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How to track down an electrical fault

We trace the source of a tractors' parasitic power drain with a multimeter

Automotive electrical systems in newer machines that make extensive use of computer processors can be very complex and confusing. But there are a lot of machines on the farm with electrical systems that won’t have anywhere near that level of complexity, especially those with diesel engines and no computer controls.

Finding and fixing electrical faults in those simpler systems can be a straight forward exercise. If you understand a few simple principles and follow some logical steps, electrical faults can easily be tracked down right in the farm shop.

A fault in this tractor’s electrical system was allowing a continuous 2.58 milliamp current to flow when engine and key were turned off, draining the battery within a couple of days.

A fault in this tractor’s electrical system was allowing a continuous 2.58 milliamp current to flow when engine and key were turned off, draining the battery within a couple of days.
photo: Scott Garvey

In this Shop Class instalment we roll up our sleeves, identify and correct a typical electrical problem in a late-model utility tractor. All it took to get the job done was a reasonably good quality, digital multimeter, some basic hand tools and a lot of patience.

The battery in this tractor would lose its charge after sitting for a few days, requiring a boost or recharging to get it started again.

Because the battery is relatively new and returned to full voltage after some time on a charger, we suspected there was a parasitic power draw when the key was turned off rather than a fault within the battery itself. But we first checked to make sure the alternator was working correctly and keeping the battery up.

With the tractor running at a fast idle, we used a multimeter to check the voltage across the battery. Set to “DC volts” with one lead on the battery positive post and one on the negative, the meter showed about 14.5 volts, which confirmed the charging system was working fine.

Now we knew both the battery and charging system were in good order. So it was time to check out the likelihood of a parasitic power drain.

With the engine shut down and the key off, the negative cable was disconnected from the battery and the multimeter was set to “DC amps.” When the positive meter lead was touched to the cable and the negative lead to the negative battery post, the multimeter indicated a steady current flow of 2.58 milliamps. Of course, there shouldn’t be current flow when the tractor is turned off; some machines will still have very small current flows for a variety of reasons. As a rule of thumb, anything above 0.5 milliamps is considered unacceptably high. So our initial suspicion of a parasitic power drain was confirmed.

Removing one fuse at a time until the current flow stopped revealed which circuit had the faulty component.

Removing one fuse at a time until the current flow stopped revealed which circuit had the faulty component.
photo: Scott Garvey

Remember, when the multimeter is set to “DC amps”, it becomes a part of the electrical circuit and all current flows directly through it (unlike when it’s set to DC volts), so it’s possible to damage the multimeter if the level of current flow is too high. Do not to touch the meter leads directly across the battery (positive to negative posts) when set to “DC amps.” Current flow that high will likely blow your meter’s brains out — figuratively speaking.

The first step in tracking down exactly where the tractor’s electrical fault lay was to identify the specific circuit it was in. To do that, we left the negative battery cable disconnected and the multimeter connected between it and the negative battery post. We then removed one fuse at a time from the fuse box and rechecked the multimeter reading until it fell to nearly zero. In this case that happened when we removed the ignition switch circuit fuse.

With the instrument panel gauge cluster disconnected, current flow fell to 0.0. That indicated the fault was in the gauge cluster and it will need to be replaced.

With the instrument panel gauge cluster disconnected, current flow fell to 0.0. That indicated the fault was in the gauge cluster and it will need to be replaced.
photo: Scott Garvey

That identified the circuit with the faulty component, significantly narrowing down the number of possible culprits. From this point on in the process it’s handy to have a copy of the tractor’s wiring diagram. It shows which components are included in which circuits and the colour of the wires connecting them. We didn’t have one, but this tractor has a pretty simple electrical system and we knew from the fuse name which components we should probably check.

With the fuse from the faulty circuit reinstalled, we isolated one component at a time by disconnecting it and continually rechecked the multimeter reading. After unplugging the dashboard gauge cluster, the meter reading fell to 0.0.

Clearly, we had found the problem. Replacing the gauge cluster should eliminate the parasitic loss and get the tractor working normally again.

About the author

Machinery Editor

Scott Garvey is the machinery editor for Grainews.

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