Hey guys,

When I bought my 300sd it lacked a belt for the AC compressor. I just put one on yesterday, but the AC system has yet to blow cold. Infact, it always blows hot as if the heater is stuck on. I have moved the dial back & forth and tried different settings and buttons but to no avail. The engine doesn't slow when I turn the system on so I believe the clutch may be bad on the compressor or their may be a problem with a switch or relay somewhere, particularly because it seems teh heat is always on. Let me know what you guys think

Cheers,
Mike

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1984 300sd auto with 246,000 miles

I'm having the same issue with my 300D. I charged the system and the compressor began working. I would get cold air for a while then it would just kick off and blow only very hot air like the heater was on.

I've been reading this site and trying to diagnose my issue. I think maybe my mono valve on the firewall that controls the hot water inlet may be defective. Or, it's control voltage is not right. I've still got some work to do to isolate the problem.

I believe the dial with the Red/White/Blue temperature settings on it is a variable resistor than controls the mono-valve voltage. I believe it fails in a way that lets hot water thru by default.

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Codifex
1981 240D ChinaBlue (Got her running with a donor engine.)
1983 300DTurbo w/sunroof.
1984 300TD manual sunroof. (Electrical Gremlins)

Start by turning the forward most portion of the compressor while the engine is not running. It should turn smoothly only feeling compression pulses. If it won't turn or feels as if it's full of rocks, then you don't WANT the clutch to engage because the compressor is toast and will just put strain on the belt.

If the compressor DOES turn smoothly, then it is likely that it simply has leaked out the refrigerant. You need to find the leak, repair it and recharge. Before chasing a leak, however, momentarily jump 12 volts to the clutch and see if it clicks. If it does, then find the receiver drier and remove the two connectors from the low pressure switch. If it does not have continuity across the switch, then the system is low or empty.

Put manifold gauges on the system is another, more conventional approach to seeing if it is empty. It all depends on the tools and equipment you have access to.

Hope this helps.

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2001 SLK 320 six speed manual
2014 Porsche Cayenne six speed manual

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