I can ALMOST feel the cold air after an extensive rebuild of my old R4 system. No power at the compressor with the pressure switch jumped so into the Klima box I went! I could smell the stink as I scraped off the silicone. Yup, she's cooked! 1985 300CD


Does anyone happen to know what this diode was so I can try replacing it and fixing the burnt tracer on the back???
I put better photos of this thing on benzworld in the W123 section on the AC clutch diode thread. That site is better for photos.

Many thanks!

That doesn’t look like a diode.

Looks more like a power resistor.

ETA pull the component. Measure its resistance. It may have survived even though the paint is gone. Looking at the bottom of the board it looks like it’s part of the relay driver circuit on the little transistor next to it. The burned trace looks like it’s on the high current side of the relay. I’m saying they’re different nets and their failures may not be related.

__________________
79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles

Good call! I'll pull it out today for a better look. It does seem to have the shape more in line with a resistor.

Thanks!

I have to say that’s an interesting klima. It’s quite a piece of history because it has a General Instruments 8 bit PIC microcontroller on it. This was before Microchip Technology acquired the architecture and made it the biggest selling microcontroller in automotive today.

My Klima had a 4 bit microcontroller made by National Semiconductor. A real dinosaur. So this think is a later design from what I can guess.

To really know what’s up you can trace out the circuit on paper and draw the schematic around the relay and transistor driver. My feeling is your burned component is some kind of current limiting or current sensing resistor on the driver side of the relay. The actual contacts are a much higher current circuit which is what got burned. Likely your clutch coil output got shorted to ground instead of through the clutch coil and pop went the trace before the fuse. Could this have happened while you were shorting stuff trying to trigger your compressor?

The burned resistor is on thinner traces upstream of the relay. This is on the coil (control) side of the relay and runs 100x lower current. Technically it’s magnetically isolated from the compressor clutch circuitry that got popped. If it had seen the same currents that popped that trace we’d see the thinner traces connected to it(mystery burned part) smoked long before the fat trace melted.

So that’s why I think they are unrelated failures. I actually suspect the resistor is still good. It has just seen a hard long life and the paint is evaporated off it. Test it. My gut feel is it’s a 100 ohm resistor used to set a collector current for the compressor relay. It stays on whenever the AC is on and likely aged out due to lots of heat over a forty years.

Just a wild guess. If I cared, I’d take your circuit board top image and I’d overlay it with a mirrored transparent version of the bottom green board image and reverse engineer the circuit. You can do this with some simple photediting software. Then you trace out exactly what the circuit is and you can determine the likely mystery component. My guess it’s about a 100ohm resistor on the collector or eimitter side of that transistor. Why? I don’t know. Likely to spread the heat load between the relay coil and the resistor to keep the coil from getting too hot.

__________________
79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles

I got the car earlier this year with a non functioning AC system. I found the clutch plug shoved up in the harness so yeah it's possible it grounded out someplace. Car is a 1985.

Have you seen that transistor standing up like this one on these boards? First I thought someone had replaced it, but I found this one is loose on the board. I'll touch up the solder when I fix the trace.

I'll get to work and report back! Appreciate your expertise!!!!
Why do I have a feeling these cars could benefit from a plug with a diode on it like every other R4 plug I've seen on chevy's etc???

I bought one of these to replace the connector. https://www.oreillyauto.com/detail/c/murray-climate-control/murray-climate-control-2-terminal-harness-connector/mry0/37201?q=diode+ac+connector&pos=16

That resistor ohm'd out to be a 12.2! So you're right, it's probably still good.
I re seated the big transistor and fixed the broken trace.
Same story, no voltage at the compressor plug with the pressure switch jumped. :-(

There is a small change on the plug when I click the push button control unit.. It reads 47mv in off and when I select something else that would flip on the compressor, it goes to 52mv. So it's doing SOMETHING I guess. It was doing that before I worked on the klima.

Just to be sure I wasn't missing something, I jumped pin 5/7 on the plug and heard the compressor click. Something is up with my klima box and I'm think I'm outta luck with this one.

Sounds like the relay is dead. Open the cap on the klima and ask somebody to look at it while you turn on ac. You should see it move. Also try manually pressing the contactor with your finger and closing it. Something is obviously happening. I think you may have a big contact resistance on the relay.

Ooh…or the controller is burned and not throwing the relay when the pressure switch closes. I think that’s how the klima works. I recall it takes a bunch of inputs like throttle switch, rpm, pressure switch etc. to close the compressor relay. It isn’t like the old school w123s that kind of wired everything in a daisy chain.

