1985 300d air condition question - PeachParts Mercedes-Benz Forum

jack avant · #1 05-09-2010, 09:38 PM

I have a 1985 300D and the air works at times and does not work at times. I have checked the freon pressure and it is ok. I ran a 12 volt line to the compressor and it works. I can be driving down the road and turn it on and at times it works fine and blows good cold air. Is there a way that I can check different things to determine the problem? Your comments and suggestions would be appreciated. Thanks in advance. Jack

compu_85 · #2 05-09-2010, 09:41 PM

Might want to look into the Klmia relay, and the over voltage protection relay. The OVP lives behind / under the glove box, the Klima relay is under the hood on the driver's side fender.

-Jason

vstech · #3 05-09-2010, 09:41 PM

I've heard the OVP relay can cause issues with the a/c and plenty of other things on the 85. I believe it is behind the kickplate on the passengers side outer footwell.
it could also be a damaged climate control board, needing cold solder joints renewed. sometimes the aux water pump will freeze and draw excessive amps, and melt the board also. others have installed a 1 amp fuse inline with the aux waterpump.

JHZR2 · #4 05-09-2010, 10:10 PM

+1 on the relay. do tach and other electronics work?

jack avant · #5 05-09-2010, 10:33 PM

The tach is intermitant. It works part of the time. Does this car have a kilma? If so where is it located? Thanks Jack

compu_85 · #6 05-10-2010, 12:04 AM

The Klma relay lives under a black cover up on the drivers side inner fender by the brake booster.

-J

#7 05-10-2010, 07:22 AM

A flaky ignition switch on these cars is a common cause of what you describe.

okyoureabeast · #8 05-10-2010, 10:37 AM

Quote:

Originally Posted by jack avant

The tach is intermitant. It works part of the time. Does this car have a kilma? If so where is it located? Thanks Jack

Jack, the tach is a part of the OVP system. Pull the foot panel and replace it.

I am 95% positive this will fix your problem

kerry · #9 05-10-2010, 10:56 AM

On the 85's, isn't the Klima relay designed to cut out the AC compressor under certain conditions such as a heavy load on the engine? I'm pretty sure on my 85 TD I can feel the difference in air temperature at the vents when the Klima cuts out the compressor. There are threads on here describing how to jump the Klima relay. I assume that means it keeps the AC on all the time. That relay is not cheap if it is failing.

Jeremy5848 · #10 05-10-2010, 11:21 AM

Klima looks at a number of inputs including engine speed, refrigerant line pressure, full-throttle microswitch (on the valve cover with the throttle linkage), and the push-button unit (PBU) in the dash. Everything must be "right" or Klima won't allow the a/c compressor's electric clutch to engage.

Although the OVP relay is not directly a part of the Klima circuitry, it does play a critical role in the '85 300D: OVP protects the EGR controller (also behind the passenger-side kick panel) and the tach signal, uniquely in the '85 (possibly '84?), runs through the EGR controller.

When the engine is started, Klima keeps a/c OFF until the engine has reached at least 600 RPM. Should the tach signal be lost, Klima turns the a/c compressor off. So your "flaky tachometer" is related to the a/c problem. OVP is therefore the first thing to check. Remove the lower dash panel on the passenger side for access. Pull down on the OVP relay to remove it from its socket. Recognize OVP by its red top and the 10A spade-type fuse in the top.

Edit - Add photo, taken during dash replacement. Removing the passenger's lower dash panel provides sufficient access but you have to get down into the footwell the see anything.

Jeremy

ANFERSERIOUS · #11 04-10-2023, 09:37 PM

Hi Guys, just got done installing a new Sanden kit in my 1985 300d and the compressor is not engaging. Jumped at the Klima relay #5 and #7 it engages. New pressure switch at new dryer, new TXV, ETR switch at evap, P flow condensor, lines, and a charge of R12 at exactly 2.6#.

A/c works great when jumped, aux fan comes on, my tachometer works perfectly.

Took the Klima relay out and opened it up and it looks new.

I have a used one on order.

What am I missing here?

Sugar Bear · #12 04-10-2023, 11:20 PM

I don't have an '85 wiring diagram to view. The questions that come to mind are which wire signals the the Klima relay to close and connect 5 to 7 AND is that signal reaching the Klima? I'm guessing it would start at the pushbutton control unit in the dash.

Good luck!!!

ANFERSERIOUS · #13 04-11-2023, 11:36 AM

Quote:

Originally Posted by Sugar Bear

I don't have an '85 wiring diagram to view. The questions that come to mind are which wire signals the the Klima relay to close and connect 5 to 7 AND is that signal reaching the Klima? I'm guessing it would start at the pushbutton control unit in the dash.

Good luck!!!

Good call, I am going to try the "good" used Klima when it arrives then Ill trace the signal back to the CCU.
Whats interesting is I am getting +/-3.5v at the A/C compressor connector from the Klima relay when the car is running with A/C buttons on.
Seems like a short somewhere either in the relay or wiring to CCU ?

BillGrissom · #14 04-16-2023, 01:43 AM

Check that you have minimal drop across the AC pressure switch (low pressure disables) when it is closed (satisfied). That is in the ground path from AC clutch- wire, as I recall. The switch may be low resistance but its ground connection poor. I think that is the same wiring in both my 1984 and 1985 (Klima) 300D. One question is why they put that switch in series with the clutch current, rather than on the low-current "logic side" of the relay. If the Klima relay outputs 12 V to the AC clutch, don't blame it. Not positive, but I recall that 1986+ cars also had the Klima switch, but don't know if an exact interchange.

ANFERSERIOUS · #15 04-18-2023, 08:13 PM

Quote:

Originally Posted by BillGrissom

Check that you have minimal drop across the AC pressure switch (low pressure disables) when it is closed (satisfied). That is in the ground path from AC clutch- wire, as I recall. The switch may be low resistance but its ground connection poor. I think that is the same wiring in both my 1984 and 1985 (Klima) 300D. One question is why they put that switch in series with the clutch current, rather than on the low-current "logic side" of the relay. If the Klima relay outputs 12 V to the AC clutch, don't blame it. Not positive, but I recall that 1986+ cars also had the Klima switch, but don't know if an exact interchange.

Thanks for the response Bill,

Klima is working fine. I am surprised the switch is in the ground path from the A/C clutch. I have been trying to track down where that harness grounds out so I can see if the ground is good. I can trace it back to the corner near the battery- where there is a main ground. Without taking the harness apart I cant see if the pressure switch ground path links up there.

I opened up the ccu and resoldered any questionable joints and now I am getting the clutch engaging and disengaging for the first 5 minutes of running. Then it goes away and runs properly. Still seems like a wiring issue somewhere.
Blower runs as it should, A/C is cold, CCU cycles through switches as expected.

This is a California car, i bought it from the original owner, Airline mechanic, its been garaged most of its life so im not expecting any serious corrosion issues anywhere messing with signal but who knows...

Anyone know where the A/C pressure switch grounds to?

When I installed the Sanden compressor I added a ground line from the housing of the compressor to the inner fender, I cant see how that would hurt?

Thanks in advance for any suggestions!