DMF noise when a/c is on

[cnut]

My 06 Saab 9-3 1.9 TID has just had its compressor replaced and since then, when the a/c is switched on what I think is the DMF starts making a typical worn out DMF racket. As soon as the a/c is off the noise stops. I thought that when the DMF starts making a noise it will do it all the time (when the clutch is engaged)? This doesn’t seem typical to me... Any suggestions?

Thanks

Phil

[GreenV8S]

I don't know that engine but I suppose it's a conventional layout with ancillaries driven from the opposite end to the clutch. In that case isn't it more likely the noise is coming from the A/C clutch/pump?

[cnut]

I thought that but as soon as you raise the revs the noise stops and the noise is definitely coming from the gearbox area

[finlo]

It was obviously replaced as it wasn't working, now that it is it will be putting a braking force on the crank?

[cnut]

Would a faulty dmf be highlighted when there is more braking force at the crank?

[GreenV8S]

Does the idle speed rise slightly when the a/c is switched on? If so, does the noise also occur when you raise the revs similarly with the a/c off?

[cnut]

The idle speed drops when the a/c is on but as soon as I feather the throttle I.e rising the idle very slightly the noise stops

[GreenV8S]

If it's very speed sensitive and coming from the clutch area that seems consistent with your original diagnosis that it is the DMF.

Doesn't your ECU normally raise the idle speed when the a/c comes on? That seems quite common.

[cnut]

It certainly does on my golf but not on the Saab but the Golf is petrol rather than diesel.

I’m not sure the idle even really drops but you can hear that the engine is under more load when the air con is turned on

[Little Pete]

When the noise is present can you feel anything through the clutch pedal?
If possible have a look at the crankshaft pulley and see if has any swarf and/or rust on it. It acts as a vibration damper and is in two parts which are bonded together. If the bonding starts to fail you can get the symptoms you are describing.
It could be a drive belt tensioner also.

[cnut]

I don’t think it’s the pulleys as the noise is coming from the gearbox side of the engine.

[Little Pete]

In that case if you lightly press the clutch pedal when the noise is present you should be able to feel the rattle through the pedal. If this is the Vauxhall engine, DMF failure is quite common.