Electric Fan Swap Questions

[ratdude747]

A couple years ago I scored a 2-speed Taurus fan for virtually free (Goodwill, of all places, $2.50), and now that I've finally gotten to upgrading the alternator (130A), I'm looking at working on the swap. I'm mainly going for fuel economy, but any power/torque gains are icing on the cake.

I'm planning on using a BMW switch (the 195/210 kind, to compliment my 195 T-stat) and doing my own relay circuit. I already have a crude schematic sketched which incorporates the A/C and when the A/C is on, keeps low speed on until the 195 switch trips, which will then turn on high; with the A/C off, the low will turn on at 195, and the high at 210, as usual. However, it makes some assumptions that I'd like to verify are true before buying more parts:

1. The BMW swiches are non-polarized? I'd like to use them as a high side switch, instead of a low side switch. This matters as it makes the A/C input a lot easier to implement (I can do all I want with three Bosch relays and a diode). All the schematics I see have the switch used on the low side, which I presume is to maintain compatibility with the often-used Volvo relay module, not because of the switches having some sort of diode protection or a chassis connection? I see the common terminal of such BMW switches on some schematics marked as "+", which leads me to believe it can be used in such a way.

2. The Taurus fan is only powered by one input at a time, right? Not both for high speed? This isn't as important, as all it changes is one relay connection: whether the low temp relay's common needs to be powered off the NC terminal of the high temp relay, or gets tied to the fused fan power like the high temp relay common.

3. For adding the BMW sensor, my plan is to get one of the aluminum radiator hose adapters with a 1/8 NPT hole, and then drill and tap it for the M14x1.5 the sensor uses. This will ensure the sensor has good contact with the coolant flow. I was planning to add it to the "over the alternator" part of the upper hose, which would allow me to also use it as a coolant fill bleed point if I so wished. Is this a good place to put the sensor (rad outlet), and is the 195/210 switch the correct one to use in conjunction with my 195 degree thermostat? If not a good place, where would be better?

If there is interest/relevance, I'll scan and upload my schematic sketch.

 

[RonD]

If you use the upper rad hose for the temp switch then it should be mounted sideways or down, not on top as thats where the "air" will be if coolant gets low
And upper rad hose is not a good place to purge air in any case, remove heater hose to let air out, its on the engine side of thermostat, thermostat is why air can't get out when refilling

Lower rad hose is better for temp switch(no air) but on/off temps would be different
There is a BMW switch for lower rad hose, it is (180/195*F) dual temp switch P/N 61318361787

You "might" be able to use the temp switch as 12v activation vs Ground activation, in theory its the same, but.....

I would just use another relay for the AC circuit to ground the low speed fan relay to activate it
The fewer 12volt wires there are the less likely there will be a short, with ground activation worse case is the fan comes on, no "smoke show", lol
So if you can keep all the 12volt wiring at the relays, except for the two wires going to fan motor, you would be better off, IMO

Yes, Taurus Fan just needs 12v on the low or high wire, not both

In 1995 Ranger you can tap the 12v AC ON at either of the pressure switches, don't have to do it at the compressor, but you can of course, what ever is the shorter wire

 

[ratdude747]

Here's what I have for a draft schematic now:

In this design, turning on AC (purple wire going to the cycling switch on the A/C dryer) forces the "L" relay on (via diode), and via the "AC" relay, switches the output of the "L" output of the temp switch from controlling the "L" relay to the "H" relay. The arrangement of the high-current wiring (using the NC side of H relay to feed into the L relay) ensures that only one motor input is on at a time, and H side wins.
I figured this would provide better cooling when the A/C is on rather than relying on the increased temps of A/C usage (passive heating via the condenser) to up the fan speed. Given the 4.0's known tendency to crack things when overheated, I wasn't wanting to take chances. I still want to run a 195 T-stat for efficiency reasons.

I'll mull over it, but I could make the temp switch ground sided and still have the same net result if I swapped the diode for one more Bosch relay (two A/C controlled relays, one per speed range)... I don't see what that would gain though; many other things are run on the truck with high-side switches (such as the A/C, obviously). If the switch is rated for it, as long as I properly insulate things, I'm not seeing the issue.

It sounds like I might want to buy both temp switches... now that I think about it, wouldn't I want the high temp switch (195/210) for the lower hose, since that's the outlet of the water pump (hot coolant coming from engine)? If I run the low temp switch (180/195) there, I'll almost never get the L range (defeating the purpose of a 2 speed switch), since once warmed up (thermostat opened), my temps (current setup) run in the 190's to the high 210's, which is what one would expect with the stock 195 degree t-stat? I thought the low temp switch was for applications using a 180 degree (or lower) T-stat?

