Chrysler 300C Forum: 300C & SRT8 Forums - View Single Post - $60 180*F T-stat/Fully Automatic Fan Mod
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post #1 of (permalink) Old 12-28-2006, 12:52 PM Thread Starter
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$60 180*F T-stat/Fully Automatic Fan Mod


For some time now I have been following the subjects of 180-degree thermostats and fan controllers for application on my 300C. Most recently I came across “The $40, Temp-Sensing Fan Mod” as posted by ‘95ttoplt1’ on “”, and it got me to thinking about taking it to the next level, by which I mean getting the system to turn off completely when the ignition is switched off. While those who take their drives in 1/4-mile blasts may be happy with an under-hood or dashboard-mounted kill switch, anyone desirous of changing the t-stat to
180-degrees F and installing a fan controller on a daily driver can not be too happy with the radiator fan continuing to run (after the ignition is turned off) until the radiator temperature sensor has finally reached its fan-off temperature, or having to remember to switch it off manually to avoid draining the battery.

As covered by ‘95ttopt1’ and ‘Jaak’s LX Garage’ on ‘’ it is necessary (to prevent trouble codes, check engine light illumination, and fail-safe high-speed operation of the radiator cooling fan) to modify the stock ‘RELAY – RADIATOR FAN CONTROL’, or similarly modify a suitable replacement relay with the installation of a single diode within the case of the relay. As I had just such a suitable relay on-hand (a stock Ford fuel pump relay, part number FOAZ14N089B), I elected to use it. An equivalent replacement is available from AutoZone for around $7.95. Do not use the Radio Shack SPST relay Cat. No. 275-226 which, as mentioned by someone and experienced by myself on another project, heated up and melted down!

Before modifying the relay electrically I had to modify it physically as the cover had a “skirt” that extended to the bottom of the lugs. A few seconds with a handsaw and the skirt was gone. Then, by inserting the tip of a small flat screwdriver alternately into the crack-of-the-cover-to-relay-base, I pried the cover off.

Next I sized up the job of inserting the diode. It is important to note here that the cathode-end of the diode (the end with the stripe around it) MUST attach to the 86 lug lead, and the anode-end MUST attach to the relay winding’s lug. Reversing this arrangement will result in a “RELAY – RADIATOR FAN CONTROL” that will not work for normal operation (fan mod switched ‘off’ for whatever reason), and will probably throw a code and illuminate the CEL. As can be seen in the picture, the leads of the diode are bent and looped as required to jumper the gap while the diode’s body doesn’t impede the mechanical operation of the relay.

The insertion point for the single diode (a Type 1N7001 available at Radio Shack as their cat.
no. 276-1101 at a price of $0.59 for two) is between lug 86 and the coil winding connected to it.
Upon inspection of the interior of the relay it was determined to snip apart the connection for the 86 lug lead at its attachment point to the relay coil’s winding lug. I snipped the 86 lug lead immediately adjacent to the tab on the coil with a small wire cutter, and opened the gap of the
cut so as to assure that when soldering the diode across this gap the solder would not close the gap.

After the diode was securely in place a 6” length of #22 insulated wire was soldered on at the joint of the diode to the relay winding’s lug. A hole was then strategically located and drilled through the relay’s cover so as to be able to pass the wire to the outside of the relay as the cover was replaced.

The key to having the fan mod turn off when the ignition is switched off is an auxiliary relay and a key-switched source of power to control the relay. To find the key-switch source of power I consulted the wiring diagrams of the CD service manual and found that the pink/gray wire from the front power distribution center’s C1 (green) plug, pin 22, has power in the “ON” and “START” ignition key positions. Perfect.
I then chose a 12VDC SPDT micromini relay by Radio Shack, catalog number 275-241 at a cost of $4.49, for the auxiliary relay. Alternately, Radio Shack’s mini relay 275-248 (same price) could be used.

I mounted this relay upside down to the top interior of the front PDC’s IPM in the “SPARE” relay position using a dab of silicon adhesive to stick it fast over the spare relay socket position on the panel with the relay’s pins sticking straight up.

For a source of coolant temperature sensing I chose to go with inserting a sensor into the
water pump near the thermostat where there is a 1/4" MIP plug screwed into the front of the pump.

For those who might prefer an adjustable capillary tube-type thermostatic switch, just substitute it and follow ‘95ttopt1’s mounting instructions. I searched the internet and called my local speed shop and ordered the Derale 16739 180-degree F single stage thermostatic fan controller kit at a cost of $30.00.

The kit comes with some wiring, a fuse, a relay, compression connectors, and a 3/8”-18 NPT plug adapter for the thermostatic switch itself. All I was interested in was the 3/8”-18 NPT plug adapter and the thermostatic switch, however it was cheaper to buy the kit rather than just what I needed, and scrap the leftovers. While the relay that came with this kit is the correct type relay for modification, its external construction made it hard to remove the cover and its internal construction made it impossible to modify.

