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  #1  
Old 05-29-2003, 11:39 PM
Hawaiian Harry
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Question A/C RPM sensor bypass

1986 300SDL. Problem: when accelerating quickly (not full throttle) or cruising at speeds greater then 50 MPH, I believe A/C RPM sender is causing A/C relay to shut off the compressor.

Possible Fix?: I understand there is a way to by-pass the compressor's RPM sender to the A/C relay.

Diagnosing and R&R performed: replaced Climate control unit (reman), Compressor Cut-out Circuit test (normal values).

Thanks for your help.
- Harry
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  #2  
Old 05-30-2003, 04:27 AM
sixto's Avatar
smoke gets in your eyes
 
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Location: SF Bay Area
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I'm having random problems with my SDL AC compressor too. I still can't make out the pattern. I rigged up a simple relay to use while I test the various inputs to the Klima relay.

The only bypass I can think of is to replace the Klima relay with a simpler relay. The Klima relay compares the AC compressor speed (2 wires) with engine rpm (1 wire). You can't disable the engine rpm feed because the engine has to be above 600 or 630 rpm before the AC compressor will come on. You can't disable the AC compressor rpm feed if you don't disable the engine rpm feed. If you put the simplest relay that turns on the compressor when the PBU says to turn it on and use the wires at the Klima relay, you'll keep the minimum pressure safety switch in the loop. Besides the difference in rpm safety feature, you'll give up the turn off AC compressor at 115C safety switch and turn off AC compressor at WOT switch. You can gain back these features with a couple more relays. I don't think you'll disrupt the fan functions if you lose the Klima relay.

A counterpoint to the AC compressor speed sensor argument is how many other cars on the road with serpentine belts have this feature? To me it's like leather and ACC in an S-class - if it were my choice, I'd have velour and manual AC.

Sixto
91 300SE
87 300SDL
83 300SD
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  #3  
Old 05-30-2003, 04:34 AM
sixto's Avatar
smoke gets in your eyes
 
Join Date: Jun 2000
Location: SF Bay Area
Posts: 19,759
I forgot to mention - in another thread, someone said that a misadjusted clutch gap or dirt in the clutch gap can cause the clutch to slip. If you had a fuel leak or changed the main fuel filter recently, you might have gotten fuel/oil/dirt washed into the clutch gap. I don't know how to clean it.

Sixto
91 300SE
87 300SDL
83 300SD
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  #4  
Old 05-30-2003, 08:57 AM
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Location: Gainesville FL
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I think the ACLU should take on this issue. Here, mighty MB has manifestly decided to take all control away from us mere mortals. What if we wish to burn up our motors when the compressor locks up. Or maybe we can do better without that extra horsepower on full throttle than without A/C.

Maybe that diesel would rather run with A/C till the battery goes dead than not, the motor will keep running.

I firmly believe corporate lawyers came up with the speed comparitor concept. They knew that the arrogant first owners of these things would burn the motors up while under warrantee rather than turn off the A/C. The device was purely self-protection on their part. If it were my car I'd ***** can the whole thing and use the simple relay. but then I am smart enough to turn off the compressor when the thing locks. Actually I am smart enough to hear it coming and fix the thing first.

BTW only the early Klimas can be by-passed with the stupid relay. The HFM cars don't like it. The HFM needs compressor on information for idle control.
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  #5  
Old 05-30-2003, 09:10 AM
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Location: Tucker, Ga USA
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As stated MB designed the "klima" relay to protect the multi-rib single belt. HOWEVER starting in 1996 they stopped using that type of protection for the belt & NONE of the newer cars have that type of protection for the belt. They use a fusible link now.
I started by-passing the speed sensor system about 8-9 years ago ONLY when replacement of the A/C compressor was required.
If you have a good compressor but are having speed sensor cut-out problems, by-pass is fairly easy.
IF you send me a private message with a FAX # I will fax that info on by-passing the "klima" relay.
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  #6  
Old 05-30-2003, 09:13 AM
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Steve - my weak spot is the electronics stuff. I have put a jumper between 7 and 10 on my Klima and now my a/c climate control system works like it is supposed to. Based on the way I understand the schematic I have bypassed the Klima by doing this. Is this right? My application is the 1985 300D and I think all the Klima does in this application is drop out the compressor during full throttle.
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  #7  
Old 05-30-2003, 09:26 AM
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It also does the low and high speed engine cut out and probably the voltage cut-out etc.

According to the gas klima on my desk pin 7 is term 87 and 10 is the PBC signal. Early 190D Klimas used power from the PBC to control. I am not sure how many models used this but I tell everyone dealing with KL to view whether it is a ground or 12v signal before by-passing. I have seen a number of PBCs with burnt circuits due to grounding the KL signal at the low pressure switch.

In order for your jump to work you have one of these rare powered KL circuits. I would use this KL as the 86 terminal in a "ice cube" relay and place 7 on the output 87 terminal of the relay. Term 15 can be used for the load carrying power (term 30 on relay) and 31 can be hooked to 85 on the relay.

I would not leave the circuit as you have jumped as the circuit is not designed to handle the load of the compressor clutch. It is a 100ma circuit at most and the clutch is at least 3amp (3000ma) circuit. That circuit is carried on the small printed circuit of the PBC.
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  #8  
Old 05-30-2003, 11:05 AM
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Location: Plano, TX
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I actually like having the compressor cutout in place. Once the system is sorted out it works just fine. Age and use introduce multiple failure points, all of which have to be identified and repaired.

Here's why I like having the system in place. I don't know what a failing a/c compressor sounds like; I would never hear it coming. I actually had the compressor lock up about a year after getting the Klima situation sorted out. In retrospect, the compressor *was* making a little bit of noise beforehand, but it was far too subtle for me to put two and two together. Now maybe if I was Steve or Donnie I'd have recognized impending doom - but I'm not, and I suspect neither are 99% of the people here. The a/c worked fabulously right up to the point where it stopped. The failure happened in remote West Texas. As much as it stunk to be without a/c out there, it would have stunk a whole lot more to be on the side of the road with a broken serpentine belt.

One man's opinion.

- JimY
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  #9  
Old 08-02-2017, 07:43 PM
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Posts: 6
My ac compressor has no speed sensor. I had an engine swap, from om603 to om606, and it came with the compressor. It is not engaging and my mechanic tells me it is because of the absence of the speed sensor. Is there a special klima relay for that compressor. Or is there a way to have it work normally?
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  #10  
Old 08-07-2017, 02:38 PM
WTB: 94/95 E320 Wagon
 
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Location: Charleston SC
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See post #30 on the second page of this thread:

Compressor speed sensor.

This will show you how to replace the KLIMA relay with two standard relays to bypass the compressor speed signal.
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'87 124.193 (300TD) "White Whale", ~380k miles, 3.5l IP fitted
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'73 Balboa 20 "Sanctification"
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