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DDEC – How to Check Phantom Electrical Load
BAttery Power As Jon mentioned above
CBs 19, 20, and 21 are all connected to a bus bar and are fed 12 volts
directly from the batteries. So they are hot all the time. CB 19 and
CB 20 go directly to the ECM of the DDEC engine and are normally the
injector driver circuits. Detroit required Prevost to keep these wires
hot so that when the engine was shut down the small amount of
“housekeeping” that the ECM had to do could be accomplished even though
the systems were shut off. CB21 feeds Relay 23 and this is the engine
run signal that is distributed once the key is turned on.Normal draw on these two circuits (CB 19 and 20, wires 240 and 241)
is about 250 milliamps. It is a very small parasitic load that should
not deplete the batteries. But we have seen on some units where these
wires begin to pull around 1 amp. This could cause you to have low
batteries over a long period of time.
To check this, cut your 12 and 24 master switches when you turn off
the vehicle and then trip the three circuit breakers, CB19, CB20 and
CB21. Let the vehicle sit and see if you have the same issue when you
go to start the vehicle.
In the XL series we also had a hot post in the front junction box (
stud #70) that was connected to the 12 power directly. It was for
radios and CB radios or whatever in the charter coaches. Check this
stud to see what is connected to it. It may be something that would
draw enough to bring down the battery level over time.
If you still get a low battery level then you will need to open
circuits to verify that they are truly open with no current draw. You
may choosse to get this type work done at a shop like Jeff’s or at one
of the Prevost service facilities.
Bill Jensen
for Prevost
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