The auxiliary fan on my 300SDL never seems to work. I've searched the various forums but haven't found an answer yet.

The auxiliary fan works fine when I jumper the load side of the auxiliary fan relay. The temperature switch for high-speed operation also appears to work. According to the wiring diagram the wire between the temperature switch and the relay should be brown/green. However, at the temperature switch it is brown/green, but at the relay there is a brown/white (or maybe brown/gray) wire on the coil contact. Testing the circuit from the temperature switch to this contact appears to show an open circuit (which explains why the fan doesn't come on).

Does anyone know where the brown/white wire comes from? My car has the automatic climate control system, so I wonder if that brown/green wire goes to that and then mysteriously changes colour to brown/white en route to the relay?

Would it be safe to connect another wire directly from the temperature switch to the relay coil contact?

Thanks

Terence Lowe

Welcome to the forum.

There are three wires on the temperature switch.:

Brown/grey
Brown/blue
Brown

The brown/grey wire runs to the coil side of the aux. fan relay and provides the ground.

You won't get the fan to run on this circuit unless the coolant gets above 105C. so, chasing this problem is probably not productive.

The real question is whether the fan operates at low speed via the high pressure refrigerant switch?

The auxiliary cooling fan has 2 speeds - low and high. When the AC is on and the pressure at the dryer reaches 20 bar, the aux. fan switch (the one with the wires built into the switch S32 ) is supposed to activate the fan at low speed.
The high speed switch S25 , is only supposed to come at 105C.
S32 closes to ground and activates relay K10 .
You can jumper the wires together that go to S32 and see if your fan comes at at normal operating fan speed - low. Your Aux fan should come when you jumper the wire going to S32.
If the fan does , then you most likely have a bad S32, wiring issues or your pressure is low.
My AC system never achieves the pressures defined by the shop manual on the high side for the S32 aux fan switch , so I must assume that there is more to the pressure dynamics at the switch or I am misreading the shop manual info.
My Aux fan starts almost immediately when I start the AC system and my pressures are never above 17-18 bar.
The FSM shows a brown and a brown / blue from S32 to K10 and I verified that on my SDL. I would look to that circuit.

__________________
1986 300SDL 201K Daily Driver
1984 300D In Progress
1989 240GL 196K Swedish Brick
1984 300SD -- Sold 289K
------------------------------

Thanks for the speedy response.

The wire at the 105C switch is green/brown (exactly as shown in the wiring diagram for an '87 300SDL). However, at the high-speed auxiliary fan relay (pin 85) it is a brown/gray (which is not as indicated by the wiring diagram). There is no continuity between the 105C switch (green/brown wire) and the relay (brown/grey wire), so testing the circuit by grounding the green/brown wire (with the ignition on) does not cause the fan to come on at high speed. Putting a jumper into the load side of the relay does cause the fan to operate at high speed. Putting a jumper into the load side of the preresistor relay causes the fan to operate at low speed. So the load sides of both relays work as they should. The problem has to be on the coil side of the relay.

Since the wire from the 105C switch changes colour before arriving at the relay, I can only assume that the green/brown wire goes somewhere else (for some other purpose?). This is not as shown on my circuit diagram. My car does have the Automatic Climate Control (not shown on the circuit diagram) so again I'm assuming that my green/brown wire goes to that (wherever it is physically in the car) and that there is some logic there which grounds the brown/grey wire when necessary and turns on the fan at high speed. I get the same response whether the AC is on or off.

So my questions are still:

Can I safely connect a new wire directly from the 105C switch and splice it to that brown/grey wire so that the fan will at least come on when the temperature is above 105C?

or...

Should I disconnect that brown/grey wire completely from wherever it comes from?

Terence Lowe.

You really should try the jumper at the fan switch on the dryer. If you jump the 2 wires that plug into the wires on the fan switch on the dryer, you should get low speed aux fan. You don't need to worry about the 105C high fan speed , that is say an auxiliary fan speed for a high heat condition. The normal operation is that S32 , the switch on the dryer with the built in leads, controls the normal operation of the aux fan which is low speed. You should isolate this problem first. K10 is the relay for normal fan operation that is connected to S32 , the pressure switch on the dryer with the built in leads.
At high speed , the aux fan draws 17.8 amps and the pre-resistor will get really hot running at high all of the time. The pre-resistor is 0.2 ohms and the load is about 62 watts. The pre-resistor body does not have a good thermal path to anything and the temp. rise on the resistor gets really large even at normal fan speed. If you run at high speed all the time , the pre-resistor will be hot enough to fail soon and could burn anything touching it. I failed to clean oak leaves from the area around the resistor in the spring and when I started the AC I saw smoke coming from the back of the hood. Of course ... the pre-resistor was, at normal ... low fan speed hot enough to nearly start a fire.
High fan speed is not intended to be continuous duty.
I would first isolate the problem ---- of the K10 relay and S32 pressure switch circuit. Really, I live in south Texas and it gets hot, , very hot,, Point... I don't see my engine temp reach 105C.
If the primary S32 - - K10 relay circuit is functional , you should not have to worry much about the S25 switch (the 105C switch you are talking about.

__________________
1986 300SDL 201K Daily Driver
1984 300D In Progress
1989 240GL 196K Swedish Brick
1984 300SD -- Sold 289K
------------------------------

Thanks. I'll look into the S32 switch on the dryer, if I can find it.

I don't understand why running the fan at high speed would affect the preresistor. Even if S32 was closed at the same time as the 105C switch, wouldn't most of the current come from the K9 auxiliary fan relay?

I'm still trying to figure out why the green/brown wire at the 105C switch becomes brown/grey at the relay.

Many thanks,

Terence Lowe.