Fanbelt Squeal and Tacho reading - Chris TD
Car: 97 Astra 1.7 TD (LPT) Vauxhall engine
Symptom: The following glitch appears occasionally on startup, normally after standing overnight or all day. The engine starts and the Tacho rises up to about 500 rpm even though I know it's ticking over at 900-1000. The steering is sometimes very heavy (pump not being driven?) but not always. After a minute or so of tickover or manoevuring round the carpark and pulling away, when the revs should be up about 1500 to 2000 the fan belt starts to squeal intermittantly for 10-40 seconds and suddenly stops. At the same time the squealling stops, the tacho jumps to the correct reading, and normal service is resumed.

I've tried starting it up with the bonnet open and a torch to check if the pulleys/alternator/steering pump are rotating, but so far Murphy's law has dictated that when I have time to do this (not running late into work). the problem doesn't appear!

The belts seem tight enough, but I might tweak them up a bit. What puzzles me is the tacho reading - anybody any info on where it's picked up from? could it be caused by a voltage drop (with alternator not turning)?

Any info appreciated.


Chris TD
Fanbelt Squeal and Tacho reading - Adam Going (Tune-Up)
Hi Chris,

Modern diesels have their tachos sensed directly from the alternator. You almost certainly have a slipping belt, although it could be that the alternator bearing is partially siezed. It is almost certainly a "poly-vee" (multi-ribbed) belt, which is designed to run VERY tight, so don't be shy about tightening it. If there are any tiny crackes accross the vee sections fit a new belt. If the belt has an automatic tensioning device this may be faulty.

Regards, Adam
Fanbelt Squeal and Tacho reading - Hew Jamsandwich
I think your Astra has two belts, one for the alternator and one for the power steering pump, if it is the Isuzu engine. Sounds like both of yours are slipping, as they seem prone to do if damp/cold. The squeal is probably the steering pump belt, especially if it gets worse when you make a big steering input immediately after starting.
Fanbelt Squeal and Tacho reading - Robin the Technician
The tachometer is driven directly from the alternator on a lot of diesels now. To establish if the fan belt is slipping is to open the bonnet armed with a jug of water. With the engine running and the belt squealing, pour water on the belt. If the squealing stops, its definitely the belt. For reference, tighten the belt until you can twist the belt just less than half on the longest stretch of belt. This represents about 5-8 mm and is about the right tension. if it twists half or more its too loose. It maybe that the belt is knackered and for what it costs, replace it. Its worth keeping the old one 'just in case of emergencies'.
Fanbelt Squeal and Tacho reading - Cyd
My own experience is that tightening old belts quickly leads to failure. I would replace both belts and clean the pulleys with solvent too.
Fanbelt Squeal and Tacho reading - Dizzy {P}
Once again I find myself agreeing with Cyd.

In vee belts, it is the cords just under the top surface that give the belt its strength and these cords are designed not to stretch. If the belt is starting to need periodic re-tensioning it is probably in breakdown mode. Tightening will put more of a load on the belt and will speed its failure.

The rubber part of the belt is there to support the cords and to transfer the driving force from/to the pulleys. It contributes very little to the strength of the belt.

What is sometimes not realized is that the drive is only through the sides of the belt, not the bottom. There should be a gap between the bottom of the belt and the vee-groove but the belt can wear to the extent that it is riding on the bottom of the groove and tightening it in this situation will not stop it slipping.
Fanbelt Squeal and Tacho reading - Cyd
Thanks Dizzy, I've learnt something today. I was speaking from experience but didn't know the theory.
Fanbelt Squeal and Tacho reading - Chris TD
Thanks all for the info - trip to the motor factors and a little job for the weekend I think.

Chris TD

Value my car