Multipoint fuel injectors
The two connections on an injector are (1) supply voltage (when the ignition is switched on, and in some vehicles only when cranking or running) and (2) an earth path controlled by the Electronic Control Module (ECM).
It is recommended that the oscilloscope is earthed through the black plug on the BNC lead and the remaining connection (the red) is used to probe either side of the two pin break–out lead.
One connection to the two pin break–out lead will show the supply voltage and the other side an injector waveform from which the injector duration can be measured. The oscilloscope must be set up correctly for voltage, timebase and trigger source to display this waveform. The TA010 20:1 Attenuator must be used if the signal voltage exceeds the input range of the scope.
Single point injectors
Some single point injection units, mainly Lucas / Rover, employ this style of electrical connector and can be connected in the same way as the multipoint version. The settings for the oscilloscope are the same as for the multipoint injection system.
Examples of injector waveforms can be found in the automotive waveforms library.
Inductive crankshaft or camshaft sensors
These sensors can be referred to as the following:
- CAS — Crank Angle Sensor
- CPS — Crank Position Sensor
- CID — Cylinder Identification Sensor
Some vehicles may employ two crankshaft sensors, both of the inductive type, one used for engine speed and the other used for crankshaft position. Both can be tested with the two pin break–out lead, assuming the connections are compatible.
There are two ways in which these sensors can be tested.
- As with the injector test, connect the black lead from the BNC cable to the vehicle’s body, engine or battery negative as an earth connection. The red lead can then be used to probe either side of the two pin break–out lead. An inductive sensor produces its own voltage and is not earthed to the engine. The two connections are the sensor’s negative and positive voltage outputs. To display a waveform, ensure the oscilloscope is set to AC voltage and has a suitable voltage scale and timebase. If the resultant waveform is lower than expected, connect to the other fly lead.
- As inductive sensors do not have a supply voltage and are not earthed to the engine, they can be tested using the two pin break–out lead connected directly to both plugs (red and black) on the BNC lead.
If the output of the test is lower than expected, the polarity of the connections may be incorrect, so try reversing the leads and re–testing. It is not always possible to get this right first time as there is no consistency between manufacturers’ wiring.
Examples of cam and crankshaft sensor waveforms can be found in the automotive waveforms library.
Coolant temperature sensor
This sensor has a voltage supply and a variable voltage output that is monitored by the vehicle’s ECM. It is recommended that the oscilloscope is earthed through the black plug on the BNC lead, and the remaining connection (red) is used to probe either side of the two pin break–out lead.
One connection of the two pin break–out lead gives the supply voltage and the other side usually a voltage that decreases as the engine temperature increases. This is the case for a sensor which has a negative temperature coefficient (NTC). The opposite will occur if the sensor has a positive temperature coefficient (PTC). Probe both the yellow and the blue connections to find the correct terminal for monitoring the voltage change with rising temperature.
The oscilloscope needs to be set up correctly for voltage, timebase and trigger source to display this waveform.
Examples of Coolant temperature sensor waveforms can be found in the automotive waveforms library.
Knock sensor
A knock sensor, if fitted, is responsible for adjusting the ignition timing when ‘knock’, ‘pinking’ or ‘detonation’ occurs. The frequency of the detonation is always the same (15 kHz), and under these conditions the crystal becomes excited and produces a voltage. As knock sensors do not have a supply voltage and are not earthed to the engine, they can be tested using the two pin break–out lead connected directly to both plugs (red and black) on the BNC lead.
If the test shows an inverted output, the polarity of the connections is incorrect, so reverse the leads and re–test.
Examples of a knock sensor waveform can be found in the automotive waveforms library.
GM ignition coils
Some of the early GM coils have an access point for the coil’s negative and positive terminals mounted towards the top of the component. This allows measurements to be made on both the positive and the negative signals.
It is recommended that the oscilloscope is earthed through the black plug on the BNC lead and the remaining connection (the red) is used to probe either side of the two pin break–out lead. One connection to the two pin break–out lead is the supply voltage and the other side is the switching signal and induced voltage. The oscilloscope must be set up correctly for voltage, timebase and trigger source to display this waveform, and the TA010 20:1 Attenuator must be used if the voltage exceeds the input range of the scope.
Examples of primary ignition waveforms can be found in the automotive waveforms library.
Austin / Rover ignition amplifiers
These are permanent magnetic pick–up varieties with the two connections being the coil negative and positive.
It is recommended that the oscilloscope is earthed through the black plug on the BNC lead and the remaining connection (the red) is used to probe either side of the two pin break–out lead. One connection to the two pin break–out lead gives the supply voltage and the other side the switching signal and induced voltage. The oscilloscope must be set up correctly for voltage, timebase and trigger source to display this waveform. The TA010 20:1 Attenuator must be used if the signal voltage exceeds the input range of the scope.
Examples of primary ignition waveforms can be found in the automotive waveforms library.
Bosch K–jetronic fuel injection system
This basically mechanical fuel injection system has several connections that are compatible with the two pin break–out lead.
The warm–up regulator: This component is responsible for the fuel enrichment during the warm–up phase and its two connections are a 12 V supply from the fuel pump relay and an earth connection.
It is recommended that the oscilloscope is earthed through the black plug on the BNC lead and the remaining connection (the red) is used to probe either side of the two pin break–out lead. One connection to the two pin break–out lead is the supply voltage and the other the earth path.
The oscilloscope must be set correctly for voltage, timebase and trigger source to display this measurement.
The auxiliary / extra air valve: This is responsible for allowing extra air into the engine during the warm–up phase to increase the engine’s idle speed. Its two connections are a 12 V supply from the fuel pump relay and an earth connection.
It is recommended that the oscilloscope is earthed through the black plug on the BNC lead and the remaining connection (the red) is used to probe either side of the two pin break-out lead. One connection to the two pin break–out lead is the supply voltage and the other side the earth path.
The oscilloscope must be set correctly for voltage, timebase and trigger source to display this measurement.
The thermo time switch: This is responsible for the operation of the cold start injector. The two connections on the break–out lead are (1) a constant 12 V while the starter motor is cranking and (2) an earth path, which is held at earth potential for up to about 10 seconds (depending on coolant temperature). It is recommended that the oscilloscope is earthed through the black plug on the BNC lead and the remaining connection (the red) is used to probe either side of the two pin break–out lead.
The oscilloscope must be set correctly for voltage, timebase and trigger source to display this measurement.
The cold start injector: This injector works in conjunction with the thermo time switch, and delivers a small amount of fuel into the inlet manifold.
It is recommended that the oscilloscope is earthed through the black plug on the BNC lead and the remaining connection (the red) is used to probe either side of the two pin break–out lead. The oscilloscope needs to be set correctly for voltage, timebase and trigger source to display this measurement.
