Any way to test the pressure switch on the evap/drier?

[TimGinCentralNJ]

Background:
My 134a-retrofitted A/C has always had the habit of cutting out on really hot days, though it works great when the temps are around 80 degrees or below. Always thought it might be some kind of temp/pressure-related issue. Recently, it's stopped working altogether.

With the help of a friend (who's a helluva lot better at reading elec diagrams than I am), we've established the following:

-System pressure is a little over 100 psi static on the high side @ 70-80 degrees ambient temp.
-Fuse #9 (hot at all times) leading to compressor control relay tests "good".
-Compressor control relay itself and all associated fuses tested "good".
-Fan clutch is "good" and aux fan low & high both work when they're s'posed to. Engine temp never gets higher than 12-noon.
-When I jumper the black/gray wire coming off the pressure switch (the one which leads to the compressor) to a 12-volt lead, the compressor springs to life, engages the aux fan, and provides 45 degree air at the vents.
-If I do this long enough, the aux fan kicks into high speed, as I gather it should. I feel like I've tested all the "right" things.

Could it be anything other than the pressure switch on the evap/drier? Do I have to evacuate the system to test this thing?

Any ideas would be appreciated,

Tim "sweaty bits" Goodell
'91 535iM

[Bellicose Right Winger]

Good analysis Tim and it could be a bad switch. But I think you might want to check a few more things to be certain. Have you checked for continuity through the pressure switch with an DMM? Resistance should be near zero. Any corrosion on switch terminals? Instead of putting 12 volts on the black/grey wire at the pressure switch connection, what happens if you install a jumper at the connection? If AC runs w/jumper then 1) switch is bad or 2) system operating pressure is less then 1.5 bar (22 psi) or more then 30 bar (435 psi)...based on my '93 ETM anyway. If system doesn't run with jumper installed, and there is a problem with 12 volt supply or AC controls or the high temp cutout on compressor is preventing operation. The high temp cutout isn't the problem in your case since the compressor runs when you put 12 volts on the black/grey wire to the compressor. I'd try to put a set of gages on the system to make sure low and high side pressures are normal under high ambient temp operating conditions. Yes, you need to discharge the system to change the switch.

Paul Shovestul

Background:
My 134a-retrofitted A/C has always had the habit of cutting out on really hot days, though it works great when the temps are around 80 degrees or below. Always thought it might be some kind of temp/pressure-related issue. Recently, it's stopped working altogether.

With the help of a friend (who's a helluva lot better at reading elec diagrams than I am), we've established the following:

-System pressure is a little over 100 psi static on the high side @ 70-80 degrees ambient temp.
-Fuse #9 (hot at all times) leading to compressor control relay tests "good".
-Compressor control relay itself and all associated fuses tested "good".
-Fan clutch is "good" and aux fan low & high both work when they're s'posed to. Engine temp never gets higher than 12-noon.
-When I jumper the black/gray wire coming off the pressure switch (the one which leads to the compressor) to a 12-volt lead, the compressor springs to life, engages the aux fan, and provides 45 degree air at the vents.
-If I do this long enough, the aux fan kicks into high speed, as I gather it should. I feel like I've tested all the "right" things.

Could it be anything other than the pressure switch on the evap/drier? Do I have to evacuate the system to test this thing?

Any ideas would be appreciated,

Tim "sweaty bits" Goodell
'91 535iM

[TimGinCentralNJ]

Good analysis Tim and it could be a bad switch. But I think you might want to check a few more things to be certain. Have you checked for continuity through the pressure switch with an DMM? Resistance should be near zero. Any corrosion on switch terminals? Instead of putting 12 volts on the black/grey wire at the pressure switch connection, what happens if you install a jumper at the connection? If AC runs w/jumper then 1) switch is bad or 2) system operating pressure is less then 1.5 bar (22 psi) or more then 30 bar (435 psi)...based on my '93 ETM anyway. If system doesn't run with jumper installed, and there is a problem with 12 volt supply or AC controls or the high temp cutout on compressor is preventing operation. The high temp cutout isn't the problem in your case since the compressor runs when you put 12 volts on the black/grey wire to the compressor. I'd try to put a set of gages on the system to make sure low and high side pressures are normal under high ambient temp operating conditions. Yes, you need to discharge the system to change the switch.

