A/C Back Pressure Causing Lockout? - PeachParts Mercedes-Benz Forum

emerydc8 · #1 05-22-2008, 01:48 AM

When it is hot outside (over 100 degrees) and my car sits in the heat after driving it, upon restarting, the a/c compressor will occasionally not engage. It will engage, however, after shutting the engine off and restarting it.

I talked to my tech and he said that it is a fairly common problem in hot climates. He said a heat-soaked car can cause just enough excessive a/c head pressure at the start up to lock out the a/c clutch. Then, when it is restarted, it will work okay. Mine leaves the a/c lockout code afterwards.

I'm not sure if it's the high back pressure causing the belt to slip or maybe it's the overpressure switch. I thought about trying a new belt or maybe replacing the tensioner. Has anyone dealt with this?

carnut · #2 05-22-2008, 09:52 PM

My experience is the system is disengaging the compressor because of low refrigerant not hi head pressure. Pressures are easily confirmed with gauges connected. Are the radiator fans working as designed? With a cooking thermometer in the center vent, whats the outlet temp with outside temp of 100? With average humidity the outlet should be approx. half of outside temp all things being equal. 90 degree day, 45 to 50 degree vent temp. Turning the car off and restarting simply causes the computer to re read the pressures and if in spec allows the comp. clutch to re engage.

speace · #3 05-22-2008, 10:06 PM

If heat and pressure are the key, would it possibly be that the system is simply, over-charged by just a bit?

Do make sure your belt is good and the tensioner keeps it tight. (No squeaks when you rev it and release.)

emerydc8 · #4 05-22-2008, 10:56 PM

I charged it to spec last year, so I don't think it could be overcharged now. The fans are working fine. When the a/c works, it works well. Temperatures are 40-44 F at the vents. This only happens occasionally and it is always when it is very hot and the car is driven, parked, and restarted. The belt seems okay, but the tensioner may be wearing due to the high mileage. There are no unusual noises when revving or accelerating.

Matt L · #5 05-22-2008, 11:34 PM

Do you have a compressor speed sensor on that car? If so, I would suspect it.

emerydc8 · #6 05-23-2008, 05:38 AM

I'm thinking that the speed sensor may be working correctly, but the excessive pressure when the system is first engaged causes it to slip just a bit and lock out for the duration of the run time. Once the speed sensor opens the clutch circuitry, does the ignition have to be turned off to reset it or does it allow the clutch to reengage after the problem is no longer sensed?

speace · #7 05-23-2008, 07:06 AM

I'm not a professional, but have a good bit of experience in A/C. I am very familair right now with my 92 500SL and my 92 400E is very similar. I would expect the 92 and 93 400E to be the same. From what I've seen, when the clutch slippage is sensed, the A/C locks out for a period of time. This seems to be several minutes before it tries again. Restarting will bypass that lockout. I would think that if it sensed high pressure, it might not retry at all. Do you know if it engages at all when you first start the car before it fails?

On the drier are two pressure switches. The one that is colored plastic and has pigtails should be the high pressure switch. (It is red on my 400E and is green on my SL.) If it were mine, I would consider that switch to possibly be faulty. I think you have to evacuate the system to change it.

I believe that a clogged receiver/drier or expansion valve could cause high pressure. The expansion valve should not care about the ambient temperature. Could the drier clog when hot? Not very likely, I would think.

Is "A/C Lockout" the only code it throws up? I think there is a separate code for high pressure.

david s poole · #8 05-23-2008, 10:47 AM

if the speed sensor is cutting out the compressor,when a/c is turned on with motor running you will see compressor engage for about 3 or 4 secs and then turn off.you mention some nice cold temps from grills when working so it might be the evap temp regulator switch.

Matt L · #9 05-23-2008, 12:35 PM

When magnetic sensors fail, they sometimes become heat sensitive. At one temperature, you have continuity. At another, you have none.

