MOTOR Magazine

A MOTOR Magazine Newsletter
December 19, 2016

Contributed by Bob Chabot
Parasitic Drain: Back to Electricity Basics

Cars always win unless you’re well-grounded

A strong foundation in fundamentals — such as basic electricity, the ability to navigate wiring diagrams, a systematic approach to diagnostics and testing and voltage drop testing — allows you to build the necessary skillset to properly approach and resolve electrical problems. The basics and how you apply them have become an industry focus, as the proliferation of software, electronics and networking in modern and emerging vehicles make proficiency a differentiator for shop profits and technician earnings.

"Use authentic wiring diagrams to help locate and remedy electrical circuit issues," Orlando emphasized. "Wiring schematics indicate all the components in the circuit required for proper electrical operation. Taking a few minutes to study the diagrams can help you determine what it can and cannot be. They're a useful resource to pre-diagnose the electrical circuit in question before the actual car comes in the bay." (All images — Peter Orlando)

Use a Systems-Based Forensic Diagnostic Approach
It’s one thing to "book-know" the fundamentals, but quite another thing to put book knowledge to work. "A technician's foundation in fundamentals must be strong," agreed Jim Morton, an industry educator, aftermarket trainer, expert diagnostician and owner of Morton's Automotive Technical Services (MATS). "But it doesn't end there."

"Shops and technicians must also continually invest in updating and applying their skillset, if they want to be more effective, efficient and earn more. Most of the time I've visited shops as a mobile diagnostician, they wouldn’t have needed me if they had technician(s) who knew and applied fundamentals. But they paid me big bucks to be there, a high labor rate predicated on my personal ongoing investment in those fundamentals and updating related skills."

"Technicians need to know ‘why’ before you decide ‘how’ to approach a vehicle," noted Peter Orlando, founder of Automotive Training Technologies LLC. "They must understand electrical basics, research vehicles before they start working on them, perform simple definitive checks and follow OEM procedures; otherwise they can create man-made problems."

"It's critical to follow a sound diagnostic plan and process,” advised James E. Wilson, an expert diagnostician and Bosch USA trainer. “Key steps in this process include verifying the complaint, researching the known issues, creating a prioritized plan to test the system, leveraging the basics when performing system tests and verifying your work before returning the vehicle to your customer.”

Here are two different ways to test voltage drop. Method #1 is easier to understand, but it requires the technician to do the math. Method #2 is easier to use, but the meter readings can be confusing to those new to this method of testing, so expect a learning curve.

Method #1 requires the technician to place the digital multimeter's (DMM) negative lead on the negative battery post and kept there, so the battery can be referenced while testing the entire circuit path. Leave the key and load turned on. Then take three system voltage measurements: (1) At the battery; (2) As close as possible to the positive side of the load as you can get; and (3) As close as possible to the ground side of the load as you can get. If everything is working correctly, then nearly all the voltage supplied should be used by the load, so expect a readings close to 0.0V to 0.5V, unless there are other sources of resistance present.

Method #2 is easier to use and takes the changing voltage factor out of the way, but digital multimeter (DMM) readings can be confusing to newcomers. The only differences are lead placement, the expected values you'll get, and how you test the positive side of the circuit to let the DMM do the math for you. Instead of referencing at the negative battery post, place the negative meter lead on the positive battery post. The meter reads the voltage potential between its leads and is going to be the same as you just learned about on the ground side in Method #1. However, any significant voltage reading here means there is a problem between the battery positive and the last point of measurement.

Voltage Drop Testing is No Longer Optional
"Be sure that you are addressing the root cause and not the effect," explained Morton. "That's why I recommend technicians perform 'general' tests before 'specific' tests. I've watched people everyday waste time and throw parts at a vehicle because they jumped to the conclusion that a service issue is mechanical in nature. That's futile and expensive, should the root cause of a problem be electrical."

The experts agreed that technicians who cannot correctly perform voltage drop tests (which are based on electrical fundamentals) correctly on vehicles today are just not going to be able to resolve electrical problems. Understanding how voltage is supposed to drop in an electrical circuit makes voltage drop testing a powerful general test for locating other sources of unwanted resistance that shouldn’t be there.

Here are some basic guidelines they shared to help interpret voltage drop test results:

  • A measurement of 0.0V on the positive side of the load indicates an open circuit between battery and load.
  • If the positive side of the load reading is good, but the ground side of the load reads 0.0V, there is an open between the test points and the load.
  • If both readings are good, the electrical circuit is fine, but there is a mechanical fault with the load.
  • If the positive and negative measurements are the same as source, either the circuit is not "on" or there is an open on the ground side.
  • If the positive side measurement is less than source, there is unwanted resistance on the positive side of circuit.
  • If the measurement on the ground side of the load is over 0.5V, there is unwanted resistance on the negative side of the circuit.

Parasitic drains are becoming more common than they used to be, in large part due to the increasing number of electronic control modules (ECUs), relays, switches and other electronics in vehicles today. Just one can take a vehicle down, so being capable of tracking down parasitic drains is another essential applied skill.

Parasitic Drain Testing Can Troubleshoot “Current Over Time” Issues
To catch a small parasitic drain caused by an ECU that won't go offline, you need to use a systematic approach. For example, mobile a/c systems today feature many ECUs that manage many relay-controlled circuits, such as compressors. Current testing can isolate where a problem lies in a compressor’s relay controlled circuit. First, identify which fuse(s) power the two sides of the relay. Second, use a fused jumper wire in place of the fuse and connect your low amp clamp. Third, operate the component you want to test.

Only one of three readings can result:

  • No current flow — Relay controlled components rely on a switch or driver to close the circuit that energizes the relay. If an ECU is acting as the switch, then typically there are criteria for turning the switch on. No current flow means there is either a problem with the ECU driver or one of the required inputs is missing.
  • Current less than 0.5 amps — A current reading of 0.5 amps or less is telling you that the solenoid in the relay is energized. That must mean the ECU driver is working and that all required inputs (like sufficient refrigerant) are there.
  • Current less than 5.0 amps — This number is for most a/c compressors and has to be adjusted for other relay-controlled devices. The reading indicates that, electrically, all is good and the problem is mechanical.

The good news is that shops and technicians do not have to be Einstein or an electrical engineer to be successful at solving electrical problems. What's needed is a mastery of fundamentals along with competency in associated testing. Without them, adjusting to new inbound technologies will be increasingly difficult. Commit to them and your competitive edge will be enhanced.

[Editor's note: Visit for the latest diagnostic and service insights.]

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