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Evaporator Temperature Regulator - 1980 300SD

Started by ScottyB, 20 February 2023, 08:19 PM

ScottyB

Does any one know where this regulator is located?  Can not find it.  Also, can it be circumvented in the event it is not working.
1978 300CD
1980 300SD
1983 380SL
1990 420SEL

daantjie

#1
There is a temp sensor to prevent freezing if I recall. This locks out the compressor but if this is the switch you mean you need to effectively strip out the dash to remove the evaporator core to replace it.  Not a small job.

This is called the "ETR switch" if indeed this is the one you mean.
Daniel
1977 450 SEL 6.9 - Astralsilber

ScottyB

That I am afraid may be the one.  Is there something that can be done to circumvent this sensor and will do any harm as I am here in California and it is highly unlikely to freeze.
1978 300CD
1980 300SD
1983 380SL
1990 420SEL

daantjie

Not sure how/if they fail, and not sure how common failure is on these.  I guess what I mean is what failure symptoms are you experiencing?  No compressor engagement?  This can also be due to low pressure in the AC system, so that the system prevents the compressor from running and lunching itself.  Best would be to take high and low pressure port readings and see if you are in range.  Best case you are just low on gas and then a regas should make the compressor engage again.

You can also bridge the terminals at the pressure switch (at the drier at the front of the radiator) with the car running to see if you can "force engage" the compressor.  Do this just as a test for a short period of time to see if the compressor kicks in. If it does then likely you are low on gas.
Daniel
1977 450 SEL 6.9 - Astralsilber

rumb

Its not the outside temp freezing, its the ac system. If it freezes internslly then you could damage components.
'68 250S
'77 6.9 Euro
'91 300SE,
'98 SL500
'14 CLS550,
'16 AMG GTS

ScottyB

Sadly it appears it is the ETR switch.  Is there any known work around given I really don't want to pull the whole dash apart to replace?
1978 300CD
1980 300SD
1983 380SL
1990 420SEL

UTn_boy

You can disconnect the wires going to it and bridge them, but you then lose the function of the ETR switch.  On cooler days or evenings the evaporator will ice up, as current to the a/c compressor clutch can no longer be interrupted when the ETR switch senses 38 degrees F at the evaporator.  As others have said, this is not exactly a part that goes bad.....but it's certainly not impossible.   There are two or three more reasons why the compressor won't come on.  I;d investigate those first, as they are common failure points.  Some things there are just no work arounds on.  The ETR switch is one of those things.
1966 250se coupe`,black/dark green leather
1970 600 midnight blue/parchment leather
1971 300sel 6.3,papyrus white/dark red leather
1975 450se, pine green metallic/green leather
1973 300sel 4.5,silver blue metallic/blue leather
1979 450sel 516 red/bamboo

JeffCullen

#7
While I agree the ETR switch is important and would not bypass it in my own car, at his request I helped a friend bypass it in his '80 SD.

We went through the whole system with the wiring diagram eliminating every other possibility.

We ran a new wire from pin 87 (red/green AND yellow/white) of the ACC relay behind the glove box straight to the pressure switch on the receiver dryer.

Both the "in" and "out" wires on the pressure switch are blue/green. We figured out which wire went to the compressor clutch by bridging it to battery positive (thus engaging the clutch), and then made sure we bridged pin 87 of the ACC relay to the OTHER side of the pressure switch so the pressure switch would still be effective.

There isn't a wiring diagram for the '80 SD in our Technical section here on The Org, but this circuit is the same as a 280S. See page 26 of this PDF: https://cdn.w116.org/cd/ETM/80s.pdf
1979 300SD #6680 - Astralsilber - 153,750km
1998 E300 Turbodiesel - Rauchsilber - 165,000km

ScottyB

1978 300CD
1980 300SD
1983 380SL
1990 420SEL