I found a good candidate dual-temp fan switch

A mechanic friend of mine lent me one of his catalogs to flip through and I found this fan switch for a mid-80s Mercedes 190E.



According to the data sheet, one circuit closes at 212F, and the other closes at 224F. This would be good for the Taurus fans since it would be in between the stock electric fan; the low speed Taurus fan would kick on at 212, the ECM would kick on the stock fan at ~217, and the high speed Taurus fan would kick on at 224. Perfect.

Some problems...

First, the only place I can find this is on NAPA and none of the other vendors have a three-pin fan switch. I originally found it in a BWD-Niehoff catalog but Advance Auto doesn't even show any matches for the part (TFS611). My guess is that nobody makes this anymore, so it's not going to be a long-term solution.

I have no idea on where to try and find a pigtail. Maybe there are MB junkyards that would have it. Maybe some of the pigtail parts makers still produce one. I haven't even started looking.

I don't know the pin-out or the switch behavior (does circuit 1 close when circuit 2 opens? dunno).

The thread is 14mm, so you will need to find a way to put a 14mm bung onto something.

Standard part number TS-508 "no applications" lol

Standard owns BWD now interesting

I don't think an adapter would be too hard to find, let us know on the switch and pigtail if you ever find one.

It sounds like the temps are too high. I would want the fan to turn on at more like 190 for low speed and 210 for high speed.

Those temps are too low. You'll be turning on the fan before the thermostat even opens fully.

Okay here is some of the thinking I went through. For one thing you need to have a place to put the switch, and the reliability of the reading will also depend on where the coolant temperature is being monitored, and if you are not at the source then a lower temperature may be needed.

The thermostat housing gives the most accurate reading. Unfortunately this sensor uses a 14mm thread and there's not an easy way to install that into the thermostat housing (I don't believe there is room on the front without hitting the center bolt).

The thermostat housing has a bypass for the heater core, so the upper heater hose gives a reliable reading as long as there isn't a heater control valve blocking the coolant flow, otherwise you will only get timely readings when the heater is on and otherwise will get delayed (unreliable) readings as the coolant temperature changes from conduction. So if you have a 1997+ or if you have deleted the control valve then you can insert some kind of sleeve in the upper radiator hose and tap a sensor there, otherwise you shouldn't do it. In my case I have already deleted the valve, but have tentative plans to install one of the metal Renix-era valves in its place, and besides this isn't portable so bad idea. And anyways it probably isn't possible to fit a 14mm threaded bung into a sleeve in that hose--it would have to be a T-fitting of some kind and would probably impede coolant flow to the heater.

The upper radiator hose is likewise reliable if the thermostat has opened, but is not timely (and unreliable) if the thermostat is closed. However the thermostat isn't a binary switch but instead opens and closes slowly, so a switch in the upper hose is going to be pretty reliable as long as you are within ~10 degrees F of the thermostat threshold (any lower and the thermostat will be closed so you will have to rely on conduction will will be wrong). There is also room in the upper hose for a sleeved bung of some kind. So all told this is the best place to put a sensor.

Given that temperature will be pretty reliable in this location, it seems important to activate the fan at the appropriate trigger--if you use 190 or less then the fan will come on all the time, basically it will come on everytime the thermostat opens, which is too cold. Instead you want low speed coming on when thermostat is fully open (about 210). If temperature keeps rising the ECM will kick on the secondary fan at ~217, but the low-speed primary fan would still be running so you'd have two fans going now. If temperature still keeps rising you would want to kick on high-speed so maybe another ~7 degrees at ~224. (BTW does anybody know what temperature the ECM trips the dash warning light?).

Those are the values I came up with. Maybe in an ideal world you would use 3 degrees cooler to allow for difference between the thermostat and a switch in the hose, however they don't make switches for every value and this is the closest I came up with.

In any case I would not want the fan coming on at 190, since it would be engaging everytime the thermostat started to open

Here's a three-pin fan switch for a 1994 Saab 900 that has good temperature values (circuit 1 at 198f and circuit 2 at 221f). However it has a 22mm thread. Where are you going to put that? You pretty much need to make a hole in the radiator tank and that's going to be completely inaccurate.

For mounting options on the 22mm I would consider redrilling and tapping out the thermostat housing, which I believe is either 1/4 or 3/8 NPT, those are both small enough that the threads should be mostly, if not entirely gone by the time you finish drilling. I'm not sure what to do with the 14mm though.

EDIT: that is... if there is enough space on the housing to put a hole that big in it.

Honestly after looking at cost for things like the sensor and fittings and timers and all that junk it's just as cheap to buy the SPAL and be done with it. It will work with the stock sensor so buy it and done.

This might work--1994 BMW 325i dual switch (NAPA), #1 on at 195 and #2 on at 205



14mm thread size is big but could work in upper hose

I am doing the two stock Jeep electric fan conversion to eliminate the clutch fan on my 91 XJ 4.0L . I saw the variable fan controllers available. Can switches like this be used to efficiently operate the two fans or is the variable controller the only way to go (expensive...).

Have you considered building something? You could easily build a device that would use a single standard sensor to control up to four different fans/relays/lights/whatever with about five bucks in electronics and a PCB.

Parts list -
* standard OBD-I/II thermostat housing mounted sensor
* 1k ohm resistor
* LM339 quad comparator
* 1 to 4 4.7k ohm 10-turn variable resistors (or whatever you can adjust comfortably, 10 turn will be more expensive but easier to adjust accurately)
* 1 to 4 IRF511 MOSFETs
* 1 to 4 relays of whatever type you want to run

Should do pretty much anything you want.

kastein said:

Have you considered building something? You could easily build a device that would use a single standard sensor to control up to four different fans/relays/lights/whatever with about five bucks in electronics and a PCB.

Parts list -
* standard OBD-I/II thermostat housing mounted sensor
* 1k ohm resistor
* LM339 quad comparator
* 1 to 4 4.7k ohm 10-turn variable resistors (or whatever you can adjust comfortably, 10 turn will be more expensive but easier to adjust accurately)
* 1 to 4 IRF511 MOSFETs
* 1 to 4 relays of whatever type you want to run

Should do pretty much anything you want.

Click to expand...

Schematic? Some of us are not that knowlegable in electronics.