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. You may instead
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
The fan will remain off until your
low temp set point is reached and the sensor
activates power to the low speed relay (Relay 4). When your
climbs higher, the second sensor will activate the next
relay (Relay 3), automatically cutting power to the lower relay.
only one circuit will be on at a time. This action works for each relay
in succession as temps rise and fan speed increases.
Why Four Speeds?
Why not? I know two speeds is definitely not enough
for a Mark VIII fan. Three speeds might be ok. Four seems about
right to me. The diagram here
can easily be converted to a 3-speed harness by eliminating relay
#4. Heck, you can even build a 5-speed
harness using these
components by adding just one more relay. But
just 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 at full
so power and ground cables for these fans should be 8-10 GA (I'm using
The 8 and 10 GA cable I'm using for my Mark VIII fan are high-flex fine
strand type. Cables in sizes 8 gauge and bigger can usually be
found sold as welding cable. These cables have much finer strands than
common cable. I'm willing
to pay a liitle extra because I have grown tired of dealing with stiff,
large strand cable that is hard to form and breaks easier after long
Here's a Wire gauge Guide that you may find useful.
OPTIONAL AC RELAY
This relay will override other lower temp activated relays. It
activates the fan at your chosen speed unless a higher speed
sensor or manual override switch is activated.
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
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 connector plug)
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 descreases it. The kit
comes with a mounting bracket for the unit, some screws, .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. That's
good, since I used 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/high speed).
Total of four
measured the resistance values for each circuit on thid board and
below. There are more testing results further below also.
the resistor coils will get hot when 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 an extra speed choice.
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%.
The diagram near the top of the page and the above bench test photo
original design with low speed 26%
for LOW. After seeing that
setting actually run on my
fan, I thought it might 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).
(Medium 1): 36%
MEDIUM (Medium 2): 55%
activates the 77% speed.
Since the 26% speed
used, it was not measured.
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. Your choice
though. It might be a good choice for a 3-speed setup using
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 has .375 inch
posts instead of small .250 inch posts like normal relays. This
box holds standard relays. The 90 degree mounting ears are removeable
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 help made this installation look nice.
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 Block Technician Terminal or Ford Mini Series Terminal. This
is the type of terminal that goes into the Ford fan connector. Dorman
fits 14-16 gauge wire and is
available on line. The
gauge version appears to be Dorman PN 85365. I have only
found the large one shown on line here: http://detroitmotorparts.com/s/1/p/22070373,
however they link you to an Advance Auto Parts link which is dead.
Dorman 85365 is not found on the Advance Auto Parts page. If anyone can
help with other sources, please contact me.
So I sucessfully used the 14-16 gauge terminals since I couldn't find
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 for this fat cable. If necessary, a little trimming of
the inside of
the housing with a hobby knife will help
it go in and click. Mine went
in and clicked without an issue.
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 resistance values in ceramic, so it may
take a lot of trial and error to get that working.
So then I began looking at these Chassis Mount Resistors. The
dimensions for what I found 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, however I have not tested
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,
you can try some that are reasonably close to that, such as 0.22, 1,
1.5 and 3 ohms. A source I found for these
Here are some DigiKey part numbers and resistance values to consider .
If you incorporate something like
this in a build, please email me. I would like to know how your project
|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
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
<<< 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.
Another angle. That relay on the left side of the Hella
box is the 80A full power relay that is triggered by an override switch
on the dash. Hopefully it won't be needed, but it comforts me to have
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.
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 parts and
tell you to mount the probe in a supplied mounting strap that is then
isolated from the radiator by a foam pad. THIS IS A 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 less of an effect when hot air from the
radiator is blowing past the probe, but there will be signiificant
delay when warming up if the car is not moving. The delay will be
such that the radiator will heat up past the setting you want for the
fan to come on, so it comes on late, and then delay the fan from
shutting down as the radiator temp reduces, causing wide swings of the
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. Obviously I
didn't want to damage my radiator by bending things, so I mounted the
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 snug againt
the radiator. Sensor activation
with this method is quick and accurate. Faith in Hayden is
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 full 12v, so I added a 220k ohm resistor to
power wire. Now it seems just right.
The final speeds I chose (posted
below) have turned out well.
FULL POWER: 100%
(AC activates 77% speed)
Your comments are welcome.
(Mostly what 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