I'm trying to get the air conditioning on a 1985 Mercedes 300D-T to work. Here's what I know-

• A/C is R-134a conversion, done by a Merc technician in 2014
• Refrigerant at correct levels, no leaks
• Compressor works intermittently (once every other week), tach would come on at same time
• Compressor otherwise is not receiving power, clutch engages when jumped, but replacing Klima relay has not changed this
• Removing EGR computer and jumping tach brought back tach signal, but not A/C
• Aux fan not working

I've brought this to 3 different shops, all they tell me is it's electrical, there is no electricity reaching the compressor. Fuse #8 is disconnected, anything that should be on it is on another line. One shop said the condenser fan "has burnt up relay and connection" (does the aux fan even have its own relay?), though from another post here with a working AC, it seems that isn't necessary to get the AC to run. Regardless, when the compressor clutch has, for whatever reason, engaged, the climate control works fine the entire drive, only to stop working after starting the car the next time. My only thoughts so far-

• replace OVP (with or without reconnecting EGR computer), I know it connects to another circuit ("circuit 30") but I don't know if that's for anything compressor related
• replace ETR switch on the evaporator, maybe causing compressor to cut out
• replace climate control unit, last resort, temperature sensor appears to function when compressor worked, turning off after getting to temperature

Thanks in advance for any advice.

Fuse #8 controls 3 things in its stock configuration. The largest load is the Blower Motor Control box under the dash. The other items it sends power to are your Climate Control Pushbutton Unit AND your KLIMA Relay. Your KLIMA Relay is fed power through a Red/Yellow stripe wire from fuse 8. Fuse 8 is fed power from circuit 15x which receives power when your key is in the Run position.

Your Klima needs to see an RPM signal (among others) to engage the Compressor clutch. That RPM signal comes from your EGR Controller. Your EGR Control Box receives its power through your Over Voltage Protection Relay.

If your OVP relay is bad, you will not have a functioning Tachometer, and your Klima will not engage the Compressor because it can’t receive an RPM signal.

When I bought my 85 300CD, my tach didn’t work. I replaced the fuse on the OVP relay and it still didn’t work. Turns out my OVP was defective. I bought a used OVP and all is well.

So does the OVP then control any other part of the AC unit besides the EGR computer? I’m going to try reconnecting it when I get a new one in but I, at least, already got the tachometer signal back by bypassing the port with two jumper wires. So theoretically the AC system is seeing a tach signal, unless the OVP does something more.

I only just tried jumping the Klima pins from 7 to 10 to enable the compressor clutch. It appears I’m not getting the 12V needed from the Klima. Would that be related to the OVP? Or is there something else upstream? Clutch still works with a direct jump of 12V to the plug.

No. I believe the OVP only protects the egr computer on the diesels. Bypassing the egr computer by jumping the pins on the connector will give you a working tach but it still won't give a signal to the computer because A: the connector isn't plugged in and B the computer isn't receiving power. I'll check my schematics when I get home. In the mean time, at least pull your ovp and check the fuse. Remote lower kick panel and reach up near the outside vent.

Passenger side kick panel.

Okay, but the EGR computer doesn't send any signal to the AC besides the tach speed, correct? I've checked the OVP, fuse is fine, already ordered another in case the internal diode is bad.

If the OVP only protects the EGR, and the EGR only gives the AC system the tach signal. Unless the computer sends any other information to the climate control, or the OVP has another connection to the compressor, it seems moot. As I said previously, it seems the Klima is getting no power.

If you have experience soldering, you may want to consider opening your OVP and reflowing the solder connections from the main pins to the board. You may also need to do this to the EGR or Klima relay, although you'll need a fine point tip on your iron for those. Sounds like a intermittent connection due to a bad solder joint. At least once done, you'll eliminate this from the diagnostics. I would recommend doing this for any old relay before proceeding with parts replacement.

Good luck

See this thread for some good info and tutorials

1987 300D bad OVP symptoms but OVP is good

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Stable Mates:
1987 300TD 310K mi (Hans)
2008 Jeep Grand Cherokee OM642 165k mi (Benzrokee)

123.0, 123.1 Electrical Chart

Since I can't jump the compressor from the Klima, tach reading aside, my thinking is the ETR switch is faulty. This chart, which might be the wrong year and could be showing the ice cube relays, seems to show the ETR switch is between the dashboard buttons and the compressor relay. Which is effectively the Klima relay in the 1985(?). I've already ordered an ETR switch, but, if there anything else that would be upstream from the Klima 12V pin besides the ETR switch, I'd be interested to know.

Attached Thumbnails

 

Correct me if I'm wrong but that diagram doesn't show the micro switch on top of the valve cover that cuts the compressor during hard acceleration. I think it removes ground when triggered or if it comes unplugged.

You're probably right, this is just the only version of that chart I've seen around online. But that's the first I'm hearing of that acceleration switch, does it have another name?
So far, I will go back and resolder the OVP I have, while waiting for the new one coming in, to see if that makes any difference. ETR switch is coming in next week.

I was going to ask whether you had looked at the switch on top of the valve cover. When that switch got wonky on my '85 300D, it caused the compressor to cut on and off very quickly, or not turn on at all. Both symptoms of course resulted in little to no cooling in the cabin. I unplugged the switch and tied it off to the side, and haven't had any issues since. I know nothing about car electrical systems. But I can say with certainty that the micro switch does not need to be plugged in for the A/C to work on a '85 W123 with KLIMA, and that the switch can fail in a manner which prevents the compressor from engaging while the switch is plugged in. Good luck!

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Christopher
'06 Mercedes E350 station wagon (silver/black)
'85 Mercedes 300D (black pearl/palomino)
'85 Mercedes 300SD (smoke silver/burgundy)
'79 Cadillac Sedan DeVille

'05 Toyota Camry (because always running is nice)

'85 Mercedes 300D sold back to orig. owner 8-1-06
'84 Volvo 264GL Diesel, owned 2000-2013

This switch on the cover, you mean the kickdown? 0125450728 (connector) and 0025450114 (the switch itself)

This post may have helpful information, beginning with post #5:
What's the switch on the valve cover do? (617)

__________________
Christopher
'06 Mercedes E350 station wagon (silver/black)
'85 Mercedes 300D (black pearl/palomino)
'85 Mercedes 300SD (smoke silver/burgundy)
'79 Cadillac Sedan DeVille

'05 Toyota Camry (because always running is nice)

'85 Mercedes 300D sold back to orig. owner 8-1-06
'84 Volvo 264GL Diesel, owned 2000-2013

Thats correct, the only signal your klima receives from the egr computer is the tach signal. The jumped pins in the female side of the plug will not send a signal to the klima since they are bypassing the egr computer, however your tach will now function.

As I said previously, your klima is powered through the OVP so its a good thing you are replacing it.

Volvo Diesel gave a reference to the only 1985 KLIMA schematic I’ve seen. Pin 2 is the tach input.

You are correct in the P/N’s for the micro switch.

Okay so I’ve replaced the OVP with one that supposedly had been tested as good. I’ve tried this with and without the EGR computer/tach jump. With and without the valve cover switch plugged in. Unfortunately nothing has changed, still no power reaching the Klima. Going to try the ETR switch next when that comes in.