That's why I would check that sensor, especially when hot. If the belt is slipping, you will indeed lose the AC immediately. But if the sensor puts out no voltage when hot, that's when you will lose the AC.

Arthur Dalton · #10 05-23-2008, 12:50 PM

That pigtail switch has nothing to do with comp cut-out. It's function is solely for the Aux Fan Low speed cut-in if high side pressures demand more airflow......nothing more.

The sw. that cuts out comp is the other one. [ spade connector one] . That is a binary sw and cuts in at 2 bar for low refrig monitoring and cuts out at 30 Bar for comp protection.

The circuit that is suspect is the blue sensor/thermistor in the water jacket. That is engine coolant temp input to the ACC panel and it's function is to cut in the high fan, @ 105C, but it also has a higher temp cut-out value to cut compressor signal if engine is heat soaked or extremely HIGH.
Next time, before start-up, look at the CT indicator on dash.
If high reading, that is the suspect.
If not , then you need gauges on the high side before start for further diagnosis..............

emerydc8 · #11 05-23-2008, 06:16 PM

I have the cool harness installed on the temp sensor that starts the high fan at 93 instead of 105. Do you think that might have any affect in this scenario?

Arthur Dalton · #12 05-23-2008, 08:03 PM

Quote:

Originally Posted by emerydc8

I have the cool harness installed on the temp sensor that starts the high fan at 93 instead of 105. Do you think that might have any affect in this scenario?

Of course.
That drops the a/c cut-out spec. too low for high ambient conditions.
That is why I recommend anyone using a resistor bridge on the termistor to have it cut-in about 100C... [vs the 105C] ...if you go lower , you think you are doing yourself a favor with early fan activation , but you are not aware that you are also effecting the a/c protection at higher temps b/c it is the same sensor input to N22 that controls comp cut-out saftey.
You are also burdening the fan motors and alternator.

..but I am sure we will hear a different opinion on that..
I have tried all bridging values way before Cool Harness times , and I find the best is to get a 100C cut-in , regardless ...
That will also give you about a 97/98 cut out, Which is fine..without shaving too much off the a/c cut out temp trigger point.

emerydc8 · #13 05-23-2008, 08:22 PM

The coolant temp is always the highest after driving the car in very hot weather, parking it for about five minutes and then restarting. If it becomes a big problem this summer, I will take the cool harness out of the system. As I said above, though, it's only happened occasionally. I like the fans coming in at the lower temp because, even on a very hot day, I rarely see 110 when driving.

Arthur Dalton · #14 05-23-2008, 08:41 PM

That is up to you....
I was simply diagnosing your problem and making you and other readers aware of this possible...and also to explain the trade off of going to the extreme/lowest cut-in when bridging/changing the sensors value.
I use thermistor/sensor bridging with satisfactory results in high ambient conditions, but I find approx 100C C/I to be fine. Many guys simply use a cabin over-ride switch that they can activate fans as needed by simply bridging the low fan circuit manually..[they bridge the high pressure sw at the drier]. This give low fan for traffic and towing conditions w/o too much fan speed and alt. draw.
...so, Cool Harness is cool...but it can be too cool in the sense that the lowest cut-in does not always equate to the best application when all aspects are considered. Which is why JF has different R values to chose from.
If you find it to be a problem, I would simply change the R value to a higher cut-in for fan rather than eliminate the Bridging.

emerydc8 · #15 05-24-2008, 06:20 AM

Thanks for the good advice. Being a control freak, I kind of like the manual switch, but I'm not the only one who drives the car, so if the current setup (93 degree high fan) becomes a problem this summer in Tucson, I'll probably get a higher rated cool harness from Jim. This car has run so cool since I modified the viscous fan clutch, installed the cool harness and got my antifreeze mixture down to about 35%. It just seems to boil the coolant temp after its been run and parked in a hot area--just like the small blocks do.

Have a good weekend.