms. This type of oil loss seems to be quite a common occurrence.
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Old 08-25-2017, 06:32 AM
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Mine was fixed by adding oil to the tensioner but an hour into a drive it came back on when the oil leaked out, I have the rebuild parts just need the time. Personally I don't rev over 3k until the engine is fully warmed up, the belt is at its loosest when cold amongst other things.
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Old 08-25-2017, 09:04 AM
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Originally Posted by FredR View Post
The consensus amongst those who have access to both the Kempf tool and the officially sancioned one is that the Kempf tool should be at the full extent of the window.
So in other words the pointer should be right at the edge of the window towards the front of the car (radiator). In which case my car is a smidge tight. I'll get outside this morning and crank the car all the way around to tdc and test again.

On the other hand in the current thread of the same subject the good Doctor advises that a loss of oil in the tensioner can induce alarms. This type of oil loss seems to be quite a common occurrence.
That was my other thought, dampening error. Well, how much risk is there if the belt is properly tensioned (and 35k miles on the belt) and throwing alerts? I'm planning on having the belt replaced in the fall just because, but I'd rather not grenade the engine.

The tensioner was rebuilt when the belt was last replaced at 70k miles along with the other stuff, how long should the little thing last?

Finally how much more of the engine would have to come off to refill the tensioner? Would that include taking out the mechanical fan and everything else?
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Old 08-25-2017, 09:13 AM
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Originally Posted by gazfish View Post
Mine was fixed by adding oil to the tensioner but an hour into a drive it came back on when the oil leaked out, I have the rebuild parts just need the time. Personally I don't rev over 3k until the engine is fully warmed up, the belt is at its loosest when cold amongst other things.
Thank you and agreed, I'm not racing this thing around right now but it does need to get up into the higher RPMs where the proverbial fun is from time to time.

As long as I am not going to either break the belt or skip a tooth I'm ok with an overhaul after the shark meeting. In the meantime a leak in the tensioner would be annoying as it was rebuilt when the belt was replaced ~35k miles ago. Still things do happen...
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Old 08-25-2017, 09:57 AM
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Originally Posted by Christopher Zach View Post
So in other words the pointer should be right at the edge of the window towards the front of the car (radiator). In which case my car is a smidge tight. I'll get outside this morning and crank the car all the way around to tdc and test again.



That was my other thought, dampening error. Well, how much risk is there if the belt is properly tensioned (and 35k miles on the belt) and throwing alerts? I'm planning on having the belt replaced in the fall just because, but I'd rather not grenade the engine.

The tensioner was rebuilt when the belt was last replaced at 70k miles along with the other stuff, how long should the little thing last?

Finally how much more of the engine would have to come off to refill the tensioner? Would that include taking out the mechanical fan and everything else?
Christopher,

The Kempf tool seems to be pretty reliable and it sounds as though you have the belt at the top end of tension if not a smidgeon too tight. As you apply the tool and raise the thing through a 90 degree arc you will see the window open up progressively and with more tension the notch passes across the window until you reach the measurement position. If the belt is too tight the notch indicator will lie outside [i.e. beyond] the window.

I trust you understand that not only is the tension to be checked at TDC, it has to be TDC on No1 firing stroke- most important you get this right as the wrong stroke gives incorrect indication.!

As I remember, on my S4 motor to get at the tensioner one has to remove the pulleys and the lower cover- presumably the same on your model year.

Regarding the aging of the belt how long has the thing been on the car? The factory recommendation is 60k miles - I personally limit the belt to 40k miles or 6 years- probably a bit conservative but then I live in a hot climate that tends to do for plastics and rubbery items [no killer colds starts though!]

Rgds
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Old 08-25-2017, 10:32 AM
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Ok, some pictures, because pictures are fun.



Close up of the belt



Another picture of the belt



Right at TDC after dragging the motor around



The tool in place



Random spot on the belt

I also have a little video of me moving the belt, it does move.
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Old 08-25-2017, 11:45 AM
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Your pic suggests the belt is on the tight side as per Kempf tool measure- you did not confirm whether the crank is on No1 TDC or the counter stroke- from what I could see of the rotor arm the motor is on the correct stroke
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Old 08-25-2017, 02:30 PM
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Originally Posted by Christopher Zach View Post
So tooling around in my 86.5 US 928S a week ago I saw the dreaded BELT_TEN light come on at about 5200 RPM. Drove home at slow RPMs, checked car later, was fine. Few days later it came on again at >5k RPM. Figured the tension light was coming on with a cold engine and might have been loose I parked the car in the driveway, ordered the Keffer tool, and waited.

