RPM occasionally fluctuates with AC Compressor on

[ac427]

Just taking a quick look and not having read all 7 pages. How does the ecu determine the effort needed to maintain idle?

I believe there maybe a separate map for when the A/C is engaged. Would the A/C pressure sensors come into play?

[Bry5on]

Just taking a quick look and not having read all 7 pages. How does the ecu determine the effort needed to maintain idle?

I believe there maybe a separate map for when the A/C is engaged. Would the A/C pressure sensors come into play?

The DME estimates a torque request from the AC compressor, it actually translates this as a number over the CAN bus (or was it D bus?) and uses this to set at least the throttle relative opening target as the revs fall to idle. I don’t think it would cause hunting, unless maybe the AC compressor clutch was slipping? Hmm.

[Andratch]

Well folks, I think I’ve reached a stopping point on this for now. I feel a combination of the Mullet Tune and a 950 rpm idle has made this much more manageable.

I’ll keep trying to find a solution, but it’s now cold out so the AC isn’t needed as much. When it warms up again outside, I may try to convince Kassel to let me swap the DME and see what that does.

[0-60motorsports]

Well folks, I think I’ve reached a stopping point on this for now. I feel a combination of the Mullet Tune and a 950 rpm idle has made this much more manageable.

I’ll keep trying to find a solution, but it’s now cold out so the AC isn’t needed as much.

Wish you the best of luck. Been years for me and never found the solution. Bet you its wiring degradation or ECU component failing....

[dukeofchen]

I haven’t recharged it, but I tested the low/high pressures and they were “in spec”

From my understanding our AC systems don't line up with the standard universal charts out there, because BMW uses a variable displacement type compressor. In my observation, the compressor instead regulates to a low side pressure in the 35psi range, and as refrigerant charge increases only the high side pressure will rise unless it is severely overfilled. In contrast, the standard charts you see show both low/high pressures increasing with ambient temperature. The implication being that if going only off pressures, at high ambient temps the low side pressure will be lower than the chart, and the "happy zone" on the high side might actually be higher than what standard charts would suggest.

The only true spec for our cars is the total refrigerant mass, so a total evac and refill does seem like something easy enough and worth checking off the list, especially if the other options are a deep dive into engine tuning or chasing electrical gremlins.

[Andratch]

Hey Folks, looking to get y'all's thoughts on one more test I may want to run...

I'm curious if there's something in my DME (and 0-60motorsports and the others with this specific problem) that's failing that is causing this issue, and want to isolate it. My car is a '05, my DME was a non-HP from a '03, converted to HP and upgraded to CSL by Kassel. Maybe something went wrong in that process.

I have a local shop that is open to letting me 'borrow' a tested, unmodified HP DME from a car they're converting to S62 for a test. Could I get that, save a backup of the factory tune, then load at least temporarily load an alpha-n tune on it to see if the problem goes away? If that fixes the fluctuation, then I buy that DME from the shop, convert that DME to CSL, put the mullet back in, and go about my merry way. If it doesn't, then I flash the borrowed DME back to factory, give it back.

Is that a viable plan? I wouldn't need to modify the borrowed DME to test it if I use a alpha-n tune temporarily, right? Am I missing anything?

NOTE: I'm holding off on replacing the AC compressor for now because 0-60motorsports and others mentioned that they replaced their compressor/dryer or did vacuum-and-recharge with no change, and it'd be odd if all of our AC compressors were 'quiet quitting' at the same time.

[0-60motorsports]

Hey Folks, looking to get y'all's thoughts on one more test I may want to run...

I'm curious if there's something in my DME (and 0-60motorsports and the others with this specific problem) that's failing that is causing this issue, and want to isolate it. My car is a '05, my DME was a non-HP from a '03, converted to HP and upgraded to CSL by Kassel. Maybe something went wrong in that process.

I have a local shop that is open to letting me 'borrow' a tested, unmodified HP DME from a car they're converting to S62 for a test. Could I get that, save a backup of the factory tune, then load at least temporarily load an alpha-n tune on it to see if the problem goes away? If that fixes the fluctuation, then I buy that DME from the shop, convert that DME to CSL, put the mullet back in, and go about my merry way. If it doesn't, then I flash the borrowed DME back to factory, give it back.

Is that a viable plan? I wouldn't need to modify the borrowed DME to test it if I use a alpha-n tune temporarily, right? Am I missing anything?

NOTE: I'm holding off on replacing the AC compressor for now because 0-60motorsports and others mentioned that they replaced their compressor/dryer or did vacuum-and-recharge with no change, and it'd be odd if all of our AC compressors were 'quiet quitting' at the same time.

Good plan, my DME is with a forum member as well for testing but we dont have a timeline when he will get a chance to check it.

I bet we have a wiring degrading issue or dme component degrading issue going on due to the age of our cars.