 

[franklin2]

How about this version?

 

[RonD]

The other temp switch(180/195) is for use on lower radiator hoses, and its temp is 180 when upper hose is 195, radiators provide 15deg of cooling, so same on and off as upper hose(195/210) , just out of the way and no way for an air bubble to cause fan to not come on


This is the normal wiring for relays with 2 speed fan: https://www.therangerstation.com/tech_library/images/volvo_dual_speed_fan_relays.JPG

You could add a 3rd AC relay(turns on relay with 12v) that would ground the low speed relay's ground wire(grey) and it wouldn't effect high speed coming on

Just don't like the idea of running 12v thru the temp switch, lol, it should work, just don't like it

 

[ericbphoto]

How about this version?View attachment 51811

Not crazy about that. If, for some reason the high temperature switch closes and you have the A/C on, you will energize both high and low speed fan motor windings. Plus, why the A/C signal going to the common supply to the temperature switch? That is unneeded.

 

[ratdude747]

The other temp switch(180/195) is for use on lower radiator hoses, and its temp is 180 when upper hose is 195, radiators provide 15deg of cooling, so same on and off as upper hose(195/210) , just out of the way and no way for an air bubble to cause fan to not come on


This is the normal wiring for relays with 2 speed fan: https://www.therangerstation.com/tech_library/images/volvo_dual_speed_fan_relays.JPG

You could add a 3rd AC relay(turns on relay with 12v) that would ground the low speed relay's ground wire(grey) and it wouldn't effect high speed coming on

Just don't like the idea of running 12v thru the temp switch, lol, it should work, just don't like it

That doesn't add up. Coolant flows from the lower hose into the radiator to the upper hose. Coolant cools down as it goes the though the radiator (obviously), thus, the lower should be hotter than the upper as long as coolant is flowing correctly.

I saw the bit on the volvo relays (where I started my design from, actually)... the volvo relay module requires ground side switching, which is why such designs (and things directly modeled after it) use such. I'm not using volvo relays, hence why I'm asking.

 

[ericbphoto]

Coolant flows from the lower hose into the radiator to the upper hose

Are you sure? On the 3.0l engines, it flows out of the thermostat at the top of the engine over to the top of the radiator and returns from bottom of radiator to engine.

If it's backwards from that on the 4.0l, then you are correct.

 

[RonD]

No, coolant doesn't flow that way
It always flows in to the top and out of the bottom on any vehicle radiator

You can use any 12v 5 post relays, rated for the fan amps(40 amps), which would be a 12v 40a rated relay, they cost under $10 WITH the base, but you can usually buy 5 with bases for $15

Like these: https://www.ebay.ca/i/352117535131?chn=ps&norover=1&mkevt=1&mkrid=706-89093-2056-0&mkcid=2&itemid=352117535131&targetid=884837763599&device=c&mktype=pla&googleloc=9001573&poi=&campaignid=9567052948&mkgroupid=99233748238&rlsatarget=pla-884837763599&abcId=1063836&merchantid=138578504&gclid=EAIaIQobChMI74OupNOX7QIVWiCtBh08MgCaEAQYAiABEgJBLvD_BwE

$13.20US


Most car makers use a Ground to activate relays because, as said earlier, its safer to run ground wires around a vehicle than running 12volt wires

 

[ratdude747]

No, coolant doesn't flow that way
It always flows in to the top and out of the bottom on any vehicle radiator

You can use any 12v 5 post relays, rated for the fan amps(40 amps), which would be a 12v 40a rated relay, they cost under $10 WITH the base, but you can usually buy 5 with bases for $15

Like these: https://www.ebay.ca/i/352117535131?chn=ps&norover=1&mkevt=1&mkrid=706-89093-2056-0&mkcid=2&itemid=352117535131&targetid=884837763599&device=c&mktype=pla&googleloc=9001573&poi=&campaignid=9567052948&mkgroupid=99233748238&rlsatarget=pla-884837763599&abcId=1063836&merchantid=138578504&gclid=EAIaIQobChMI74OupNOX7QIVWiCtBh08MgCaEAQYAiABEgJBLvD_BwE

$13.20US


Most car makers use a Ground to activate relays because, as said earlier, its safer to run ground wires around a vehicle than running 12volt wires

Duh on the cooling direction... I forgot that water pumps aren't cetrifugal impellers. Not super keen on putting the sensor on the lower hose though.

