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Having successfully fixed the over temp sensor in my car (photos soon) we realised my wife's car has an issue.

Even with the inhibit switch pressed with the height set to Standard her compressor is running every 20-30 minutes and the car sags right overnight.

What are the first things to check in order of probability?

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What, the pump runs every 20 - 30 minutes with the ignition off and parked up?

Sagging suspension overnight is normally air leaks. Air springs get perished or the valve block seals get old.

Craig.

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You've got a leak somewhere. Pull fuse 44 (pre-99) or 29 (99 or later) and leave it overnight. That will show you which corner is leaking and dropping. Then you will need to get the soapy water spray out and check the air springs, connections to the air springs and where the pipe comes out of the valve block for bubbles.

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Craggle wrote:

What, the pump runs every 20 - 30 minutes with the ignition off and parked up?

Sagging suspension overnight is normally air leaks. Air springs get perished or the valve block seals get old.

Craig.

The compressor only runs when driving with the Inhibit switch pressed. The sagging is overnight, ignition off, Inhibit switch still pressed.

The air springs were absolutely buggered when we got it two years ago. God knows how it passed the MoT they gave it at the dealer. They are all new.

Car is low mileage at 68k and garaged for most of its life, but I guess standing can do as much harm as high mileage.

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Gilbertd wrote:

You've got a leak somewhere. Pull fuse 44 (pre-99) or 29 (99 or later) and leave it overnight. That will show you which corner is leaking and dropping. Then you will need to get the soapy water spray out and check the air springs, connections to the air springs and where the pipe comes out of the valve block for bubbles.

I assume that disables the entire system, including the checks triggered by the door switches?

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It kills the power completely to the EAS system when the ignition is off so nothing will change with doors being opened or closed. All the inhibit switch does is stop it from dropping to motorway height at speed or staying at motorway height when you slow down.

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Fit a pressure gauge so you can see what is going on.

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Thinking about this, is there any real world benefit to leaving the system powered with ignition off, or is removing the fuse a sensible step?

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I don’t think there’s any benefit to EAS being powered with the vehicle off. I think the designers didn’t want to have the vehicle sitting unlevel when parked so they had it wake up at regular intervals to level itself.

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If you remove the fuse the EAS won't be powered even with the ignition on, or at least not powered enough for it to do anything. Removing the fuse takes the feed off the timer relay so when it wakes up every 6 hours or so to stabilise the heights, it won't do anything. That way, if the car is sitting on the floor after being left overnight, you have no way of knowing where the leak is as it will drop 3 corners to match the lowest. Taking the fuse out, or removing the timer relay stops it from self levelling so you can see which corner is dropping and causing the other 3 to drop to match. Some have suggested taking the timer relay out and replacing it with a standard 4 pin relay so that disables the self levelling and any other movement once the ignition is off. Downside to that is that it also stops you from being able to connect diagnostics (ignition on or not) and can lull you into a false sense of security (well, it can;t be serious, only one corner is dropping so the other 3 must be fine) resulting in an overworked and worn out compressor. Pull the fuse or relay to isolate a leak, then deal with it.

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Excellent contributions from everyone. Thank you!

I realised the 15 percent duty cycle of the compressor amounts to 9 minutes in every hour. I'm pretty sure my car exceeded that by more than double while driving in Cardiff today.

I'm pretty sure the speed triggered changes alone would be over 9 minutes in some conditions. Of course the motorway miles would see it dormant.

I'll pull the fuses and start leak checking both cars.

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Where did the 15% duty cycle figure come from?

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The spec on the OE Thomas pump.

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I’ve used the 4 pin relay in place of the timer relay for several years now. It only takes a minute to swap relays when I need to connect the Nanocom. Maybe now since I’ve sorted all the leaks, the 4 pin relay isn’t needed(?). When the weather gets cold (-10C), my rear new Dunlop Air Bags leak to the bump stops over night. When warmer they don’t leak at all.

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Is the relay everyone is talking about the black one under the passenger seat?

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Yes, left hand seat (driver’s side for me).

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Okay. Left Sooz car (2001 Vogue) in high overnight.

Both rear springs have dropped around an inch.

Comments, please...

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Either both rear springs are the same age and have both started leaking around the same time or your valve block needs a rebuild.

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Okey dokey. I'll get a spare and a kit from X8R.

Springs are only about 3 years and 15k miles.