Using a high 0 to 600 amp current clamp
How
to connect the oscilloscope - diesel glow plugs
Plug
the current clamp into channel A and turn on its power supply. The
clamp should be positioned on one of the two battery connections (live or
earth), which ever allows the easiest connection or directly onto the glow plug
supply from the timer relay. The current clamp needs to be facing the correct
way as if reversed an inverted picture will be seen.
With
the example waveform displayed on the screen you can now hit the space bar to
start looking at live readings.
Fig. 6.1
Figure
6.1 shows the amps clamp connected to the supply wire of the glow plugs.
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Example diesel
glow plug waveform
Glow
plugs / timer relay waveform notes
This
test is conducted to evaluate the condition of the glow plugs (this example is a
4 cylinder engine) and to measure the 'on time', which is controlled by the
timer relay.
A typical glow (or heater) plug will have a high initial current draw that will
gradually drop, stabilising at a constant amperage. The current draw will be
dependent on the wattage rating of the glow plug. This data is available in the
appropriate diesel data books.
Once
the wattage has been ascertained, multiply it by the number of cylinders and
then divide by the voltage to calculate the expected stabilised current.
Example:-
Each
glow plug = 150 watts so 4 glow plugs = 600 watts
current = watts divided by volts: 600 watts divided by 12 volts = 50 amps
The
length of time that the glow plugs are operational can be measured from the
initial drop in current to the switch off point, in this case it is around 17
seconds.
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Technical
information - glow plugs
Glow
(or heater) plugs are utilised to aid cold starting, being activated in certain
engine conditions. The glow plugs can operate in several ways the most simple
being switched on during cranking or while the ignition is switched on. The glow
plugs are wired in series and are fed with battery voltage for a set period of
time, which is determined by the 'glow plug timer relay'.
The
components soon heat to their operating temperature in a matter of seconds and
can seen to glow 'white hot' when tested out of the engine. The plug should heat
from the tip backwards, (as illustrated in figure 6.2) failure to do this
means that the plug needs replacing.
Fig. 6.2
Other
systems may remain on until the engine's water temperature reaches a
predetermined temperature, while others may operate in a similar way but will be
pulsed alternatively in sets of two.
The
testing of the glow plugs can be achieved in one of two ways, by either leaving
the plugs in-situ and monitoring the current draw of them all, or by removing
them and analysing their performance visually as they heat up and measuring
their individual current draw, at the same time. It is possible to use the Pico
scope to measure this either with this set up or by selecting 'view' then 'new
meter'.
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