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Old 05-10-2003, 05:12 PM
Arthur Dalton Arthur Dalton is offline
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Join Date: Aug 1999
Location: Florida / N.H.
Posts: 8,768
One wants to remember that there are two fan circuits...
They both share the same fan motor/s, but have nothing to do with one anothers basic operation..each circuit has it's own fuses, relay. sensors..

The high fan is switched by the ECT circuit and is an aid to the radiator in controlling high Engine running temps.

The low speed is switched on by a high pressure switch at the reciever/drier of the a/c unit and it purpose is to aid the a/c systems high side pressure from getting too high by pushing some air throught the a/c condenser..

So, a couple of easy test and what they tell you..

Does the Fan motor work???
By removing the ECT connector, the fan should come on.
[ this will not work if one has installed a temp lowering resistor bridge, see JimF site]
If the fan comes on, you have tested the integrity of the high speed fan circuit. But the main reason for this test when looking into a/c fan circuit is just to verify that the fan motor/s are good..

So, you now go to the pressure sw at the a/c drier. jump this sw and look for fan run. If good , do NOT assume you have a bad
pressure sw. This sw only comes on when the high side reaches
a hefty reading [ as in high ambient temp/load].
The #1 cause here can be LOW freon [ enough to cool, but not enough to get the high side up to cut-in pressure called for by the sw specs]. The common symptom here will be crap cooling at stop/idle [ no air flow]..better cooling at speed BC you have air flow from car movement
Jumping the sw also test the rest of the low fan circuit. No fan- start looking at fuses
Another easy test [ this applies to both circuits]:
If you dissconnect the ect for high fan test or jump high pressure sw for low fan test and hear the relays CLICK, but NO fan.. you more than likely have a fuse blown on the load side of the circuit
[ these are the large amp ones , usually on top of the relays or outside the fuse box]
The exception to this is that the low fan also has a dropping resistor [ behind driver headlamp] that can be open [ common to see wires burnt off here]..
A note here .. Benz has allowed increasing the value of the low fan fuse , as the original was under-rated. [ I always check this first on No Low a/c fan BC of this]
You would assume low fan to draw less , but BC they are using a dropping resistor in the circuit to accomplish the lower speed, it actually draws more..
They allow 25A replacement..

Last edited by Arthur Dalton; 05-10-2003 at 05:49 PM.
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