MACS Monthly Newsletter - 2017 - JUL6

EMI Good COP - Bad COP Case Study

to do now is remove fuses one by one until
you've reached the fuse powering the culprit
EMI emitter. When the problem is also present in the ignition accessory position your fuse
pulling isolation procedure is shortened drastically to just fused circuits live in Accessory.

COPS (Coil On Plug) can actually fire a spark plug just
fine while still producing excessive conducted or radiated emissions capable of creating a variety of symptoms.
One particular EMI case study shared by John Thornton
comes to mind regarding EMI transients caused by a bad
defective ignition COP. The case involved a late model
Dodge. The customer complaint was a jerky feeling engine and a MIL illuminated. A key DTC was for an electrical problem with one of the MDS (Multiple Displacement
System) solenoids. The shop had replaced a list of parts
related to that system to no avail. John established that
the ASD (Auto Shut Down) relay was turning on and off
very rapidly as the engine ran. In the end, he determined
that the PCM was undergoing rapid COP (Computer Operating Pulse) interruptions. The PCM as it very quickly
was 'knocked out' then brought back on line would see
the status of one of the MSD relays and flagged it as being off when it should have been on. In reality, the PCM
should have been awake when it actually was asleep.
With a multi trace lab scope, John correlated one particular cylinder's ignition coil transient spike with the vehicle's ASD rapid chatter. Not a good thing for any relay.
Definitely not a good thing for a tech without knowledge
of EMI. A new COP was popped in the cylinder and the
car was fixed. HVAC heads, clusters, radios, ABS modules, BCM, and pretty much any module you can name
could be plugged into a case study like this.

3. OK - I've FINALLY Found My EMI Emitter - What's
The Fix?
a. Try running a redundant ground, then an
alternate ground or ground strap. (Fig. 6A and
6B)
b. Run an alternate B+ power source to the
component.
c. Run an alternate ignition switched circuit
to the component.
d. Connect a capacitor between the power feed to the EMI emitting component and
ground. Trial and error are sometimes the key
to find an effective capacitor value. (Figure 4)
e. Connect a noise filter (Figure 5) in series
with the power feed leading to the emitting
component.*
Note: noise filters have a fairly low current rating.
*Do not use a noise filter (coil) in series with a circuit
rated for higher current than the filter can handle - typically around 15 amps.
f. Relocate the component (if possible) or at
least move wiring away from it as
much as possible.
g. Replace the component. Sometimes it's
not a wiring issue. Sometimes the component
is simply noisy due to an internal problem (i.e.
cracked circuit board, faulty internal capacitor,
etc.).

Photo & Drawing - Dave Hobbs

2. EMI Present with Key On AND Engine Run? How
about EMI Present in ACC Only?
a. It's getting easier now. If you've got to this
step you don't have to worry about being McGyver to keep the engine running while you
determine if the EMI induced logic lock up
or other symptom has stopped. All you have

Figure 6A and 6B: Adding a redundant (extra) ground can sometimes be the simple fix for unwanted crazy symptoms in the world of automotive electronics. Braided ground straps (left) have greater skin effect (more exposed surface area) than normal twisted multi strand wire. This is essential when
trying to shield an electronic module. Stray electrical signals conducting through thin air (RF - Radio Frequency) are attracted to the braid's skin effect
and travel harmlessly to ground (right). A braided ground strap, however, tends to handle current less efficiently than a similar size conventional multi
strand wire so you'll need to up the size a bit to prevent the strap from becoming too hot / causing a voltage drop. Fleet vehicles with sensitive aftermarket
electronics such as 2-way radios are especially popular applications for braided redundant grounds. The process of putting the right strap in just the right
place is sometimes referred to as "RF bonding."

July 2017

MACS Service Reports

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