You will require a PicoScope to perform this test. A list of suitable accessories can be found at the bottom of this page.
Plug the high current clamp into Channel A of the scope (set to its larger range if applicable). Place the clamp around the main battery positive cable from the alternator to the battery. Switch the clamp on and adjust the zero setting.
Plug a BNC test lead into Channel B of the scope. Connect a Back-pinning Probe to the positive (colored) plug on the test lead and place a black clip on the negative (black) plug. Place the black clip on a suitable earth connection in the engine bay. Probe pin 1 of the alternator multi-plug, or refer to the manufacturer's wiring diagram.
Plug a BNC test lead into Channel C of the scope. Connect a Back-pinning Probe to the positive (colored) plug on test lead and place a black clip on the negative (black) plug. Place the black clip on a suitable earth connection in the engine bay. Probe pin 2 of the alternator multi-plug, or refer to a manufacturer's wiring diagram.
The connections are illustrated in Figure 1.
It is very important that the clamp is placed on the positive cable from the alternator to the battery. If the clamp is placed on the battery negative, for example, it will show just the balance between the current generated by the alternator and any current consumed by electrical loads. In this case the current reading may not change greatly when additional loads are placed on the system.
In our no-load state the alternator is changing at approximately 14 amps. When we turn on the loads—heated screen, main beam headlights and full speed heater blower—the output current increases to approximately 70 amps.
This signal is fed back to the engine Electronic Control Module (ECM) and remains a constant square wave or pulse-width-modulated signal. The duty cycle of the signal changes as the output of the alternator increases.
This signal comes from the ECM and changes with load demand. Again it is a square wave or pulse-width-modulated signal. In the no-load example waveform, the signal is idle as the ECM has not detected that any consumer devices are switched on other than the normal engine running. In the full-load example, the signal is active and the alternator's regulator reacts to it by increasing the field current, thus increasing the output current.
The duty cycle on the green trace (alternator feedback) remains virtually unchanged regardless of electrical demand.
Most manufacturers now employ an electronically controlled alternator system by using the engine's ECM, also referred to as the Powertrain Control Module (PCM), but Ford were one of the first to introduce such a system which they refer to as ‘Smart Charging’.
The concept of Smart Charging is actually fairly straightforward. A battery has the capacity to take a slightly higher-voltage charge when cold, so the ECM charges the battery at a slightly higher capacity when cold and balances the charge rate against consumer loads.
The alternator works at maximum capacity only when absolutely necessary and the battery is kept in a constant and healthy state of charge. Battery temperature is estimated from the intake air temperature sensor, which is a good indication of the temperature under the hood.
Battery type warning: Ford specify a silver calcium battery for use with this system. A conventional lead-acid battery is not suitable.
This help topic is subject to changes without notification. The information within is carefully checked and considered to be correct. This information is an example of our investigations and findings and is not a definitive procedure. Pico Technology accepts no responsibility for inaccuracies. Each vehicle may be different and require unique test settings.
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