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Dirty electrical signals

Frequency variations and electrical surges are examples of this.

There is probably not a person who doesn’t like clean clothes, a clean bed, and clean water. You’d also be hard-pressed to find a farmer who doesn’t strive for clean fields and clean grain going into the bin. 

But what about clean electricity coming out of your machinery’s alternator and into the onboard electronics? I am not talking about a solar- or wind-powered tractor, but the cleanliness of the electrical power created and delivered by the vehicle. 

The term clean describes electricity that is free from voltage variations, is the proper voltage, and is absent of frequency variations and surges. The success of modern production agriculture is rooted in advanced electronics in engine controls, combines, sprayers, grain dryers, and irrigation systems.

The circuitry of such equipment is designed to work properly with high-quality electricity. If dirty power is introduced, it would be no different than feeding dirty fuel to an engine.

If the power delivered does not match the needs of the circuit, then it can malfunction, suffer premature failure, or have unexplained readings or response. 

Since the alternator is the source of electricity for any machine, the greatest possibility of dirty output begins there.

Whereas a generator creates direct current (DC), an alternator’s internal output is alternating current (AC). The problem is that machinery, for the most part, runs on DC electricity.

An AC signal is a sine wave and alternates between positive and negative, while a DC signal is a straight line.

An alternator employs diodes that can be considered one-way electrical check valves. The purpose of the diode is to chop the AC signal and, for our purposes, only use the part of the output that is on the positive side of the zero crossing (where the polarity changes). It can be likened to a combine spitting the chaff out the rear and putting the clean grain in the hopper.

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Effects of a Faulty Diode

A weak or failed diode can create two issues: either the voltage output will be too low or its frequency will change. Either problem contributes to electrical noise.

Think of this as an electrical version of an engine with a rhythmic misfire that pops at the tailpipe. If a diode completely fails, then the output of the alternator will be low. The electrical signal will be dirty and, depending on its function regarding chopping the signal, the battery may back-feed and drain when the machine is shut off.

Often a weak or failing diode may pass current both ways when it reaches a certain temperature or the electrical load on it is higher. This is important to keep in mind if a problem pops up under certain machine-operating conditions. Recognizing this will make it easier to diagnose.

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Aging Components 

Though the alternator has the highest possibility for noise creation, it is not the only component that causes this problem. An often-overlooked aspect of electrical circuits, components, solenoids, and relays is that as they age, they can get weaker and may leak. For this discussion, we will only be concerned with electrical leakage.

Contrary to conventional wisdom, electrical leakage does not identify a bare wire. It is a wire or component that allows the magnetic waves that are created as current passes through to escape. This is identified as EMI, or electromotive interference.

It is especially problematic when machinery gets older and the insulation on wires becomes dried out and porous. The same phenomena is evoked with the plastic housing that is common on solenoids and relays. For the voltage to become dirty or exhibit EMI, the leaking circuits need to be near another leaking circuit. The stronger EMI signal will then penetrate the leaking wire that has the weaker signal and will cause havoc.

When a solenoid or a relay, which is an electromagnet shuts off (discharges), the collapse of the internal field windings acts like a mini ignition coil. For this reason, many circuits that employ an electromagnet (such as an air conditioner compressor clutch) incorporate a diode in the wiring. This diode prevents the discharge spike from going back into the electrical system and creating a surge. Understand that a leaking wire or component is different than dirty output from the alternator. In this case, the signal is clean but getting polluted by crosstalk from a compromised wire or part.

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Diagnosing a Dirty Circuit 

Another aspect of DC voltage is that it only has one goal: get to ground. If there is a weak ground circuit somewhere on the machine, then, in essence, the traffic jam of electrodes is being denied its desire to find ground and will do what is necessary to achieve that. This has the potential to dirty up a circuit and very possibly one that is not even connected to the poor ground.

The first step in diagnosis is being suspicious of dirty power in an advanced piece of electronics or one with complicated circuitry (even a radio) that is displaying strange or intermittent behavior. An example would be a monitor that suddenly flashes or resets or an auto steer system that self-cancels for no reason.

A very common mistake by the novice as well as an experienced diagnostician is to try to rationalize where electricity will go. EMI will take a route and influence circuits that are completely divorced from its source. There is no logic you can apply to this other than a suspicion of dirty power.

Now you need to qualify when the event occurs. Does it occur when the air conditioning, headlights, or any other high-current demand load is on? Does it seem to happen at a certain air or engine temperature? If you can get a handle on when the event occurs, it will make finding the source a much easier task.

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Apply Logic to the Problem 

At first blush, the effort may seem overwhelming due to the complexity of modern equipment. Just persist and apply some logic to the problem. 

Let’s say the issue occurs with a yield monitor on a combine when you turn all the headlights on. There could be a leaking wire up near the lights on the top of the cab. It’s possible that leak is influencing the yield monitor. Yet, that probability is not that great when you consider that no other wires going to the yield monitor are in close proximity to the lights. 

EMI that dirties voltage in most, but not all, instances can only travel so far. With the example of the combine yield monitor, first go to the most likely source of the problem, which would be the alternator and the wiring harnesses that run near it. There may be nothing wrong with the diodes. It can just be the wiring is getting older and the integrity of the insulation is slightly compromised. In a case like this, grabbing the harness and repositioning it slightly to move it away from a possible EMI source may be all that is required.

If you can duplicate the problem in the shop, try running the engine with the field circuit wires disconnected from the alternator. If it is a modern electronic engine, you will want a battery charger connected.

Electrical engineers will tell you that the proper way to confirm dirty voltage is with an oscilloscope. Since most farmers do not have an oscilloscope, if you follow the tips provided here, there is a good chance you can confirm and find why the circuit is dirty – but don’t get too comfortable. As more electronics are added to farm equipment, an oscilloscope will be as essential a tool in a farm shop as a ratchet set!

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