FOUR SPEED FAN CIRCUITRY SIMPLIFIED
This setup will work for any 1 or
2-speed cooling fan. If a 2-speed fan is
used, I suggest using the high circuit only. I opted for using multiple
temperature probes as shown. The probes can be mounted on or
inserted into radiator fins or into a coolant hose for direct coolant
allows you to custom set each speed setting to
a precise temperature set point for your car. Starting and fan speed
should be smooth if starting at a lower speed. Of course you may
use screw-in temp senders
somewhere, such as the radiator or engine, however I don't know of any
that are adjustable.
Relay Function Theory
I have designed this to allow the fan to remain off until your
low temp set point is reached and the sensor
activates power to the low speed relay (Relay
4). In theory this
low speed could be used to maintain cooling at idle with minimum fan
climbs a little higher, the second sensor will activate the next
relay (Relay 3), automatically
cutting power to the lower speed
only ONE CIRCUIT will be
powered at a time. This function works for
in succession as temperatures rise and fan speed increases.
Why Four Speeds?
Why not? I know two speeds is definitely not enough
for a Mark VIII fan, because the full speed is overkill most of the
time. Three speeds might be ok and the above diagram can easily
be adjusted to make it into a 3-speed controller by simply eliminating relay
speeds just seemed about
right to me. Heck, you can even build a 5-speed
harness using these
components by adding just one more relay. But
I chose to do four speeds.
The amperage capacity for the high
speed relay (Relay 1) will
depend on the
fan you use. Most 15 inch or smaller fans (such as Volvo 960, 850,
can get by with a 40A relay for the high speed, however I suggest at
least 50A. A larger fan,
such as the 17-18 inch Ford Taurus/T-Bird/Lincoln Mk VIII fans, should
use a 70 or 80A relay. These big fans can pull 40 amps continuously at
so power and ground cables for these fans should be fat, such as 8-10
GA (I'm using
The 8 and 10 GA cables I'm using for my project are high-flex
strand type. Cables in 8 gauge and larger can usually be
found sold as welding cable.
I found them sold in reasonably short lengths in Amazon so you don't
need to buy them in large spools. Fine strand cable has much finer
common automotive cable. I'm willing
to pay a liitle extra because I've grown tired of dealing with stiff,
large strand cable that's hard to form and breaks easier after years of long-term
Here's a Wire gauge Guide that you may find useful. If you can
read it I think you'll find I used cables larger than required. That's
ok in my book.
OPTIONAL AC RELAY
This optional relay will override all other temp activated relays
(except for the full power relay). It
activates the fan at your chosen speed unless there is a higher speed
sensor/relay or unless a manual override switch is activated.
preference was NOT to use 100% for the AC, my idea was to wire this
relay to the "high" relay, which runs below full power (turned out to
be 77% in my case).
The 12V switched wire at terminal 85 gets connected to the power wire
activates your AC compressor.
NOTE: Or if you have an '84 to '89 Volvo 240 you may use the AC power
which is the Red/White wire going to the AC microswitch (attached to
knob switch in the center dash).
ISO Relays used for this project:
1. Full Power: SPDT rated
at 80 amps.
2. High: SPDT rated at 40
3. Medium: SPDT rated at 40
4. Low: SPDT or SPST rated at
5. AC: SPDT or SPST rated at
15 to 40 amps.
Mini ISO Relay
Connections (View from bottom of relay or bottom of connector
SPST (Single Pole, Single Throw): This type relay will have an 87b
SPDT (Single Pole, Double Throw): This type relay will have an 87a
30: Constant 12V (battery), unswitched
85: Signal, 12V switched (fuse panel)
86: Ground (completes coil circuit to close connection)
87: Consumer 12V (to fan or other device to be powered)
87b: Extra consumer (same as 87)
87a: Opposite of 87 (changeover relay only)
SPST vs SPDT Relays. What’s the
Single Pole, Single Throw (SPST): This relay will be
having a middle 87b spade (or no middle spade at all). This is
most common relay used for fog lights or other simple circuits. If
there is a middle 87b pin, that pin will have power whenever there is
power to the 87 pin (whenever the relay is “activated”). This way
middle 87b pin may simply be used as an extra power output.
