Eliminating Engine Interference in car radios

ID: 420156
Eliminating Engine Interference in car radios 
29.Jun.17 01:44

Dan Healy (USA)
Count of Thanks: 4

Eliminating Engine Interference


There are three basic levels of what is called EMI (electro-motive interference) or RFI (radio frequency interference).  Most of us call it “engine noise” or “engine interference”.


Level number 1.


         The most objectionable “engine interference” comes from the ignition system.  It is caused by the opening and closing of the distributor points or the “firing” of the electronic ignition and the corresponding high voltage “pulses” that are generated by the ignition coil that then travel from the coil through the rotor and distributor cap, through the spark plug wires to the spark plugs creating the spark in the spark plugs which ignites the fuel/air mixture or combustion.  This is such a high powered pulse of electricity that a small amount of the energy is radiated (it literally flies off of the wires) into the air.  The spark plug wires act as small transmitting antennas that radiate this interference or “noise”, which is picked up by the antenna and fed into the radio.  Even other cars that, for instance, pull up beside you at a stoplight, will have their radio reception disrupted from your engine.  This interference radiates quite strongly for about a hundred feet in all directions.  There are several methods for eliminating this interference.  The best and most popular method is by using resistor spark plug wire and resistor spark plugs.  Without going into heavy technical and mathematical theory the reason why this works is because, due to a phenomenon known as “induction”, the resistor wire and plugs cause the spark to go directly to the spark plug rather than being lost or absorbed and radiated from the wires.  The high tech “Indy” and “Nascar” race cars that use complicated computers and high tech ignition systems all use resistor wires and spark plugs for best performance which also helps eliminate a tendency to foul the spark plugs.  It is an electronic and scientific misnomer that solid wire provides higher engine ignition performance.  Actually it’s the other way round.  (There was a time when “in-line” resistors were sold at auto parts stores that could be installed in solid spark plug wires but they are no longer available and are very difficult to find.) 




The next “layer”, so to speak, of ignition interference requires what is called a “condenser” just like the type used in distributor “points & condenser” tune up kits for mechanical or non-electronic old style distributors.  These condensers have a metallic body with a mounting tab. Mount the metal body onto the spark coil mounting bracket and connect the center lead wire to the coil terminal marked “Bat” or “Ign”.  This capacitor absorbs the electronic “impulse” created by the opening and closing of the distributor points.  Make sure the coil mounting bracket is solidly bolted to the engine and all paint, grease or corrosion has been removed and the metal is “bright” between the two surfaced that are mated


There are also ignition shielding kits that shield the spark plug wires, distributor and coil.  These work well but are very complicated and are expensive and mostly used in aircraft or cars with non-metallic bodies.


Level number 2.


         This level deals with the generator and/or alternator.  This is the second most active culprit of electrical noise interference.  Typically, a generator causes more interference than an alternator.  The graphite “brushes” that continue to make and break contact with the rotating armature journals for a generator or the graphite brushes that “wipe” on the commutator ring for an alternator cause a “whining” noise in the radio.  This interference uses the vehicle’s “wiring harness” to radiate just as the spark plug wires do for the ignition noise.  Part of this noise is also picked up by the antenna and fed into the radio and part of it travels through the electrical system and is fed into the radio through the power lead.  NOTE: car radios are made with very sophisticated and complex filtering circuitry at the power inlet source of the radio just to filter unwanted interference from entering the radio through the car’s electrical system.  Nevertheless there are standard suppression devices known as “condensers” or “chokes” or combined “condenser / choke” units and are made specifically for dealing with alternators / generators and any other electrical devices that may generate unwanted interference.  However the condensers you will start with are also from standard tune-up kits available at auto parts stores.  Note: try to find tune-up kits, or if sold separately, condensers for cars with 6 volts electrical systems (they have greater capacity than 12 volts versions although either will do).  In this application the condenser is connected to the voltage regulator at the output terminal sometimes marked “battery” or “ignition”.  The metal case is grounded at the mounting bolts of the regulator.  A second one is often connected to the “armature” terminal on generator with the metal body grounded to the case of the generator.




Level number 3.


This level deals with everything electrical in the vehicle other than the engine ignition and alternator / generator.  This includes the head light dimmer switch, the stop light switch, the heater motor, the windshield wiper motor, electric fuel pump if there is one, the gas gauge, temperature gauge and any other electric gauges or the turn signals.  In other words anything that operates from the vehicle’s electrical system.  For the most part, noise from these can all be eliminated by connecting a simple distributor type condenser to the offending item.  This is done by connecting the “body” or ground part of the condenser to a well grounded place on the body, frame or motor / transmission near the power inlet to the offending device and the center lead to the power inlet lead of the offending device.  For instance for the fuel gauge it would be on the fuel tank at the gauge sensor.  Fuel and temperature gages have a regulator or “ballast” that supplies current to the gages that can also require suppression.  The condenser “body” would be connected to a grounded surface near the gauge regulator and the center lead would connect to the power source feeding the regulator.   The windshield wiper motor would be at the switch; the heater motor also at the switch, both where the supply voltage is connected.  The fuel pump would be where the power lead connects to the pump etc.  NOTE: a fully suppressed automobile could have as many as a dozen of these capacitors suppressing noise from even lighting such as the dome and courtesy lights.


Extremely Important Grounding and shielding.


