Keeping Track of the Little Things: Wrangler Sensors & Sending Units

The oxygen sensors used on a Wrangler play an important role in engine efficiency and environmental pollution. Their purpose is to read the amount of oxygen content left in the exhaust and report back to the ECU, which then, in turn, will change the fuel mixture in order to maintain a proper air-to-fuel ratio (the ideal ratio is 14.7 parts air to 1 part fuel - 14.7:1). Through the feedback loop of the oxygen sensors, the ECU is able to keep the AFR within tight limits which promotes engine efficiency and power delivery.

The oxygen sensors equipped on JK and JL Wranglers use a Zirconia element (coated both inside and out with a layer of platinum to act as an electrode) that is capable of conducting oxygen ions when at or above 350 C. With the sensor installed in the downpipe, the exterior element is exposed to the exhaust stream whereas the inner area is exposed to air. When at operating temperature, the remaining oxygen molecules from the exhaust are able to pass through the zirconia sensor element and deposit a charge on the interior platinum electrode which then generates a voltage. This voltage is measured by the ECU and depending on its value, fuel trim is either increased or decreased. 

A low voltage indicates that the engine is running lean and the AFR is too high whereas a high voltage indicates a rich mixture and that fuel should be removed from the combustion process.

A faulty or inoperative oxygen sensor will illuminate the Check Engine Light and can have detrimental effects on fuel economy and power delivery as now the ECU will not be supplying the proper amount of fuel in order to maintain an AFR of 14.7. On top of this, due to the improper mixture, combustion pollutants will increase. Oxygen sensors do wear out due to the heat cycles and road grime they are exposed to. Most manufacturers call for an oxygen sensor to be replaced every 100,000 miles. Jeep Wranglers use a four-wire ISO-HEGO (isolated heated exhaust gas oxygen sensor) unit.

The oil pressure switch on your Wrangler works on a very basic principle but has a vastly important function. Oil is the essential life blood of the engine and losing oil pressure and/or volume can quickly cause irreversible catastrophic damage. The oil pressure switch uses on Jeep Wranglers is an electromechanical type. The switch is composed of two sections - the mechanical section (metal part) and the electronic sender (plastic part). Inside of the switch is a simple mechanical diaphragm that moves in relation to the pressure of the oil that is coming through the entry port. In turn, the diaphragm pushes on a circuit which creates a voltage. The ECU is calibrated to receive this voltage and corresponds to a certain oil pressure. If the pressure signal drops below a certain threshold, the check oil light will illuminate. 

These switches do commonly fail due to grime, corrosion, and oil simply leaking out past the diaphragm. In fact, both TJ and JK Wranglers are known to have flawed OEM oil switches that fail prematurely. TJ owners will find their oil pressure switch next to the oil filter and fairly accessible. JK owners, unfortunately, need to pull the intake manifold off in order to get at theirs. Lack of oil pressure or erratic oil pressure readings can be indications of a faulty sensor, however, it could also indicate a bigger problem along the lines of the pump. Thus if you suspect an oil pressure problem (and there is no physical evidence of oil leaking and the volume is correct) you should verify via a mechanical gauge.

Jeep Wranglers use a 3-wire crankshaft position sensor that operates based on the Hall effect. Situated close to the crankshaft, this sensor reads the position of the crankshaft and outputs this data to the ECU which can then use it to calculate fuel and ignition timing and engine revolutions. Integrated on the crankshaft are notches. As the crankshaft spins, each notch passes the crankshaft position sensor and disrupts the magnetic field that is produced by the sensor itself. With each disruption, the sensor switches on and off, thereby creating a square digital signal that the ECU can interpret to determine crank position.

Crankshaft position sensors are often the culprit behind a no-start condition on higher mileage Wranglers, as their constant heat and road grime exposure leads them to deteriorate or the electric wires to break. Without a proper crankshaft signal, the engine will not start (but will always crank) as the ECU cannot determine the position and thus will not enable the ignition or fuel systems. A check engine light will also illuminate.

