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| AlanMcR Expert Date registered: Apr 2006 Location: US, CA, Los Altos Vehicle(s): G300DT E300DT 230SL Posts: 3500  | AC fix for mid-90's 463 where AC fails on fan speeds 3 & 4 If your AC works fine on blower speeds 1 and 2 but quits working when you set the blower to 3 and 4 then you may have an interesting little manufacturing defect. The test is pretty simple: Measure the voltage going to the compressor clutch at each fan speed. It should be pretty stable. If it drops down with each fan speed (example: 12.8 -> 11.3 -> 10.1 -> 8.5) then your G is probably wired up wrong. The basic problem is that the power for the compressor clutch is incorrectly sourced through the resistor pack for the blower. A one -wire fix corrects this. The attached PDF's show the problem and the fix. (The PDF creator rotated some of the images). Page one shows the original circuit diagram. Page two traces the worst case flow of current. That is with the fan switch in position 4 and the compressor engaged. The MB drawing shows connections in the blower switch that don't actually exist. If the switch worked like drawing shows then the circuit would work, though there is no need to drive compressor clutch current through the fan switch. The last page for the actual function of the blower switch. Page three shows which wire to cut and re-route. This fix brings 12V to K58, the compressor relay. Page four shows the function of the switch that was installed and called for in the EPC - A6675450204. Attachments ----------------  AC-Fix.pdf (419KB - 88 downloads) | |
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hipine Date registered: Jul 2006 Location: US, CO, Bailey Vehicle(s): 460 1980 280GE w. 617A  | RE: AC fix for mid-90's 463 where AC fails on fan speeds 3 & 4 Man Alan, that original schematic is strange. I thought that idiot POs were the only ones who'd put in a relay and connect the same wire to the primary as the secondary, but it looks like that's what MB did with the K-58. That seems like the real problem to me. When I first looked at your schematics I thought, "K58 is just a coil, it should energize just fine on the fan (reduced) voltage, and the contacts should feed the compressor 12v regardless." But then I noticed that the feed through the secondary contacts of K-58 was the same as the feed to the coil. Strange..... Wait, so doesn't your new red-lined schematic result in K58 being "on" all the time the ignition is on, regardless of whether the fan switch is on or off? If so, K58 is serving no purpose whatsoever. What about just moving the wire from pin 5 on K-58 over to pin 2 of K-58 and then runnning a new IGN12V to pin 5 of K-58. That would seem to be a more realistic use of K-58 to me than the way MB did it, and would preserve the functionality of K58. I must be missing something. I don't mean to second guess what you did, I just suspect that I'm missing your point and that maybe anyone who go in to try this might miss it the same way (anybody who gets this deep should know why the mod works, not just blindly do it, IMHO). Can you enlighten me a little? Thanks! -Dave G. PS - it also looks to me like the way you've re-drawn the switch, that position "4" is the lowest speed (most resistors between 12v and fan motor). Is that how it is? The way your switch diagram has it, you switch in more and more resistors in series as you move from 1 to 4, increasing resistance. The way MB's has it drawn you'd switch in more and more resistors in parallel, decreasing resistance for higher speeds as you move from 1 to 4. I guess the observation of lower and lower voltage at the compressor verifies that position "4" is the low speed position and "1" is high...Or maybe that observation at that point is a function of some other aspect of the circuit..... Curious.... Sorry if I'm muddying the waters with my "460-level electrical ignorance".  PPS - no, 4 must be high speed because that's the one that runs a wire direct from the switch to the motor bypassing the resistor pack entirely... I'm lost.... PPPS - Looking a little more I think your (Alan's) switch diagram is correct. The dropping voltage at the compressor is due to the line to K-58 being "back fed" through more and more of the resistors in the pack as fan speed goes higher from position 1 to 4. This is probably not a problem for the coil of K-58 (pin 2) and would be sufficient to pull it in, but IS a problem for the compressor clutch which requires more current. That's the reason for K-58 to be there. Yep, I think I'd move K-58/5 to K-58/2 and put IGN to K-58/5. Okay, it's official...I'm second guessing you Alan.  But still (and always) only in the spirit of transparency of thought process. PPPPS - Okay I'll stop here with one last question. What's "X53/1"? The reason I ask is I'm trying to wrap my head around whether a blown (shorted) diode at that location could cause your symptoms. The fan will work the same regardless of whether the switch contacts are per your diagram or MBs. Your description of things works if X53/1 is just a connection. I'm trying to wrap my head around what happens if X53/1 is a diode passing current only from the switch towards the resistor pack. But my brain is tired.... PPPPPS - I CAN'T STOP!!!!  X53-1 looks like just a connection. So I'm back to where I was at PPPS above, noting Alan, that your suggested mod still leaves K7 fed by the "droopy" voltage, and it seems to be workign okay - further justification to let K-58 serve a purpose, to use the droopy voltage to close contacts, but feed the contacts with 12V IGN via the terminal swap described in PPPS.Edited by hipine 7/5/2011 3:30 PM | |
