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Aux Fan controls 280E/CE
In the 78 280E/CE, your aux fan will be energized by one of two conditions, either engine temp sensor above 212F or refrigerant above 144F.
The refrigerant switch is one of the two found on your A/C receiver-drier, down in the RF corner of the engine compartment. Of the two, the other is refrigerant (low) pressure sensor, which is the final control on the compressor clutch activation chain, and actually not invoved with your aux fan. The temp switch should have just a blue (to relay) and a brown (ground) lead. If in doubt, from a cold engine start, select Temp 65 on the Climate Control, compressor switch "on" and select Auto Hi. Pull the blue lead off that blue/brown lead switch - the aux fan may stop. If the A/C clutch de-energizes, it's the other switch (I believe the pressure sw has one lead which is red and blue and another with two wires on one lead - shouldn't confuse with the blue only). If the aux fan did stop, you've found your problem, since that switch shouldn't close until the system has been running for a while and the refrigerant has got too warm [part number 001 821 73 51 - but CAUTION, you likely need to depressurize system to change it out]. If pulling the refrigerant temp switch wire did NOT turn the fan off, then the problem must lie in either the engine temp switch OR one of two relays - either the aux fan control relay (which integrates the changeover switch function) or the aux fan relay itself. (I'm looking for the numbers which you can find around the relays' wiring bundles in the "ice-cube" relay box, LF fender black covered box - I only remember that 21 is the fuel pump relay; the others are 5, 6, 8, and 12) Of course, since it's coming on as soon as you turn on the ignition switch, you can "easter-egg" the relays in the box until the fan stops - but there may in fact be two involved in the fan chain, as well as other possible involvements - but look for two to affect the fan.
[I'll also attempt to provide exact location of the temp sensor switch (likely in the head sensor box, RH side of the head, although the texts refer to a "thermostat housing temp switch", which I can't see on the actual thermostat outlet area), when I find that - pending that, you could also try pulling their leads]
Courtesy of whunter, that pic nicely sorts out the switches in the sensor box - the single-lead brass-based one (second switch back, first electrical vs vacuum) must be the engine temp 212F switch. The lead pulls straight up and off, with a little plastic catch on the plug, and should provide the other side of your logic troubleshooting. When cold, it should be "open", if you have a meter and want to ensure that it's the switch, not the relay. AND, as he also points out, if the wiring to either switch is grounding out somewhere, it will give the relay a "false positive" that causes it to energize and switch power to the fan. For that matter, you could also meter the refrigerant switch if that's "closed" when cold - to verify that blue (wrongly) goes to ground.
But your early 280E/CE Climate control wiring is NOT the same as the later 300's, according to the diagrams. CAN ANYONE PROVIDE WHICH WIRING BUNDLE NUMBER(S) IN THE 280E/CE FEED THE APPLICABLE RELAY(S), in case both switches check out okay???
The wiring diagram on the D-B CD is Diagram 4 - and located just above the Title, in the lower RH corner. When chasing diagram - make sure you note the parts/locations applicable to the 280E/CE, vs SE (diff engine switch wiring) and the 300's (different energizing wiring and an additional drier switch).
If you don't have one and have Adobe, email me and I'll try to cut and paste you a copy of the applicable .pdf.
With two switches and two relays involved, I guess it's lucky they work at all...
Happy easteregging!
Stan
Last edited by Tristar1959; 07-10-2006 at 03:35 PM.
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