cambelt warning light

johntom · 02-04-2011, 02:20 PM

Hi
my recently aquired 1990 GT has the cambelt warning message on sometimes, however i checked the belt tension a few days ago (with the kempf tool, timing at 0 degrees) and the reading showed the belt to be slightly tight ? If the belt is tight enough then what would be the likely cause of the warning message ?
The car is undergoing some refurbishment and isn't in daily use, on a trip to the local garage (about 6 miles) the light didn't show, when i collected the car to come home, the light came on straight away. I didn't rev the car above 2500rpm during either journey?

Landseer · 02-04-2011, 02:29 PM

What did they work on?
Sounds like on the front of the engine they disconnected or damaged the wire, causing the light.
Its a single wire with a round plug that attaches to the cover just to the right (UK Drivers side) of center.
Educated guess.
BTW there is a 2 minute delay or so on the warning.

Bigger question now is are they competent to work on a 928?

johntom · 02:42 PM

Hi
the car went for it's annual MOT test after being stood for a few years, no work was done by the garage. The warning message did come on when i first drove the car a few weeks ago, the previous owner said the light did come on occassionally. He changed the belt a few years ago/a few thousand miles ago and thought it might just need re-tensioning after the breaking in period. With this in mind i expected the belt to be loose not tight?
Could a wiring fault be random or is it likely to be a tensioner issue?

Landseer · 03:05 PM

It could be either. The connector inside the cover could be dirty or the wire frayed.

Lots of times when the belt is changed the shop omits a washer that's necessary for maintaining the continuity of the circuit and it changes the sensitivity, so that's another potential solution. That's a little hard to get at for inspection.

Removing the cover, reviewing belt tracking ( post pictures) and rechecking / adjusting tension are things you should do first. If its a little loose, some moderate acceleration when cold has tripped mine on the 32V earlier cars (same setup). (Actually check the wire first!) I;ll find a picture for you.

Check at TDC (cam gear aligned on mark when crank is at O/T) then rotate engine crank a full two times to TDC and measure again. Never back-track, if you pass O/T, go two more turns.

I've found it tight, then loose on that next check.

Yes, they should be retensioned at 1000 to 1500 miles after replacement.

The replacement is a bit of a symphony. Just enough pieces to mess it up a few ways. Every mechanic represents that they can do a timing belt job. Its really hard to trust somebody who hasn't focused deeply on the nuances of this car. And, lots of cars get some maintenance just before sale, but the parts are skimped on just a little by the owner (ie, "just put a belt on it" or "Ok, belt and pump" --- but they leave the rollers and the guide bushings alone and trouble occurs.

My guess is you are a little loose or the wire is damaged.

What tool are you measuring tension with?

Landseer · 04:40 PM

Some pictures, mainly from an 87 which I'm nearly positive is like your GT.
Last couple pictures of the whole belt might be from similar 86.

NOTE --- In this first picture I'm measuring the belt tension for an 86. The loose belt triggered the light. The tension on this device shows below "bottom of window" ie, the belt is too loose. For this 32V car it should be nearer the top of the window. This belt has about 20K miles and 26 months since new and has been retensioned before.

Service bulletins by Porsche state that the 928 belt tension should be checked at EVERY service interval.

stepson · 02-04-2011, 08:34 PM

I had issues on my '89 with a new belt that I had installed. I replaced that belt (Conti brand) with a Gates branded belt and have not had any issues since. The Gates belt was much more substantial than the Conti belt.

BTW, there is a 3 minute delay before the TB warning light will come on. I believe that delay is built in so that the engine will have a chance to warm up and expand before it registers a problem.

Landseer · 02-04-2011, 08:37 PM

I've had to retension the Conti belts twice on the 85 and twice on the 86.

johntom · 02-05-2011, 12:51 AM

Hi
many thanks for the great pictures.

I used the Kempf tool.

Will i need to remove all the covers to physically see the tensioner?

After i checked the tension i noticed the bleed screw was loose, so i pumped in some gear oil and tightened both screws. The following day I drove the car a short distance about 6 miles in traffic (so more than 3 minutes) and the message didn't come on. However when i drove the car back the message was on straight away (the car had been running at the MOT garage, for about 30 minutes, but no work was done).

Would adding the oil effect the tension ie allow it to loosen when warm (unless it has leaked out)?

Landseer · 02-05-2011, 03:38 AM

Not sure. The oil is supposed to help dampen the movement. Both screws are removed and oil is fed in one side til it comes out the other.

On the kempf, I've needed to be tight, near top of window. If less than half tight with kempf, I get the warning. I think first picture above shows very loose.

If your belt is tracking well (a little back from the front edge of the cam gear) and the tension is proper, then you might have a wiring problem.

