4-Speed Mark VIII Cooling Fan
Controller/Harness Project and Diagrams

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  4-Speed Cooling Fan Controller/Harness Project
I am NOT offering fans, controllers or harnesses for sale here. You may use this info to build your own.
I began a forum discussion about this project before publishing this page.  You may find it here.
During the summer of 2016 I began looking into creating my own relay-based cooling fan control project after experiencing a succession of failures over the years of popular high-tech variable speed fan controllers that are on the market.  I came to the conclusion that most of the popular variable controllers have some nice technology (ie: DCC, Flexalite, Derale), but you should not expect them to be reliable for long periods of time if using a high current fan like the Mark VIII. They weren't for me.  My experience over many years has been that relays are much more reliable and as long as you take care when building a high-current relay component harness, it will last nearly forever. 

Why am I saying bad things about DCC?  I'm glad you asked. There are lots of people who love the Delta Current Control fan controllers. I loved them too once. Their line of PWM variable boxes are enticing in their web site and the cost is pretty reasonable. The first problem I had was when two of them failed on my car (they lasted 1.5 to 2 years average). At the time I was using a Ford T-Bird fan, which pulls lots of current. I had their largest controller at the time (65 amp) which was installed according to their specific instructions.  DCC has steadily increased their current capability in their controllers over the years and as of this writing their most powerful has 95 amp continuous capacity. 

I can handle failures. I was prepared to just buy another. What I cannot deal with was my experience with DCC's non-existent customer service. When the last DCC controller failed, I ordered another on their website and paid with Paypal. I received no acknowledgement of an order, no follow-up email, and as a number of weeks went by I emailed the owner (Brian Baskin) a number of times.  My car was down the entire time!  After six weeks had gone by with nothing, I filed a Paypal dispute and eventually got my money back.  I never heard from Baskin. What kind of place does crap like this?  Not a place I ever want to deal with again. They offer a 90 day warranty, but never answer emails? They have the following statement in their site: "In a time when profit margins govern allowable failure rates, product features are determined by the opportunity of market exploitation, and the consumer is seen as no more than a short term item of commodity, the value put upon interim revenue is clear. We see things a bit different at Delta. We appreciate the value of our customers not as commodities, but as our best means of long term market expansion. There is no better advertisement than word of mouth."  Ha!  I have seen a number of forum posts and received emails from many other people who have had the same experiences as I had with DCC. I think I'll stay away from them.

Also, I've been disappointed that so many of the high-tech fan controllers only offer threaded screw-in temp sensors designed to go into your engine (or maybe so you can have a custom bung welded on your radiator?).  I have used simple radiator fin probes for many, many years and they work pretty darn well when placed in the correct position.  Why don't more companies offer more options? Many do not.

While I'm registering complaints, I also noticed that if you want a fan to increase to a higher speed when your AC comes on, the ONLY option is 100%.  Really? That seems short-sighted. What if I want only 70, 80 or 90% of full speed for my AC?  If you're going to the trouble of building a high-tech PWM controller, isn't that a simple extra to add?  Maybe  AC is not really an important thing to fan controller engineers.

As you may have noticed, I'm now using a Lincoln Mark VIII fan and shroud (1997 model) mounted on a large Griffin aluminum radiator.  I decided to try building my own fan controller using reliable relays to see if I could make one work the way I wanted. 

More general info on the installation of my Mark VIII fan and Griffin radiator in my Volvo 242 Turbo can be found in my Cooling Fan Project Page at: http://www.davebarton.com/ElectricCoolingFans.html#markVIIIfan

Your feedback is appreciated. If you have any comments or if you decide to use this info to build something like this for yourself, I would like to hear from you.

Click on photos for larger images
<<< Diagram for 4-speed configuration.
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 adjustable temperature probes as shown. The probes can be mounted on or inserted into radiator fins or into a coolant hose for direct coolant contact. This allows you to custom set each speed setting to a precise temperature set point for your car. Starting and fan speed ramp-up should be smooth if starting at a lower speed.  Of course you may instead use screw-in temp senders mounted somewhere, such as the radiator or engine, however I don't know of any that are adjustable.

4-Speed Relay Function Theory
I have this 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 the low speed could be used to maintain cooling at idle.

When your temperature climbs a little higher, the second sensor will activate the next relay (Relay 3), automatically cutting power to the lower speed relay.  This way only ONE CIRCUIT will be powered at a time. This function works for each relay 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 #4.  Four 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, S80) 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 full power, so power and ground cables for these fans should be fat, such as 8-10 GA (I'm using 8 GA).

The 8 and 10 GA cables I'm using for my Mark VIII fan are high-flex fine strand type. Cables in 8 gauge and larger can usually be found sold as welding cable. This type of cable has much finer strands than 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 vibration.

<<< Here's a Wire gauge Guide that you may find useful.

This relay will override other lower temp activated relays. It activates the fan at your chosen speed unless a higher speed temp sensor or manual override switch is activated.  Since my 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%).

The 12V switched wire at terminal 85 gets connected to the power wire that activates your AC compressor. 
NOTE: Or if you have an '84 to '89 Volvo 240 you may use the AC power 'ON' wire, which is the Red/White wire going to the AC microswitch (attached to the AC switch in the center dash).

