Hi Paul,

I had been plagued with the bet warning problem you have for years, and as my post said I just got it fixed.  If you haven't yet check where my last post said to, in the fuse box there are plugs with wires that go all along the bottom of the fuse box.  Check ALL those plugs and make sure that they are ALL tight.  The one that was loose on my car was the right-most plug, closest to the brain.  In my car('85) this is the "W" plug.  The letter is listed above the plugs.  In your case I would check all the plugs because it may be in a different plug in your MY!!

That's the first place to check. Restart car if any are loose and hope light stays off!!!!!!  If light comes on see below....

Pull the sensor (carefully, I broke the retaining clip) where it goes through the timing belt cover.  Run the end of the sensor to ground and then start your car.

1) If the light still comes on trace the wire to the sensor and check for a break in the wire (I know it is very hard to trace)!  If no break is found then the warning control unit is bad!!  If break is found repair and warning light should be repaired!!

2)  If the light does not come on then there is a problem by your tensioner somewhere.  Bring the car back to your mechanic and get it looked at again.. you may have to replace tensioner!!

I can clarify the fuzzy points if you need me too!!
Good Luck!

The belt tensioner for all 928s is mechanical

--it does not self adjust , does not use oil pressure , is not connected to the engine oiling system ---

the "warning System" is a wire attached to the tensioner metal arm - as long as the arm is grounded to the pin sticking out of the tensioner the "system" is happy . Or if the plastic shoulder bushings where the arm pivots have disintegrated it is grounded and happy or if a prior "technician" became frustrated and attached the wire to any , any , any ground the "system " is happy. 

Yes the tensioner is oil filled but only if someone fills it !! yes it does compensate for thermal expansion using 35 to 50 cup shaped bi-metallic washers . Yes , later versions act as a vibration damper with a baffle and valve , if they have had oil added to them . The tensioner only tensions the belt from the point where it leaves the crank shaft and travels to the passenger camshaft drive gear - it is basically just picking up the "slack" on the portion of the belt where it is being pushed from the crank gear up to the passenger cam gear - the rest of the belt is being pulled around it's path by the energy from the crank shaft gear . Since the belt has teeth and the crank gear and camshaft gears have teeth the length of the belt between them never changes unless it jumps teeth . The tension of the belt between these gears varies with the compression and relaxation of the valve springs but has little to do with how the "tensioner" is set . If you want a simple example of how it works examine the chain on a bicycle with gears -- the chain is tight only from the front sprocket to the rear sprocket -- the rest of the chain is under little tension

...... so repeat after me "the tensioner is mechanical , the tensioner is not supplied with oil by the engine"  The tensioner warning system is very unreliable -- if you get a warning always get it checked but do not rely upon it to warn you before the belt breaks . Just my observations on this subject . The belt is affected by use , age , oil contamination etc.

-- Which brings up another question if I drive around in 2nd gear at 4,500 rpm does my belt wear out twice as quickly ??? Do GTS with a 2.75 ring and pinion need belts changed more often than an 86 with a 2.20 drive ratio ??  The 86 spins at 80 % of the speed in 60,000 miles it may have only traveled the equivalent of 48,000 miles or is the Porsche 60,000 recommended interval just an estimate of how long most belts will last for most people ???? I recommend 45,000 mi. as a safer alternative
Jim Bailey
928 International

Last night this warning started flashing on my 89/AT, TOOTHED BELT SERVICE, Is this the Timing Belt? and if needs to be tightened is it same as earlier models (83-85), i do have the timing belt aftermarket tool that i purchase when i had my 83. when i turn off the car and restart, the light warning is gone but after driving 20-30 miles it starts flashing. Thank You.
Since you say the light comes on after 20-30 miles, than that tells me the belt still has memory.

I thought I remembered a time lag involved, before the pod warning lights up?
IIRR there is 3 minute time lag before warning circuit is activated.
3 minutes delay at startup. After that, if the 'sensor' looses ground, the light will immediately go on.
IMHO, the delay was likely put in because with the stock tensioner, belt tension varies wildly for a short time after startup of a cold engine, and there were too many spurious lights in testing.