A/C Temps higher with vehicle speed. help and posturing! - PeachParts Mercedes-Benz Forum

kerry · #1 08-08-2012, 09:33 PM

I'm having issues with the AC in my Fuso and am wondering if the problem could be similar. At high speeds on hot days the temp at the vents is 66.

Whiskeydan · #2 08-08-2012, 10:27 PM

Quote:

Originally Posted by kerry

Hmmmm? How did you figure out this was the problem. I'm having issues with the AC in my Fuso and am wondering if the problem could be similar. At high speeds on hot days the temp at the vents is 66.

Simple test Kerry. Lightly mist the condenser with water and watch vent temp. If the vent temperature drops to an acceptable level then you need more condensing surface area, more air flow...
Bare in mind it's a delicate balancing act with the amount of freon once everything is fixed. So, by misting the condenser the pressures will drop, now the system appears to be low on refrigerant.
You should reach a point where the evaporator temp switch opens the compressor circuit preventing liquid from slugging the compressor.
The MB is adjustable and I've found a few to be set to shut off the compressor too soon. Better safe than sorry though. Liquid at the suction side is a bad thing.

kerry · #3 08-08-2012, 10:49 PM

Thanks. I've given it some more thought and I'm not sure it's my problem. If I drive 65mph on the highway, air is 66 degrees. Around town at 20 mph it will be 48 degrees. But I can try the test anyway.

Whiskeydan · #4 08-08-2012, 10:54 PM

Quote:

Originally Posted by kerry

Thanks. I've given it some more thought and I'm not sure it's my problem. If I drive 65mph on the highway, air is 66 degrees. Around town at 20 mph it will be 48 degrees. But I can try the test anyway.

It sounds overcharged if you have cooler vent temps when slowing down.
Have you checked the system pressures at fast idle, 2000rpm? If the hi side is above 275-300psi then you need to recover some freon out of the system.

kerry · #5 08-08-2012, 10:56 PM

I'm AC incompetent so I haven't checked the pressures.

Ken300D · #6 08-09-2012, 09:25 AM

Quote:

If I drive 65mph on the highway, air is 66 degrees. Around town at 20 mph it will be 48 degrees.

Higher vent air temperatures at highway speed is sometimes related to the "outside air" vs. "recirculate inside air" flap in the HVAC system. In the MB, this is controlled by a vacuum motor that often goes bad. On the hottest days, I pretty much need complete recirculation (no outside air) to stay the coolest at the vents.

So I have tie-strapped the recirculation control (in the summer) to "recirculation only". Before that I noticed that the A/C got cooler at slow speed and then warmer when at highway speed. And I found the control was pretty much stuck at the halfway point (some recirculation and some outside air).

Ken 300D

dcotejr · #7 08-09-2012, 11:38 AM

Quote:

Originally Posted by Ken300D

Higher vent air temperatures at highway speed is sometimes related to the "outside air" vs. "recirculate inside air" flap in the HVAC system. In the MB, this is controlled by a vacuum motor that often goes bad. On the hottest days, I pretty much need complete recirculation (no outside air) to stay the coolest at the vents.

So I have tie-strapped the recirculation control (in the summer) to "recirculation only". Before that I noticed that the A/C got cooler at slow speed and then warmer when at highway speed. And I found the control was pretty much stuck at the halfway point (some recirculation and some outside air).

Ken 300D

Sounds like what is happening to my 350SDL. Can you provide pics of what to tie strap??

Whiskeydan · #8 08-09-2012, 07:06 PM

Quote:

Originally Posted by dcotejr

Sounds like what is happening to my 350SDL. Can you provide pics of what to tie strap??

Pull the glove box and you should see the recirculate flap and it's vacuum pods.
You can observe it in action and see how to tie it if this is the problem.

vstech · #9 08-09-2012, 09:39 PM

Quote:

Originally Posted by Whiskeydan

It sounds overcharged if you have cooler vent temps when slowing down.
Have you checked the system pressures at fast idle, 2000rpm? If the hi side is above 275-300psi then you need to recover some freon out of the system.

not really... if it's cold air at 20mph, engine RPM is going to be around 2K... at 66MPH engine RPM is going to be around 2K also, (maybe 2500...) so the compressor is doing the same work, but the HEAT LOAD on the cabin is drastically increased at 66... massive heat from airflow around and through the car. not to mention the air mix without pure recirculate.

kerry · #10 08-09-2012, 09:54 PM

I was referring to my Fuso truck. At 20mph around town rpm's are much lower than at 60mph. I don't get how the heat load on the cabin increases at speed. Isn't it reduced by the movement of the air? It's cooler on a motorcycle at speed.

Brian Carlton · #11 08-09-2012, 10:22 PM

Quote:

Originally Posted by vstech

but the HEAT LOAD on the cabin is drastically increased at 66... .

The heat load on the cabin is dramatically reduced at highway speeds because the airflow over the cabin will keep the surface temperature at ambient, despite the radiant heat from the sun.

At low speeds, the heat load climbs significantly due to the inability of the steel body to shed the radiant heat load from the sun.

No, there is no heat load from friction over the body............it's not an XR-71...........

vstech · #12 08-10-2012, 12:22 AM

Quote:

Originally Posted by Brian Carlton

The heat load on the cabin is dramatically reduced at highway speeds because the airflow over the cabin will keep the surface temperature at ambient, despite the radiant heat from the sun.

At low speeds, the heat load climbs significantly due to the inability of the steel body to shed the radiant heat load from the sun.

No, there is no heat load from friction over the body............it's not an XR-71...........

... obviously our HVAC training and experience levels are different...

vstech · #13 08-10-2012, 12:27 AM

Quote:

Originally Posted by kerry

I was referring to my Fuso truck. At 20mph around town rpm's are much lower than at 60mph. I don't get how the heat load on the cabin increases at speed. Isn't it reduced by the movement of the air? It's cooler on a motorcycle at speed.

actually, the body feels cooler on a motorcycle because the air passing across our bodies has sweat evaporating faster... the car does not have sweat glands.

a vehicle will have a much higher heat load, due to the vehicle's cabin air exchange rate. also, the air across the vehicle is constantly transferring heat into the cooled air and cabin structure.

Brian Carlton · #14 08-10-2012, 01:01 AM

Quote:

Originally Posted by vstech

a vehicle will have a much higher heat load, due to the vehicle's cabin air exchange rate. also, the air across the vehicle is constantly transferring heat into the cooled air and cabin structure.

This is a factually incorrect statement.

If the vehicle is sitting in a parking lot and not moving, the heat load from the sun will cook the exterior skin and the interior temperature will climb to very high levels (130F).

If the vehicle is moving, the skin sheds the heat load because of the constantly moving air across the skin. The temperature of the skin is ambient temperature.

Take any vehicle for a drive without a/c. It's always cooler if you're moving...........

Codifex Maximus · #15 08-10-2012, 01:35 AM

If the cabin is hotter at speed, it sounds to me like there is either a flap open and letting hot air in OR there is more water circulating thru the heater core. Need to either tie strap that vent closed and/or restrict that heater core water.