I then proceeded to figure out how to adapt the 3/8”-18 NPT plug adapter to the 1/4" tapped hole in the water pump. With the IAT sensor and its wire harness located just above the tapped hole, I figured a 90-degree turn upward would minimize the forward projection of the thermostatic switch and its wires, and help align the thermostatic switch’s wires with the IAT’s wiring harness. I went to a local hardware store and came up with a 3/8”-18 FIP to 1/4"-18 MIP brass reducer, and a 3/8”-18 MIP to 3/8”-18 FIP 90-degree brass elbow. These two pieces came to $3.45.

After locating the radiator drain hose protruding below the bumper and the drain valve at the bottom right corner of the radiator, I placed a clean oil drain pan beneath the bumper and carefully* opened the drain valve 1/2-turn CCW using a 19mm deep-well socket and ratchet. I left about one gallon of coolant drain out (removing the radiator cap allowed air into the system as the coolant drained out), then carefully* returned the valve to its closed position.
*The valve is plastic, so don’t over-torque it in either direction!
Prior to starting physical work in the vicinity of the IAT I removed it from the CAI tube, laid it aside for safe keeping, and laid the IAT wire back out of the way. I then removed the 1/4" plug in the front of the water pump (some say this could be difficult, I had no problem with the engine temperature at about 100*F), bench-assembled the reducer and the elbow, and screwed the assembly into the water pump with the open end of the elbow on top.
Below: Reducer/Elbow assembly screwed into water pump.

This was now the time to change-out the stock thermostat with a 180-degree unit. Based on the information developed by ‘meister’, and my own study of Stant’s on-line catalog, I went with the
Auto Zone #4128 for $13.

At Left: The Auto Zone #4128 180-degree T-stat
At Right: OEM 203-degree T-stat
The AZ unit is 3/16” longer, 0.5 mm bigger in diameter at the top, and 1/4” smaller in diameter at the bottom than the OEM unit.

After removing the two thermostat housing bolts the stock thermostat is accessible. To remove it and its gasket I carefully inserted the tip of a long screwdriver into the top area of the t-stat, leaned the screwdriver back on that stud visible in the picture, and gently pushed on the screwdriver handle like a pry bar until the t-stat/gasket assembly popped out of the well. I carefully removed the gasket from the stock t-stat and installed it around the 180-degree t-stat. Because of the slightly larger (+0.5 mm) diameter of the top of the AZ t-stat the gasket produces a snug fit when the assembly is inserted back into the well. I sprayed it with some silicon lubricant, then with the jiggle pin valve toward the top, started with one side of the assembly just hanging on the edge of the well and went around the gasket pressing it with my thumbs so that the gasket slowly squeezed its way into the well.

Then I placed the thermostat housing over the t-stat and replaced the bolts.

Early on I had decided that there was a possibility that there was a slight chance that air could be trapped at the top of the water pump, and while this might not interfere with coolant flow, if I could minimize the amount of trapped air it sure wouldn’t hurt anything. So, with the newly installed thermostatic switch hardware still open to the air I decided to refill the cooling system at the expansion tank while keeping an eye out for coolant rising in the elbow. I added coolant to the expansion tank up to the cold fill max. line and no coolant showed at the elbow! So I decided to start putting coolant back into the system through the top of the elbow. This took some doing as the ingoing coolant tended to bubble from the air coming out. While I was doing this the level of coolant in the expansion tank was also going down some.

Below: Coolant Filled To Top Of Unplugged Elbow

Below: Elbow With Adapter Plug Installed

When the level of coolant flushed out at the top of the elbow (left above) I screwed in the adapter plug (right above) which I had prepared with Teflon tape. I then poured the balance of the coolant remaining into the expansion tank. This exceeded the maximum cold fill level line by about 1/2", which after running the engine went down to about 1/2” below the maximum cold fill level line.

That completed the t-stat swap and physical installation of the thermostatic switch.
Now for the wiring!

First: The thermostatic switch. In preparation for wiring the thermostatic switch over to the auxiliary relay I followed the IAT wiring harness back to where the wires to the spark coil tee’d out of the harness. At this point I carefully cut away the tape closing the tee connection in the harness cover so as to allow the IAT harness cover to slide toward the IAT plug connector.

The thermostatic switch came with two 6” +/- long wire leads. I cut one of them back by 2” and then spliced 24” of #22 stranded insulated wire to each lead, and finished the splices off with heat-shrink insulation. The 2” stagger between the splices minimizes the cross-section of the two wires whose loose ends, after CAREFULLY (that 10-32 stud is scarey stuff!) screwing the thermostatic switch into the adapter atop the elbow, were then slipped into and through the wire harness covering of the wires from the IAT.