Paul Shovestul

[Bill R.]

could very well be the problem that causing whats happening now but I can almost guarantee that the cutting out on hot days is caused by the high pressure switch... which leads me to my next question. When the retrofit was done was it done correctly with the bmw conversion kit which has the replacement filter drier and the replacement high and low pressure switches since the pressures are different on r134a ? If the new conversion pressure switch was not used then that will cause it to cutout at high temps. If it wasn't switched over then you'll need to get that switch anyway.
Sorry my mistake, the primary reason that the switch was redesigned for r134a was not because of pressures it was because the seal materials in the switch are not compatible with r134a. The pressure cutout on the high side looks like its the same as the r12 pressure but since the 134 system with the r12 condensor will operate at higher pressures its much more likely to shut down on high pressure on hot days.

[ryan roopnarine]

if callen isn't gonna read this thread soon.....does anyone know how much the retrofit kit is from the dealer, or perhaps bma?

[winfred]

i've never had a problem using the stock switches on e32/34 conversions, the high switch cuts on the fan around 275 and off around 225 if i remember right, cars with seiko compressors have a temp switch on the body of the compressor. i always change the oil to ester and replace the dryer, bill what's your opinion on pag? i've seen a few books say it corrodes copper when combined with r134a, fact or ********?

If the new conversion pressure switch was not used then that will cause it to cutout at high temps. If it wasn't switched over then you'll need to get that switch anyway.

[Bill R.]

fact, only what actually happens is it causes copper to move from one surface to another... such as it removes it off the copper tubing in the condensor or evaporator and plates it right onto the valves, pistons,cylinders and bearing surfaces of the compressor... During one of my tired of dealing with automechanic customer phases I worked at a shop that rebuilt compressors for a/c from 2 ton up to 500 ton onsite and in the shop...we did a lot of 125 to 250 ton 12 cylinder compressors and copper plating was extememly comon on systems that weren't kept clean and all the moisture was not evacuated with new filter driers changed often...It looked exactly like you had had these parts plated at a copper plating shop
except on these it would build up to the point where seizing would occur and other wear related problems since copper isn't a good bearing material in itself.. this was caused by the formation of acids in the system and 134a is supposed to be far worse at this than r12 ever was.. Sometimes when you pull a compressor off a newer 134 equipped car you'll notice copper plating if you pull the service plate off and look inside..Here's some stuff I had posted before...

i've never had a problem using the stock switches on e32/34 conversions, the high switch cuts on the fan around 275 and off around 225 if i remember right, cars with seiko compressors have a temp switch on the body of the compressor. i always change the oil to ester and replace the dryer, bill what's your opinion on pag? i've seen a few books say it corrodes copper when combined with r134a, fact or ********?

[Bellicose Right Winger]

Bill, I wonder why the retrofit kit changes the pressure switch. ETK says my '90 w/R12 and my '93 w/R134 from the factory use the same switch, PN 64 53 1 390 070.

An incomplete evacuation during changeover would cause high side pressure problems due to non-condensables.

Paul Shovestul

could very well be the problem that causing whats happening now but I can almost guarantee that the cutting out on hot days is caused by the high pressure switch... which leads me to my next question. When the retrofit was done was it done correctly with the bmw conversion kit which has the replacement filter drier and the replacement high and low pressure switches since the pressures are different on r134a ? If the new conversion pressure switch was not used then that will cause it to cutout at high temps. If it wasn't switched over then you'll need to get that switch anyway.

[Bill R.]

material on the switch that fails not the difference in pressure as the reason that bmw changed it..It was my mistake in thinking that they would have raised the cutout pressure. Condensable's can very easily cause high pressure problems as well, since I see very few a/c automotive shops that do anything even remotely resembling an adequate job of evacuating a system on cars..most vacum it for 30 minutes or less with some cheesy air power vacum pump that won't pull 20 inches much less 500 microns
Also on r134a its very easy to overcharge the system which will also cause it to shut down on high pressure when hot.

Bill, I wonder why the retrofit kit changes the pressure switch. ETK says my '90 w/R12 and my '93 w/R134 from the factory use the same switch, PN 64 53 1 390 070.

An incomplete evacuation during changeover would cause high side pressure problems due to non-condensables.

Paul Shovestul

[winfred]

you talking bout this turd!
http://www.harborfreight.com/cpi/cta...temnumber=3952

[QUOTE=Bill R.]most vacum it for 30 minutes or less with some cheesy air power vacum pump that won't 20 inches much less 500 micronsQUOTE]