Zwoobah

Yesterday I recharged my AC with the help of one of the techs here at work. We evacuated 2.55lb of refrigerant - it's only supposed to hold ~1.71lb, so it was clearly way overcharged for quite some time. We recharged it with 1.71, and it still was not cold, so we evacuated and rercharged again, and put in 1.81lb, which made it somewhat colder.
It worked OK for the 1st 10min of my drive, then no cold air. I ran the diagnostics this morning:
No errors for the high pressure switch or the cycling of the high pressure switch
The compressor will kick on occasionally at low speeds, for 1-3sec max, then back off. At highway speed it rarely kicks in at all, and when it does it's also for only 1-3sec. The air is never significantly colder than ambient.
When the compressor is off, channel 52 shows it being switched off due to the kick down switch - which should only switch it off for 12sec max. Does this point to the kick down switch being bad? Where is it and how do I replace it?
Initially I assumed the high pressure switch was nuked by the long term overcharge, but it's not showing any errors.

kday

I've never come across a reference to a kick down switch. That's more of an automatic transmission thing, isn't it? I believe the climate control unit gets the tachometer signal from the ECU, it might use that.

Can you get your tech friend to put a gauge on the charge port while the system is running?

I guess you could try disconnecting the high pressure switch that cuts the compressor (left side of radiator with red connector as I recall). Probably not wise to leave it disconnected though...

Zwoobah

<a href="https://www.audiworld.com/tech/int5.shtml">scroll </a>down to the bottom to see the chan 52 diagram

"c2 Kick-down switch, compressor off for 12secs max."
I'm assuming this is the switch that cuts the compressor when system pressure exceeds 400psi. So, potentially the system is over 400psi pretty much all the time.
We were thinking that the expansion valve may be blocked, resulting in constant system pressure over 400psi. Unfortunately, there is no way to easily determine this since these cars are not equipped with a low-side connection. The pressure gauges did show a slight "bump" down in vacuum pressure toward the end of the evacuation process, but other than that there are no obvious expansion valve symptoms.

kday

I don't think that document applies 100% to the S-cars. The climate control was similar for a number of years but it did change during that time.

There is a schrader valve on the high pressure switch, FWIW. The bentley says to replace the O-ring and lube it with PAG oil on removal. If you pulled that switch (or just disconnected it -- I don't remember if there is a connector) and the problem went away, that'd be a pretty good clue...

Zwoobah

FYI, don't try to interpret CC diagnostics before your morning coffee...
Anyway, so now that I know the low pressure switch is causing the compressor to shut off, what can I do about it? I'm told the switch is on the compressor and can be replaced, but should I check anything else first? Could this be why the previous owner overcharged the system - to build up pressure enough that the faulty low-press switch wouldn't trip?

kday

There are two electrical connectors to the compressor -- at least on mine: an RPM output and a clutch engagement input.

I'm not sure where the low pressure switch is. Maybe by the restrictor orifice or whatever it's called near the evaporator...

kday

Switch opens (compressor OFF): 1.45-1.6 bar (21-23 psi)

Switch closes (compressor ON): 2.9-3.4 bar (42-49 psi)

Removing and installing

- Remove plenum tray.

- Remove electrical connector and unscrew low pressure switch -F73-.

Refrigerant system remains closed (Schrader valve under switch)

- Install new O-ring -A- (8.9 mm x 1.8 mm), lubricate with refrigerant (PAG) oil and ensure proper seating in groove.
- Torque switch to 5 Nm (44 in lb).
- Install plenum tray.

repeater

If it didn't do it before the re-charge, then it's not a cycling switch.

Zwoobah

Awesome. Thanks Kevin

Zwoobah

it didn't do it before, but it had an extra 3/4lb of refridgerant in there. I'm thinking maybe the low pressure switch has been faulty for some time, so the previous owner assumed the system was low before and really overcharged it. potentially that forced the faulty switch to work.
We evacuated and charged it twice. I am 100% sure that the right amount of refrigdgerant is in the car, I watched the reading with my own eyes. There could be something else wrong with the system causing the pressure to drop, but it's not the refridgerant level.