Single Pole, Double Throw (SPDT): If you have a relay with an 87a pin
in the middle spot, it's an SPDT relay, also called a "changeover
relay." In a changeover relay, the 87a pin will be “HOT”
87 pin is "OFF," so long as the relay is connected to power. So when 87
is "OFF", 87a will be "ON". When 87 turns "ON", 87a will turn
The Hayden 3653 adjustable
temperature probe sensor pictured here can
from Amazon or Summit Racing for about $20 each. It's only rated
at 16 amps, so it really should never be used for any fan without a
relay. This sensor may be used
to trigger a 12V circuit or
ground circuit, either one. For my project I chose ground
circuits. The terminals on the side are typical .250 inch (6.3
to Hayden, the temperature range is 32°F to 248°F (0°C to 120°C). Turning the trim pot clockwise
increases the temp set point; counter-clockwise
decreases it. The kit
comes with a mounting bracket for the unit, some screws, some .250 inch
insulated terminals, some wire and
some mounting parts for the radiator probe. Dimensions for this
are about 2 x 1.4 x 1.2 inches (51 x 36 x 30 mm).
The resistor board for this project can be a simple auto AC/heater fan
resistor. The one I chose is
Dorman 973-018 for 1990-96 Chrysler/Dodge (4-speed fan). It's easily
found on-line. Cost is
under $10.00. This is designed to use .250 inch (6.3 mm) female
crimp terminals, but I found that .312 (7.9 mm) female terminals are a
better fit and they have a much better (larger) contact surface.
I just happened to have those on-hand. Better contact is
good, since I'm using 10 gauge wire here.
This resistor board offers four speeds
to choose from. I am only using
three of them (plus a full power relay for the fourth/full
measured the resistance values for each circuit on this board and
below. There are more testing results further below also.
the resistor coils will get hot when this board is in use.
1. Input: High Speed
2. Input: Medium 2
3. Input: Medium 1
4. Input: Low
5: Output to fan
It would be best to place this thing where your fingers won't be
tempted to touch it.
I mounted mine in the lower left fan shroud.
I have taken temp readings from
this resistor board when fans are running and have found it so far to
be not much higher than the shroud ambient temperature.
It was important to me to get some good
test results for this project before committing to installing it
and being on my way. It needed to actually work as expected, so I
bench-test version with all the needed components so I could test the
circuits and get some real performance measurements. It's important to keep in mind that the fan
was tested fully installed with a radiator, intercooler and AC
condenser in front of it. It would likely pull less amperage with
higher RPMs if it was tested out of the car. That would not be
Mark VIII Fan Speed/Current Testing
(static 12.7v battery with engine
I was a bit surprised that the high
power circuit on this resistor produced a number so much lower than
100%, but that offered an opportunity for a speed choice below
these speed figures, you can decide which circuits you
want to use for which fan speeds. The 26% speed might seem ok on
speed and the 77% speed might look good for high, but until it's tested
in a car,
you won't know for sure.
Peak Spike/Constant Current
spike amps when ramping from 77% to 100%, not when ramping fron OFF.
The diagram near the top of the page and the above bench test photo
original design using 26% for
my LOW speed. After seeing
setting actually run on my
fan, I thought it would be
a bit too low to be useful.
So I decided to change it to
the following four speeds below. Turned out to be a great choice.
FINAL SPEED SETTINGS CHOSEN
Another test was done after
installation with the engine running
and alternator charging (voltage readings below).
activates the 77% speed.
Since the 26% speed
used, it was not measured. I have a 188 degree F thermostat in
the car and the low 36% speed seems to do well to keep things nice and
cool at idle on a warm 85 degree day. If the climate is warmer then the
medium 55% speed comes on occasionally. Keep in mind that every
car is different. My car has a very large aftermarket AC condenser
(with dual fans) and an intercooler in front of the radiator. Those
items obstruct the flow to the radiator a lot. Cars without those
things will be very different.
Dorman 973-032 is a 3-speed
GM fan resistor (1977-94) that may be used as an alternate, although
after testing I think the low speed will be too low to use. It's your
though. This board might be a good choice for a 3-speed setup
medium (around 50%), high (around 70-75%) and then a full speed relay
for 100%. It's cheap
also, less than $10.
Dorman 973-430 is the same 3-speed resistor with an included plug.