Located on the throttle body itself, the throttle position sensor measures in what position the throttle blade is in relative to the closed or wide-open position. Knowing the position of the throttle blade is useful for the ECU in order to help it calculate and monitor fuel trim, ignition timing and even transmission response (if equipped with an automatic transmission). Like the crankshaft position sensor, the throttle position sensor (TPS) uses a disruption in ​a magnetic field to produce a signal that is sent to the engine control unit. Typically, a part of the throttle body spindle shaft has a particular magnet or ferrous portion to it, which rotates with the blade, disrupting the magnetic field which is then picked up by the sensor. Some symptoms of a bad TPS include rough, low or surging idle, low acceleration and possibly an inability to upshift (automatic transmissions). If you suspect your TPS is going bad, it should throw a code that can be read by an OBD2 reader.

The idle air control valve outfitted to a Jeep Wrangler (well, outfitted to any vehicle) is there to help provide enough air for the engine to maintain a smooth and constant RPM at idle and during deceleration when the throttle blade will normally be closed. The purpose of the idle air control valve is to bypass the throttle body blade and inject air directly into the intake manifold in order to maintain a smooth idle RPM. It is simply an electric solenoid with an attached plunger that retracts to let air pass when the throttle blade is closed. 

Idle air control valves are prone to carbon deposits and can become jammed. Symptoms of a bad or faulty IAC valve are poor idle, engine surging and perhaps even stalling. Again, with most sensors, the Wrangler computer does monitor the function of the IAC and should throw a code if it is found to be faulty. If your Wrangler's IAC is found to be acting up, you can try cleaning it first before outright replacing it - oftentimes a good internal scrub to remove the carbon buildup is that is needed.

The manifold absolute pressure (MAP) sensor plays a vital role in overall engine performance and response. Located on the rear of the intake manifold on JK Wranglers and on the firewall for TJs, the MAP sensor is used to determine the mass of air going into the engine in order to deliver the correct amount of fuel. Composed of two parts - a vacuum tube and a sealed housing - the way the sensor works is very simple. Inside the housing is an air chamber that is set to a known pressure and sealed at the bottom by a flexible silicone chip. This chip has an electric current running through it. On the outside and at the bottom of the sensor is a vacuum tube that protrudes into the intake manifold and measures the vacuum inside the manifold. The tube is pushed up and down with changes in vacuum, which in turn flexes the silicone chip above and changes the resistance of the chip. The PCM (powertrain control module) is able to interpret this change in resistance and work in conjunction with an air temperature reading and engine speed in order to deliver the proper amount of fuel.

MAP sensors occasionally do fail and this failure is typically a result of exposure to the constant high heat found under the hood. A bad MAP will illuminate a check engine light and an associated code. Symptoms of a bad MAP can vary significantly but the most common are poor engine response and excessive fuel consumption.

The coolant temperature sensor is an electric probe that is found on Jeep Wranglers screwed into the block next to the thermostat. With the probe protruding directly into the water jacket, the coolant flows over it which changes the electrical resistance of the probe (the ECU supplies a regulated voltage to the probe at all times). The electrical resistance of the probe will change with temperature and consequently the computer (calibrated as such) can interpret these resistance changes and calculate what the actual temperature is of the coolant. Based on this signal, the ECU can then regulate the cooling system as necessary (such as turning the fan on and off) and the fuel.   

Like all sensors, the coolant temp sensor can deteriorate over time. A bad temperature sensor can lead to reduced engine performance, increased fuel consumption and most critically, internal engine damage. Early model JK Wranglers are known to have issues with leaking and faulty thermostat and coolant temperature sensors. Jeep has issued several bulletins on this matter. But otherwise, generally speaking, a coolant temperature sensor does last a long time and there is no specific maintenance interval specified for one.