Air charge temp sensor.

Engine coolant temp sensor.

Both the ACT and ECT sensors are very similar. Before replacing either one, make sure the rest of the coolant system is in good condition. All of the following items will affect the ACT and ECT:

• Coolant level
• Radiator Fan
• Engine Temperature
• Water pump and Fan Belts
• Thermostat
• Engines general condition
• Harness and wire general condition

The following variables are exclusive only to the ACT:

• EGR System
• Ambient temperature
• Air Filter
• Air Filter to Throttle Body Duct

Both the ACT and ECT are thermal transistors, which means they allow less electricity to pass through the sensor the warmer it gets. The ACT & ECT receive the signal return voltage from the EEC, and then allow a certain amount to return back to the EEC. The ACT is mounted into the incoming air stream and changes resistance in response to the temperature of that air. The ECT is in direct contact with the engine coolant flow and changes resistance in response to the temperature of that coolant.

Both sensors are very important when calculating fuel ratios and timing curves. This is because of simply chemistry, fuel and spark are constants in the equation. Air is the biggest variable in combustion, it changes density greatly over a range of temperature. So keeping track of the changing temperatures of the incoming air and the engine in which it is burnt becomes very important. But due to their simplicity of design, the ACT and ECT are rarely at fault when problems occur.

Before testing the ACT and ECT or any other EFI component perform a self-test. Trouble codes received during test can be used as a diagnostic tool along with other indicators. To test the sensors you will need a volt meter. You can test the ACT and ECT by back probing the harness while reading the voltage returning to the EEC. Or you can remove the connector completely and test the resistance between the 2 pins.

• To test the ACT the ambient temperature should be above 50F (10C) to receive acceptable input from the Air Charge Temperature (ACT) sensor during the KOEO and KOER Self-Test. To accomplish this, the engine should be at normal operating temperature.
• To test the ECT the engine temperature must be greater than 50F (10C) to pass the KOEO Self-Test and greater than 180F (82C) to pass the KOER Self-Test. To accomplish this, the engine should be at normal operating temperature.

F	C	Volts	K ohms
302	160	0.12	0.54
267	131	0.20	0.80
250	120	0.30	1.18
230	110	0.36	1.55
212	100	0.47	2.07
194	90	0.61	2.80
176	80	0.80	3.84
158	70	1.04	5.37
140	60	1.35	7.60
122	50	1.72	10.97
104	40	2.16	16.15
86	30	2.62	24.27
68	20	3.06	37.30
50	10	3.70	58.75
32	0	3.97	65.85
14	-10	4.42	78.19
-4	-20	4.87	90.54
-22	-30	4.89	102.88
-40	-40	4.91	115.23

Values were calculated for VREF=5.0 volts.These values may vary 15 percent due to sensor and VREF variations.

Article & images courtesy of fordfuelinjection.com.