NOTE
When the voltmeter is connected in parallel
with a cable (meter leads and cable ends
start and end at the same point), the meter
will measure the voltage potential at both
ends of the cable. With no current flowing,
the values will always be the same. Thus the
voltmeter will read zero.
2. Figure 1-7 shows the voltmeter being used with
current flow (starter pulling 500 amps).
NOTE
The V1 reading (See Figure 1-7) means that
with 500 amps flowing through the positive
cable there is a 1.0 volt drop in that cable.
There must be current flow in order to
produce a voltage drop. The V2 reading (See
Figure 1-7) of 0.3 volt means that with 500
amps flowing through the negative cable
there is a 0.3 volt drop in the negative cable.
3. Figure 1-8 illustrates the digital voltmeter
reading in a DC system with both correct and incorrect
polarity. Whenever the minus sign is read, reverse the
meter leads to obtain the correct polarity.
1-17. MEASURING CURRENT FLOW WITH THE
AMMETER. The ammeter is used to measure
current flow. The unit of measure is the ampere or
amp. Unless the induction pickup (clamp-on)
ammeter is used, the circuit must be opened and
the ammeter connected in series in order to take
measurements.
1-18.
AMMETER SELECTION.
With but few
exceptions (See Figure 1-10) it is desirable to make
ammeter readings without opening up the circuit and
thus disturbing a circuit defect and prevent it from
being discovered. Therefore the use of a clamp-on
induction type ammeter is recommended for most of
the measurements described in this manual. This type
is easy to use, cannot be damaged by misconnection
and will not damage the vehicle wiring. A clamp-on
ammeter operates by measuring the magnetic flux
produced by current flow through a conductor and
translating that into amperes which are displayed on
the meter readout.
1-19. AMMETER USE.
Figures 1-9 through 1-11
illustrate the use of various types of ammeters.
1. Figure 1-9 illustrates the used of an in-line
digital ammeter. Note that the circuit must be
disconnected in order to use this type of meter. The
unit shown is an internal shunt type and generally used
for loads of 10 amps or less. Care must be taken not to
measure current greater than the meter rating. This
type of ammeter is not recommended for most of the
measurements described in this manual. However, if
this is the only type available, be sure to follow the meter
manufacturer’s instructions for connection and use.
2. Figure 1-10 shows an in-line digital ammeter
used to measure an unknown current draw such as
may result from a parasitic load. On the 10 amp scale
the current would be too low to be measurable. However,
with the meter on the 300 ma scale, the current draw
in milliamps can be read.
1-4
-12.5
DIGITAL VOLTMETER
CONNECTED CORRECTLY
IN RELATIONSHIP TO
POLARITY
DIGITAL VOLTMETER
CONNECTED
INCORRECTLY IN
RELATIONSHIP TO
POLARITY (VALUE
CORRECT BUT MINUS
SIGN WILL SHOW TO
ALERT OPERATOR THAT
LEADS ARE REVERSED)
V1
V1
12.5
Figure 1-8. Correct and Incorrect
Polarity Readings
DP-1008
10 AMPS OR LESS LOAD
Figure 1-9. In-Line Digital Ammeter
10A
DP-1009
Figure 1-10. In-Line Ammeter
UNKNOWN
CURRENT
DRAW
DP-1010
new diagnostic manual 4/5/05 9:47 AM Page 1-4
This document, and more, is available for download from Martin's Marine Engineering Page - www.dieselduck.net
