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More Mercedes A/C fun

curtis73 on Tue July 07, 2009 12:24 AM User is offlineView users profile

Year: 1999
Make: Mercedes
Model: E300TD (W210)
Engine Size: 3.0
Refrigerant Type: R134
Ambient Temp: 109
Pressure Low: 34
Pressure High: 325
Country of Origin: United States

Ok, more on the Mercedes saga Thanks to you all I finally have (or I should say had) the A/C blowing ice cold on the wife's E300. Now we're having more fun issues. It has been crapping out on us at various times and I assumed it was something to do with the A/C; compressor shutting off due to high side spikes, or the evaporator freezing, but that isn't the case.

I had the guages on it to try and diagnose. It was 109 degrees in Austin TX that day. I set a brick on the accelerator to maintain 1500 RPMs and just waited. Compressor stayed on, the water reached 90C, the fan clutch locked and unlocked, no high side spikes, nothing. So I walked back to feel the air coming out of the vent and darn near burned my hand. The outside vents on both sides as well as the rear vent in the back of the console were blowing 180+ degree air. The center two vents above the radio were blowing 60 degree air. (tested with a laser thermo)

It comes and goes. It varies from center ice cold, outside vents cool-ish up to center vents cool and outer vents screaming hot. My first thought was a fault with the temp sensors, but that would make one side hot and the other side cold. This is inner and outer.

I posted on several Mercedes forums, but the answers I get range from "take it to the dealer" to "did you try armor all on the tires" Matt, you've been so helpful on W210s, hope you might have some insight where to start. I figure it has to be a vacuum line not operating a door or something, but I can't find it. It would be nice to know what part of the dash I should rip out

-------------------------
1966 Pontiac Bonneville
1973 Impala Wagon
1987 BMW 325 convertible
1996 Impala SS
1997 Toyota Tercel

Matt L on Tue July 07, 2009 9:54 PM User is offline

Doors are not used to control the temperature. The compressor runs all the time (above 3C ambient) and the heater cores are activated to rewarm the air to the proper temperature, as determined by the climate-control unit.

It must be reheating the air after it passes through the evaporator even when there is no demand for more heat.

To completely disable this reheating, clamp off the hose going between the duovalve and the engine. The duovalve controls the water through the heater cores. It is located right beside the module box, rear passenger side under the hood, between the box and the inner firewall. It looks like two tiny soda cans, with an electrical connector at the rear and three water hoses (which aren't as obvious as the cans). The hose from the duovalve to the engine goes through the inner firewall, so you can clamp it off on the engine (front) side.

The duovalve is a medium-failure part on this car. It's easy to change, but it's pretty expensive for what it is.

curtis73 on Wed July 08, 2009 12:45 AM User is offlineView users profile

So the valve would cause the outer vents to be hot but the center ones to be cold? I just assumed that there was a valve for the passenger and one for the driver.

I'll try your diagnosis this week and report.

-------------------------
1966 Pontiac Bonneville
1973 Impala Wagon
1987 BMW 325 convertible
1996 Impala SS
1997 Toyota Tercel

curtis73 on Thu July 16, 2009 10:53 PM User is offlineView users profile

More symptoms that might help...

Now the vents kinda randomly change from hot to cold. It started as just the outsides and rear vents getting hot, but now it seems like everytime you hit a bump or pick your nose they switch; sometimes one is hot while three are cold, sometimes the left is hot while the right is cold. I would describe it as completely random.

-------------------------
1966 Pontiac Bonneville
1973 Impala Wagon
1987 BMW 325 convertible
1996 Impala SS
1997 Toyota Tercel

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