Mini ISO Relays used for this project:
1. Full Power: SPDT rated at 80 amps.
2. High: SPDT rated at 40 amps.
3. Medium: SPDT rated at 40 amps.
4. Low: SPDT or SPST rated at 40 amps.
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 center pole.
SPDT (Single Pole, Double Throw): This type relay will have an 87a center pole.
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 difference?
Single Pole, Single Throw (SPST): This relay will be identified as having a middle 87b spade (or no middle spade at all).  This is the 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 the 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” anytime the 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 "OFF". 

<<<  The Hayden 3653 adjustable temperature probe sensor pictured here can be purchased 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 mm).  According 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 item 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 speeds. I measured the resistance values for each circuit on thid board and included those below. There are more testing results further below also.
Dorman 973-018 Pinouts
1.  Input: High Speed
2.  Input: Medium 2
3.  Input: Medium 1
4.  Input: Low
5:  Output to fan

Resistance Values
0.3 Ohms
0.8 Ohms
1.4 Ohms
3.1 Ohms
Yes, the resistor coils will get hot when in use.
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 built a 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 very realistic.

Lincoln Mark VIII Fan Speed/Current Testing (static 12.7v battery with engine off).
Dorman 973-018 4-speed resistor.
Resistor Pinout
RPM (Percent)
Amps Peak Spike/Constant Current
Fan full power
1800 (100%)
1. High
1400 (77%)
2. Medium 2
1000 (55%)
3. Medium 1
  650 (36%)
4. Low
  470 (26%)
**Peak spike amps when ramping from 77% to 100%.
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.  Using these speed figures, you can decide which circuits you want to use for which fan speeds.  The 26% speed might seem ok on paper for a low speed and the 77% speed might look good for high, but until it's tested in a car, you won't know for sure.

The diagram near the top of the page and the above bench test photo shows my original design with low speed 26% being used 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.
LOW (Medium 1):       36%
MEDIUM (Medium 2): 55%
HIGH:                           77%
FULL POWER:          100%
AC engagement activates the 77% speed.
Another test was done after installation with the engine running and alternator charging (voltage readings below).
Since the 26% speed was not used, it was not measured.
Resistor Pinout
Amps Constant Current/@Voltage
Fan full power
40 @13.3v
1. High
29 @13.6v
2. Medium 2
15.7 @13.9v
3. Medium 1
8.6 @14v

Optional Items

<<< 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.
Dorman 973-032 Pinouts
1.  Input: High Speed
2.  Output to Fan
3.  Input: Medium
4.  Input: Low

Resistance Values
0.4 Ohms

0.9 Ohms
3.3 Ohms

Hella 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 high-current 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 side.

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 had to do some grinding on the inside to make everything fit inside. A larger box 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 have. A google 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 or Ford Mini Series Terminal. This is the type of terminal that goes into the Ford fan motor connector. Dorman PN 85367 fits 14-16 gauge wire and is available on line. The 10-12 gauge version is more appropriate and appears to be Dorman PN 85365, but it cannot be found.  I have only found the 10-12 gauge terminal mentioned here: http://detroitmotorparts.com/s/1/p/22070373, however this links you to an Advance Auto Parts link which is dead. Dorman 85365 is not found on any Advance Auto Parts page. If anyone can help with any real sources, please contact me.

<<< So I improvised and 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.

<<< For 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 Resistor.  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 it may 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 watt capacity. 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 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 (I'm only using 0.3, 0.8 and 1.4), 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 is DigiKey Electronics: http://www.digikey.com/product-search/en?keywords=resistor.  Here are some DigiKey part numbers and resistance values to consider . . .
PN A102367-ND  0.1 ohm
PN A102368-ND  0.22 ohm
PN A102369-ND  0.47 ohm
PN A102106-ND  1 ohm
PN A102185-ND  1.5 ohms
PN A102360-ND  2.2 ohms
PN A103830-ND  3 ohms

If you incorporate something like this in your build, please email me. I would like to know how your project goes.  CONTACT ME.

Completed Project
After almost a year of use, this controller has worked perfectly. Zero glitches or worries. 
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. 

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 also.

<<< Here's another view of the Dorman resistor board mounted in the lower left corner of the fan shroud. 
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 AREALLY 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 the 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 fan from 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 force.  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 probes tightly 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 fins for good contact. Sensor activation with this method is quick and accurate.  Faith in Hayden has been restored.

<<< 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 to the power wire.  Now it seems just right.

The final speeds I chose have turned out well.
LOW:                    36%
MEDIUM:              55%
HIGH:                    77%
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). Qty: 4.                Available here:  http://www.davebarton.com/volvorelays.html#1259926-101gray

Relay plug socket, interlocking, 5-pole. Qty: 4.                      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. Qty: 1.                       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 IN5408. Qty: 1.                        I have some extras 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: 3.               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 Dorman resistor terminals using 10 gauge wire). Qty: 4.     Available here:  http://www.davebarton.com/blackvinyl.html#overcrimps

Crimp terminal - .250” (6.3 mm) female, 10-12 gauge (for relays). Qty: 7.                                 Available here:  http://www.davebarton.com/blackvinyl.html#.250inchplugs

Crimp terminal - .250” (6.3 mm) female, 14-18 gauge (for relays). Qty: 12.                               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