If your mechanic told you that you need to replace the aux water pump then I
strongly suggest that what needs replaced is the mechanic.
The Aux Water Pump is part of the climate control system, not the cooling
system. It insures delivery of hot water so you get hot air out the
I do not know what year your 300E is, therefore I do not know which engine
you have. I have a 93 300E with the M104 engine. I suspect you probably
have the M103 engine.
Here in Arizona with ambient temps hovering aroung 110F, even in stop-and-go
traffic with the air-conditioner running full blast my temp gauge almost
never exceeds 105C. Yes, 85C to about 110C is normal.
It sounds like your problem relates to the electric aux radiator fan(s).
Depending upon year you may have one or two. These fans are two-speed fans.
They come on at low speed when when the high-pressure side of the
air-conditioning system reaches a certain pressure. They switch on at high
speed when the coolant temperature reaches 110C.
When you pull into the driveway at the temp gauge is reading above 110C, pop
the hood and check the fans before shutting the engine off. Are they
Also, a lot of times a pile of dead leave can accumulate between the fan and
the ACC condensor, or between the condensor and radiator. Shine a bright
light through there and check.
BTW, competant MB mechanic should have no difficulty isolating problems like
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The 300E is very similar to my 260E. I had a simialr problem and I had to
replace the viscous fan clutch, as this was not engaging at around 110
degrees, so the engine was not getting any forced air cooling whan at a
My car does not have air con (rare in the uk) so there may be an additional
probelm to do with the aux fan for the air con with your car.
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