Follow the instructions given in the paragraph "Testing the battery charging system".
A negative value means that charging system is not powering the loads and a significant amount of current must be supplied by the battery, which is discharging at the time of the measurement.
Alternatively, you can use a multimeter (Diagnostic instruments): connect the multimeter probes to the battery terminals, select the DC scale on the instrument and check for the presence of 14.5 V±0.5 at an engine speed of 3000 rpm.
Caution! If polarity is reversed when clamping the ammeter onto the cable, the sign of the readings will also be reversed, giving rise to incorrect diagnosis.
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