AC has a loud short bang at startup then cools normally

[sdgusman]

96 328i sedan Air Conditioner has loud bang at startup, then cools very well.

My AC wasn't cooling too well so i had a neighborhood shop do a recharge.

Afterward recharge, was told low and high pressure gauges looked good, system blew quite cold BUT a very loud bang when I start the AC. I don't even start the AC while driving right now. I leave it on and only comes on when cranking the car. Much quieter noise during engine and ac startup at the same time.

Mechanic troubleshot then replaced compressor and accumulator. After he pulled a vacuum and recharged, had warm air. Gauges looked normal. Turned out he put 2/3 air and 1/3 freon for the recharge. Told me the gauge set he had was bad. He then took it to another shop and ran a vacuum on it and refilled freon by weight. AC is blowing cold again BUT the loud bang at startup is still there. Hi and lo gauges read normal range.

Any ideas what could be causing this loud bang at startup?
Could it have been overcharged both times and this is the culprit?
Is there a high pressure relief valve on the compressor that is releasing?
Could air still be in the system causing this?
Could the accumulator have gotten messed up? First time on the initial charge when the bang started; and then the NEW accumulator being messed up after install because he recharged the system with faulty equipment.
Should i have another vacuum and recharge done by weight hoping this eliminates the bang at start up.

Thanks,

Gus in Vegas
USN retired

[bohica2xo]

BMW used a hydraulic tensioner for the belt in that car.

Open the hood, and locate the tensioner. Have someone start the car, then turn on the A/C while you watch the tensioner.

[sdgusman]

Thanks for the reply. I was wondering if I should replace the belt, hydraulic tensioner and pulley before i assume anything wrong with the shop recharge to spec. I did replace all 3 of those in 2014....will definitely have a look at it when the ac is being turned on.

[bohica2xo]

Gus:

You asked about Sin City. I have never heard a bad thing about them, and I think Chris keeps about half of the E30's in Las Vegas on the road.

If you replaced the tensioner 6 years ago, it might be time. Sometimes aftermarket replacements just don't last as long as OEM.

[sdgusman]

Had Sin City BMW look at the tensioner, belt and other things that could be moving around when the AC is under load. They determined it was the compressor clutch engaging with a bang when I start the AC. Mentioned if the PAG oil is off in the compressor, it can cause this. I specifically asked about the tensioner, and they said both the belt and tensioner appear to be operating normally when I start the unit.

The compressor itself is brand new; so I will be returning it and putting another in. I'll post back in a few weeks when the new compressor gets installed.

[bohica2xo]

Too much oil (or a really tight compressor) could cause a high torque issue at startup - which would cause the belt tension to go sky high & bang the tensioner stop.

If the clutch itself is actually making a banging noise, it is not right. So yes, swap it out before it does something worse.

[sdgusman]

Mechanic swapped out compressor, pulled vacuum and recharged. Blowing cold. Still have loud engagement bang at start up.

I'm thinking about swapping out the ac hydraulic tensioner, pulley and belt? Any thoughts?

r,

Gus in Vegas

[bohica2xo]

Well, 2 compressor, same tensioner. Same noise.

I hate to just throw parts at things, but given the price of a tensioner assembly (under 50 bucks) and the ease of replacement ...

Still sounding like the hydraulic tensioner leaks down over time when not in use. Once it is cycling it is ok.

[sdgusman]

According to shop I ended up taking it to, it was because of too much PAG oil in the system. They evac'd and recharged system again, no more problem.....I did have to pulley, tensioner and belt replaced as well.

[ice-n-tropics]

Bang is liquid slugging.
hotrodac