1. Input: High Speed
2. Output to Fan
3. Input: Medium
4. Input: Low
H84988001 Four Relay Sealed Box. Available on-line for about $40
to $50. Pricey, yes! If you're concerned about moisture messing
with relays or maybe you just want them to look nice, this may be a
solution. Keep in mind it is NOT waterproof. There is an o-ring
sealed lid, but the bottom is not sealed at all. NOTE: The
80A (full-power) relay will NOT fit in this box, since it uses .375
terminals instead of small .250 inch terminals like standard
box holds only standard relays. The 90 degree mounting ears are
and supposedly you can connect two or more of these together side by
I found an ABS project box to fit the
three Hayden sensors and to help make this installation look
There are a lot of sizes out there in a couple colors, including black.
This box is 4.7 inches long x 2.6
inches wide x 1.4 inches tall O.D. (found on Amazon). I
to do some grinding on the inside to make everything fit inside. A
would have been easier, but I wanted it as compact as possible. Cost
was about $6.00. The box was painted black before completion.
Ford Fan Motor Connector. Fits a
variety of Ford/Lincoln/Mercury fans, including the Mark VIII fan I
search will find them in a few places. I have only found them available
with 12 gauge wire pigtails. In my opinion that's a bit small for
such a fan. I used 8 gauge welding cable.
Dorman Conduct-Tite Terminal,
AKA: Ford Standard Block Technician Terminal. This
is the type of terminal that goes into the Ford fan motor connector.
offers PN 85367
which fits 14-16 gauge wire and is
available on line. The
gauge version would be more appropriate, however so far it has NOT been
help with any real sources, please contact me.
So I improvised and sucessfully used the 14-16 gauge terminals since I
larger ones. I added one of my large
terminal overcrimps to make a solid crimp over the 8 gauge cable I
used. Inserting this into the back of the Ford connector housing
can be a little tight. I opened up the holes on the back of the
connector to accommodate this fat cable. If necessary, a little
the inside of
the housing with a hobby knife will help
it go in and click. Mine went
in and clicked without an issue.
those of you who are curious about what crimper I use, this is it for
90% of my crimps. Thomas & Betts model WT 112M. It runs about
$45. I've probably had this one for 20 years.
This is an aluminum Chassis Mount
It is NOT something I have tested or
used yet. The reason this info is here is because It has
been suggested for durability (particularly in corrosive environments)
that some of you might consider a different style resistor instead of
the Dorman resistor board. Most of the suggestions have been
pointing to ceramic automotive resistors, such as those used for
ignition, AC and
cooling fans in Euro cars. The problem I encountered is that
there are very few choices for varied resistance values in ceramic, so
take a lot of trial and error to get that working well.
So then I began looking at these Chassis Mount Resistors. The
dimensions for what I found below are 65
x 47 x 26 mm (about 2.5 x 1.9 x 1.25 inches), they run about $10
each and they are
rated for WET CONDITIONS.
Most cermamic resistors have
only a 30-40 watt capacity. The chassis mount types
I have listed below all have a 100
You would need three of them to come close to duplicating the three
speeds I have with the Dorman resistor board I used, however I have not
any yet to see what speeds they will actually generate. Since the ohm
values I found in the Dorman resistor board were 0.3, 0.8, 1.4 and 3.1
(I'm only using 0.3, 0.8 and
you can try some that are reasonably close to that, such as 0.22, 1 and
1.5 ohms. A source I found for these
Here are some DigiKey part numbers and resistance values to consider .
|PN A102367-ND 0.1 ohm
PN A102368-ND 0.22
PN A102369-ND 0.47
PN A102106-ND 1 ohm
PN A102185-ND 1.5
PN A102360-ND 2.2 ohms
you incorporate something like
this in your build, please email me. I would like to know how your
This setup has been road tested in
95 degree plus Texas climate
and aside from a little fine tuning of temperature settings as I
encounter different climates, it
works great. The 77% speed I chose for the AC works very well
After almost a year of
use, this controller has worked perfectly. Zero glitches or
That's nice for a change.
<<< Here's the
full setup installed in my 240. As you can see I opted for the fancy
Hella relay box. Those four relays in the box are for AC, Low,
Medium and High speed.
Another angle. That relay on the left side of the Hella
box is the 80A full power relay. It is triggered by an override switch
on my dash. Hopefully it won't be needed, but it comforts me to have
it in case.
view of the
Dorman resistor board mounted in the lower left corner of the fan
That's 10 gauge high-flex wire and .312 inch terminals. To those
of you who cried out that it would melt, sorry, but you were all
wrong. 6 months later and all is working perfectly.
This is worth mentioning if you decide to used the Hayden 3653 temp
sensor I used. It concerns mounting the radiator probe on the radiator.
