MACS Monthly Newsletter - 2017 - JUN5

Don't even think about using a test light in series
with the battery cable to detect an excessive parasitic drain. The high content of electronic modules
on vehicles built over the last 30 years have put that
test into obsolescence. In previous MACS Service
Reports we've covered techniques for pulling the
battery cable w/o losing battery power to the vehicle by connecting a boost box and back feeding
power into DLC's B+ and ground. To save time,
the last thing you want to do with any intermittent
problem is to do anything that makes the problem
quit while you are working on it. That's why we
stress - don't let the battery power to the vehicle
drop to zero while you connect test equipment!
Keep the system powered up.

power circuit (with blower running) and you've diagnosed a voltage drop problem. But why do this
test with the key off and everything dead? Here
is the reasoning behind doing voltage drops (looking for mV, not volts) at each fuse; it aids you in
identifying an offending circuit with too much parasitic drain. (Fig. 11) When a circuit is completely
off / inactive, there should be 0.0 amps of current
flow and therefore 0.0 mV of voltage drop. If a pirate's motto is "Dead men tell no tales" then a circuit's motto must be "dead circuits have no voltage
drop." Fused circuits that are live but only with
a normal amount of current draw (i.e. 5 mA) will
have a mV size voltage drop even across the most
clean and tightest fitting fuse terminals. For example, you might only see a voltage drop of say, 0.1
mV (that's more than zero) which calculates to 22
mA of parasitic current draw when measuring that
voltage drop across a 15 amp mini fuse (for example). For just a single fused circuit that is slightly
high but still acceptable as long as the entire vehicle's parasitic current draw is lower than around
50 mA. That same 15 amp mini fuse could be feeding an HVAC head and a couple of other items on a
particular circuit. If, however, with the ignition off
and doors shut, etc. you measure 1.0 mV of voltage
drop across that fuse, that fuse's circuits are drawing in excess of 200 mA. 200 mA is enough to drain
the battery to a no crank condition in a few days or
even less depending on the battery's condition and
ambient temperatures.

Narrowing Down the List of Suspects
After waiting a few minutes (or an hour or more
on some vehicles) if you observe your ammeter
showing 300 mA instead of a more tolerable parasitic draw of 30 mA, it's time to use your meter in
mV mode to check for a normal amount of voltage
drop. Any voltage drop (including only a few mV)
indicates that a fuse's circuits have some kind of
battery drain. There's a big problem with the old
method of removing a fuse to install a Fuse Buddy / ammeter or removing fuses (one at a time) to
see if the drain measured at the battery has gone
down. The offending circuit with the excessive
parasitic drain may be supplying power to solid
state electronics. Whether the electronic device is
a HVAC control head or a BCM, the result can be
more minutes / hours of waiting for those modules
to 'wake up' and then go 'back to sleep' once the
fuse that was removed in the diagnostic process is
reinstalled into the fuse panel. A passive method
for measuring the mV of voltage drop at individual
fuses to identify the offending circuit has existed
in a few OEM dealer circles for years. Unlike removing fuses, this method allows for intermittent
battery drains to keep on 'doing their thing' while
you move your voltmeter 's probes from fuse to fuse
looking for the circuit with an excessive drain.

The Handy Battery Drain 'Decoder' Chart
The folks at PowerProbe have provided a chart
on their website for calculating actual current draw
in a fused circuit by how many mV of measured
voltage drop across each fuse. http://info.powerprobe.com/fusechartsdownload The chart lists
various fuse types and sizes along with the actual current draw that would be present at the mV
voltage drop you are measuring. Remember: Even
though 0.5 volts of voltage drop is fine for an entire
circuit that is powered up, a circuit that is either
powered down or sourcing a normal parasitic draw
(typically mA in single digits) should have either
0.0 mV of voltage drop across each fuse or up to 1.0
mV of drop depending on the fuse type and size.
The type and size of fuse are very important factors
as you can see from the small sample of this chart
shown in Fig. 13.

Millivolt (mV) Drop Testing on Fuses
If you're not familiar with this method, you might
recall that conventional voltage drop testing is testing for MORE than 0.5 volts of voltage drop when
measuring from one end to the other of a fully powered circuit. Measure 1.5 volts on a blower motor

June 2017

MACS Service Reports

http://info.powerprobe.com/fusechartsdownload http://info.powerprobe.com/fusechartsdownload

Table of Contents for the Digital Edition of MACS Monthly Newsletter - 2017