Date registered: Dec 2010
Vehicle: 1973 450 SL
Mentioned: 15 Post(s)
Quoted: 369 Post(s)
There is a relay for the aux fan in the relay rack above the fuse box. Best access is achieved by removing the passenger side lower dash panel, the little silver right angle trim on the lower A pilar, and the passenger side kick panel. Another option is removing the glovebox liner and going in from the top. The lower dash panel has screws at the forward edge near the firewall. Make sure they are removed or risk damage to the panel.
The '78/'79 ETM that I have does not define a key code for that circuit so I resort to wire colors going into the connector that the relay plugs into. If you find a little tag with a "6" on it wrapped around wires leading into the connector, check for the following.
Your aux fan relay is powered by a 2.5mm red lead from fuse 2 at relay terminal 30 (pole 3 of the connector). The power lead for the aux fan is a 2.5mm black wire at relay terminal 87 (pole 1 of the connector). The solenoid of the relay is powered by a .75mm black with red hash wire from fuse 8 at relay terminal 86 (pole 4 of the connector). The solenoid is grounded by the 100 degree aux fan switch via a .75mm blue wire at relay terminal 85 (pole 5 of the connector); this pin bushing has another .75mm brown with yellow hash wire that leads to another relay that separates the aux fan from ignition change over. If you find those colors bundled together, you've found your aux fan relay.
Fuse 8 is on the start run bus so your ignition has to be in position I or II for the relay to work. The lead at your aux fan switch is poorly protected and usually in poor shape - suggest a continuity check of that blue wire from the connector at the aux fan switch to the pin bushing at pole 5 of the aux fan relay connector. If you find a bad wire, pull back the brittle protection to the point where it enters the body harness (at the alternator on most cars) and see if you have a break. If you do, I can help - PM me.
73 450 SL