I've got a '79 450 SL and since I've owned it, the Air Conditioning has never worked. Not important to me, but thanks to another thread on this forum I have successfully removed my leaky radiator, and it's now at a local shop. Meanwhile, I am looking into the engine compartment and the A/C compressor is staring me in the face, and from looking around, I see that the first thing in removing/replacing the compressor, is to remove the radiator.

The question is, how to determine if the compressor is likely a problem...it may be fine, but something as simple as a fuse could have the whole thing out of commission. (Actually I did already check the fuse and it's good!)

Here are some observations:

Forward on the compressor, is a two pin connector. I've checked the resistance accross these pins on the compressor side, and they read zero or near zero ohms. Conclusion: The coils or some protection inside is not damaged.

Turning on the A/C (Ignition on, engine not running) does not result in 12 V at the line side of that 2 pin compressor connector.

I read somewhere regarding a newer model, that the aux fan comes on when the A/C is turned on. Today, switching on the A/C (ignition on but not running engine), did not put 12 V at the aux. fan, as is the case when the temp switch is grounded at the thermostat housing.

The clutch assembly turns smoothly, albeit with slight resistance. I presume this to tell me the bearings are not seized.

When the engine was running (before the radiator was removed), switching on the A/C in the cockpit gave no response as far as we could see at the compressor, and the auxiliary fan did not come on either (but that is broken).

I understand there may be an A/C relay that may be the culprit. I have no idea where to look for that.

I expect any R12 is long gone.

I'm thinking the best strategy would be to confirm as best I can that the compressor is not bad, before I put back the radiator, identify any electrical problems, and then get the car to an A/C shop for vacuum test and new refrigerant with dye.

Any advice as far as the compressor or the electrical checkout is much appreciated. It's been a result of prior threads that I was even able to make (what I hope are somewhat intelligent) observations.s

Thanks much.

Stu

Okay, where to start. First off, I'm no A/C expert, but I can give some minor advice.

If you're not getting voltage to the A/C clutch with the A/C turned on, then indeed there may be an electrical issue. That issue could reside in a relay or in the climate control unit. There may also be a low-side pressure sensor that is preventing the clutch from coming on because the refrigerant charge is too low.

You really need to get a hold of a manifold set. Hook up the manifold to the low side (if you don't know about high and low side then stop now and call a pro). Start the engine. You need to have at least about 35-45 psi on the low side for the system to be happy. Too far below that and the compressor will never turn on. If you have adequate pressure on the low side, then the problem could be the pressure switch. Bypassing this switch (temporarily!!) will allow you to see if the compressor will kick in. Don't bypass this switch unless you have adequate low side pressure. If you have inadequate low-side pressure, it may be as simple as adding some R-12. If your charge is completely gone, then air has probably infiltrated the system and a complete leak check, evacuation, new receiver/dryer, seals, etc. are in order.

Report back your findings. Hopefully by then somebody more knowledgeable about MB A/C systems will have joined the thread at that point......

You really should get a hold of a shop manual for your A/C system, or have somebody with WIS print out the troubleshooting steps and the wiring diagram.

__________________
-tp


1990 300SE "Corinne"- 145k daily driver - street modified differential - PARTING OUT OR SELLING SOON - PORTLAND OR. AREA - PM ME FOR DETAILS
1988 560SEL "Gunther"- 190K passes anything except a gas station
1997 S420 - 265k just bought it with a rebuilt trans. Lovely condition

If you are not getting voltage to the compressor clutch with the a/c on, this does NOT necessarily indicate an electrical issue? The most common cause of this situation is a low refrigerant charge which opens the low pressure switch, interrupting the compressor circuit.

I would advise that you do not replace the compressor until such time that you get the radiator back in the car and filled with coolant so that you can run and properly diagnose the a/c system. In the vast majority of a/c problems the compressor is NOT the problem. If you remove the compressor at this point, you will contaminate the system and require a relatively involved procedure to get everything back as it should be.

I suggest that you take things one step at a time and properly diagnose the system. After the car is running again, connect manifold gauges, and go from there.

Larry and TP,

I will heed both your warnings. Will wait for the cooling radiator repair to be complete, re-install and run the engine.

Will then check voltages at the compressor at least, and if not present, I'll bring it to a shop for diagnosis, and probably spring for a charge if they say it will take it.

I must say I wouldn't have even thought to investigate the system, much less invest in it if not for the success I had in following advice on the cooling system (big thanks to Benzguy). But now it's got my attention, and if I can get the pro work done (pertaining to the lines and refrigerant), I know I can handle the electrical component side.

I do pledge to keep my progress updated here.

Your 1979 450SL has the very troublesome climate control II system. To verify this, take note of the fact that you have buttons vertically oriented, and a separate AC rocker switch to the right side of the vertical row of buttons. This system depends on NO vacuum leaks anywhere in the vacuum pods, as a single leak will lower the vacuum and prevent the vacuum switch that should turn on the compressor from operating. Try pushing the defrost button in and turn the AC switch on, this bypasses some of the vacuum stuff. If your compressor will rotate in this configuration you have vacuum leak problems. To properly troubleshoot this setup you will need the vacuum diagrams. Also, the engine must be running so as to provide vacuum to the climate control unit. Does your servo operate correctly? It is the black unit under the hood on the right side close to the firewall. Usually they are rusted up inside and do not work. I ended up purchasing the digital servo for my car, it works great! (I also have a 1979 450SL and have been through this exercise myself.)

__________________
Richard Wooldridge
'01 ML320
'82 300D 4.3L V6/T700R4 conversion
'82 380SL, '86 560SL engine/trans. installed
'79 450SL, digital servo update
'75 280C

[QUOTE=Richard Wooldridge;2283726]Your 1979 450SL has the very troublesome climate control II system. To verify this, take note of the fact that you have buttons vertically oriented, and a separate AC rocker switch to the right side of the vertical row of buttons. QUOTE]

Yes, precisely.

Apparently all roads lead to waiting for my radiator to come back from the shop, re-installing and starting the car to check the A/C operation.

Regarding the vacuum system, it seems to be working fine. Automatic Door/Gas/Trunk locks all operate, and vent controls seem to be fine too. Perhaps a bit sluggish, but I don't know how fast top-notch operation is / was in the first place.

Regarding the servo, I will open it up and inspect, but it looks fine from the outside...however when I got the car I remember the mechanics at my shop pointing at it and saying to each other something like, "Oh yeah...it's got one of THOOOOOSE".... I guess they are truly known for their bad reputation.

Appreciate all the comments...

Stu