[W126 Coupe] more a/c questions

[Dick Spellman]

Hi Jack:

I would recommend you run the tests in 83-601 to see where the fault(s) 
lie. If no air is coming out the center vents it's being diverted to the 
defrost vents and/or to the leg area. If you can access the vacuum 
switch valve to the right of the center console and have a Mighty Vac 
(MV) handy, you can test each of the individual vacuum elements one more 
time without having to get at each element directly. Just unplug the 
respective vacuum line and run the connected MV up to 15 hg or so and 
let it sit for a minute to see if the needle creeps lower. (I wouldn't 
go much above 15hg to protect the rubber diaphragms from rupturing) If 
all the elements and their respective vacuum lines check out, then one 
remaining possibility is that one of the element linkages is stuck or a 
pivot on say the leg flaps is broken in the heater box. I found that 
83-601 done in an orderly fashion pretty much gave me the details of 
what needed fixing.

By the way, I just replaced the complete ACC head last weekend in the 86 
300SE with a rebuilt (warranty 2 or 3 years) from Ken at 
<www.thebenz-store.com>. This was the sedan that I rebuilt the ac and 
vac elements on 2 years ago. Worked fine for 2 years but, started 
behaving in such a manner a few weeks back that I once again ran the 
tests in 83-601 and determined that the head unit this time round was 
all done. An important note found on the new unit instructions says 
"check that there in no reading at or close to 0-ohms resistance on both 
the mono-valve head and the coolant recirculating pump as an external 
short on either of these components will damage and void the warranty on 
the acc push-button unit". So, as an additional test you can and should 
check these two devices with an ohm meter with electrical disconnected 
to see if they are working showing and open vs. a short as the 
instructions recommend. I tested both my units before installing the new 
head unit and found them to be in working order. I took the added step 
of running 12VDC across each unit to see/hear the operation was as it 
should be... Clicking on the solenoid element for the mono-valve and the 
buzz and sounds of coolant being pushed though the heater core on the 
recirc. pump.

The behavior I just dealt with was in fact the vacuum switching valve 
changing air distribution locations erratically along with the 'Auto' 
button having been selected causing wild shifts in the fan speed. Every 
time I'd hit a bump in the road the system would start mis-behaving. 
Again, I diagnosed using 83-601 before laying out $155 to effect the 
repair. I didn't bother withthe cold solder joint repair where I had 
done this 2 years earlier and felt that after 20 years a reman was in order.

Good luck, keep at it.

Dick

Jack wrote:

> Hi again,
>
> I’ve got the vacuum elements switched out, no more leaks. But the air 
> won’t come out the center vents.
>
> Anything other than max a/c also blows hot air. Not good when it’s 
> already 100 degrees out here. I changed out the mono valve too.
>
> What the heck is going on with this thing? The center vent (40) 
> element didn’t leak but I don’t hear any movement when adding vacuum.
>
> Looks like a pain to get in to that one!
>
> Jack
>
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