This image is from the instructions. They supply the shown probe
mounting parts and
tell you to mount the probe in a supplied metal/rubber mounting strap
that is then
isolated from the radiator by a foam pad. THIS IS A REALLY BAD
IDEA. What this does is delay radiator temps from quickly
transmitting to the probe. The delay with mounting it this way is
substantial. It has slightly less of an effect when hot air from
radiator is blowing past the probe, but there will be signiificant
delay when warming up the engine if the car is not moving fast.
The delay will be
such that the radiator will heat up past the setting you want for the
fan to come on, so then it comes on late, and then it will delay the
shutting down as the radiator temps reduce, causing wide swings of the
fan on/off cycle. You'll curse Hayden because you wont be able to
accurately dial in your on/off settings. It's just bad. Don't do that.
DO THIS: The probes that come
with these units
are a bit fat and would not slip into my radiator fins without a lot of
This may be why Hayden wrote those instructions that way. I
didn't want to risk damage to my radiator by bending things, so I
against the radiator as shown in this photo using thin zip-ties. While
it might be hard to tell, the full length of each probe is very snug
the radiator fins for good contact. Sensor activation
with this method is quick and accurate. Faith in Hayden has been
Here's the override switch I placed on my dash. The small LED below the
switch illuminates green when the fan is on.
The LED was too bright with a full 12v, so I added a 220k ohm resistor
power wire. Now it seems just right.
The final speeds I chose have
turned out well.
FULL POWER: 100%
(AC activates 77% speed)
Your comments are welcome.
Update 2017: The dorman version of the Lincoln Mark VIII fan is available
in Amazon here cheap: https://www.amazon.com/gp/product/B001SGHK1W/ref=pe_850140_225176060
(Mostly what parts I can
remember. Some items are
available in my Relay Page or Harness Parts Page if needed).
Relay - Picker High Current 80A relay
(SPDT). Qty 1. Available here: http://www.davebarton.com/volvorelays.html#highcurrentrelay
Relay - Tyco gray “101” 40A relay (SPDT).
Available here: http://www.davebarton.com/volvorelays.html#1259926-101gray
Relay plug socket, interlocking, 5-pole.
Available here: http://www.davebarton.com/volvorelays.html#relayplug
Optional - Hella H84988001 Four Relay Sealed Box. Qty: 1
Relay high-current plug socket, 5-pole.
Available here: http://www.davebarton.com/volvorelays.html#highcurrentplug
Fuse holder (mini fuse) with 5A fuse (18
gauge wire). Qty: 1
Fuse holder (maxi fuse) with 50A fuse (8
gauge wire). Qty: 1.
Hayden 3653 adjustable temp sensor. Qty: 3.
Resistor pack - Dorman
973-018 4-speed fan resistor. Qty: 1.
Diode - PN
I have some
because the minimum buy quantity was 20. Email me.
Wire - 8 gauge (high flex welding cable
recommended). Qty: about 8 feet red.
Wire - 10 gauge (high flex marine grade
recommended). Qty: about 2 feet red.
Wire - 16-18 gauge auto primary wire. Red
and black. Qty: about 8 feet red and black.
Heat-Shrink Tubing - Varies sizes (maybe
3/8 to 1/2 inch) in black or red and black.
Toggle Switch - On/Off
for high speed relay override wire. Dash mounted. Qty: 1.
Crimp terminal - .375" (9.5 mm) female,
6-10 gauge (for high-current relay). Qty:
Available here: http://www.davebarton.com/blackvinyl.html#.375inchterminals
Crimp terminal - .312" (7.9 mm) female
straight, 10 gauge (for Dorman resistor). Qty: 4.
Available here: http://www.davebarton.com/blackvinyl.html#headlightplugs
Terminal Overcrimp (optional) - Large (for
resistor terminals using 10 gauge wire). Qty:
Crimp terminal - .250” (6.3 mm) female,
10-12 gauge (for relays). Qty:
Available here: http://www.davebarton.com/blackvinyl.html#.250inchplugs
Crimp terminal - .250” (6.3 mm) female,
14-18 gauge (for relays). Qty:
Available here: http://www.davebarton.com/blackvinyl.html#.250inchplugs
Terminal Overcrimp -
Large (used for Ford terminals in the fan motor connector). Qty: 2.
Available here: http://www.davebarton.com/blackvinyl.html#overcrimps