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1983 300CD
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Discussion Starter #1
Sorry for yet another thread about AC trouble shooting...

Car is a 1985 300CD california that I pieced together all of the AC components because a PO ripped it all out at some point.

I found all the hoses, compressor, condenser, and installed new all O-rings, drier, and expansion valve.

I had a local shop leak test and fill the system with freon. The shop claims the system is solid and ready to go, but needs a new AC relay (Klima) to work. They wanted about $450 for the relay so I paid them for the freon and pressure testing and took the car home. They also were able to bench test the climate control unit and it is supposedly working fine.

I installed a used but working AC relay, but cannot get the compressor to kick on.

Is there a simple test to check the compressor? Is there a constant voltage that should be present at the compressor? I'm assuming the switches on the drier are good, but is there an easy way to test those? I'm also assuming no freon leaks, since it was just tested and filled.

Thanks!
 

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1985 300D - 1997 E320 - 2004 E55 - 2007 ML320
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You should be able to trip the AC compressor on, it engages when the relay closes to engage the clutch to power the compressor. Just jump it out, the it should supply power to the clutch and engage the compressor. The relay could be bad. They're easy to test thou. Hook a test like up to the output, ground the light, power the input, and then supply voltage to the relay (positive and ground). It's either going to close or open the circuit depending on the configuration of the relay ie, Normally Open or Normally Closed. Don't quote me but it should be a NO circuit.
 

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1983 300CD
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Discussion Starter #4 (Edited)
I'm starting to think it must be compressor that is bad...I jumpered the Klima, hooked a battery straight to the compressor, hooked a battery charger straight to the compressor, and nothing made it kick on.

Am I doing this test wrong? Should I be applying the voltage directly to the compressor when the car is running or when the car is off? What exactly should it look/sound like when the compressor successfully kicks in?

The compressor spins freely when the car is running, so I know it isn't seized. The compressor supposedly was working fine on the donor car...

I really don't want to have to replace the compressor, because I just paid the shop to charge the system, so I'd like to avoid cracking it open at all costs.
 

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1985 300D Gone 1985 230CE Perfect, 1984 300TD Driver, 1981 300TD Bad engine
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It sounds like that you need to measure the resistance of the compressor clutch coil. If it shows an open circuit the coil is bad. If it is about 3.6 ohms it is good.

Also measure the voltage of the clutch coil when the A/C should be running. It should be around 12 volts.

The air gap between the clutch and the drive should be about .030".

If all of these are good the clutch should engage and spin the compressor. You can tell when the compressor is spinning because you can see the clutch plate spinning.
 

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79 300td
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5 Posts
Does a 79td have a "klima" relay? I've been trying to locate where I'm loosing the power to energize my compressor and not able to follow the wire through the spaghetti. Thanks
 

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1985 300D Gone 1985 230CE Perfect, 1984 300TD Driver, 1981 300TD Bad engine
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1983 300CD
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136 Posts
Discussion Starter #9
Using the Klima wiring diagram I was able to confirm that my compressor is good (jumping pins 5 and 7), however it seems the wiring to my AC pressure switch must be the issue. I pieced this whole system together, including the wiring, so I'm trying to reverse engineer some of the wiring (a PO just clipped wires, so I'm kinda in the dark).

Can someone please tell me the color of the wires labeled "d" and "c" coming off the pressure switch (27) as shown in the diagram below? I was able to find the other wire (blue) coming off the pressure switch that goes to 10 on the Klima.

If someone could provide an actual picture of this wiring setup on a 1985 300/CD, it would be extremely helpful. Thanks!

 

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1985 300D Gone 1985 230CE Perfect, 1984 300TD Driver, 1981 300TD Bad engine
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They are two blue wires going to one connector.
 

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1983 300CD
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Discussion Starter #11
Thanks! I connected the two stray blue wires I had to the other side of the switch, but still no luck.

What does it mean by "anti icing switch"? Is the the same as the ETR under the passenger's dash?
 

