1968 Shelby GT500

Air Charge Temperature (ACT)

Before you start blaming the air charge temperature sensor and replacing it make sure the rest of the coolant system is in good condition. All of the following items will affect the ACT:

•Coolant level
•Radiator Fan
•Engine Temperature

•Ambient temperature
•Air Filter
•Air Filter to Throttle Body Duct
•Water pump and Fan Belts
•Thermostat
•EGR System
•Engines general condition
•Harness and wire general condition

The Air Charge Temperature (ACT) sensor is a thermal transistor, which means it allows less electricity to pass through the sensor the warmer it gets. The ACT receives the “Signal Return” voltage from the EEC, and then allows a certain amount to return back to the EEC. Because the ACT is mounted into the incoming air stream it changes resistance in response to the temperature of that air.

temperature graph

The ACT sensor is 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 its simplicity of design, the ACT is rarely at fault when problems occur.

Before testing the ACT 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 an ACT sensor you will need a volt meter. You can test the ACT by back probing the harness while reading the voltage returning to the EEC. Or you can removing the connector completely and test the resistance between the 2 pins on the ACT. The ambient temperature should be above 50°F (10°C) 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.

graph

Air Charge
Temperature Sensor

Temperature

Voltage

Resistance

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

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