I have a problem with my rev counter,on my w201 190 2.5 diesel.
I took out the cluster already to see if there's something wrong with
the rev counter but I didn't notice anything,I bought a used one from
internet to see if there is a fault in the tacho it self but it still
I changed the rev per minute sensor located at the bellhousing and
still no signal.
I took the car to my electrician and told me the rpm sensor is ok,(is
sending impulse).The problem is that the impulse is not reaching the
tach.there is no impulse to the tacho.We checked all the fuses and they
are all fine.
Is there a relay related with the tach?
Some thing that my electrician is suspecting is that the klima relay is
not letting the impulse going to the tach,
Is that True?Does the Klima relay playing part with the tach?
Not familiar with that mode, but on the earlier 116 diesel MB had an
electronic widget that converts the pulses to the proper signal for the
tach. It is located on what appears to be a test cap inside the
engine compartment, the driver side fender area. I guess the test
connection is so MB could hook up maybe a tach, timing light, etc. The
top of the cap has the widget and screws off. Mine went bad and the
tach started acting eratic, then finally died. Got a new one at MB.
I don't recall the wiring diagram and am not sure if Klima relay is up
or down stream of the tach, but it is a possiblity.
Klima relay controls A/C compressor. The A/C design incorporates a
logic to stop compressor if it is found to be "frozen". What does it
mean? There is a rpm sensor at the end of the compressor. Klima relay
constantly compares the rpm of engine and compressor (after a half
second delay after both start turning). If it is found to be more than
20-30% (don't recall the exact value mentioned by manual) different than
spec, the relay cuts off the compressor clutch and the A/C can never get
truned on until engine is restarted. This is to protect the single
belt. AFAIK, all MBs in the 80s with single belt have the same logic.
Later models are probably the same.
Sometimes this design causes no A/C when the pulley slips due to
oil/dirt, or when the tension/belt is loose. If A/C works only for a
brief period affer start and it can not turn on again until restart, it
is typically this reason.
Does the A/C work?
This turn off can also be triggered by two other sensors... one is pressure
switch which if you don't have enough pressure, compressor will stay off.
The other sensor on modern car will sense ice at the evaporator and will
shut off the compressor too.
The one Tan mention is to protect the engine belt in the event the
compressor clutch is siezing.
If the 190D has a tach amplifier that is most certainly the problem. It can
(sort of) be "repaied" with a hack:
But that may only last a year of three. A new tach amp fixes it for
a couple of decades... agai IF this car has one. I'm guessing...
Need Mercedes parts? http://parts.mbz.org
Richard Sexton | Mercedes stuff: http://mbz.org
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