W124 A/C after a few hours driving - PeachParts Mercedes-Benz Forum

Q-Ball · #1 06-12-2004, 03:44 PM

1993 300E M104 148K

Drove car from Atlanta to Tampa yesterday in 90 degree heat. Had A/C on low fan setting and was very comfortable, and after 2.5 hours, it gradually got warmer, did not blow ice cold, and no matter what wheel was set to, did not change temp of air coming out. Shut A/C off for an hour or so, turned back on, blew cold and set wheel to 75 degrees, and put on Automatic, and had no further problems after that for the next 3 hours or so.

This is the second time it happens to me on prolonged highway driving, in the summer. The first time I shut off a/c waited an hour or so, put on high, and a few small ice chips actually came out of center vents. Seems as if something is freezing, or not allowing proper flow.

I will continue to leave in automatic mode,and let it do its magic, in the Fl. heat, but just curious why after hours of driving, the cooling capability of the A/C diminishes.

Just had 150k service completed, a/c checked out fine, was even topped off. Car is 100%

stevebfl · #2 06-12-2004, 06:13 PM

Sounds like the system isn't cycling after a while. Get a thermometer (preferably digital) and put it in the center duct and turn the blower down and the temp to "min". The temp should go down to somewhere in the high 30's to the mid forties. If one is driving on the road the incar temp drops and the ducts cool. The temp in the ducts can not get to 34 or so or the evapaporator will ice up. The cycling temp should generally be the same continuously.

It is important to use a digital as they react quicker. The time of a cycle can be so quick an analog thermometer will hardly notice the change. There will usually be a 3-6deg change of duct temp during a cycle.

Q-Ball · #3 06-12-2004, 07:38 PM

Steve,
Thanks. I am glad you chimed in since you are in Florida and know these cars better than anyone. I actually thought of stopping by to see you on my way down, but figured on a Friday it was not a good idea. Eventually i will have the car your way in the future since I have moved to Tampa.

I will get a digitial thermometer in the morning and give it a whirl. Should I be driving while conducting this test? Am I looking for drastic temp. changes between cycles? And what is an acceptable change? What else should I do? Also, my drivers vent, nearest window, puts out very little air. I can see flap and it is more than partially open, but it will not move anymore.

Thanks again,
Update later.
FQ

stevebfl · #4 06-12-2004, 10:18 PM

Yes drive the car. The condition you state is after an extended drive. It is easy to see if you can cycle, but it easiest if you creat the most oportune conditions. Engine speed over 2000, airflow over the condenser (50mph works), and a cooler time of the day using low blower speed. This will give your coldest duct temp posible and you should cycle. Cycling means the compressor shuts off just like your home AC does when it reachs desired temp. In the cars case the refrigeration systems is always going maximum, temp change is effected by mixing heat with the AC.

Q-Ball · #5 06-13-2004, 03:11 PM

Bought a digital thermometer, and set out for a drive. Car was in garage, outside temp 91, cruise control set to 60mph, and set a/c to lowest temp setting, and low fan speed. Started out after 10 minutes blowing 44.8 & 42.1. After 15 minutes, and 10 or so cycles, ( i am counting cycles as the hissing sound the a/c makes every minute or so ) temp dropped to 37.1 & 41.4.

On the way back home I set the wheel to 75, and placed it in auto mode. Blew cold all the way home at around 41-43.

Seems my problem is not short distant driving, air blows rather cold and comfortable. Twice in 2 years I have made the Atlanta/Tampa journey in 90 degree heat, and after hours of driving the cooling efficiency of the a/c diminishes. Should I leave it alone, and let it run its course???

Thanks,
FQ

stevebfl · #6 06-13-2004, 03:24 PM

You have verified that the system cycles. Your options for solutions are changing. You have likely a intermittant problem rather than a systematic problem.

From a statistical approach I would guess that you have a clutch gap problem. With a large gap and reduced magnetism due to the heat the clutch just does grab after one of the cycle events. This is a natural occurance as clutches wear, especially if they were set wide on creation. The gap increases with wear. They are adjustable.

The next statistical choice is the Klima relay or a heat related belt issue.

Check your gap and continue to watch your duct temp. You will get to know it. You also must realize that if you bring the temp up you will mix heated air and you are then no longer in touch with evap temp.

MB_FanAddict · #7 06-14-2004, 11:28 AM

From your description Q-Ball, It sounds like after awhile the evaporator is freezing up causing cooling to diminish (especially since ice chips blew out at you when you tuned it back on - NOT normal).

A bad evaporator temperature sensor?

Chris

Q-Ball · #8 06-14-2004, 02:11 PM

I definitely agree with it freezing up. It has happened twice under prolonged drives. Do you know where it is located? I rarely drive the car more than 2 hours, but when i do, it would be nice if all was cool.

I will also do some searches on the topic.

Thanks

FQ

jcyuhn · #9 06-14-2004, 03:26 PM

To me it sounds as if the evaporator is freezing up as well. There are a couple of easy ways to deal with this problem. First, leave the fan speed on automatic, if that seems to work. It moves more air across the evaporator than low fan speed, should help prevent freezing up. Second, if it does freeze up, you should be able to remedy things by switching to "EC" mode for about five minutes, no more. This switches off the compressor, with no refrigerant moving through the evaporator it warms up, the ice melts, voila!

I don't understand why you had to switch the a/c off for an hour. Was it that long before it functioned again, or is that just how long you waited to try it again?

- JimY

Q-Ball · #10 06-14-2004, 07:14 PM

Its just the time period i had it off, between driving with windows open, and stopping for fuel. You are probably right, 5-10 would have done in 90 degree heat.

I am going to check into the switch next.

thanks

FQ

Bzoo · #11 06-16-2004, 11:30 AM

I have a 400E with the exact same issue. Driving from the Tampa area to Atlanta it freezes up after 4-7 hours or so..every time I make the trip although this only started last year. Never before. When I got out of the car to look under the hood to see if the compressor was engaging, I saw the low pressure side in a case of ice. Maybe the evaporator sensor is at fault although my system also cycles under normal circumstances. I wonder if the cycling I'm getting under normal driving is because of the high pressure switch cycling it as opposed to the evap sensor causing it to cycle. I'll be interested in your results.
Brian

Q-Ball · #12 06-16-2004, 05:52 PM

it is interesting. Even just last night,after major thunderstorms on the I-4 corridor between Tampa and Orlando, My compressor was freezing up after 45 minutes or so, in the humidity. I have ordered the evaporator sensor, and will replace in the next 3-5 days. I will definitely report my findings.

Q-Ball · #13 06-16-2004, 06:41 PM

Now confusion on which part is correct. Fastlane has an ACC temp sensor. I don not think that is the correct part. I understand i need the evaporator temp sensor. anyone have part number. Fastlane is closed for now, so i hope i can catch them before they ship part.

FQ