I then re-installed the IAT sensor into the CAI tube and plugged in the wire connector. After sliding the wire harness cover back to the tap point of the coil wires I re-taped the tee joint in the wire harness cover.

Next I trailed the two thermostatic switch wires down along the coil wire harness, across the
valve cover (remember, the engine cover sticks down here), and up through the slit of the IPM’s
cover hinge. I then (because I had it on-hand) covered the thermostatic switch wires with split/corrugated plastic wire harness. Wrapping them with electrical tape would have done just as well. Finally, I tie-wrapped things at a few places to keep them from flopping around.

Second: Power for the auxiliary relay’s coil. As mentioned above, ignition key switched power is available from the pink/gray coming out of pin 22 of the green connector C1 of the IPM. This connector is located on the bottom of the IPM. Refer to the bottom view photograph of the IPM at the end of this article. To access it, first extract the positive battery cable holder (located back along the positive battery cable about 6” from the end) from the inner fender, release the latch adjacent to the positive battery cable connection on the side of the front PDC, and hinge the entire front PDC upward. Be prepared with a medium length screwdriver to be used as a prop to hold the front PDC up while doing the wire tap on the pink/gray wire. I removed about 1/4” of insulation from the PK/GY wire about 1&1/2” from the green connector (remove wire harness tape as required) and spliced and soldered on a 12” length of #22 wire that would go to the auxiliary relay on the upper side of the IPM. I tape the splice, and the harness as required, fed the loose end of the wire upward between the IPM and the FCM, and lowered the front PDC back down and latched it in place. I then re-install the positive battery cable holder to the strut tower. Now then, with reference to the top view of the IPM at the end of this article, I neatly route the added wire over to the auxiliary relay along the surface of the IPM carefully bending the wire around relays and the end of the wire upward along the side of the auxiliary relay, and solder it to the coil pin at the corner of the relay.

Third: Now, back to the two wires from the thermostatic switch. Because of how the top edge of the IPM fits into the groove around the edge of the cover it was necessary to notch the edge of the IPM so that the wires do not interfere with closing of the cover. I simply nipped the edge with my wire cutter a few times. Then I finished routing these two wires to their respective pins on the auxiliary relay, and soldered them to the pins.

Fourth: I plugged in the modified radiator fan control relay, carefully routed the wire to the auxiliary relay, and soldered it to its respective pin (refer to the Schematic) on the auxiliary relay.

Fifth: Depending on personal preference the ground wire from the auxiliary relay may simply be run to the ground stud on the strut tower just behind the front PDC, or to an on/off switch somewhere in the engine compartment, or somewhere in the passenger compartment within reach of the driver. A switch in the ground wire will permit turning off the fan mod with the flick of a switch. I installed my ground switch, a Radio Shack Cat. No. 275-614 ($3.99), in the passenger compartment so that I could easily switch the fan mod system off for operational comparison, or in the unlikely event of a problem with the system. I connected one end of a 10’ piece of #22-stranded wire from the auxiliary relay’s common pin, and routed the wire over the edge of the IPM and beneath the PDC, into, along, and out of the covering on the positive battery cable, and continued up, back, and across to follow along the wiring I previously installed for the MDS Indicator Modification as it crossed the firewall and entered the passenger compartment through the hood release cable boot. Once inside the car at the left end of the dash I brought the wire over to the area of the emergency brake release handle where I had previously installed my MDS Indicator LED ground switch and planned to install the fan modification ground switch. In order to be able to locate and mount the switch from behind the panel I had pull the dash panel below the steering column away from the dash. To do this, at the left end of the dash is a closure trim piece, about a foot long, that needed to be removed first. I gripped it at the top with my fingers and pried to pop the upper clip, then I slid a finger down and pry the lower clip to pop the cover free. This exposed a single mounting screw, which I removed. Then, along the bottom of the panel about 8-9 inches from the left end is another screw, which I also removed. Then, by prying with my fingers at the left end of the panel, I pulled it rearward until it popped loose. I then pried at the bottom right corner and the upper right corner to pop the clips retaining the right end. With the trunk release button/wiring and brake release cable intact, I then flipped the panel around to expose the area behind the brake release handle. There are a couple of ribs below the handle. I drilled a hole centered between two of the ribs closest to the center of the handle next to the MDS Indicator LED ground switch. I then soldered a 12” piece of #22-stranded wire to one terminal of the switch, and the wire from the engine compartment to the other terminal of the switch, and mount the switch through the drilled hole. Now then, with the 12” piece of wire hanging out toward the door, I placed the lower dash panel back in place. The emergency brake cable needs to be lifted a bit to clear the fiber closure panel beneath the dash. I gave the lower panel a whack with the palm of my hand to snap the clips back in place. Finally, I loosened the screw holding the doorjamb trim in place and place a bared/soldered/looped end of the wire under the screw head and tighten the screw. I then re-installed the closure trim piece by aligning the clips in their holes a pressing them into place.


