For an 88 300 SEL can anyone refresh my memory on the operation of the operation of the condenser on a 126? As I recall head temperture something to do with this. When does it operate, (temperature on and off operating points, which temperature locations head, block, ambient etc.)

I can speak at least to my 85 300SD, which should be sufficiently similar.

There are two switches to activate the aux fan both are activated by ground contact.

1. The first in on top of the coolant thermostat valve. A single wire is grounded upon the heat tempurature.

2. The second is on top of the A/C drier. There are two wires which, if crossed when the ignition is in the on position and the A/C is on, will complete a ground circuit. If you jump these two wires while the A/C is on then you are possibly looking at a bad aux. fan. Try jumping the wires quite a few times with the fan completely stopped between to ensure the brushes don't have bad contact points.

Hope this helps.

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80 280SL
85 300SD
87 300TD
92 300D 2.5 Turbo
92 300TE 4Matic

pwogaman, NOT! The two systems are indeed different in how they achieve the end result. Yes they are temperature controlled and AC pressure controlled, that is the extent of the similarities. The 88 300 SEL has a blue two pole switch that is mounted in the middle of the cylinder head between the valve cover and the air cleaner assembly. This signal is sent to the AC pushbutton assembly for temperature control. When the predetermined value is met the AC panel activates the K9 relay for high speed fan operation. Low speed fan operation is governed by AC pressure, when the red/green pressure switch is closed due to high pressure in the system it activates the K10 relay.

Quick Tests:

1) Disconnect blue two pole switch-Electric fans should run at high.speed.
2) Bridge red/green switch on rcvr/drier-Electric fans should run at low speed.

I stand corrected, but did in fact qualify my response. As I've said before, verify, verify, verify.

__________________
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Peach Parts W124.128 User Group.

80 280SL
85 300SD
87 300TD
92 300D 2.5 Turbo
92 300TE 4Matic