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1983 300CD
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Discussion Starter #12
So it seems that of the two blue wires that connect to a single wire on the AC pressure switch, one goes to the anti-icing (ETR) switch under the passenger dash, and the other is grounded to the ACC.

The anti-icing connection gets 12V when the key is on.

Can someone with more electrical knowledge than me tell me why it makes sense to connect 12V to a switch AND straight to ground at the same time? Or am I missing something here?
 

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1985 300D Gone 1985 230CE Perfect, 1984 300TD Driver, 1981 300TD Bad engine
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Thanks! I connected the two stray blue wires I had to the other side of the switch, but still no luck.

What does it mean by "anti icing switch"? Is the the same as the ETR under the passenger's dash?
ETR means Evaporator Temperature R?????. So, yes.
 

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1983 300CD
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Discussion Starter #14
Reading through tons of other posts here...

Does the wiring KLIMA pass through both sensors on the drier or just the pressure sensor? According to the diagram, it only includes the pressure sensor, the ETR, and the ACC...but, I've been reading a few posts on other sites that claim it also passes through the temp sensor...?
 

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So it seems that of the two blue wires that connect to a single wire on the AC pressure switch, one goes to the anti-icing (ETR) switch under the passenger dash, and the other is grounded to the ACC.

The anti-icing connection gets 12V when the key is on.

Can someone with more electrical knowledge than me tell me why it makes sense to connect 12V to a switch AND straight to ground at the same time? Or am I missing something here?

If understand what you are asking, it makes perfect sense. Are you measuring at the two connectors for the ETR? If yes, here is the explanation.

The switch completes a circuit. A circuit has power and ground. When the switch is open one of the connectors goes to ground, the other goes to power.
Keep in mind that it is not a direct path to ground or power. Since when measuring with a high impedance DMM there is near zero current, the resistive elements of the circuit create no voltage drop. That is why you measure full voltage.

While you are there measure the switch. It should be near zero ohms down to about 40 deg F. Below that the switch opens and caused the power to the compressor clutch to stop. This allows the evaporator to stay above the freezing point of water. Hence the name Anti-Icing Switch.

If you want to get really cold air for a while, add a switch in parallel to the ETR. This will keep the compressor running when the evaporator gets be low the 40 degs F. If it runs too long the evaporator will ice-up and stop air flow. When I make this modification I'll use the timer relay that is used for the rear window defogger. This will keep the ETR from being bypassed for too long. Keep in mind that the additional switch will not provide colder air when the system is not able to produce it, such as at idle.
 

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1985 300D Gone 1985 230CE Perfect, 1984 300TD Driver, 1981 300TD Bad engine
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Reading through tons of other posts here...

Does the wiring KLIMA pass through both sensors on the drier or just the pressure sensor? According to the diagram, it only includes the pressure sensor, the ETR, and the ACC...but, I've been reading a few posts on other sites that claim it also passes through the temp sensor...?
I did this test: Jumpered the wires going to the temp switch. The fan is running. Pulled the Klima relay. The fan continues to run. The drawing is correct.
 

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1983 300CD
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Discussion Starter #17
I did this test: Jumpered the wires going to the temp switch. The fan is running. Pulled the Klima relay. The fan continues to run. The drawing is correct.
Thanks for all your help...is there anyway you could post or email me a couple photos of the wiring setup on/around your drier? I'll PM my email.
 

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1983 300CD
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Discussion Starter #18
Please Help!

Another day troubleshooting with no luck. I'm 99.9% sure that my problem is in the wiring to the AC pressure switch and back to the Klima but I cannot pin point the issue. I've been through all the obvious issues about 10 times and I'm almost certain all of the other components are working individually (ACC, Klima, pressure switch) but the circuit is not correct.

If someone could either post pictures or just a thorough description from a 1985 300CD/D/DT of the drier switch connections, particularly the pressure switch, it would be extremely helpful (which color wire, from where, connects to what).

Imagine someone clipped all those connections and you had to rewire it with only a multimeter and some very scant wiring diagrams.

Thanks!
 
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