The fool's gold of truck service
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The fool’s gold of truck service

Jake Schell is the associate product manager for Mitchell 1’s Commercial Vehicle Group.

More decades ago than I care to remember, some movie or folklore led me to start panning for gold in the dirt road outside our home. Slushing a pan full of dirt around in a pan with water from the garden hose rather quickly revealed the road to be a mother-load of GOLD.

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The bright flecks floating to the top of the dirt in the pan glistened up at me in the sunlight. Carefully, I scooped the gold off the dirt and into my jar. As my gold dried, it turned black. No longer sure about this gold, I sought out the expertise of a higher authority, my Dad. With a chuckle he told me that it wasn’t gold at all but fool’s gold, or iron pyrite. My illusion as a rich gold miner quickly evaporated. “But how do you tell the difference between fool’s gold and gold?” I remember asking in despair. My Dad responded, “They test it.”

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Spring forward a few decades. A service truck comes into the shop with a transmission shifting issue. A quick check reveals several shift solenoid low voltage codes. Going through the preliminary electrical diagnostic checks fail to identify what or where the low voltage condition originates.

I clear out the codes and take the truck for a drive. Sure enough, before long the problem happens again. Clearing the codes again, I monitor the voltage to the shift solenoids. Both solenoids operate correctly–cycling on and off with the corresponding gear–at least until the air conditioning cycled. Then the voltage dropped, and the low voltage codes reset.

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The problem source was narrowing. Arriving back at the shop, I started looking at the wiring to the recently installed air conditioning system. Whoever installed the air conditioning system found a wire with key-on power and tapped into it to power the air conditioning compressor clutch.

While the circuit voltage provided enough power to illuminate a test-light, it was not enough to power the transmission shift solenoids and the compressor clutch. Finding a hot wire under the dash can be fool’s gold; it looks good but it’s really just creating a new problem.

Gone are the days when the technician could just tap into a hot wire to power radios and other add-on accessories. The voltages present in many circuits are less than battery voltage and power sensitive electrical components. Splicing into circuits can cause any number of other problems that can lead to failures of costly components or assemblies.

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Now more than ever before, technicians must consult wiring and circuit information before using a circuit to power other equipment. Manufactures generally provide additional fused ports to accommodate electrical accessories.  So the takeaway is this: Power in a wire can be fool’s power. Make sure it’s the right power intended to be used for accessories.                  

Additional tips for repair and maintenance of Class 4-8 trucks may be found in the Mitchell 1 ShopConnection Truck blog.

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