Charging AC system that is severely undercharged and only cools under active RPM's

[Durden]

I'm sure somewhere deep in a thread this has been answered but I cannot find it.

I realize the E39 AC systems are interesting and perhaps I'm overcomplicating this. I have a lot of experience in handling AC systems from my E28 days but I digress. Issue is that the AC only works when I'm driving (and not too well at that). I've diagnosed it as a low system because everything seems to be working as it should (clutch, secondary fan, sensors) and I'd like to charge it to see if that does what I believe it should. Only issue is....at idle the compressor isn't kicking in. Or at least the pressure isn't changing from static. Not until I get in and rev to around 1600 (540I) at which point the high and low start looking a little more normal.

Anyway - can I simply open the can into the non-running system as many other car owners do? I'd really prefer not 'jump' the compressor but I also want to do this the correct way. Ready to hear any thoughts on the best way to do this.

[Mike WW]

I believe on E39s there is a low side switch, which prevents operation if the system is too low on either pressure or charge. I'd figure out which one and jumper it for testing. Be sure, you don't want to jumper the high side and risk blowing things up. That would account for the symptoms. On the other hand, you should be able to hear and see the compressor kick in, grab someone to help if you need. I don't have an explanation for why it starts working at higher RPM unless perhaps there is a compressor problem.

Dumping another can if unneeded into the system isn't going to make the Antarctic hole much bigger, but with the lack of a sight glass like earlier cars I'd probably try to throw some gauges on there before I did so. Basics, don't dump it into the high side and start the car with the can still connected.

[Durden]

Thanks for the response Mike! I remember gleaning a lot of good info from you on mye28.com

So I've got the gauges on and am using the low-line for adding (though I've yet to add anything or try till I get more info) so we're good on the basics. I was attempting this on my own so no one to run the AC button while I'm on the engine side; but I moved the gauges to where I could see them in the gap under the hood which is what allowed me to measure the oddness. To be specific:

- Without car on gauges read a static pressure around 130.

- With car started nothing changes (AC not on)

- With AC button pressed nothing changes still. I wait a good 5 minutes to see if some sort of process needs to kick in. Nothing ever happens.

- I rev the engine and as soon as it hits 1500 or so I see immediate movement on both the gauges and on my vent thermometer. Once I let off the gas the gauges start to move the other direction and the thermometer gets hot again :-/

I'll give it another day or two to see if anyone else has something wise to add. I certainly appreciate it. I'll also look into jumping the low pressure switch.

[jmc1590]

I just spent a bit of time chasing compressor issues on an E53 X5. Similar system, with a few minor differences. If I'm not mistaken, you've got the combo 3-wire pressure switch attached to the dryer. Older systems used a 4-wire unit that had a separate high & low switch, which could be "bypassed" for troubleshooting. In my case, the 3-wire unit was faulty, occasionally providing valid values, shutting down the system more often than not. Replacing it resolved my issues. (Note - there are a handful of references stating that the E53 AC pressure switch has a check valve under it, allowing for easy replacement. I can say that for a 11/2001 build date, this is not the case! If you need to replace your pressure sensor, evac the system 1st!)

Do you have access to INPA or DIS? The ability to turn on the compressor via software, as well as read the values the pressure sensor was delivering allowed me to maintain progress in tracking down the fault.

As far as manually jumpering the compressor clutch, as long as you have a set of guages attached, and can monitor your high-side pressures, I think you'll be ok. 130 static sounds reasonable, depending on abient temperature. What kind of high / low pressures were you getting once it engaged?

James

[geargrinder]

Read the codes read the codes read the codes. The IHKA (HVAC module) may very well have some very clear ideas about why it is not working. Good stuff from jmc above... if you have INPA you can force things on and off as well as read the codes, that would go miles towards helping your diagnosis I think.

It may very well be the clutch is crapped out and only partially grabbing so you're getting a gear-reduction effect, and that's why revving makes something happen? Could also be compressor failure or switches etc. but the best thing is getting some software hooked up to the car...

[Ed CT]

- I rev the engine and as soon as it hits 1500 or so I see immediate movement on both the gauges and on my vent thermometer. Once I let off the gas the gauges start to move the other direction and the thermometer gets hot again :-/.

I had the same symptoms on my '98. Adding a slight charge did not help. Replacing the compressor was the fix. IIRC, the compressor started making some slight non-normal noises, too. If you decide to R&R the compressor, also R&R the dryer, and maybe the expansion valve as well. While you're doing the expansion valve, the Final Stage Unit is right there, so, ... And also, if your heater fan/ motor is chirping, here's your chance. And the heater core is nearby, too. It never ends with these E39s! Seriously, tho, just the compressor & dryer.

[Durden]

Hello again!

Firstly - appreciate the great advice. Due to jury duty and Memorial Day plans I've just now been able to dig into it.

I hooked up INPA and - outside of a surprisingly high number of Jet Heater faults - no issues there.

I do however have an issue of a different nature. INPA seems unwilling to allow me to override the compressor. I get an ApiCheckJobStatus: Job-Status error which I've attached. It gives this same message for several of the other activations as well. The heater valve and fan I can activate just fine, but those Digital Commands and Compressor Override are giving me fits.

Sort of at an impasse until I get that figured out of find a different piece of software to perform these overrides. The upside is...I had no idea INPA could be so much fun re: IKHA. All the sensor outputs and other items are great data points to have.

Thanks again for the previous - and any future - guidance

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[Durden]

Giving this a pre-weekend bump as I'm hoping to find some time to hook things up again in the next couple days.

I'll also post the different pressure numbers I was receiving to see if I can determine anything without having to manually kickoff the compressor.

[edjack]

The E39 HVAC system is charged by weight, not pressure (unless you can read and apply psychrometric charts). There's a sticker on the underside of the hood that specifies the quantity.