[W126 Coupe] A/C Not cooling

[Dick Spellman]

Hi Robert,

If your system is charged with R-12fine it is the best at cooling most say.
If your system is running R134a it should be charged at 80% of the volume of
the charge capacity label for R-12.
A set of gauges on low and high side of the refrigerant circuit plus ambient
temp reading at the condenser area will tell you what the pressures are
based on a ac chart.  Pressure fluctuations will also indicate any blockage
in the system in either case.

I know many folks complain about R-134 not cooling well.  I can only provide
you with my 8 years history of running my black sec which makes the car
hotter after sitting in the sun, when local temps are well into the 90's.  I
have had no complaints.  The car runs cold at the vents in a matter of
minutes and stays that way regardless of highway or stop n go.  I run MAX
and recirc for a few minutes then dial it back to 75.

Here was the checklist I followed when I rebuilt the system in 2002-3.
Refrigerant Circuit-New A-6 compressor (the A-6 Harrison/GM compressor was
used in early model MB's and later changed to Nippondenso) with oil drained
overnight and PAG 150 (4 ounces) added directly to compressor before
install, new expansion valve, new receiver dryer (with 2 ounces of PAG 150
added to the low pressure valve immediately after vac'ing the system down.
System was vac'd down and then I let it sit for an hour with gauges attached
to be sure there were no leaks.  With no changes noted, I added R-134a
refrigerant until the pressures indicated a full charge.

Presumably all of the above is fine after your shop did their repairs to
your system.  The only way YOU can be 'somewhat assured' all is well with
the refrigerant circuit, is to raise the hood and with engine/ac on observe
the compressor clutch is running continually and not cycling.

The other components that must be working properly regardless of the
refrigerant type in use are:

1. viscous clutch - this serves to keep the cooling of condenser and
radiator fluid in stop n go traffic.  It would help if you could tell us
what the temp gauge is reading when you experience loss of cabin cooling and
if the temp gauge is stable and around 80C-90C most of the time or if you
are noting it is going higher.

2. the auxiliary electric fan - is this kicking on at any point? - it is
temp controlled and will kick on low and high at 90 and 105C.   Your shop
needs to test the fan and related relay/sensor controlled electrical circuit
if you are seeing high temps on your cooling system.

The above two issues can be easily diagnosed by YOU in observing your temp
gauge on the instrument cluster.  A proper working cooling system will ride
around 80C all of the time unless you sit for a long period in stop n go
where 90 or so might be reached.

3. the mono-valve if leaking will compete with your cooling by introducing
hot coolant to the heater coil that sits right next to the ac evaporator in
the cabin blower box.  The mono-valve is best visually inspected for a
blocked screen or small tear in the rubber diaphragm.  A replacement of the
head is only $20-30 for the part and it is a 30 minute job.

4. I'd say this is the most ignored problem in our MB's when it comes to
cooling.  The vacuum pods that control the air distribution in the cabin
will fail after 10 years of use.  In addition to regulating air flow they
also determine just how much outside fresh air is introduced to the cabin.
I found all the vacuum pods in all the 126 MB's I've driven had failed.
They should be tested with a mighty-vac and replaced if found to be
leaking.  A DIY test is to set your ACC pushbutton to AC, set dial to MAX
and recirculation switch to ON if so equipped as is the case on early MB's.
The system will attempt to close the fresh air door (in recirc mode). A leak
in either of the two pods that control this door will leave it hanging wide
open introducing all that HOT outside air to again compete with your systems
ability to cool properly.  YOU can see this activity by removing the glove
box interior paper liner and looking behind there where the fresh air door
is clearly visible.  This is visual inspection easy on the early MB's and
not so with the passenger air bag equipped later year models.

Hope this helps some.  I am willing to bet that you have one or more failed
vacuum pods given the shop work done where they would/should have tested
both the visco and electric fans as a part of the focus on the refrigerant
circuit.  They will have ignored the vacuum system in my opinion.

-\Dick




On Sat, Aug 13, 2011 at 2:13 PM, btb management
<btbmanagement at hotmail.com>wrote:


>  My 1985 500 had it's a/c repaired and works great up until about 80

> degrees

> outside.  Once that temperature rises the a/c starts to blow warmer

> air.  At 90 degrees it is just a fan and not helping with the cooling down

> at all.  Any suggestions?

> Robert

>

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