Sometimes it comes loose just inside the center cover, which you can see. Sometimes the tensioner has been assembled wrong. Sometimes the wire or connector gets frayed.

You can meter the wire for continuity from the plug at the motor to the correct plug at the CE panel,

And, Check the wire attachements just inside cover using continuity meter (at least it will show if the wire inside cover is grounded via the tensioner apparatus during normal state at rest).

If ok, then something is breaking the ground during driving, chances are the tensioner washer is installed wrong or missing or been substituted with the wrong replacement

harborman · harborman's Garage

Hi Landseer, What actually controls the T-belt tracking? Not the tension, but the tracking on the cam gears. I have noticed quite a few pictures of posts showing the cam gears and belt. It seems most of the pictures show the belt more towards the front side of the gears, towards the gear covers.

Landseer · 02-05-2011, 07:27 AM

Nothing controls it from an adjustment perspective.

All comes down to the play in the plastic bushings of the tensioner bolt and the straightness of that bolt. Plus the play in the bearings of the tensioner pulley.

harborman · harborman's Garage

Quote:

Originally Posted by Landseer

Nothing controls it from an adjustment perspective.

All comes down to the play in the plastic bushings of the tensioner bolt and the straightness of that bolt. Plus the play in the bearings of the tensioner pulley.

Ok, so then it's important to replace those plastic bushings and look at the bolt holding that tensioner pulley when doing a t-belt change. Thanks!

riber.bentsen · riber.bentsen's Garage

Well, the waterpump must have something to do with it? Because when I changed the waterpump last, the belt moved about 1 mm forwards on the cams..

The old WP was an aftermarket knock-off installed by the idiot pennycounter who owned the car before me.

harborman · harborman's Garage

Quote:

Originally Posted by riber.bentsen

Well, the waterpump must have something to do with it? Because when I changed the waterpump last, the belt moved about 1 mm forwards on the cams..

The old WP was an aftermarket knock-off installed by the idiot pennycounter who owned the car before me.

Yes, someone here posted a picture showing two water pumps with different pulley heights. One was the old pump and a new one he had just bought. Being that the pump pulley is up top I could see how it could move the t-belt position quite easily. Like I posted before, you can do a fast job or a good neat job. One needs to really double check everything to avoid doing more work!

Landseer · 02-06-2011, 06:29 AM

That's the other thing that can make the difference.

johntom · 02-12-2011, 11:40 AM

I removed the covers today and the terminal bolted onto the tensioner, inside the cover (that the wire pushes onto) was loose enough to wriggle about. Hopefully that is the cause of the warning light but i haven't had chance to drive it and find out? When i metered the continuity through the tensioner wire to ground and from the plug to the fuse box all was fine which i expected as the light isn't always on. The tensioner didn't appear to have lost the oil i pumped in last week. Re the tracking, I did notice the belt on the driver's side cam pulley was slightly closer to the front than the otherside, the belt tension remains on the tight side with all the window covered (kempf tool) ?

Landseer · 02-12-2011, 01:18 PM

Remember to measure only on the approach to TDC, never reverse a tad to achieve it, go around two more crank turns of you overshoot.

Also, kempf placement is in a belt tooth as far away from the cam gear as possible without hitting the center cover.

Just wanted to say that for completeness.

johntom · 02-12-2011, 02:03 PM

I measured the cambelt tension bang on the crank pulley TDC mark (with corresponding cam to block marks) and with the tool 1 tooth off the centre cover, as per the instructions. I rotated 2 turns and checked a couple of times, with the hole still covered, the readings did seem consistant.
If the circuit is broken for say 1 second, then remade, in the case of a bad contact, will the warning message go off or stay on until the next restart ?

Landseer · 02-12-2011, 02:39 PM

I honestly don't know. And I don't know if its different during the 2-3 minute initial running period in which there is a delayed light, and the normal running period. I can try that test tommorow for you. (maybe tonight later if I go out)

If you have tension and oil, and the warning started at the completion of the last TB service, I'm thinking you may be fine and its either this looseness (hopefully fixed) or a missing washer at the tensioner.

So long as you have about a 1/16 inch from the edge of the camgears you should be ok.

Is the front edge if the belt showing any fraying or rubbing?

Pete R · Pete R's Garage

Quote:

Originally Posted by harborman

Yes, someone here posted a picture showing two water pumps with different pulley heights. One was the old pump and a new one he had just bought. Being that the pump pulley is up top I could see how it could move the t-belt position quite easily. Like I posted before, you can do a fast job or a good neat job. One needs to really double check everything to avoid doing more work!

I noticed this once too. I think it happens with rebuilts. The rebuilder may have to resurface the gasket area and take it down a couple mm. This will ultimately change tha mounting depth and pulley location.. just a theory