Fig. 7.26 Testing the coolant temperature sensor

In this test the voltage drop across the sensor terminals is being measured. For this test the ignition is switched on, but the engine is not running. The sensor wiring remains connected and the signal voltage will be high because the sensor element is cold. Next the engine is started and as it warms up to operating temperature the sensor voltage will change as shown in Fig. 7.27. To check the sensor performance across the operating range, from engine cold to engine hot, is likely to take some time. This means that the time base of the scope must be set accordingly.


In this case the preliminary visual inspection is very important because it is sometimes the case that the vacuum pipe from the intake manifold to the MAP sensor has become loose or damaged. Figure 7.28 shows the oscilloscope connections for this test.

Figure 7.29(a) gives an impression of the voltage trace from an analogue MAP as observed over a period of 3 s or so. In Fig. 7.29(b) a section of this trace is enlarged and the following points should be noted:

• high engine load

• high voltage (low manifold vacuum)

• as the throttle opens the vacuum falls and the voltage rises

• a low voltage indicates high vacuum.

Figure 7.30 shows the voltage trace for a MAP sensor that gives a variable frequency digital signal output. The signal frequency changes with manifold vacuum and the range of frequency is approximately 50-110 Hz. Points to note are as follows.

1 Temperature HOT

2 Temperature is decreasing, causing the resistance to

1 Temperature HOT

2 Temperature is decreasing, causing the resistance to increase 3 Temperature COLD

Fig. 7.27 A voltage trace from a coolant temperature sensor

• The top line should be very close to the reference voltage that is supplied to the sensor.

Fig. 7.28 Testing a manifold absolute pressure sensor
Fig. 7.29 Signal voltage from an analogue MAP

1 The upper horizontal lines should reach reference voltage.

2 Voltage transitions should be straight and vertical.

3 Peak-Peak voltage should equal reference voltage.

4 The lower horizontal lines should almost reach ground.

Voltage drop to ground should not exceed 400 mV.

If the voltage is greater than 400 mV, look for a bad ground at the sensor or ECU.

Signal frequence increase as the throttle is opened (vavuum decreases). As the throttle closes the frequency decreases. The T ranges from approx. eq 50 Hz to 120 Hz.

Fig. 7.30 The voltage pattern for a variable frequency (digital) type MAP sensor

• The rise and fall lines should be near to vertical.

• The peak-to-peak voltage should be very close to the reference voltage.

• The lower lines should be very close to the earth voltage level. Any voltage difference greater that 400 mV at this point requires investigating.

The MAP sensor may be tested with the aid of a vacuum pump, to simulate manifold vacuum, and a voltmeter as shown in Fig. 7.31.

Frequency measurements can be made on voltage (V dc, V ac or mV dc) or current inputs {mA/A ac or dc) for MAR MAP, and ABS sensors. An adjustable trigger point allows toggling between positive and negative trigger slopes.

Fig. 7.31 Testing a MAP sensor with the aid of a vacuum pump

This test requires that the vacuum pipe from the manifold to the sensor should be disconnected and the manifold end of the pipe must be connected to the vacuum pump. The positive lead of the voltmeter must be connected to the signal output from the sensor and the negative lead connected to a good earth. The actual means of making a good electrical contact between the signal cable of the sensor and the voltmeter lead is dependent on the means available to the operator. However, any form of connector that impairs insulation or connector reliability must only be used with caution. The advantage of using the vacuum pump is that the sensor output can be checked accurately against measured vacuum and this has advantages over other methods.

Do It Yourself Car Diagnosis

Do It Yourself Car Diagnosis

Don't pay hundreds of dollars to find out what is wrong with your car. This book is dedicated to helping the do it yourself home and independent technician understand and use OBD-II technology to diagnose and repair their own vehicles.

Get My Free Ebook

Post a comment