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Parasite on Board

By on March 1, 2016

After parking your truck for two or three days, have you ever returned just to learn that it would not crank? It could be a simple case of forgetting to turn off the lights or leaving some accessory on in the cab, or could it be something more sinister? This article focuses on ‘parasitic drains’ or unwanted electrical demands when the ignition is turned off.

Modern trucks usually come well equipped with eight or more computers that operate systems such as your climate control, engine fuel, anti-lock braking, and lighting. Sometimes the list can continue well beyond 10 controllers. This somewhat new phenomenon arrived with potential new problems in tow.

The majority of these computers must go to sleep when the vehicle is not in use. Unfortunately, one or more on-board computers simply may not enter sleep mode, resulting in a continuous electrical demand. This draining of the batteries, while the engine is not running, prevents the alternator from recharging them. As a result, your truck may crank slowly or not at all.

To further complicate this condition, technicians may struggle to locate the source of the problem. It takes a very highly skilled technician to understand the troubleshooting techniques necessary to efficiently isolate the parasitic drain. A technician should use an amp meter with an induction clamp to facilitate the troubleshooting process.

This type of meter can be complicated, but is absolutely necessary. The technician simply isolates one main battery cable coming to the battery box and places the amp clamp around the cable without unhooking the cable. With the key off, the tech can observe the amperage flowing through the cable. Most systems will allow up to .3 amps, but anything over that  likely indicates a problem. Upon determining that the amp reading exceeds the allowable limit, the tech must l
ocate the source of the parasite by disabling possible suspects.

At this point, an experienced technician is needed. Experience will likely direct the tech to any onboard devices such as refrigerators or inverters, which tend to be very needy from an electrical standpoint. The tech can simply remove fuses or breakers from suspect circuits, while observing the amp flow on the meter. When the defective circuit is located, the amps will drop to an acceptable range. This process of elimination is a very effective method for identifying parasitic drains.

Finally, pinpointing these
unwanted electrical demands must be a high priority for any truck owner or operator. If your batteries continue to move from fully charged to a low-charge condition, the life of the battery will be greatly diminished! To add insult to injury, the longer a battery stays in a low-charge condition, the shorter the battery’s life. Identifying theHomerHogg
somewhat elusive electrical parasite can be a challenge, but it must be pursued and conquered, if you are to control your maintenance costs and keep your truck on the road and out of the shop.

Homer Hogg’s Maintenance Matters airs on the Dave Nemo Show (Road Dog Trucking, SiriusXM 146), 8 a.m. ET, the first and third Thursdays of each month.

Homer Hogg, Technical Development Manager for TA and Petro, has worked as a truck technician for more than 30 years. He is ASE Master-certified, a Daimler Certified Trainer, and a member of the Nashville Auto Diesel College Hall of Fame.

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