Truck Service Fool’s Gold


More decades ago than I can remember, a movie or folklore led me to start panning for gold on the dirt road in front of our house. Pouring a pot full of dirt into a pot with water from the garden hose fairly quickly revealed that the road was a mother load of GOLD.

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The bright specks floating atop the dirt in the pan shone towards me in the sun. Carefully, I picked up the gold from the ground and into my jar. As my gold dried, it turned black. No longer sure of this gold, I sought the expertise of a higher authority, my father. With a chuckle, he told me it wasn’t gold at all, but fool’s gold, or iron pyrite. My illusion of being a rich gold digger quickly evaporated. “But how do you tell the difference between fool’s gold and gold?” I remember asking desperately. My dad replied, “They’re testing it.”

Fast forward a few decades. A service truck comes into the shop with a transmission change problem. A quick check reveals several low voltage shift solenoid codes. Preliminary electrical diagnostic checks cannot identify where the low voltage condition is originating.

I clear the codes and take the truck for a spin. Sure enough, before long, the problem reoccurred. Clearing the codes again, I monitor the voltage of the shift solenoids. Both solenoids work fine – turning on and off with the corresponding gear – at least until the air conditioning kicks in. Then the voltage dropped and the low voltage codes reset.

The source of the problem was shrinking. Back at the store, I started looking at the wiring for the newly installed air conditioning system. Whoever installed the A/C system found a wire with key power and tapped into it to power the A/C compressor clutch.

While the circuit voltage provided enough power to illuminate a warning light, it was not enough to power the transmission shift solenoids and compressor clutch. Finding a hot wire under the dash can be a breeze; It looks good but it just creates a new problem.

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

Now more than ever, technicians need to review wiring and circuit information before using a circuit to power other equipment. Manufacturers usually provide additional fuse ports to accommodate electrical accessories. So the conclusion is this: the power in a wire can be crazy power. Make sure this is the correct wattage to be used for the accessories.

Additional tips for repairing and maintaining Class 4-8 trucks can be found in the Mitchell 1 ShopConnection Truck blog.


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