This is a 1972 280SE 4.5. My AC seems to have electrical problems. I am observing the following:

• The clutch on the compressor is not engaging.
• No voltage at the compressor clutch when AC is on.
• In static state the system has about 100PSI freon on both high and low side.
• When the ac is switched to max fan and max cooling the fan blows fine but the green light does not come on.
• I replaced all the fuses in the three locations passenger side firwall of engine compartment.
• Some of these fuses do not have wires on both sides of the fuse so not sure if this is an issue or just an option not installed, see pictures.

I am used to freon leaks being the universal problem. Can anyone help me troubleshoot this?

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1972 280SE 4.5

You might try using a jumper wire directly from the battery to the compressor to see if the compressor clutch engages. If it does, then it would seem like a matter of tracing back to where you are loosing the 12 volts to engage the compressor clutch. It's been a while since I have owned a 280se, but if I remember correctly the a/c system uses a low pressure switch that prevents the compressor from engaging if you have low pressure. I would think with 100 psi of pressure that should do the trick. You also might want to try to jump across the terminals of the low pressure switch. If the compressor comes on while jumping the low pressure switch then you probably have a faulty low pressure switch. This switch is normally installed in one of the a/c lines or near the receiver/drier. Could also be part of the receiver/drier.

A possible relay failure? A number of the relays in these cars are interchangable. Try switching the AC relay with another identical one.

Incidently. These old relays can be easily "freshened", since there are old fashioned contact points in them that can be burnished to restore electrical continuity.

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Berfinroy in CT
Present vehicles:
1973 300 SEL 4.5
1959 Rolls Royce Silver Cloud I
1959 Ford Thunderbird convertible/430
Past vehicles;
1958 Bentley S 1
1976 ex-Max Hoffman 6.9
1970 300SEL 2.8
1958 Jaguar MK IX
1961 Jaguar MK IX
1963 Jaguar E-type factory special roadster
1948 Plymouth woody
1955 Morgan plus 4
1966 Shelby GT350H Mustang

This relay would be located next to the fuses?

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1972 280SE 4.5

I regret that I am unable to determine from the Service Manual Section 83-12 exactly which of the numerous relays pertains to the magnetic clutch for the 3.5/4.5 108s. The electronics for the AC system between the various 108-109 models vary.
Perhaps someone else can ID the compressor clutch rely.

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Berfinroy in CT
Present vehicles:
1973 300 SEL 4.5
1959 Rolls Royce Silver Cloud I
1959 Ford Thunderbird convertible/430
Past vehicles;
1958 Bentley S 1
1976 ex-Max Hoffman 6.9
1970 300SEL 2.8
1958 Jaguar MK IX
1961 Jaguar MK IX
1963 Jaguar E-type factory special roadster
1948 Plymouth woody
1955 Morgan plus 4
1966 Shelby GT350H Mustang

If the green light does not come on at any fan speed it is the switch that is bad. I had this problem and replaced the switch and it worked fine.

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Brad



1966 230 SL
2005 CLK 500 Cabrio
2013 C 300 4 Matic sport 3.5

As with any auto function, it is best to go back to basics and work backwards. The suggestion to test the compressor clutch with a jumper wire from the battery is a basic first step. If your compressor clutch functions when jumped then you can next use a test light to determine if you have 12 bolts at the compressor when both the ignition switch and a/c switch is on. If no 12 bolts at the compressor then you work backwards to the low pressure switch, no power there, then you go back to the relay and then the switch. Replacing electrical parts without the proper testing gets expensive quick, especially on a Benz. You cannot harm any of the parts mentioned by testing as outlined.

Badali is correct, if the green light is not coming on, then the output side of the switch has no power. You are getting power to the switches, as the fan comes on.
I had the same thing happen on mine, and since I did not want to replace the temp switch (has a capillary tube that goes to the evaporator) I just wired the compressor wire to the fan switch, so that once the fan is on it turns on the compressor. Since the AC is not that strong, I do not really need to be able to regulate it. The only thing that is lost is being able to use the AC fan without the AC compressor being on.

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Csaba
1972 280SEL 4.5, silver

UPDATE.

Turns out the compressor clutch is faulty. On my W115 the clutch will engage without the engine running, but it seems on the W108 12 volts only gets sent to the compressor if the engine is running.

I had my AC guy test and drain my freon, 1.5 lbs of 100% R12. I'll replace the compressor and clutch. Also if it's available I'll change the receiver drier then see what happens.

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1972 280SE 4.5

Hi I get no response at all from the AC system when I turn it on. Before I start troubleshooting, I wanted to get any feedback to help. I'm assuming at this point it is a fuse(s) - on firewall. Any feedback would be appreciated. Thanks. John

What do you mean no response? Does the fan run? Does the compressor clutch not engage? Do you have freon?

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1972 280SE 4.5

Check to see if you have power to the switch first if nothing happens when you turn it on. The fan should run if the switch has power and the green light will come on. The switches usually go bad with age.

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Brad



1966 230 SL
2005 CLK 500 Cabrio
2013 C 300 4 Matic sport 3.5