AC Wire Cut. spliced but still no AC!

[drewusmaximus]

Hi all. 1995 525i

when I purchased the car I noticed the wire that connects to the AC compressor was cut. I managed to splice the wire but, still not getting cold air. Sound like the compressor isn't kicking on. Do I need a new compressor or is there something else I can look at that might prevent the compressor from kicking on?
thanks in advance!

[Tre0Tre34]

If there is not sufficient pressure from the freon in the system, the compressor will not kick on.

I would start by checking this

[GregT53]

Just get a refill kit from a parts store and add refrigerant. I have used a product called A/C Pro made by STP with good results. I think it was about $30. It comes complete with an easy hose connection and gauge. Make sure it's compatible with your system which should be R-134A. Low refrigerant causes the compressor to "lock out" as a fail safe because the refrigerant lubricates the compressor. The recharge should last a good, long while (like a couple of years at least), otherwise you have a leak.

[drewusmaximus]

Thanks guys! I have a refill kit with a gauge and it shows the pressure is low. 15%. The process I know is when the compressor kicks on, you can add refrigerant because the pressure drops when the compressor kicks on, allowing you to add refrigerant but, the compressor doesn't kick on. So, is it ok to add refrigerant when the compressor doesn't kick on? Do I keep adding freon until the compressor kicks on?
Thabks in advance!

[ronald1855]

I would add freon with the engine off first until it doesn't accept more. Then start the car and the ac, then add a second time until you reach the correct pressure.

Adding gas with the engine off is the same as adding gas with the engine running but with the compressor off. The only "pump" in the system is the compressor.

- - - Updated - - -

The compressor will probably run with a certain minimum pressure like maybe (purely guessing here) 70% of the rated pressure. There must be some kinda valve that opens/closes when the pressure falls below this, and cuts the power to the compressor automatically. The reverse should happen when you add gas with both the engine running and ac activate but compressor not runinning right now.

But I would add gas first until it won't take more, then remove the kit, start the engine and compressor, and if it runs then add more gas until the right pressure is hit.

[mrgraybeard]

The usual behavior for an AC compressor with low freon is for the clutch to engage briefly (like for one second) and then to disengage when the pressure switch fails to detect an adequate charge. It'll re-engage seven or eight seconds later, run for a second and disengage again. That cycle will repeat as long as the AC switch is on and the compressor is undercharged. That's how the system protects the compressor.

If the compressor isn't engaging at all, there's likely another problem with the system that prompted a previous owner to cut the wire. Bad compressor clutch, perhaps.

[Layne]

The usual behavior for an AC compressor with low freon is for the clutch to engage briefly (like for one second) and then to disengage when the pressure switch fails to detect an adequate charge. It'll re-engage seven or eight seconds later, run for a second and disengage again. That cycle will repeat as long as the AC switch is on and the compressor is undercharged. That's how the system protects the compressor. If the compressor isn't engaging at all, there's likely another problem with the system that prompted a previous owner to cut the wire. Bad compressor clutch, perhaps.

That's only true if it's low but not completely out. The suction side of the compressor pulls the pressure down below the switch setting when the compressor runs for that 7-8 seconds. It won't come on at all once the resting pressure is below the switch setting.

Most likely scenario is that it has zero pressure and will immediately leak out if you add more.

[mrgraybeard]

That's only true if it's low but not completely out. The suction side of the compressor pulls the pressure down below the switch setting when the compressor runs for that 7-8 seconds. It won't come on at all once the resting pressure is below the switch setting.

Most likely scenario is that it has zero pressure and will immediately leak out if you add more.

Interesting. I guess I've never seen a system with a total leakdown -- even some pretty boogered ones had some residual pressure.

Maybe the OP could try borrowing a vacuum pump from Autozone and see if the system holds a vacuum. If it doesn't, there's obviously a gross leak somewhere.

[Tre0Tre34]

^ or possibly a boogered pressure switch?

[ronald1855]

OP reports pressure at -85% of normal.

[MIKYZZ4]

If not running at all, isn't compressor fuse protected or triggered by a relay?

[drewusmaximus]

I'll check the fuse and relay, if any. Then try to fill while engine is off.

[ross1]

I'll check the fuse and relay, if any. Then try to fill while engine is off.

Fuse 29(7.5a)serves the compressor relay, #9 (15A) serves the compressor clutch, both are in the driver's side E-box as is the relay.
If both are good try jumping the relay, connect a jumper between the sockets that correspond to pins 30 & 87 on the relay. There ought to be a diagram on the relay itself. If the compressor does not come on you either have a wiring problem between the relay and compressor or a duff compressor clutch, perhaps the reason it was disabled before.
If the compressor DOES come on then you do not have an "on" command from the control system. Perhaps low refrigerant.
Several conditions must be met before the DME will command the compressor ON, or more specifically not off. A system pressure of at least 1.8 bar but not exceeding 30 bar, ambient temp of at least 9*c and engine NOT at full throttle.
A wiring diagram will help you understand how the current flows.
If all conditions and met and sensors functioning and still no juice to the clutch you could have a problem with the IHKR, the controller on the dash.
Start with the simple stuff, it usually is just that.
You mention a 15% charge. Not sure what sort of gauges you are using but if that doesn't correlate to the pressures mentioned it's not going to come on. You need to see 30 psi or so.
Adding gas with the engine off is a fool's errand, suggested by same.