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| AlanMcR Expert Date registered: Apr 2006 Location: US, CA, Los Altos Vehicle(s): G300DT E300DT 230SL Posts: 3500 | RE: AC fix for mid-90's 463 where AC fails on fan speeds 3 & 4 Dave, Comments are definitely appreciated. Figuring out this mess was quite the adventure. I remember looking at this thinking "These guys do this for a living ... there must be some good reason they did it this way". I never figured out that "good reason". X53/1 is just a connector. All the "X"s are. I've spent so much time over the years looking at MB circuit diagrams that the translations are automatic. As you point out, I'm still feeding K7 with droopy voltage, that seemed to work and I wanted to change the minimum number of connections. The cuts and connections are in a particularly dense bundle of wires behind the relay panel. Best accessed by laying upside down with the head in the foot well. The other fix for this would be to find the fan switch that the circuit designer thought was going to be put in and install that. But then, where is the MacGyver-ish fun in that? My AC blows cold and it makes me smile every time I turn it up. That is enough for me | |
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| hipine Date registered: Jul 2006 Location: US, CO, Bailey Vehicle(s): 460 1980 280GE w. 617A | RE: AC fix for mid-90's 463 where AC fails on fan speeds 3 & 4 AlanMcR - 7/5/2011 1:57 PM ... My AC blows cold and it makes me smile every time I turn it up. That is enough for me I'm all for the end justifying the means, but is it okay for K-58 to be on all the time the ign is on? The only time K-58 was off before was when the fan was "off", now K-58 can be on even though the fan is off. Maybe K-58, and the "fan on" permissive requirement was there to prevent icing of the evaporator if AC was left on with the blower fan turned off? I dunno, that's just a guess. I know what you mean about "they must have had a reason". I used to work manufacturing HVAC control modules for cars and the regulatory requirements were nearly as strict as for engine and airbag controls because HVAC controls the defroster which is a safety requirement. So those circuits are pretty highly scruitinized. If they'd really wanted the AC to be off with higher fan speeds for some solid reason I'd think they would have wired it that way rather than just relying on the lock-up threshold voltage of the compressor clutch pack. There's some chance the jumpering of 2&5 on K-58 was an undetected oversight. So I think its okay to modify to keep the compressor running at all fan-on speeds. Personally, I'd choose an implementation that didn't also defeat the "K-58 off with fan speed set to 0" aspect of the original circuit, just to minimize potential for unintended consequences. But that's the beauty of a little discussion like this. Folks following along can think about it for themselves and go from there. Good on ya for digging this strangeness out into the light of day for us! Oh yeah, I was also curious, did you ohm out the fan switch contacts to verify your revised diagram? Or did you conclude "it must be that way" based on the voltage stepping you observed downstream? Thanks again! -Dave G. PS - I see that K7 has to also be on to feed the compressor, so no danger of the compressor being fed with fan off as K7 is still a limiter for that. I also hear what you say about X53/1 being just a connector, but it also seems strange to me that three wires come FROM the same place (fan switch plug) and TO the same place (resistor pack) and one of the three wires goes through a connector that the other two don't.... "there must be a reason...." Fun stuff Edited by hipine 7/5/2011 7:33 PM | |
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| AlanMcR Expert Date registered: Apr 2006 Location: US, CA, Los Altos Vehicle(s): G300DT E300DT 230SL Posts: 3500 | ||
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| hipine Date registered: Jul 2006 Location: US, CO, Bailey Vehicle(s): 460 1980 280GE w. 617A | RE: AC fix for mid-90's 463 where AC fails on fan speeds 3 & 4 AlanMcR - 7/5/2011 5:51 PM ...The asymmetric use of connectors can be observed in other places and probably results from cobbling together a limited production vehicle with production parts from other vehicles. Great point, and a very likely explanation. I wonder what the German forums say about this (can't read it). I'd think that every one of these would come straight back to the dealer if half the fan speeds didn't work w. AC and would expect a service bullitin of some kind. Oh well, we don' need no steenkeen service bullitins. We have Alan!  All the best and thanks again for sharing, -Dave G. | |
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