AC Fix, 1985 300D - PeachParts Mercedes-Benz Forum

BillGrissom · #1 09-02-2019, 02:50 PM

This problem is specific to 1985+ 300D and perhaps some 1986+, though I recall reading some have even more involved AC compressor controls.

The AC compressor stopped engaging. I noticed the tachometer wasn't working. I recalled reading about that correlation, so skipped the under-hood KLIMA box (links) and went upstream. Several posts mention that if the KLIMA box doesn't get a tach signal, it won't engage the AC clutch, which was my scenario. The last link below explains all, with photos. Following that, I jumpered pin 16 (from pickup signal -) to pin 14 (gnd) and jumpered pin 20 (from pickup signal +) to pin 25 (to tachometer). The tachometer then worked and AC compressor engaged.

My value-added is that I opened the EGR Control Box ("1985 617.95X Control Unit" to M-B) and took photos. The large component in clear plastic labelled "Bosch" has a copper bellows, likely to measure barometric pressure. I saw no obvious damage. The conformal coating and multi-layers made it too hard to follow the traces. Most likely the raw mag pickup signal (sensor on transmission bell-housing, coming in on pins 16 & 20), goes thru an amplifier before being output (pin 25, rel to pin 14 gnd) to the tachometer.

Finally, since I saw no damage, I went further upstream and wondered if the car was being nice to the EGR Box. It should get +12 V power on pin 1. Nothing. That comes via the Over-Voltage Protection (OVP) relay above it. The 10 A fuse was OK. I measured 13.4 V with the engine running, so the alternator's voltage regulator is working perfectly. I'll post back after I resolve the OVP issue. It appears the OVP relay feeds only the EGR Box. Since, like most, my EGR is no longer functioning, the car is still as functional.

www.peachparts.com/shopforum/diesel-discussion/359300-ac-compressor-wont-come-85-300d.html (note that funola mis-typed, jumpered pins 7 to 10 at KLIMA)
https://www.benzworld.org/threads/1985-klima-relay.2466569/
http://www.peachparts.com/shopforum/diesel-discussion/317518-85-300td-tach-issues-dont-worry-ive-searched-forum.html

BillGrissom · #2 09-02-2019, 03:53 PM

Yes, the OVP relay supplies just the EGR Box. That is shown in several of the links. Schematic from the last link is attached.

If you don't mind simplifying/molesting your M-B, if you already removed the EGR vacuum stuff (about half the underhood vacuum clutter), plus EGR valve because none of the above controls still work (no parts avail) which also allows easier access the alternator bolts as does re-routing the HP AC run (could be life or death issue if alt/fan belts fail in Death Valley), then you could discard the 1985's OVP and EGR Box. Replace the later w/ 2 jumper wires. Or you could pay $$$ for replacement parts and try to keep it all working.

vwnate1 · #3 09-02-2019, 07:11 PM

Thanx Bill ! .

I've saved the last image, it looks useful .

BillGrissom · #4 09-02-2019, 10:31 PM

I plugged the OVP Relay back in, heard it click as key goes to "run" position, and now the EGR Box gets power, dash tach works, and AC clutch actuates. Probably an erratic OVP. Not terribly expensive, so might replace it if problem recurs, though I don't need the EGR Box which is all it powers per diagram.

vwnate1 · #5 09-03-2019, 10:11 AM

My 1982 240D's windshield wiper relay box is just now beginning to die, I guess they have a finite lifespan .

The '82 is the last year they used the early style relay with the larger pins so finding a good used one won't be easy .

I checked on new ones some time back , they're not cheap .

funola · #6 09-03-2019, 12:38 PM

Quote:

Originally Posted by BillGrissom

I plugged the OVP Relay back in, heard it click as key goes to "run" position, and now the EGR Box gets power, dash tach works, and AC clutch actuates. Probably an erratic OVP. Not terribly expensive, so might replace it if problem recurs, though I don't need the EGR Box which is all it powers per diagram.

Compared to the Klima, the OVP is pretty simple, per the diagram: a fuse, a relay and a zener diode. What can get erratic other than the relay contacts?

Note b on the OVP says it goes to fuse circuit 15, note c says it goes to circuit 30 (I would assume also a fuse circuit). I think more likely the erratic behavior is due to dirty fuse contacts.

BillGrissom · #7 09-09-2019, 03:43 PM

Quote:

Originally Posted by funola

... note c says it goes to circuit 30 ...

Yes, many people have wondered what "circuit 30" means. I don't recall I have 30 fuses, and the schematic would have said "fuse ##" as they do for fuse 15. It is obviously a 12 V power source, since it goes straight to the integral fuse. My guess is they tap that from the 12 V feed to the "Blower Speed Relay Box" which is above the glove-box liner. I measured the 12 V source was fine, but doesn't mean it isn't erratic or could keep supplying 12 V under a load. More likely, a problem on the coil side wiring, since I don't recall hearing the OVP relay click originally. Even if its contacts were bad, I should still have heard it try to actuate.

vwnate1 · #8 09-09-2019, 06:34 PM

I don't have the diagrams to look at but a circuit labeled #30 in a German car means it's direct battery current, un switched and un fused .

Always a red wire and usually 1.5 diameter .

This is according to the DIN codes .

15 will be un fused current in the basic ignition circuit, always black .

58 will be the park lights, fused .

And so on .

funola · #9 09-09-2019, 10:43 PM

Here's a power distribution diagram. Circuits 15 and 30 pertain to the circuits on the ignition switch. 30 is hot at all times- unfused. 15 is switched unfused.

funola · #10 09-09-2019, 10:47 PM

Power distribution

vwnate1 · #11 09-10-2019, 09:58 AM

Thanx ! .

I'm glad my 50 year old information was correct .

I don't much like these newer typ of schematics, they're not helpful in the normal diagnosis .

I wonder why I'm banned from posting any attachments no matter how small ? .

my123ca · #12 09-14-2019, 11:25 AM

You can mount the OVP Relay incorrectly if the guide isnt there anymore.