MACS Monthly Newsletter - 2017 - JUL5

July 2017

MACS Service Reports

Photo & Drawing - Dave Hobbs

1. Determine if the logic lock up or other EMI
symptom occurs in one or all positions of the
ignition switch to find the emitter.
Figures 4: No mystery here; you've seen capacitors on blower motors for years. But
a. Problem Only While Engine is Rundid you know how they are wired into the circuit and how they work? One leg of the
ning?
capacitor is typically connected to a circuit that you are trying to rid of interference.
Think about what's unique to that scenar(far right) The other side is connected to ground. If the capacitor has + and - markio. If the problem is not occurring in the
ings on it, make sure you connect it accordingly. The idea is that DC that is supposed
to be flowing to a 'victim' device (in this case an HVAC control module) charges the
key on / engine off or accessory mode, the
capacitor on its way to power up the module. After that first charge of the capacitor,
fuel pump, injectors, alternator and ignithe capacitor is for all practical purposes invisible to the circuit. If a voltage transient
tion system are prime suspects. The alter(spike) or noise (AC voltage) are on the circuit that the capacitor is tied to, the capacinator can be disconnected (voltage regulator allows that unwanted noise / transient to travel through it to ground, thereby protecting the module. Choosing the correct size is sometimes a matter of trial and error.
tor plug) to quickly determine (hopefully)
The rule of thumb is; the higher the frequency / shorter duration of the transient / AC
if the problem is still present. Keep in
signal, the smaller the capacitor size should be.
mind your time window before the battery
goes down. That's usually just a few minutes. If you can discover a suspicious looking
pattern with a multi trace lab scope on the
power or ignition feed to your victim device
(i.e. HVAC control head) and can correlate
it to an injector pulse or fuel pump current
ramp waveform (pretty advanced diagnostics) you've got your culprit. If your scope
skills are not the best or you just are unable
to unearth some clues with your scope, you
can disconnect the fuel pump and then the
injectors (fuses are the recommended quick
method) one at a time while supplying an
alternative source of fuel (propane, injector cleaner, etc.) in an entry point into the
intake after the MAF (if equipped). If the
alternator and fuel related components, the
assumption now points to either the ECM
or an ignition system component. The use
Figures 5: We unwrapped the black tape sealing up the components in this GM noise
of the vehicle's radio AM band (outside
filter (p/n 1224205) to see what makes it tick. There is a capacitor and coil. The two
your shop and away from the building)
wires coming out of the coil impede the flow of AC on power circuits. The coil is
connected in series with a module / device that is either susceptible to interference or
can give a clue as to whether the problem
guilty of making excessive interference via a power circuit. The capacitor is connected
is ignition related. Rev up the engine.
in parallel to ground, with the power wire the filter is connected into, in order for AC
If you can hear an excessive amount of
and transients to be pulled off the circuit to ground. Caution must be taken not to
RPM variations in the radio on a medium
exceed 10-15 amps for circuit being filtered. Therefore this filter would be great to
install into the ignition feed to an electronic HVAC control head suffering from crazy
strength AM radio stations, you've found
symptoms or a radio with back way noise (noise on the power circuit).
your culprit system.

Photo Dave Hobbs

module's power, ignition and ground circuits with
a fast sampling rate set for a high speed capture
can sometimes help locate the circuit that
either needs spike suppression (via a diode
or EMI filter - see figures 4 and 5), shielding or replacement of the component causing the transient. Other forms of EMI are radiated emissions and magnetic fields. EMI
is usually conducted through the air (RFI &
magnetic fields) or through wiring (electrical
noise). The component creating the problem
is referred to as the emitter and the victim
device referred to as the receptor. Identifying the emitter can be achieved with a simple
process of elimination.

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