I located mine in the finger well beneath the emergency brake release handle, out of sight, out of harms way, and easily accessible.

Left: 180-degree F Thermostatic Switch Fan Controller ON/OFF Switch
Right: MDS Indicator LED ON/OFF Switch (Previously installed)

Another personal preference might be to be able to switch the fan on manually before the coolant has warmed up enough to activate the automated system. Such a manual “ON” switch would be wired in parallel to the thermostatic switch’s wires (see the Schematic below). This can be done by attaching the wires from a simple on/off switch, mounted anywhere that is convenient for the purpose, to the same two connection points on the auxiliary relay as the thermostatic switch’s wires. If finger-tip access from the seated driver’s position is preferred, a DPDT Center-Off minitoggle switch, Radio Shack Cat. No. 275-620 ($4.99) could be used instead of the 275-614 mentioned above, and when appropriately wired, would provide system off, automatic system on/off via the thermostatic switch, or fan-on operation. The backside of the dash panel may need to be modified to accommodate such a switch as it unlikely will be physically larger than the switches I used, and won’t fit between the ribs referred to above.

Note: Regardless of the control switching schemes chosen (or the total lack thereof!) the system will be switched off when the ignition key is in the ‘OFF’ or ‘ACC’ positions. If, for some reason, it is desired to run the fan to lower the coolant temperature it will be necessary to do so with the engine idling. This saves the battery and circulates the coolant through the cooling system to keep temperatures throughout as even as possible.


PHYSICAL ARRANGEMENT (Top of front PDC, IPM cover removed):



In a word: PERFECTLY!

My daily commute is 60-miles roundtrip, about 40-miles highway / 20-miles city. Morning ambient temperatures have been 20-30*F with afternoon drive-time temperatures in the 30-45*F range.

With the fan mod system switched off while cruising at highway speeds the coolant temperature swung +/- 2*F of 185*F. In bumper-to-bumper, stop-and-go, 10-15 MPH crawls (I get to do 4 - 5 miles of that almost every afternoon!) I’ve observed the coolant temperature climb to 195*F. There just isn’t enough air flowing at low speeds to keep the temperature down. I turned the fan mod system on at this point and, in less that 5 minutes, the coolant temperature was down to 185*F. So that works as it should.

As for fuel mileage, I have experienced no perceivable difference. I have been keeping continuous records of miles driven, gallons consumed and prices-per-gallon for the last seven months. The running average until the install was 18.62 MPG. Since the install (just six fill-ups) the average (of the six fill-ups) has been 18.65 MPG. The implication is that the cooler operating temperature of the engine has not lead to an air/fuel ratio problem. If such a condition were occurring the on-board diagnostics would have indicated as much by turning on the check engine light.

The only possible negative operational aspect of the 180*F t-stat/fan mod that I can perceive is the possible premature failure of the radiator fans due to the extended amount of time that they will be operating. Oh, for sure, they are going to run more than they ever would without the fan mod. That’s a given. Gotta have air flow to cool things. That’s what this all about! Technically, once the fans are switched on automatically the coolant temperature will never get down to 170*F during engine operation to allow the fans to be automatically turned off. This is just something that functionally comes with this type of modification. Recently, with the lowest ambient temperatures of the year available, I have found that it is possible, when cruising at sustained speeds in excess of 25 MPH, to switch the fan mod off and still have coolant temperatures hover around the 185*F mark. Even with the fans switched off there is probably an amount of ‘windmilling’ that continues to exercise the mechanicals of the fans. All factors considered, once one accepts that the fans must run more to provide the benefit of lower operating temperatures, this possible negative aspect is dismissible as a necessary side effect to achieve the main effect: MAKING MORE HORSEPOWER!

2005 300C BRILLIANT BLACK, DRL, Mesh Grill/Vents, Chrome Pillars & Fender Trim, MDS Indicator, Quad Brake Lights, Tinted Windows, De-Molded, K&N Aircharger, 180 T-Stat, Flashing Sidemarkers, Predator Cust. Perf. 93 CAI Tune
1997 MUSTANG COBRA 'VERT, TRIPLE BLACK, Vortech S/C'd., ECT Mod., 4.10 Rear, Lowered 1&1/2", 18" Saleen Wheels
1950 FORD TUDOR FLATHEAD V8 - Sold 6/27/14 after 16 years of fun.

Last edited by DESIREE; 01-30-2007 at 06:35 PM.
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