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Troubleshooting Article Archive:  May 2005          
Import Chassis Converter Relays
The use of import chassis for tow trucks causes two special equipment troubleshooting challenges:  taillight converters and equipment mounting.  I'll talk about taillight converters in this issue, and equipment mounting in my next column.  

When you use an import chassis with a domestic tow truck bed, you must use a taillight converter to make the lights work properly.  Domestic beds have taillights in which the same bulb element serves the dual purpose of turn signal and brake light.  Import chassis, on the other hand, are made to operate two separate bulb elements, one for brake lights and one for turn signals.  The taillight converter, which is in the shape of a small box, contains electronics which take in current on five wires from the import chassis and sends it out on four wires to the domestic bed.  (A few converters take in current on four wires and send out on three, leaving out the parking lights, as import/domestic issues don't affect the parking light circuit.)

Like all electronics, taillight converters sometimes need to be replaced.  The important lesson to remember about taillight converters is when you replace one, you need to make sure that the new one is equipped with relays.  The converter needs relays because it was designed to operate only one or two turn signal/brake lights, and most tow trucks beyond the dually pickup type have at least three sets of turn signal/brake lights:  one set at the back of the bed, another on the light bar pylon, and a third in the tow lights.  Add duplicate lights on the pylon, and duplicate lights at the back of the bed -- some tow trucks have six or more sets of lights -- which consume too much current to go through the converter box, and the box burns out, as is shown in the photograph.  

Wire the relays on the load side ("downstream") of the taillight converter to ensure the heavy current goes through the heavy side of the relays and that only the light current to run the relays goes through the converter.