dinosavva_no1@hotmail.com
dinosavva_no1@hotmail.com
By Dino Savva
The CTS is a device that monitors the coolant temperature. On the '91 and later Jeep 4.0L engine, it is normally mounted in the thermostat housing (see photo). As the coolant temperature increases, the resistance of the sensor decreases, providing a varying output voltage to the engine's computer. The computer uses information from the CTS to adjust air/fuel mixture, ignition timing, and to switch on the electric cooling fans.
The resistance (R) of the CTS changes with temperature as follows:
Temp.(*C)...R unmodified (Ohm).....R modified (Ohm)
....0...............36000......................3600
...10..............23550......................3420
...20..............15630......................3190
...30..............10520......................2900
...40...............7170.......................2570
...50...............4950.......................2210
...60...............3450.......................1850
...70...............2440.......................1520
...75...............2050.......................1360
...80...............1740.......................1210
...85...............1470.......................1080
...90...............1250........................950
...95...............1060........................840
..100................910........................740
..105................770........................650
..110................660........................570
The electric fans normally switch on when the coolant temperature reaches ~105*C (~220*F) and off again when it falls back to ~95*C (~200*F). The engine comes with a thermostat that opens at 91*C (195*F) so the fans would switch off at just above this temperature. If the fans' off temperature was set too low i.e. at or below the thermostat opening temperature, the fans would stay on all the time once activated at the on temperature (or by A/C operation) until the engine is shut down. The factory boys definitely knew what they were doing.
My Jeep has several performance modifications and I have a thermostat that opens at 82*C (180*F). I wanted to make the fans switch on at ~100*C (~210*F) and off at ~85*C (~185*F) to keep the engine running cooler. That meant that I needed to lower the sensor's resistance at 100*C from 910ohm to 770ohm, and lower the sensor's resistance at 85*C from 1470ohm to 1060ohm. The idea is to trick the engine's computer into believing that the coolant is hotter than it really is so that the electric fans switch on/off at lower temperatures.
To lower the resistance of a sensor, you need to splice in a resistor across the wires going into the sensor so that it is in parallel. The formula for change in resistance with two resistors in parallel is:
1/R = 1/R1 + 1/R2
Where R=Total resistance across both resistors, R1=Resistance of CTS, R2=Resistance of spliced in resistor.
Let's do the maths for the CTS at 100*C:
1/770 = 1/910 + 1/R2
1/R2 = 1/770 - 1/910
R2 = 5005ohm
Let's also do the maths for the CTS at 85*C:
1/1060 = 1/1470 + 1/R2
1/R2 = 1/1060 - 1/1470
R2 = 3800ohm
So I needed to splice in a resistor of about 4000-5000ohm to lower the fans' on/off temperature to where I wanted it. I elected to splice in a 4000ohm (4Kohm) resistor. The results were as follows:
With the 4Kohm resistor in place, the resistance of the CTS was altered as shown in the chart above. The cooling fans now switch on at just under 100*C (~210*F) and off at about 85*C (~185*F), so I had achieved my aim.
This mod did produce other side effects.
The engine runs less rich during warm-up with the idle speed reduced from 1100rpm to 800rpm. The transition from the rich air/fuel mixture during engine warm-up to a normal A/F mixture when the engine reaches normal operating temperature now occurs at about 55*C (~130*F) instead of 75*C (~165*F). The only downside is that the engine cranks over for slightly longer before firing up from a cold start when the weather is cold.
To overcome that problem, I soldered in an on/off switch in series with the 4Kohm resistor so that I can disable the resistor in the colder months.
In conclusion, I recommend this simple mod but only if you have a 180*F thermostat installed and if you live in a warm climate. If you live in a cool climate, keep the switch in the off position and turn it on only in the summer.