trouble shooting the switch control unit - PeachParts Mercedes-Benz Forum

sdparizona · #1 07-02-2005, 07:00 PM

hello all,
so I am not getting any juice to the compressor.... here's what I have done so far. this is a conversion to 143A and so have flushed the system and replaced the compressor, the switch control unit (but it was a used one!!

) the temp switch for the aux fan, the low pressure switch, and the expansion valve under the dash. As some of you might know I am still trying to find the A/C compressor relay which is somewhere other than the fuse block on the drivers wheel well (must be something specific to the '85 300TD). I thought maybe it could also be the ETR valve so I pulled the glove box and bypassed the ETR valve circuitry and still nuthin, anyway so I was thinking as I do some research on where that relay is, it could also be something wrong witht the used switch control unit I got. everything works fine with the entire system except the compressor wont turn on. while I was bypassing the ETR switch I did some checks for power there and there is no power at the ETR switch, there is also no power at the pressure switch (neither side) so i am now questioning the switch control unit. so with the background said here's my question........ does anyone know how to test the pushbutton switch unit to see if it is actually sending out the power to the compressor?? thanks all for sticking with me on this. specifically I would like to know which terminal on the control unit I should be testing...

sdparizona · #2 07-02-2005, 07:08 PM

oh, forgot to mention, the compressor works fine and is hooked up correctly. with the engine off I con take a lead directly from fuse 8 to the compressor and I can hear it click on loud and strong, so that is not the problem. the problem is before the compressor, before the pressure switch, before the ETR valve....

sdparizona · #3 07-02-2005, 07:10 PM

hey does anyone know if the A/C compressor relay is before or after the pressure switch? that might shed some light on whether it is the relay or something else. I am thinking......... or trying to.......

sixto · #4 07-03-2005, 12:37 AM

The compressor relay coil is powered through the evaporator temp switch. Power to the compressor comes directly from a fuse.

Sixto
95 S420
87 300SDL

sdparizona · #5 07-05-2005, 02:21 PM

thanks for the info,
so where does the power to the evap temp sensor come from? I am not getting any power to that switch. so starting at the end this is what I am picturing. power to the compressor comes directly from fuse 8. control of the compressor comes from the pressure control switch on the receiver which in turn comes from the ETR (evaporator temp) switch under the dash but where does it come from before that??
thanks
Scott

sixto · #6 07-05-2005, 02:31 PM

In defrost, total ventilation and normal modes the PBU sends +12V to the ETR which opens below 40*F.

Sixto
95 S420
87 300SDL

leathermang · #7 07-05-2005, 02:32 PM

" control of the compressor comes from the pressure control switch on the receiver which in turn comes from the ETR (evaporator temp) switch under the dash but where does it come from before that??

From the " ON " button on the AC controls ?

sdparizona · #8 07-05-2005, 03:29 PM

thanks for the info again. so which connector on the bpu should i be testing to so see if the pbu is sending 12v to the ETR? I want to confirm that the PBU is busted before i buy a new one.
thanks
Scott

sixto · #9 07-05-2005, 05:08 PM

Connector 1 pin 6, the black wire with red stripe. I don't know which is connector 1 (left or right).

I led you astray in the previous post. In an 85 the relay is not a simple coil and switch but a solid state device known as a Klima relay. Lots of things to go wrong with a Klima relay simply from the number of inpupts. The Klima relay also has a history of solder joints failing over time.

I'd look for the Klima relay and test there. Do a search on my name and Klima relay for the pin outs of the Klima relay... assuming an 85 Klima relay is the same as an 87.

Sixto
95 S420
87 300SDL

sdparizona · #10 07-05-2005, 08:55 PM

thanks Sixto I will check both the PBU and the Klima relay is this the Klima relay for my 1985 300TD?
http://catalog.eautopartscatalog.com/buymb/quote.jsp?header=header.jsp&footer=footer.jsp&product=0015457405&partner=buymb&baseurl=http://www.***************/&clientid=buymbparts&cookieid=1K51820G21K518MCIN

sixto · #11 07-05-2005, 09:06 PM

That's a Klima relay. I don't know if it's the right one for an 85 TD.

Looking at the pins in that image:

Upper row left to right:

GM+ = engine rpm
VL = WOT switch
87K = AT kickdown switch
[gap]
KL = compressor signal from PBU (ground?)
[gap]

Lower row left to right:

31 = ground
[gap]
15 = +12V
87 = compressor clutch
[gap]
[gap]

It's appears to be a simpler version of the Klima relay in the SDL. No compressor rpm signal, no coolant temp input, etc.

From this info you can fit a simple relay as I did. Ground signal at KL bridges 15 and 87. The only major interrupt you leave out is the minimum engine rpm for the compressor to engage.

Sixto
95 S420
87 300SDL

sdparizona · #12 07-05-2005, 09:29 PM

thanks again Sixto for all your help. I'll check now both this relay and the PBU.
you have been a trmendous help
Scott

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