 Often radio interference is caused by parts of the vehicle’s body, frame and engine not properly grounded.  The hood should have grounding straps connected to the hood hinge bolts between the bolts on the hood and the bolts on the firewall (body).  The hood hinge is not a good electrical conductor.  All grounding straps should be made of ½ inch to ¾ inch woven or braided wire that is commonly available. 


The fenders, especially the front fenders, are frequently not grounded well enough.  This has double trouble because usually the radio antenna is mounted on one of them.  What usually happens, especially on a restoration, is the “tabs”, where the fenders bolt onto the cab / body, often have been painted along with the rest of the fender and body parts.  Remember the golden words for wiring and body connections: “Bright-Is-Right”!  These areas should be scraped and burnished so that when they are mated there is a good electrical connection.  There are preparations that will prevent oxidization (rust) maintaining continuous and long lasting




electrical contact.  Any metallic surface that is not properly grounded will act as an antenna to radiate interference.  


Battery Cables


The battery (return) cable that is connected to the engine / frame / body of the vehicle should be connected as follows:  First from the battery to the engine block.  Then from the engine block to the body and from the body to the frame for vehicles with separate frame and body parts.  Motor mounts are made of rubber and do not provide a solid electrical connection between the motor and the frame / body.  The same is true where the body mounts onto the frame.  This is absolutely important not only for radio interference but equally for a good working vehicle electrical system.  This will provide the proper electrical path and will have a noticeable effect on the performance of everything from the starter to the brightness of the tail lights.  It will determine the efficiency of the alternator / generator and the power performance of the engine.  It will help provide good working wipers, heater and gauges and most certainly improve the diminishing of the car’s electrical interference in the radio.


Wheel Static Electricity


“Static Collector Springs”


Wheel static collectors are installed inside the wheel bearing dust cover between the cover and the spindle.  This drains static electricity that is generated by the spinning of the front wheels and the contact of the rubber on the pavement. This is another common source of noise interference.


There are a couple of things to know about how important these so-called "static collector springs" are.  First, the theory is; sometimes the axel bearing grease can cause insulation between the front spindles (and in turn the frame of the car) and the wheel bearing, rotor, or drum and the rim and tire.  What happens is the rubber meeting the road causes static electricity to collect on the tire which is transmitted to the rim and rotor or drum where the wheel bearing is.  Because the grease may cause insulation this static electricity builds up then is "drained off" through the grease causing a kind of swirling noise when listening to very faint radio stations in extreme rural




areas.  This applies only to AM reception; not FM.  These "static collector springs" are a spiral shaped piece of flat hard drawn copper with a button in the center.  The button rides against the spindle and the skirt of the spring rides against the spindle dust cover.  This provides a complete circuit between the wheel and the car's frame thus draining off any static electricity.  This is also a product of cars and tires from many years ago.  Nowadays we use radial tires which are made of a different material which tends to not produce the static electricity phenomenon.  These tires use what is called lamp black and other compounds that conduct electricity which prevents the building up of static electricity.  Also, disc brakes have lining fabric that also conducts electricity which also helps prevent this static.  While it is always possible this type of emf (electro motive interference) can exist in any car it no longer is as large a problem as it once was.  There was a time when every radio store, auto parts store and auto parts wholesale houses carried these springs.  They are no longer commonly available.  You can also make them if it is really a problem.  Generally, these springs were not necessarily included in factory productions unless customers lived in extreme rural areas where absolute maximum noise suppression was necessary. 


NOTE: on a scale of one to ten; this type of noise is closer to a one.


To thank the Author because you find the post helpful or well done.

Eliminating Engine Interference in car radios 
01.Aug.17 05:39
218 from 2119

Matt Endicott (USA)
Articles: 10
Count of Thanks: 5

Excellent article Dan.

I once built and still own a Heathkit SW7800 general coverage receiver.  It has an on board inverter and can be run off 12vdc for car use, but the rf interference is terrible.

Resistor spark plugs have the perception of being bad for performance and racing purposes for two reasons I believe.  First, most people associate the resistor with ohm's law believing less current is available to the spark plug.  Of course they forget that ohm's law applies to a closed circuit and by definition a spark plug is anything but a closed circuit.  A resistor embedded in the spark plug does nothing until after the breakdown voltage has been reached and the arc has initiated.  The second reason and one that may have some merit is the perceived reliability failure of the resistor.  I've never known of a spark plug resistor to fail, but I assume it is possible.

Although caps, rotors, and wires are a thing of the past with most vehicles built within at least the last 10 years, most rotors have a resistor embedded in them and many distributor caps from the 1980's on have rf shields.  Many older European makes such as air-cooled VW's used a shielded resistor spark plug connector boot so using a resistor plug was redundant.

A bit off subject, but recently I was cleaning an old NOS distributor cap and noticed that when I blew compressed air on it a coating flaked off the interior.  As the cap was old and out of production I cleaned the remaining interior surface, masked off the terminals, and applied a new coating of clear Krylon acrylic.  It was then I learned about the interior coatings applied to the inside of distributor caps used to prevent nitric acid deteriorating the phenolic cap material in high humidity environments.  Nitric acid might be an issue with other high voltage applications outside automotive applications. US Patent 3217113.

Somewhere in my archives of old books I have a Bosch yellow jacket tech guide to RF suppression.  I could scan it, but I don't want a lawsuit so I think I will refrain.


To thank the Author because you find the post helpful or well done.