Very quiet humm from compressor

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Mogman

Member
Posts
64
Location
Scotland
Hi folks,

While under our 2004 Td5 Disco I happened to be very close to the air suspension compressor when I heard a very muffled buzzing noise coming from it. The ignition had been off for over an hour so I wouldn't expect any electrical activity but something was buzzing. Anyone else had this?
 
No but you could pull the fuse or relay that controls it.
I f it stops you have proved the point, but why it would be runing with engine of not a clue.
If air bags and pipe work are all good it should only be kicking in now and again
 
Hi Mormond,
I'll do that.

The sound is not the pump running, the pump makes much more noise than that. With the electrics disconnected I'll know if it is the motor or an air valve making the sound. I want to know because we also have a very loud noise coming from the compressor sometimes, although it does seem to be doing its job OK. After parking the compressor will sometimes pulse on for a few seconds then go off then pulse on again, and do this four or five times after you've pulled up and turned the ignition off. It's loud enough for folks to turn and look what is going on. It's clearly ECU controlled because opening a door while it's happening stops the pulsing but closing it again starts it up again. I'm wondering if this quiet sound is a clue? More on this when I can get the vehicle in the dry on the drive.
 
Mogman said:
Hi Mormond,
I'll do that.

The sound is not the pump running, the pump makes much more noise than that. With the electrics disconnected I'll know if it is the motor or an air valve making the sound. I want to know because we also have a very loud noise coming from the compressor sometimes, although it does seem to be doing its job OK. After parking the compressor will sometimes pulse on for a few seconds then go off then pulse on again, and do this four or five times after you've pulled up and turned the ignition off. It's loud enough for folks to turn and look what is going on. It's clearly ECU controlled because opening a door while it's happening stops the pulsing but closing it again starts it up again. I'm wondering if this quiet sound is a clue? More on this when I can get the vehicle in the dry on the drive.
Click to expand...


Activity on the SLS post engine stop and disembarkation is perfectly normal as the system needs to compensate for the loss of weight of the passengers once they have got out of the car.

Opening any door will stop all SLS functions.
 
Hi thebiglad,

Thanks for getting back. I know the SLS works as you say but after 6 years it suddenly started doing it very noisily and quite often. Could it be a compressor bearing going or piston wear? The suspension seems quite happy and it gets worked hard up forest tracks every day. No leaky bags, both changed not long ago.
 
Mogman said:
Hi thebiglad,

Thanks for getting back. I know the SLS works as you say but after 6 years it suddenly started doing it very noisily and quite often. Could it be a compressor bearing going or piston wear? The suspension seems quite happy and it gets worked hard up forest tracks every day. No leaky bags, both changed not long ago.
Click to expand...


Have you changed the sls air filter hiding behind the rear nearside light unit?

Have you checked the black plastic cylinder silencer (just aft of the compressor) for cracks/breaks?
 
Thanks again.

I've checked and cleaned out the filter behind the rear light unit, mine is one of the later versions that doesn't fall apart or flood with rainwater. I haven't checked the silencer, hadn't thought of that, will do it next.
 
Mogman said:
It's clearly ECU controlled because opening a door while it's happening stops the pulsing but closing it again starts it up again. .
Click to expand...

Good point ... it's not normal... so... take out the comp. housing cover and pull the plugs one by one to see which component is doing it... then take a multimeter and measure voltage on that plug... IMO it's a residual/false feed on some valve ... it could also be a capacitive reactance... or even an ECU missbehaviour.... after u identify the part u can unplug the ECU and clean the plug ... maybe there is a water ingress there which gives a false small valtage on one of the outputs.... it's quite common the water ingress in that area due to leaking windscreen seal but usually it affects the BCU.. which is in the same place with the SLABS ECU....... i can't find other explanation
 
Thanks Sierrafery,

I'll work through that but in the meantime I disconnected the SLS Fuse and pulled out the relay, the very quiet sound is still coming from the compressor. Without a stethoscope I can't say where from.

Thinking in terms of air instead of electricity, could this be a leaky valve allowing air back through and rotating the compressor vanes, or making a sound as it leaks back through the valve? Is this a piston or vane compressor, any idea?

The quiet sound is not a problem but it might lead me to the cause of the big noise I get when I pull up.
 
Mogman said:
Thanks Sierrafery,

I'll work through that but in the meantime I disconnected the SLS Fuse and pulled out the relay, the very quiet sound is still coming from the compressor. Without a stethoscope I can't say where from.

Thinking in terms of air instead of electricity, could this be a leaky valve allowing air back through and rotating the compressor vanes, or making a sound as it leaks back through the valve? Is this a piston or vane compressor, any idea?

The quiet sound is not a problem but it might lead me to the cause of the big noise I get when I pull up.
Click to expand...

Interesting if untenable argument cos in this case IMO the sound must be there even with the doors open or battery disconnected(check again)...coz u said
...opening a door while it's happening stops the pulsing but closing it again starts it up again
Click to expand...
..... also take into account that the SLS fuse/relay is feeding only the compressor .. the valves are commanded directly by the ECU.... so the electrical issue still stands ... quite eldritch i must say... but dont abandon m8 u'll get to the end of it
 
Last edited:
Ahh, the valves are commanded directly by the ECU, OK, so fuse and relay out leaves them alive.

I think I'm confusing you, and myself (confusing me is not hard to do!!). The loud sound you hear when we pull up and can be started and stopped by opening and closing the doors is what i would like to find the cause of. The very quiet sound I can hear with my ear against the compressor housing when all should be quiet is a very different sound but one that I wondered might give a clue to the cause of the loud one.

Anyway, while underneath looking a few minutes ago just spotted oil splatterd around the front offside wheel bearing area! Hmmm! That wasn't there two days ago.
 
Mogman said:
Anyway, while underneath looking a few minutes ago just spotted oil splatterd around the front offside wheel bearing area! Hmmm! That wasn't there two days ago.
Click to expand...

D2 has a wheel hub which incorporates the bearing(unfortunatelly) ... but the leak is through the oil seal from the axle casing(not a killer to replace)
 
>>leak is through the oil seal from the axle casing<< I had a feeling that might be how it is, thanks, that's one for when our Landie mad son is here to give me a hand.
 
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