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Old 06-21-2000, 11:32 AM
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My name is Jerry Kaidor, and I just got this '85 300D.... It turns out that the car is a bit of a bastard child - somebody had installed an engine from an '81 300TD wagon. The engine compartment is a pseudo-random mix of '81 stuff and '85 stuff - usually the worst stuff from the respective time.

For example, it has that fine California '85 Trap Oxidizer. And the superior early vacuum pump. And the improved early transmission. And an fuel injection pump from a non-turbocharged car ( no ALDA ).

...So I have been forced to learn rather more about mid-eighties Mercedes technology than I had planned. Oh well, that's the fun part, right?

The current project is to make the AC work. The car came to me with a missing high-side hose, so there is no R12 to get rid of. Oh, and it had the wrong condenser - the existing condenser was correct for the earlier "York" compressor
setup. $125 later, a local wrecking yard
provided me with the correct part - I have
new O-rings for the whole system, a rebuilt
compressor, a new receiver/drier, the missing
hose, a dual-guage charging manifold, a
powerful vacuum pump, some "ester" refrigerant oil, and a couple cans of R134a.

So I expect that, barring unforseen circumstances, in a few days I will have a complete and working AC refrigerant circuit.
Then the real fun starts....

....I'm sure I will have to do some electrical/vacuum troubleshooting in the ACC
(auto climate control) system. Because it
doesn't control temperature properly in "economy" mode. As you know, economy mode attempts to control cabin temperature with heat and exterior air. There doesn't
seem to be any proportional control happening
WRT the temperature wheel on the console.

Having done my research, I understand that
there's a certain number of standard things to check. For example, the circuit board behind the multi-button array tends to get cracked solder joints where it's soldered to
a couple of other boards. And the aux water pump can short out and destroy part of the board ( because Mercedes didn't use an inline fuse ). And the "mono valve" can be problematical. Then, of course, one checks in general to see if everything is plugged in.

Since I used to be an electronic tech, nothing in there is particularly scary EXCEPT that I have no schematic!

I bought the Mercedes tech info CDROM,
and the climate control schematics therein are completely unusable. It looks like they did a careless conversion to 1-bit (black & white ) graphics, and half the wires disappeared!

At any rate, if anybody has a good climate control schematic they could copy off or scan for me, I would be most grateful.

Thanks in advance,

- Jerry Kaidor ( )
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Old 06-21-2000, 07:54 PM
Join Date: Mar 1999
Location: Tucker, Ga USA
Posts: 12,153
Since 1985 cars used Compressor cut-out relay & one very important imput is flywheel speed, you will have to adapt relay to turn compressor on. Shouldn't be too hard though as all of imputs are already in place.
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Old 06-23-2000, 01:08 PM
Posts: n/a

I don't think crank speed is an issue with these cars. I understand that this appeared with the W124 because it has a serpentine belt, and a compressor failure
can break the engine! So those compressors have a built-in speed sensor, and if the
crank speed and the compressor speed are too far apart, the compressor is disconnected.
Unfortunately, the speed sensor makes those particular compressors Mercedes-specific, and therefore pricy.

Update: I have successfully charged the refrigerant circuit with R134a. With a clip lead to the compressor clutch, I get cold air out the ducts. Yay! But the control system still won't turn it on without help from the clip lead .

I've half a mind to scan this climate control schematic into a GIF and post it on my website, so I can show people just what I have to work with....

- Jerry Kaidor ( )
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Old 06-23-2000, 01:55 PM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
Ahh, but Pat is right about the 85. I can't verify the point but 85 617 motors began using the engine speed sensor for the tach and EGR control and for some reason A/C clutch control even though the belt wasn't the problem and the compressor was still a GM R4 with no speed sensor.

The earlier 617 TDs used a sensor on the front balancer. The 85 used a sensor on the flywheel. I have bypassed these control relays when necessary. Does the tach work?

Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician
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Old 06-23-2000, 03:50 PM
Posts: n/a

I guess the "engine speed" input came in coincidentally with that blue vacuum servo thingie on the L/H firewall. The one that mostly produces an improved simulation of
manifold vacuum for the transmission.

Here's where the plot sickens, as my aged mother used to say. My car is a bit of a bastard child. At some point in time, somebody installed an engine from an '81 300TD wagon in it. This is a turbocharged engine - the wagon got the turbo one year before the sedan. The under-hood environment is a pseudo-random mix of '81 stuff and '85 stuff. For example, the existing condenser was appropriate for a 240D! Took some research and a trip to the junkyard to get the right condenser....

No, the tach doesn't work. And it looks like they installed the '81 transmission along with that '81 engine, because I have been unable to find an engine speed sensor on the transmission. Best I can tell, this
sensor should be up on the L/H side of the bellhousing. Nothing. Another thing needed to make the blue servo work, is an injection pump with a rack position sensor, which mine does not have. In fact, my injection pump does not even have a proper ALDA - it has the earlier, round "altitude compensator". And even if I got the blue
vacuum control unit working, I don't know if it would do a good job with the '81 transmission.
When I bought the car, it did teeth-jarring
shifts. It turned out somebody had blocked off the vacuum to the tranny. I researched the system, understood the problem ( transmissions need manifold vacuum, diesel cars don't have it ), and hooked up the Vacuum Control Valve ( VCV ) on top of the injector pump per the early '80's system. This gave good shifts at low power levels, but the simulation still breaks down when you stomp on it. Close enough for government work....

Another thing that's missing is the "engine cold lock" sensor. The connector dangles at the end of its cord.

I don't even know if the wires that I connected to the compressor clutch were the right ones. The brown ground wire, that's
a slam dunk. The other wire was green/blue/black, and I can't find that in any of the schematics.

Assuming that it's not reasonable to retrofit the sensor to the transmission, I may try to adapt a harmonic balancer sensor to do the job.

But I see no reason to have engine RPMs affect the action of the compressor. Hopefully, I can downgrade this system to work properly as an '83 or so. With 12V from the pushbutton box through the aux fan and compressor relays, and input from the ETR switch. O if only I had a proper schematic!

- Jerry
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Old 06-26-2000, 04:37 PM
Registered User
Join Date: Jul 1999
Location: Tampa, Florida, USA
Posts: 214
I just read your post on about your now-working AC. Congratulations! It's stuff like this that gives the 123 diesel series its charm ...

- Nathan
'83 240D, 252k miles
'00 New Beetle TDI, 7000 miles (and counting, vrmmmm)
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