Arrived over the weekend, took off the passenger cam cover and took a look at the belt. Looks ok. Cranked the engine with a 28mm socket (toooough) clockwise from the front of the car, stopped, checked tension.

According to the tool, fully on the belt, and twisted so the top of the tool touches the back of the cam tower it looks like the indicator is just outside the window to the *front* of the engine, which would indicate a belt too tight. Does that make sense?

I'll try it again tomorrow morning with the engine rock cold from sitting overnight (was in the 80's today, haven't run it since last week) turning the engine through two complete cycles to TDC and mark on the passenger cam sprocket and see how it looks, but from what I can see the belt doesn't look loose. Would this be normal, anything else I should do to check what's going on?

C
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Old 08-25-2017, 02:33 PM
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Old 08-25-2017, 06:37 PM
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Originally Posted by FredR View Post
Your pic suggests the belt is on the tight side as per Kempf tool measure- you did not confirm whether the crank is on No1 TDC or the counter stroke- from what I could see of the rotor arm the motor is on the correct stroke
Yes, TDC cylinder#1. It's right on the outside edge of the window, I assume from your analysis that I have it in the right place and am turning the belt the right amount (tool touches the metal back behind the belt).

Measuriing at other places on the cycle I noticed that the tensioner reads a bit tighter, is that normal and expected (ie: not being at TDC#1 get you a false tight reading)?

Meantime car is back together and running smoothly, I'll take it out for a drive tomorrow and see what's what. I also noticed the wire coming off the tensioner has a small bit of play in it and the plastic keeper doesn't seem to connect to anything. Maybe it's slightly loose and the vibration of the motor at higher RPMs disconnects it from ground briefly (I'm guessing they use SCRs and inverters in the computer head to lock an alert in).

I wonder if I could put a schmidt trigger on there to clean up the signal or an op amp and a capacitor to ride out very short transients. Or clean the connector.

Whoops, forgot to re-plug in the line to the AC compressor, will do that before bedtime. Also now the trunk isn't latching, so I have that problem to deal with as well.....

Thank you for the advice. See anything else amiss with the belt?
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Old 08-26-2017, 12:09 PM
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Originally Posted by Christopher Zach View Post
Yes, TDC cylinder#1. It's right on the outside edge of the window, I assume from your analysis that I have it in the right place and am turning the belt the right amount (tool touches the metal back behind the belt).

Measuriing at other places on the cycle I noticed that the tensioner reads a bit tighter, is that normal and expected (ie: not being at TDC#1 get you a false tight reading)?

See anything else amiss with the belt?
It is critical to measure tension at No1 cylinder because that is the reference point upon which the system is calibrated by Porsche and yes, measuring at different points will cause different readings. Your measuring technique seems fine.

Given you have the belt too tight it needs to be backed of a little to get it right. That of course does not explain why you got an alarm. Based on your photos I cannot see anything obviously untoward but then maybe others might.

Trust you noted the comment in my first post to your thread suggesting you check for oil in the tensioner as we are advised that this can cause premature alarms.
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Old 08-26-2017, 01:07 PM
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*nod* I can see that. It seems that at other places aside from TDC#1 the tool reads about a pointer false too tight, so I could see someone putting a belt on and thinking it was tensioned when it was not.... Likewise I assume checking the tension when the outside air is 80 or so would read a bit tighter than if you did a retension in the winter when it's 40 out.

In order to check the oil the whole front of the engine will have to come off, and I think what I will do is drive it for a month or two, then take it in for a full rebuild with a new belt, pulleys, and have the tensioner rebuilt (again...) in the fall when it is cooler and I can focus on some other things. I won't adjust the tensioner at this point, it's probably ok "enough" for me right now and I won't replace the water pump because I really don't want to deplete the world's supply of pumps :-)
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Old 08-26-2017, 02:23 PM
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Originally Posted by Christopher Zach View Post

It seems that at other places aside from TDC#1 the tool reads about a pointer false too tight, so I could see someone putting a belt on and thinking it was tensioned when it was not

.... Likewise I assume checking the tension when the outside air is 80 or so would read a bit tighter than if you did a retension in the winter when it's 40 out.

In order to check the oil the whole front of the engine will have to come off, :-)
Christopher,

We recently had an example of exactly this happening when one of our friends did not understand tension had to be set on No1 cylinder at TDC.

I suspect ambient temperature does plays a role but you possibly have it the wrong way round. If the tension is set when the engine is colder, the belt tension is going to be greater after the motor warms up because of the greater differential temperature. This may explain why I have no problems when setting the belt at mid range of the Kempf tool- I do my work during our "cool season" when daytime ambients are typically around 25C to 30C.

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.
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