New AC Compressor a drag

Isn't the idle speed control supposed to work (more or less) regardless of load? I would expect the throttle/fuelling to be adjusted automatically to take account of the AC clutch being engaged. Doesn't this happen? Did it happen with the old compressor? Perhaps something hasn't been connected properly to tell the engine management system that the AC is running?

Try cleaning out your idle air control valve and throttle body, and make sure the TB stop and idle needle valve are adjusted correctly. AC load will vary depending on ambient temp, the ECU should automatically compensate.

Thanks for the suggestion! I'll check into it this evening once I research what/where "TB stop and idle needle valve" are.

cheers! B

TB = throttle body. The stop is a setscrew, the needle valve is a black knob on the underside.

No knob / needle valve on an 850!!

Tim..

Oops, for some reason I was thinking this was a 240.

Well I've been driving and monitoring the behaviour of the compressor and the near-stalling has stopped happening. The weather has been consistently hot and humid necessitating AC on even the shortest drives and since yesterday the problem seems to have cleared itself up.

Perhaps the computer has reset itself or "learned" the compressor load to adjust to it. Or perhaps the throttle body or idle air control had a spot of schmunge that has burned off/blown away/melted. Or perhaps it's Volvo gremlins at work while I sleep.

If the problem comes back I will inspect more closely. I know the engine was power-washed after the work was done... maybe it was just something that got wet that needed to dry out.

Thanks for your quick advice (again). blurp

Actually, it's not out of the question that the computer has "learned" how the engine should act. Your throttle openings (and thus air flow or manifold pressure, whichever is measured on your engine) are different now with the AC on. It usually takes a hundred miles or two for the kinks to work themselves out as the ECM remaps. This is especially likely if the weather is significantly different (hotter!) than it was when the AC failed in the first place or if several hundred miles have passed since it worked last.

Well there has been trouble-free operation except one incident a few days ago when I was accellerating up a hill (not just idling) and the engine started giving out like the fuel pump was failing. I flicked off the AC switch and full power was restored and everything was fine. I turned the AC back on a few minutes later and again no troubles since.

Weird.

blurp

Basically, the way this works is that there are combinations of airflow (or manifold pressure) and temperature. Think of a grid with airflow values across the top and temperature values up and down the left side. In each box is a value. The value represents the starting injector pulse width for that combination of temperature and flow (or pressure). The initial value is adjusted using feedback from the O2 sensor(s). The O2 sensor feedback loop is fairly fast but not instant. As the adjustment is made to the injector pulse width, the value in the box is changed.

This is sometimes called the "mapping" of the engine's operating conditions by the computer.

(This is an oversimplification. In reality RPM is also considered. But it suffices to illustrate the concept.)

With the AC on, the throttle opening (and thus the airflow or pressure) changes. This puts you into different boxes that might not be used during operation without the AC very often. Those boxes now contain the wrong initial values for operation with the AC on. The computer will adjust but it has to hit all the boxes you'll notice before the car seems to run well.

This also happens after an ECU reset. Typically (for me, anyway) this takes a couple of days and over a hundred miles. YMMV...but if this is what's happening (I can't be sure at all), then it should continue to improve and eventually run right. If it takes over a week or 300 miles, there is most likely a real problem of some sort and not just the computer remapping.

JRE

Hi Blurp, What are the steps to replacing a A/C Compressor? My clutch is not kicking in and I need to replace the Compressor. I did the blower motor last week, so now I think I can do about anything. Thanks, Joseph

Wow, that is both fascinating and informative. It appears that what has been happening is exactly as you describe it. Since that last posting I have had no stalls or problems. The last time was under a somewhat unusual load condition (a steep hill climb from a virtual standstill) and it makes sense that the expected value in that part of the "map" would not often be called on.

By remedying the problem by shutting off the AC, however, have I somehow interrupted the re-mapping process by returning conditions to match the expected value rather than allowing the computer to "learn" the new conditions?

Like I said it seems to be running great now but my wife is a bit hesitant to drive the car in case it starts "acting weird".

Thanks for the descriptive info! blurp

Sorry, Joseph, I didn't do the work myself. I would say, however, that you need to be VERY cautious with this job because even a seemingly discharged system may be hiding a very highly pressurized section (400+psi) which can release somewhat violently as you disconnect the compressor and spray you with oils, dyes, and refrigerants, all of which are likely pretty toxic. You don't want to get that stuff in your eyes.

Also, once you get it all together you need to have the proper AC vacuuming and charging equipment (you can't just do it with a few cans of RedTech). In all I think this may be one of the least "do-it-yourself"-friendly jobs you can tackle.

Good luck, blurp