strange 300SDL A/C issue - started blowing hot during drive - PeachParts Mercedes-Benz Forum

largelybored · #1 08-28-2008, 12:25 AM

Hi all,

300SDL... had the compressor and related bits replaced 2 years ago. 3 Weeks ago I had them fix the vent controls so more than just the far left vent would work.. It didn't really blow as cold when I got it home. A few days later it died completely - big puddle of goo in the driveway. I figure its 134.. Speaking of which, it was probably 105 outside when this happened here in Austin.

So I take it back to the shop, they say there was low/no 134, but they couldn't find the leak (they used dye) They put coolant back in and all is fixed... until tonite.

I'm cruising down the road and all of a sudden it starts to blow hot. I didn't get a sense that the compressor stopped, just that it all of a sudden started blowing hot. Immediately hot, faster than if you flipped the wheel all the way. I turned the A/C off and got it home. Popped open the hood, looked for leaks, didn't see any except my usual fuel leaks.

Hit the button to on and it started blowing cold... About 15 minutes between it blowing hot and getting home and it working again...

Any thoughts on what the problem might be? What could have caused last weeks burp of fluid over my driveway? It seemed to be all at once, not a slow leak. But it was parked for a day or two.

134 is cheap, do I just buy some and learn how to refill it myself? I've never done any A/C work and would be starting at point zero.

-M

TylerH860 · #2 08-28-2008, 12:38 AM

Do SDLs have a monovalve?

Thats a similar problem I had with 2 of my cars. If it has one, try unplugging it/plug it in and see what happens.

JimmyL · #3 08-28-2008, 01:08 AM

A strange 300SDL AC issue is to have it work!

ascalise · #4 08-28-2008, 01:18 AM

my 300sd did this, i resoldered everything in the CCU and it was cured.

Matt L · #5 08-28-2008, 11:14 AM

Quote:

Originally Posted by largelybored

134 is cheap, do I just buy some and learn how to refill it myself? I've never done any A/C work and would be starting at point zero.

134a is cheap and cheap refill kits are available about anywhere. AVOID THEM!

There is one reason that these kits are on the shelves, and it's not because they're a good idea or work well for most people in the long term. The store doesn't care how long your system lasts or whether you need to replace EVERYTHING in there. They only care if they sell you something.

AC systems are not very complicated, but they are very sensitive. Please learn the proper procedures and follow them.

daw_two · #6 08-28-2008, 11:20 AM

Quote:

Originally Posted by ascalise

my 300sd did this, i resoldered everything in the CCU and it was cured.

Trying resoldering the circuit board. That "mostly cured" Coda's issue.....although some times, I still get the center vents "try" to blow out of the defrost vent. I think I also have a vacuum pod with a small leak because my issue occurs at very low vacuum state (start up, low idling, etc.).

LUVMBDiesels · #7 08-28-2008, 01:03 PM

#1 Check the monovalve to see if it is getting 12volts when youwant the heat OFF (temp wheel at min)

#2 REPLACE fuse #5

Did this happen when you floored the pedal and had the transmission kick down?

The kick down switch is on the same fuse as the monovalve and the wiring for the kick down switch can short out.

Replace the fuse and don't floor the pedal for a couple days and see what happens.

lowriderdog37 · #8 08-28-2008, 05:01 PM

I've been having the same problems with my SDL. Does yours turn back to cold if you turn the knob all the way and give it a minute or two?

I will have to try these tricks from you guys...thanks!

Turbopugsleylx · #9 08-29-2008, 01:43 AM

Mine blows hot air even with the air off...and the a/c is not working...my #5 white fuse is new...what is the monovalve...where is it at and what does it look like?

Matt L · #10 08-29-2008, 11:12 AM

The monovalve regulates the coolant in the heater core. It looks like a miniature soda can (1" or so in diameter) sticking out of the heater line. It's under the hood. Follow the heater hoses.

tangofox007 · #11 08-29-2008, 11:23 AM

Quote:

Originally Posted by LUVMBDiesels

#1 Check the monovalve to see if it is getting 12volts when youwant the heat OFF (temp wheel at min)

The monovalve gets 12 volts (or whatever is available) any time the key is not in the "off" position. It operates on the basis of a switched ground, so the presence of system voltage at the valve does not tell the full story.