Testing for Parasitic Draws and Intermittent Electrical Problems

February 3, 2018 | Reply More

It’s best to have a game plan before you start. Before you even break out wiring diagrams or test equipment, it’s a good idea to ask the customer under what circumstances the problem occurs.

Parasitic draws can be the most challenging automotive problems to diagnose. So it’s best to have a game plan before you start. Before you even break out wiring diagrams or test equipment, it’s a good idea to ask the customer under what circumstances the problem occurs. For example, does it only happen when ambient temperature is hot? Or cold? Only when the engine is hot? Or cold? Things like that can help you save time. It’s also a good idea to always check for TSBs before you dive into a problem.

The most important tool you have is not in your toolbox, it’s your senses and your brain. Obviously tasting isn’t a great idea, but your other senses are a good place to start. All diagnostics should start with a thorough visual and audible inspection. For example, don’t waste a lot of time looking for an electrical problem when the floor mats are soaked or there’s aftermarket equipment that might be causing the problem. You may have to pursue that first.

Sometimes the best “second step” is to call the customer and explain the condition. If they don’t want to address underlying issues, you’re done with almost no investment in time. Make sure you are authorized for your time before you start. It’s best to log and document every step you take to justify the bill. Take pictures. Print wiring diagrams from bmwtechinfo.com and write your notes on them. Print scan tool results. Besides helping to justify the bill, documentation makes it easier to return to where you were if you are interrupted.

It’s a “best practice” to perform a “charge/start system test” as part of initial testing. The test includes a battery test, alternator DC and AC voltage tests, alternator loaded output along with the starter current draw test. Any basic electrical system problems need to be repaired before going further. There have been times when computer related problems are actually caused by excessive AC leakage in the charging system corrupting the memories of the computers.

The same is true for induced signals inside a harness, especially a harness that has been modified. Always look for “previous work” and damaged wiring or connections. Maybe it has corrosion or broken wires that affect all or part of the electrical system. Sometimes that is the cause of the parasitic draw and the diagnosis is done. Also, current only flows when a circuit or system is “on” and under load. So testing only voltage available with the system off can cause you to believe everything is good when it is, in fact, breaking down under load. When testing voltage drop to powers and grounds, go to the load first. When testing computers, test at the computer plug.

The other area to address before actually getting to the “problem”

 

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Category: Electrical

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