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can-do 07-07-2002 11:28 AM

a/c still working for no benefit on '86 300E
Greetings All,

Although I mentioned it before on another post I have kind of ignored the problem with to many other things to do in my spare time. This is quite a weird problem for an a/c system to exhibit. I can push the button for the a/c to come on and the compressor runs fine, and doesn't cycle off but there is absolutely no cooling air coming out of any interior vent. The center vents blow air, but that is ambient temp as well. Looking in the sight glass it has bubbles starting up but quickly clear out and the freon appears mostly clear to off yellow in color. When touching the intake and discharge lines coming off the compressor, there seems to be little difference in temp. After going through the archives it seems like any possible problem could be used as an excuse as to why this unit is not functioning. Any way to troubleshoot the unit to narrow down the selections of possible problems?
Thanks for any input or ideas or even related incidents that you can provide.


Gilly 07-07-2002 12:10 PM

First few guesses would be either a worn out but non-seized compressor, or a bad expansion valve. The system should be hooked up to a manifold gauge set and the high and low pressures monitored. Other possiblilty would be that you are only seeing the pulley turn and the compressor clutch isn't actually engaging the pressure, although you seem to think you see some flow or movement in the receiver-drier window, whch would indicate the compressor is in fact doing "something" although a worn compressor is still a possibility.

can-do 07-08-2002 01:16 AM

Pretty sure the clutch is engaging the compressor
Greetings Gilly,

Pretty sure the compressor was turning as freon did show movement in the sight glass and there was a slight temp difference between the intake and outlet of the compressor lines. I had given the idea of a worn compressor some thought, but that one to seems unlikely because it only saw use most likely three months out of the year. I also gave thought to the expansion valve not functioning correctly. Any ideas on how to tell if the valve is operating properly? As far as taking pressure reading of the system, do you know what pressure I'd be looking for? If you do let me know and I'll hook up my gages to the system and see what they are.


Gilly 07-08-2002 01:19 AM

Rough numbers with compressor running on a hot day, maybe 60 or so on the low side and a few hundred psi on the high side.
ps here's a few scenarios i can think of, if there are any other posiblities, others can chime right in, I haven't personally had to much problems in this regard:
If the expansion valve were blocked, you'd have very high high side and little or no pressure on the low side. If the expansion valve is stuck wide open, i'd assume both the low and high side would be near equal. If the compressor "compression" were low (worn out compressor) I'd assume both pressures would be too low, although there would be a difference, the high side should still be higher than the low side.

can-do 07-08-2002 01:34 AM

Expansion Valve result

And if the expansion valve wasn't functioning properly, what would the result be?


Gilly 07-08-2002 01:45 AM

I believe covered both possibilities in my previous post.

can-do 07-08-2002 01:57 AM

Sorry, no PS when I read it the first time
The ps wasn't there the first time I read your post.


Gilly 07-08-2002 08:27 AM

Yeah, I know. I'm always pulling that stunt, don't know why I do that. Better late than never I guess.

can-do 07-08-2002 08:28 PM

Here's the numbers Gilly

I hitched up the gauges to the system and here's what I got. With engine at operating temps and compressor on I had 160 psi on the high side and about 20psi on the low side. After shutting off the compressor I had about 140 psi on the high side and about 40 psi on the low side. The temp outside was about 90-91F Any ideas what to look at? The suction side seems a bit low, any thoughts on this? Could I still be low on freon even if the sight glass shows very little bubbles? I was going to tap on the expansion valve but didn't find it where I thought it should be, guess it's well hid. The temps on the compressor pipe intake and outlet. Hot and luke warm.

Any suggestions on the problem area?


Gilly 07-08-2002 08:48 PM

Maybe low on freon. Someone possibly has jumped the low pressure switch to make the compressor engage, or the low pressure switch is bad, or a bad Klima relay is making the compressor run. Maybe the freon level in the tank is so low you can't see the freon, making you think it's full.

can-do 07-08-2002 09:28 PM

Testing it again
Greetings Gilly,

I went back out to the car gauges still hooked up and decided to remove the pressure switch wire to ensure I was seeing freon in the sight glass. Well, each time I removed the wire and replaced it, the suction pressure continued to drop till I basically had no suction pressure on the gauge and about 110-125 psi head pressure. Got any more ideas as to what's going on here? Freon was evident in the sight glass, it wasn't empty.


can-do 07-09-2002 09:49 PM

Readings, and update on a/c situation
Greetings All,

This thread is becoming a mystery novel. Today I went out and hooked up the gauges again after checking all the sensors inside, outside, evaporator, etc. for proper ohm readings. All checked out within spec. Here are some of the pressure readings I got and all are psi not bar.

Before start up on cold engine

low side 78psi
high 85 psi

at start up

low 11
high 135

operating temp

low 10
high 145

compressor off engine running

low 37
high 125

compressor running 15 minutes

low -20
high 125

filled 12 ounces of freon into system and low pressure came up to
45-60 psi and high side to about 230 psi cool air present for test drive but only last about 15 minutes.

Vehicle shut off for about an hour and started back up with low side around 45psi on start up and high around 125 psi fan was bypassed to continue running and temp in vehicle cooled for about 15 minutes till suction pressure dropped to about 15 psi and high side to about 140 psi, bubbles still evident in sight glass.

Can someone tell me what the heck is going on here? Is there a leak I overlooked or what component is malfunctioning? I'm just trying to cool off the wife so please have a little mercy here. I can't believe the compressor has gone tits up but think the aux fan switch probably has but the overall temp has remained pretty close to 85-90 c so there really shouldn't be a problem. Tell me your thoughts I'm all ears.


pmizell 07-09-2002 10:30 PM

You need to take high/low side pressure readings at high idle -- between 1500 and 2000 rpms should do it. Have you been doing this?

Also, you shouldn't add freon to an idling engine......... rev it up and let the suction valve feed it in.

Good luck


'91 300E, 215K miles
"Smoke Silver"
Palomino interior

bdlefferts 07-10-2002 01:43 AM


Just wanted to let you know I have the same exact symptoms and problem on my wife's 1991 190E 2.6L.

I don't have any guages to read the pressures on the system but the compressor runs constantly, no cold air, high side lines are hot and low side lines feel like ambient temperature. Also bubbles in the sight glass.

I haven't been able to figure it out either. I know this reply doesn't help you at all. But i feel for you...

Keep me posted and I'll do the same,

jeffsr 07-10-2002 07:08 AM

You need to add more R whatever (12 or 134a) to you system. It is normal for high and low sides to be nearly equal after system has been off for a while and cool. If you are pulling negative pressure on the suction side, you need to continue charging the system. At operating temp, the high side should be at least 250psi or better. Low side should be at least 35-45psi (with compressor running). If you are losing pressure, have a leak. Depending on the state of the refrigerant charge, the ambient temp and the system temp, you may see bubbles in the sight glass or you may not. Remember, if system is hot, pressures will be higher on both sides and vice-versa. Overall, you pressures are way too low. Doubt the compressor is failing. There is a good differential between high and low sides, so expansion valve seems to be with you.

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