The temp switch on the head for 88 and up 124s/2nd gen 126s is really a variable resistor sensor. It is read by the climate controller to tell the aux fans to come on and if the temperature keeps climbing to inhibit the A/C. The low speed relay is driven by the dryer switch and the high speed relay is driven by an output of the climate control box in the dash.

The diagram mxfrank posted is accurate for your car according to the factory manual.

Yes the fans are driven straight from the battery with no overcurrent protection. That is just nuts. I really wonder sometimes what they were thinking. Lots of things like that on the 124.

Note for anyone coming along that has an 87 see this post instead:http://www.peachparts.com/shopforum/diesel-discussion/370870-w124-c-new-pressure-switch-auxillary-fan-worked-twice-now-nothing.html#post3506733

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech