Need Advice Please.
My '88 124(Euro)300TD Turbo has these two (expensive) relays just behind the battery. My AC compressor turns itself off regularly while running, and the only way to reactivate it is to kill the ignition and restart the engine, sometimes more than once, before I get it cooling. What works more often is when I flick the ignition switch quickly off/on while running and before the engine shuts down. This does'nt feel right, but it works faster. Mechanic says the "Klima/Kickdown" relay needs replacing.
Right behind this is another relay which the mechanic says controls the Tach. This one's been acting up too. Sometimes my tach just goes dead resulting in the rpm's decrease while idling. There's also a noticeable engine stutter and if the AC was cooling, it stops cooling. If the car is running, usually the tach will jump back on when I reach higher rpms and the AC may start cooling again. When you wiggle this relay in the socket, the Tach'll work for a while before acting up again. Tried contact cleaner, no help. Mechanic says replace this too.
Anyone out there know better?

Most Mercedes relays have the part number right on them, what are the part numbers? You might be able to buy them here at http://www.peachparts.com/catalog but you would have to type the part number in under search since the Fastlane catalog dosn't apply to gray market cars.

Check the belt tension first before replacing the relays. When the belt slips, the compressor will be cut off. Search this forum for more info.

David

The air conditioning cutout is a common problem on this
model. The Klima relay implements a function which
compares the a/c compressor RPM to the engine RPM. If
the two differ by more than a small margin, the a/c is
cutout until the ignition is cycled. This is what is
occuring to your car.

There are several possible causes, so don't be to
quick to replace the Klima relay. Other possible causes are:
1) Compressor clutch gap. The gap is specified as 0.5mm
+- 0.15mm. As the compressor ages, the gap increases.
By the time it reaches 0.8 or 0.9mm, compressor
engagement becomes quite erratic.
2) Grease/oil on the compressor clutch. Sometimes a simple
degreasing can cure the cutout problem.
3) Corrosion on wiring. Any problems in the wiring which
carries the compressor speed signal back to the Klima
relay can trigger the cutout function.
4) Not certain on this one, but I believe problems in
the engine speed feed to the Klima relay can also
cause problems.
5) Worn serpentine belt.
6) Defective belt tensioner.
7) Missing plastic splash shield. The plastic tray below
the engine prevents water from getting on the serpentine
belt. Water can cause enough slipping to trigger the
cutout function.
8) Defective Klima relay :)

The other relay your mechanic is referring to is the
overvoltage protection relay. These are known to fail
with age. However, I don't think it affects the operation
of the tachometer. Rather, I'd look for corroded connections
in the path from the engine speed sensor to the tach. If
the engine computer (EDS) cannot see the engine speed, it
will not attempt to regulate the idle speed, resulting in
the low idle speed. A failing OVP relay can also shut
down the EDS, so it could be the culprit. The OVP is not
too expensive (about $35, I think) so it may be worth
replacing.

jcyuhn,

What you described is my current failure mode ... when tach goes to zero the compressor declutches. When (this is happening less and less often) the tach starts up again, the clutch activates, as well.

Sounds like I should:

1. Check wiring from Klima back to sensor pickup
2. Check Klima connection - would a failing relay give the same indications over a several month period of time and if the relay itself is fried, how do I tell?

Thanks!

jcyuhn,

You may have just helped solve my problem too. My compressor seems to be on all the time, even when the Econ. buttom is pressed on the ACC. Everything else works fine, heat and cool, just as they should. It's just that the compressor seems to be running too much. I'm wondering if the relay may be stuck. Does this seem plausable? Thanks.

Similar problem on my '85 300SD. Tach works -- AC compressor engages. Tack stops working -- AC compressor disengages. Both finally died competely. This forum helped me trace the problem to a bad engine speed sensor. The sensor is located on the bell housing, drivers side of the engine. Tach and AC now working fine.

Great Forum!!!

Bill