A/C Not Engaging - PeachParts Mercedes-Benz Forum

JayK · #1 03-18-2010, 10:37 PM

I recently purchased this car, the A/C didn't work when I got it. The previous owner said he replaced the evaporator. The compressor doesn't want to kick in. I'm assuming that it needs a recharge (don't have a gauge though), but I found some switches disconnected and have a few questions. Attached is a photo of the receiver as I found it. Both the temp and pressure switches where disconnected. I'm not sure if the PO left it that way or my mechanic. The pressure switch is reading a short across the terminals and the temperature switch is reading open. What's normal with the engine off? Should disconnecting the pressure switch and opening the circuit allow the compressor to kick in (which it doesnt)?

Also, my dash is telling me that the outside air temp is 20 deg F. Does the climate control system use this information? Would that contribute to the compressor not kicking in?

macdrone · #2 03-19-2010, 01:06 AM

The testing of the switches I am not sure of but as far as my escapades went i had to replace them because with a full charge my A/C compressor would not engage.

I thought I had a leak which I am pretty sure I never did.

Those 2 gages read pressure and a seperate pressure(same as temp which is funny from what I have read) that is safety for the compressor so it never runs without Refrigerant to keep it from burning up.

Its silly he replaced the evap but left the dryer old in the system.

I suggest you order the 2 sensors (about 25 bucks total) and charge it up and see what happens.

I did it best A/C ever in my car as things go.

JayK · #3 03-23-2010, 06:23 PM

Thanks mac drone. Here's an update:

I just made a code scanner, and I'm getting strange results. When I hook it up to pin 7 (climate control pin on my model) of the 16 pin diagnostic header, my led lights up dimly. When I press the switch, I get a single blink at regular brightness, indicating 'all ok'. I confirmed that the pin is floating at ~8.5 volts, and not at 12 as it should be. Even weirder, when I hook up to pin 7, the blower cuts out and the air recirculating indicator on the climate control console lights up.

I confirmed that the static refrigerant pressure is well above cutoff. And I do hear a relay near the fuse panel click when I connect/disconnect the cutoff pressure switch on the drier. Also, I did conform that my blinker box operates normally on the two other diagnostic pins.

I have my suspicion on the problem, but I've never troubleshot on these cars before. Any opinions? Has anyone seen something similar?

JayK · #4 03-24-2010, 12:31 AM

I traced the clicking sound of a relay while (dis)connecting the low pressure cutoff switch. The relay, which is blue, has a burnt 15A fuse on it. Is there any particular reason why it could have blown which I should check first? The schematic on the side of the relay doesn't make it clear to me whether the fuse protects the relay, or the circuit it's connected to.

macdrone · #5 03-24-2010, 03:08 AM

Probably load on the relay, putting in a new fuse wont make it any worse try it. It will just burn out if its under too much load. Could be the compressor is bad and when it engages it pops the fuse.

Hit Man X · #6 03-24-2010, 06:54 AM

First, is there any refrigerant in your system? Do you have gauges? Toss them on and check your pressures with the system off... they should be balanced. They WILL change depending on temperature. You could be simply low on refrigerant or the system was reassembled improperly.

Fuse popping could be due to all that wiring being incorrect...

From what I remember, those two wires with the 90° looking ends go to the pressure switch... the dude with the bent over ears.

The two wires with a sort of plastic jacket covering them go to the red thing... that is your aux fan switch for when head pressure becomes too high.

Hook stuff up properly and see where that gets you.

I usually give the clutch a spin to see if the compressor is locked up or not.

Hell a bad belt will keep the compressor from staying on, at least on the 603s. There should be a speed sensor that detects a change in RPM and will turn it off so you are not stranded.

JayK · #7 03-24-2010, 03:14 PM

I hooked up the gauges and they read well above cutoff. The aux fan now (after replacing the aux fan relay fuse) comes on immediately upon turning the key, and stays on. I was able to get the compressor to come on briefly by bypassing the red pressure switch. Does that indicate that I have too much of a charge in the system? Also, shouldn't the fan go off while bypassed? Or is it the opposite? I'm doing my best to follow the MB troubleshooting guides, but a lot of it seems to be different in my model year.

Also, attached is a photo of another lonely connector I found. It's under the intake manifold, and I can't get a good look of where it's headed.

macdrone · #8 03-25-2010, 02:06 AM

Sounds like over pressure if the fan is on as soon as you turn the key. Or the sensor is bad, if it happened while you shorted the wires then thats why the fan came on. The switch is normally open and closes when pressure is applied to kick fan on (thats why the fan comes right on). I would suggest replacing them (have to recharge system) and try again.

JayK · #9 03-25-2010, 06:57 AM

Macdrone, thanks for all the advice. I tested the switch, and it's open. That's whats baffling me. The static pressure is 4.5 bar. The fans to cut out when I disconnect the low pressure switch. I guess that just turns off the whole system, and not just the compressor?

Assuming the previous owner is telling the truth, I wonder if my shop missed something when installing the new engine wiring harness. There is that one mystery plug in that last photo above. The fact that the compressor finally did come on if only for a few seconds tells me something. Not sure what though.

Can anyone tell me what that thick two pole plug/receptical combo next
to the drier is for? It was also disconnected.

JayK · #10 03-25-2010, 07:04 AM

macdrone, i forgot to say. I'm going to take your advice and replace those switches and do a recharge. Drive to harbor freight to buy a cheap vac pump, and they were out. As soon ad I find a VP I'll do it. If for some reason there is air in the system that could be setting of the HP switch right away.

JayK · #11 03-25-2010, 10:13 AM

I feel stupid. I read hit max x's comment backwards and hooked the cut
out circuit to the red HP switch and the aux fan circuit to the LP switch. It's so obvious now given the symptoms. The only thing I'm still curious about is that I thought one of the two fans were suppused to stay on, and the other was the aux. Are they both considered "the aux fan"?

Hit Man X · #12 03-25-2010, 12:20 PM

LOL, oh well.

Yes, both fans will run when commanded either by the temp switch (probably on the cyl head for the 606) or when the red switch commands it. I guess the switch closes and power flows.

Anyway, both run.

Hit Man X · #13 03-26-2010, 10:11 PM

I was workin on my SDL here earlier and from what I could remeber, that large black plug is for the aux fans.

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