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How to deal with electrical concerns related to EMI

Tuesday, January 17, 2017 - 08:00
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Electrical problems can be tough. Sometimes the most intermittent and hard to diagnose electrical problems aren’t due to the classic faulty ground, computer glitch or software anomaly. Phantom electrical problems caused by EMI (Electro-Magnetic Interference) while somewhat rare, can lead to a great deal of frustration, misdiagnosis and customer dissatisfaction. Examples vary tremendously; your road warrior customer drives past a busy airport each day and his cruise control kicks off. The 2-way radio in the local game warden’s truck intermittently causes its transmission to downshift when the radio is used to transmit. Your favorite customer’s favorite FM radio station has static but only when the engine is running. These are just a few of the problems that can occur in the world of electrical interference. It doesn’t have to be a complete frustration for you as a technician however. In this article we’ll give you practical information on grounding, shielding and proper wiring so you can put an end to high-tech menaces like electrical transients, magnetic fields and Radio Frequency Interference (RFI) on vehicles you encounter.

Antenna coax varies in center conductor diameter and distance from the braided shield. Always use the type the OEM used originally. The ground comes from the antenna’s end and should be less than 0.1 ohms. Center conductors from end to end should be 0.1 ohms or 3-4 ohms with the type using a smaller conductor on long runs such as the radio to rear back glass mounted antenna.

EMI/RFI/EMC – Meanings and examples

Starting out with a basic knowledge on this very tough topic is the first step. Electromagnetic Compatibility (EMC) is the science of making sure every electronic component on a vehicle gets along with everything else including the occupants and their favorite gadgets. A classic example of course is the use of resistor spark plugs/plug wires to combat AM radio noise and a diode across the A/C clutch coil to prevent the voltage spike induced when the compressor clutch coil is turned off from wreaking havoc on other electronics on the vehicle. EMC engineers even take into consideration the static electricity (ESD) customers can create. Opposite of EMC is Electro Magnetic Interference (EMI), which is the science behind fixing the lack of compatibility. A subsection of the entire electromagnetic spectrum is the slice of frequencies referred to as the radio spectrum. Any problem that involves any of these frequencies is referred to as Radio Frequency Interference (RFI). For the sake of simplicity, we’ll just refer to it all as “electrical interference” and get right to diagnostics.

Making twisted pair only requires a drill and a vice to twist the wire.  If replacing existing twisted pair such as with 2-wire serial buss circuits, always duplicate the number twists per inch.

Electrical Interference diagnostic essentials – modes of inference propagation

Two terms apply to all types of electronic component interference situations: Radiated Emissions and Susceptibility. If a component on a vehicle is producing too high of a level of radiated emissions for another component on the vehicle to operate correctly, you can think of that component as the “culprit.” The component not working correctly is referred to as being susceptible. In the 27 years I’ve worked for an OEM electronics supplier, there have been tremendous successes with EMC. However, nothing is perfect and components do wear. Connections can begin to be effected by rust and corrosion or an ignition coil’s primary winding’s resistance can begin to break down. Wiring can get moved during repairs and aftermarket accessories are sometimes added. Some of the smallest changes can foul up the delicate balance of EMC.

In most cases, the braid or foil grounds at ONE end ONLY.  Attaching the shield to ground at both ends only ‘sounds’ like a good idea.  In reality, we don’t want regular DC ground current flowing through the shield.  Slightly different from RF ‘bonding’ we want the shield’s skin effect to attract radiated emissions near the cable and send them harmlessly to ground.  In the case of this HID headlamp, we want the RFI produced from the bulb start up’s high voltage to STAY inside the shield!

As with any other diagnostic approach, duplicating the concern, performing a general inspection complete with an all module vehicle DTC retrieval and searching for any applicable TSBs are the first steps. Subsequent steps would be:

  1. Determining more about the “victim” component and “culprit” components by operating various switches and accessories, observing the problem in all modes of ignition switch positions and disconnecting known sources of EMI such as the alternator, fuel pump (fuel the vehicle with an alternate method such as propane via the intake/throttle body during this test) and blower motors. 

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    Adding grounds is a great method for combatting interference problems. In the case of problems of a radio frequency nature, braided ground straps are preferable due to their skin effect. The surface (or skin) of the braid is like fly paper for electrons in the air (radio waves) and brings them to ground. Many 2-way radio installers play around with the right number and placement of braided grounds straps adding RF grounds to various body panels such as trunk decks and hoods so they will be tied to the chassis ground in a more effective manner. This is sometimes called “bonding.”  

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