Mapping Ground Currents in a Car0

By David Navone

Unless Lady Luck is your shop manager, one of the most difficult options to consider when installing a car stereo sound system is the choice of electrical grounds. Car stereo differs from home and pro audio in that the component electrical grounds typically use the chassis of the car as a return to the negative battery supply.

Noise problems arise in sound systems due to the interaction of vehicle accessories and the autosound components. Power supply filters are provided in car stereo components to attenuate noise on the + 13.8 Volt DC leads, but what about the ground connections?

Where The Problem Started

Way back around 1895, some economy minded automotive technician decided that since the chassis of a vehicle was made of metal, it could serve as a cheap electrical accessory return to the battery. Rather than run two wires from a horn, for instance, only one wire was necessary; provided the horn was connected to the chassis and the negative battery post was also connected to the chassis. The process of making a connection to the chassis of the vehicle became known as “grounding.”

Autosound components are also accessories that use the chassis of the car as the negative ground return. This may be convenient as well as economical, but when the engine is started and other vehicle accessories placed into operation, the effect on sensitive autosound components is all too well known. Noise appears in the form of alternator whine, brake light pops, ignition ticks, etc.

A Typical Scenario

Consider the example of a headpiece installed in the dash and an amplifier mounted in a trunk of a 1984 Olds sedan. With all specifications optimized, the engine recently tuned, and a new battery in the car, it would be very unusual to NOT hear at least some alternator whine creeping into the sound system on a blank track (all low bits) at a moderate listening level.

Now, let’s pull the headpiece and the amp and strap them together on a test bench. Connect the amp to a couple of test speakers and then make the necessary +12 Volts DC and ground connections back to the car’s battery via jumper cables. What happens to the noise level?

The noise will be greatly reduced because the chassis of the car no longer serves as a common grounding connection. (Don’t try substituting a small wire to make long ground returns because the noise on the chassis of the car will simply couple to the wire.) Just how much noise is flowing over the chassis of a typical car?

Well, first of all, there are no typical cars. If you were to run a CRAY computer for a week, you still would not be able to reliably predict the path, amplitude, or interactions of all the ground currents on any particular vehicle. Variables such as the strength of the spot welds, tightness of the bolts, and thickness of the chassis determine the intensity of the noise.

To actually measure the noise level between any two points on the chassis of a car, a spectrum analyzer would be useful. A spectrum analyzer is a sophisticated oscilloscope capable of displaying a very broad range of frequencies at one time. A typical spectrum analyzer that could measure DC to 2 GHz would cost at least $5000.00 and could cost over $ 50,000 for the better models. However, there is another way to begin our study of chassis noise that is quite a bit less expensive — if we make a couple of compromises.

An $13.00 Spectrum Analyzer?

The Radio Shack catalog has a nifty little audio amplifier speaker (Part 2771008). The cost is around $13.00. You will need a 9 Volt DC Transistor battery, a mini plug and some spare shielded cable for this experiment.

One of the compromises for our test is that we listen to the noise rather than see the noise. Our ears are very responsive and can respond to both small and large sound levels. The second compromise is that we only examine the narrow section of bandwidth in the 100 Hz to 10 KHz audio range. Our ears are capable of responding to over 10 octaves of sound, so this should not present a problem. Anyway, this is the audible range that is most important when considering car audio interferences.

The probe for our tests can be 20 feet of shielded cable such as RG58 left over from Cellular Phone or CB antenna installations. I also used a three foot length of # 20 AWG wire with an alligator clip on one end for my test probe. This wire was connected to the outside shield of the cable at the mini plug on the amplifier. The center conductor of the opposite end of the shielded cable should have a capacitor in series with an alligator clip installed to offset any DC levels experienced during test. The value of the capacitor is not important. Try using a 1.0 to 4.7 mfd, 100 Volt DC crossover cap.

Let me recommend placing a 1 Amp in line fuse between the probe’s alligator clip and capacitor in the event you want to listen to the charging current flowing to the battery or to some other accessory. Safety always!

Ok, so we’re ready to listen to the alternating currents flowing all over the chassis of a car. Turn on the little amplifier and clip the fused alligator clip to the positive battery post. Now quickly touch the other alligator clip to the negative battery post. That loud whining noise is the alternator’s charging ripple.

Alternator Whine

Alternator whine is primarily composed of two components. The higher frequency tone is the three-phase alternating current output ripple and the low frequency hum is produced by the heavy battery current in the field windings. Turn on the headlights and notice that both levels increase in intensity. Step on the accelerator and notice that the higher frequency tone will change pitch and become even higher. This is alternator whine.

Let’s take the little amplifier and move back to the trunk of the test car. Clip one alligator clip to a nice, shiny point on the chassis of the trunk’s interior and then touch the other alligator clip to the same spot. Start the engine and listen to the little amp’s speaker. The small noise level you are experiencing is the noise floor of the amp and probe.

Checking Specific Grounding Points

Leave one alligator clip fastened to the same spot in the trunk and bring the little amp and other alligator clip up to the front seat of the car. Touch the probe to the outside of the cigarette lighter receptacle and listen to the noise. The increase in noise level over the noise floor can be directly attributed to AC ground currents, superimposed over DC ground returns, flowing between the two ends of your probe. This noise is a direct result of a voltage drop associated with resistance between the two points on the chassis of the car.

Turn on the headlights, speed up the engine, hit the horn switch, move the power seats and close the doors. Notice the change in noise level due to changing ground current paths.

Run your own experiments between any two points on the chassis. Try the negative post of the battery. I think that you will find that the negative battery post would not be a very wise choice for an electrical grounding point. There is way too much charging, as well as load currents flowing in the vicinity of the negative battery post.

How about the case of the alternator? I have been asked many times over the past several years if perhaps the case of the alternator might not be a good place to ground an amplifier or a deck. A quick test with your little amplifier and probe will emphatically demonstrate that the case of the alternator is probably the noisiest grounding point on the entire vehicle.

So What’s the Solution?

After a few minutes with the little amp and the probe it should be evident that the car’s chassis is made up of very, very noisy grounding points. However, some ground paths are definitely quieter than others. Sometimes moving the probe just a couple of inches will noticeably change the noise level.