A/C Recharge Low Pressure Port

[Old M3 Driver]

I bought a R134a recharge kit which needs to be connected to the low pressure port. I found 2 ports on the passenger side:

1) just after the firewall
2) right behind the passenger headlamp, beside the coolant tank

Can someone tell me which is the low pressure port? Thanks.

[cek2]

The low pressure line is the larger diameter of the two lines. If I recall correctly, I charged at the connector at the firewall.
The r-134a system has unigue connectors on the low and high side, so the charging hose should only fit on the low side connector. Get a second opinion. Good luck.

[DesmoBob]

this is by the firewall:

for the wal-mart/k-mart DIY recharge kit, connect it to number 2.
it will not fit on number 1.

[arkie6]

Originally posted by DesmoBob
this is by the firewall:

for the wal-mart/k-mart DIY recharge kit, connect it to number 2.
it will not fit on number 1.

My 95 M3 doesn't look like that. It has one connector by the firewall (similar to that pic) and one behind the passenger side headlight near the AC dryer. I haven't had to mess with mine yet since the AC works so well, but in general, the larger diameter hoses (not necessarily the connectors, but the size of the hose/pipe) are on the low pressure side where you add refrigerant, and the smaller diameter hoses are on the high pressure side of the AC system. Also, typically, the low pressure suction hoses are insulated since these will be cold and any heat they pick up in the engine bay will result in a loss of efficiency. The high pressure side hoses are generally not insulated since they tend to run hotter than ambient and any heat they lose will improve efficiency. When working on different models of vehicles, those are some tips you can use to sort out which AC hose is which.

[Old M3 Driver]

>My 95 M3 doesn't look like that. It has one connector by the firewall (similar to that pic) and one behind the passenger side headlight near the AC dryer.


Thanks for the help although it didn't solve my problem. My 96 doesn't look like the picture above but appears to be similar to Alan's 95. The charger kit hose will only fit the low pressure port.

When I hooked it up, my pressure gauge showed that my A/C has enough pressure (140 was the reading). However, it is still not cold. I checked the fuses and they are fine and the aux fan is running. The compressor seems to be working and when I turn on the A/C switch I can hear it kick in with a slight blip of the tach. Any clues?

[RogRacer]

Originally posted by Old M3 Driver
>When I hooked it up, my pressure gauge showed that my A/C has enough pressure (140 was the reading). However, it is still not cold.

\


That doesn't sound like a low pressure reading. It should be around 35 psi IIRC.

[BS05ZHP]

whoa...

140 would HAVE to be the high-pressure side.

Low side should be around 35 like Rog said.

My 95 and 99 both have the high and low at the firewall.

Whatever you do BE CAREFUL!!! Larger diameter pipe is always low-side.

[RogRacer]

Yeah....whatever you do...DON'T HOOK CAN OF R134A TO THE HIGH PRESSURE SIDE!!!!!!!!!!!!

[Old M3 Driver]

>140 would HAVE to be the high-pressure side.

That was my reaction too and the instructions on the kit say I should see a "green" reading and charge to "blue". 140 is in the "red" zone and that was my suspicion that the firewall port, which is larger in diameter and insulated, is actually the high pressure port.

When I tried to hook it to the port behind the passenger headlamp, which is smaller in diameter, I could not clamp the hose tight enough to get a reading. So, I suspect this must be the high pressure port.

So, my conclusion is:

1) I hooked it up correctly to the low port but my A/C has too much pressure (don't know why)
2) I hooked it to the high port and got high pressure reading, but this doesn't explain why I couldn't hook it into the other port.

Guess I'll leave this to the pros but than you all for your input.

[RogRacer]

Hey...you've got the engine running and the AC is on when you take the pressure right????

[BS05ZHP]

just check the diameter of the pipes.

if it's the larger diameter pipe connection giving you the high reading, I'd let a pro check it out.

[Old M3 Driver]

>Hey...you've got the engine running and the AC is on when you take the pressure right????

Yes I followed the instructions on the InterDynamics kit which called for running the A/C and engine for 3 minutes in a well ventilated place. As soon as I got the high reading, I disconnected the hose and did not even put in any R134a. Just didn't want to hear a big kaboom!

>if it's the larger diameter pipe connection giving you the high reading, I'd let a pro check it out

It is indeed the larger diameter insulated pipe and I will let a pro do just that.

[arkie6]

How long have you owned this car? Has the AC system worked correctly at any time while you have owned it?

Was the AC compressor actually turning when you hooked up the hose/gauge? If the compressor cycled off before you actually checked the pressure, then the low side and high side will equalize. You can check to see if the compressor is actually turning by looking at the center of the pully/clutch. It should be spinning right along with the outer pully if the compressor is turning.

140 psig is an odd pressure - too high for the low side (expect 20-50 psig), too low for the high side (expect 200+ psig unless it is very cool outside), and too high for a static system (compressor not turning - expect 50-100 psig). Could there be an improper AC charge from the previous owner? Maybe a faulty expansion valve orifice (pressure break-down device)? Maybe blockage in the system? If there is any measureable amount of moisture in the system, this can form ice in the suction line and possibly block the flow between the expansion valve and the suction of the compressor. Air in the AC system can also cause abnormally high pressures on both the low and high side ports. Another possibility is a faulty internal compressor valve. This would typically result in an abnormally high pressure on the low side and an abnormally low pressure on the high side. You would also expect to see significant fluctuations in the pressure gauge indication with this condition.

Just a few ideas thrown out there.

[Old M3 Driver]

>How long have you owned this car? Has the AC system worked correctly at any time while you have owned it?

I have owned it for 7 years and the AC has always worked fine. The last time I turned it on was probably 11/02 and it was still adequate but not super cold. It started getting hot again and when I turned it on this time, all I got was air that is no different than if I just turned the fan on.

>Was the AC compressor actually turning when you hooked up the hose/gauge? If the compressor cycled off before you actually checked the pressure, then the low side and high side will equalize. You can check to see if the compressor is actually turning by looking at the center of the pully/clutch. It should be spinning right along with the outer pully if the compressor is turning.

I'll need to check that. I couldn't check the pressure on the smaller diameter port by the passenger lamp since the hose won't clamp to it.

>140 psig is an odd pressure - too high for the low side (expect 20-50 psig), too low for the high side (expect 200+ psig unless it is very cool outside), and too high for a static system (compressor not turning - expect 50-100 psig). Could there be an improper AC charge from the previous owner? Maybe a faulty expansion valve orifice (pressure break-down device)? Maybe blockage in the system? If there is any measureable amount of moisture in the system, this can form ice in the suction line and possibly block the flow between the expansion valve and the suction of the compressor. Air in the AC system can also cause abnormally high pressures on both the low and high side ports. Another possibility is a faulty internal compressor valve. This would typically result in an abnormally high pressure on the low side and an abnormally low pressure on the high side. You would also expect to see significant fluctuations in the pressure gauge indication with this condition

Since I hooked it to the larger diameter insulated port by the firewall and assuming that it is the low pressure port, the high pressure at the low pressure port could be any one of those things you mentioned. I'll get it checked by a pro. Thanks for your input.