One idea I just had (probably another stinker) is to get another T-stat housing and drill a hole and tap it (or somehow bond a bung/nut to it) and use that for the sensor point? I can't remember if there's anything above the thermostat housing that would get in the way though. From what I'm seeing online, they're all thin wall aluminum, and I sadly don't have TIG (let alone AC TIG) to my personal disposal

I was planning on using these relays:

https://www.amazon.com/dp/B01KVZ2MU4/?coliid=I1OYVRUARG1MQL&colid=SJR3F4QNS2UY&psc=1&ref_=lv_ov_lig_dp_it

I've seen automotive relays and actuators switched both ways... If high side switching is so "evil", why did Ford use for for the A/C clutch circuit? That wiring goes all around the engine bay...

 

[rubydist]

One required thing your circuit seems to be missing: When the relay commanding "high" speed closes, it must also open the relay commanding "low" speed, otherwise you burn up the fan motor. If you look at the circuit on the tech page for "Volvo fan install" it will show you how it should be wired. To add the command for "low" speed when the a/c is on, you just add a relay that grounds the "low" speed side of the BMW temp sender circuit as was suggested above.

 

[ratdude747]

One required thing your circuit seems to be missing: When the relay commanding "high" speed closes, it must also open the relay commanding "low" speed, otherwise you burn up the fan motor. If you look at the circuit on the tech page for "Volvo fan install" it will show you how it should be wired. To add the command for "low" speed when the a/c is on, you just add a relay that grounds the "low" speed side of the BMW temp sender circuit as was suggested above.

Whose schematic are you referencing?

My schematic (post #3) takes care of that issue by feeding the "L" relay contacts from the NC of the "H" relay. If both turn on (which in my design, will often happen with the A/C on), the fan will get high only due to the NC of the H relay cutting power to the L relay's contacts. Which is why I was verifying that the motor inputs are indeed 1 at a time.

Franklin2's schematic does indeed have that problem... but that's not the schematic I'm using.

 

[RonD]

12v switching is not "evil", lol, it comes down to wiring exposure
If you just need to run one 12volt fused wire to a "relay pack" and then the rest of the wiring can be Ground controls it reduces exposure to shorts because a ground wire short won't blow a fuse or melt a wire

One is not better or worse, as far as the "control", but ground control is "smarter" because it reduces the places the wiring might get a short that blows a fuse or melts a wire

If you have choice of running 4 12volt wires or 7 12volt wires use the 4 its "smarter", not better

You don't need to drill thermostat housing, just use a hose fitting on the heater hose that comes out next to the thermostat housing
Upper rad hose is fine, as said, but orient the temp switch bung on bottom of hose or side of hose to avoid the switch sitting in "air", it only works if its sitting in coolant, and any "air" in the system will end up in upper rad hose and upper radiator

 

[ratdude747]

12v switching is not "evil", lol, it comes down to wiring exposure
If you just need to run one 12volt fused wire to a "relay pack" and then the rest of the wiring can be Ground controls it reduces exposure to shorts because a ground wire short won't blow a fuse or melt a wire

One is not better or worse, as far as the "control", but ground control is "smarter" because it reduces the places the wiring might get a short that blows a fuse or melts a wire

If you have choice of running 4 12volt wires or 7 12volt wires use the 4 its "smarter", not better

You don't need to drill thermostat housing, just use a hose fitting on the heater hose that comes out next to the thermostat housing
Upper rad hose is fine, as said, but orient the temp switch bung on bottom of hose or side of hose to avoid the switch sitting in "air", it only works if its sitting in coolant, and any "air" in the system will end up in upper rad hose and upper radiator

It would only be 4 12V wires...one is unavoiablable (the A/C connection), and the other three are going to the switch. IMHO it's better to go with the 12V switch layout which is more reliable (3 relays and diode vs 4 relays).

I thought about using a heater hose sensor adapter... my main "worry" there is that it would be reading coolant temp in the engine, not coolant temp entering the radiator. But that's kinda moot since the thermostat and the switch are set to the same temperature point. I'll look around... I'll gladly chop heater hoses over rad hoses any day.
Cost is about the same, but the wall thickness looks better:

https://www.amazon.com/GZYF-Motorcycle-Temperature-Aluminum-Universal/dp/B07HQHMJZQ/ref=sr_1_4?dchild=1&keywords=18mm+sensor+adapter&qid=1606181785&sr=8-4

My only concern is that there isn't enough meat on it to safely drill and tap to M14x1.5. Such is substantially bigger than the 1/8NPT that's in out of the box.

 

[rubydist]

The ones I have used are the Glowshift radiator hose adapters - they have plenty of meat to allow being drilled out to M14 for the BMW sensor.