The transistor tilted up looks like sloppy assembly. These old boards were hand loaded then wave soldered by humans in Germany. I’m a little stunned the part was crooked but it seemed soldered in correctly.

I’m not familiar with the diode on the plugs. Are these devices to keep the inductive spike from destroying the relay? Looking at your board both relays are protected by flyback diodes on the board.

You may have to trace the wire back. But as I said if you are seeing a small change on the compressor voltage that likely means the relay is burned. It’s like a huge resistor right in series with your compressor. You may need to grind the contacts a bit. At the end of the day the clutch is just seeing a relay switch closure to 12v. The relay is either on or off so if it only goes to 42-52mv that screams burned contact. If you shot enough current through to melt the board you may have melted the relay contact too. Likely your switch (relay) is the weak link.

I forgot if I mentioned it before but I simply wired my own relay in place of the klima. There are a number of posts here on how to do it. It doesn’t perfectly replicate the klima function but when I hit the ac button my cold comes on. All it takes is a lamp relay and some wire. I broke my old klima board out of the box and reused the old pin contacts and black plastic enclosure. Nobody knows by looking at the outside that the whole thing is hot wired.

__________________
79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles

Ya know, I didn't think about manually pushing that relay. I'll have to give that a shot.

Giving this some more thought, I see lots of crap will turn OFF the compressor, but WTF turns it on? Is it setup to go all the time unless one of the handful of things is wrong?
My thinking is it's GO signal is the KL (10) but it must have something to do with the little d or c. Little d is to temperature control unit (terminal 10)

Do you know if there is a way to verify any of that? I've got my control unit out of the car at the moment. I hate like hell dealing with 40year old german plastic, but I'll rip it apart if you think it's worth a peek...

I did buy another klima on fleabay, suppose to be NOS and was a hundred bucks, but knowing my luck, I'll install it and nothing will change. That's why I'm still in the fight at this point. lol

Yes, that is correct. Every old chevy I've come across has a diode in the plug for the purpose of protecting everything up stream. I think VDO just stuck the diode inside the Klima box.

BIG OH CRAP!!!! I just pulled the silver temp box and lets say I don't see any burnt traces at the moment! But every cap looks to have a problem. Visit https://www.benzworld.org/threads/ac-clutch-diode.3162105/?post_id=18931352#post-18931352
to see the carnage!

Anyone know where I buy a buttload of these funky yellow caps???

The box in the pdf is NOT the same box I have. ughh..

https://trythistv.com/wp-content/uploads/2024/02/climatefeb2024-1.pdf

Uhh, I don’t think the silver box is your problem. Or at least it isn’t your only problem.

There is nothing special about those caps they are just electrolytic capacitors. You can order regular radial leaded capacitors at Mouser or find cheap ones elsewhere online. The specs are on the side of the cap. Make sure your voltage, capacitance and lead spacing matches. Take photos and make notes of where each one came out.

Your relay on the klima should send either 12v or nothing to your compressor clutch. The fact it’s putting out a few mV tells me there’s a bad connection somewhere between the relay and end of the plug.

Yeah the caps look leaked out in your silver box. I think that means the temperature control won’t work well. The push button unit should still send a command signal to turn the compressor on when it is on max cool. The silver box doesn’t turn your compressor on and off. On these cars the compressor is turned on then the silver box does things like modulate the fan and hot water monovalve to stabilize the temperature. The compressor is always on.

__________________
79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles

I did the jump thing between 5 and 7 at the Klima plug and did get a full 12vts measured at the compressor plug.

I believe you are right, my temp box isn't my ONLY problem here. Multiple failure can be a real ***** to sort out. What I still can't wrap my head around is I know there are 4 or 5 things that the klima monitors to shut off the compressor, but what tells it to turn ON the compressor?

I'll get the replacement klima this week, but if that one doesn't turn on the compressor, I've got something else still interrupting the circuit. I've checked 96,97,26,27 and 31.

The d and c on 27 are the only ones I haven't dug into. That's the anti-icing switch and temperature control unit that feed the low pressure cut out switch which lets the power go through to the Klima. More digging required.

I talked to Tom at Trythis and he's going to take this info from my temp box repair and add it to the pdf. So all this work will hopefully help someone else.

Useful further reading.

https://www.peachparts.com/shopforum/diesel-discussion/185801-klima-relay-options.html

Did you try manually closing the relay(s) with your finger?

__________________
79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles

I thought about that. The are 2 relays in this thing. One is NO and the other is NC. So i guess close the one that's open?