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Ground Circuits: Part 11

Measuring ground side voltage
Thursday, July 19, 2018 - 07:00
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From our previous discussion we see the importance of the accessory ground cable in the proper functioning of each electrical circuit. All electron current supplied to every vehicle circuit grounded to sheet metal passes through the accessory ground cable on this vehicle when you see two -BATT cables. Understanding how electron current flows through the ground circuit reveals the importance of various cables and connections on the ground side of the vehicle’s electrical system.

Notice that the red DMM test lead is touching the sheet metal at some point. It does not have to touch the sheet metal at the exact location of G101. Making good electrical contact with the red test lead at any point on the sheet metal will provide a valid accessory ground cable voltage drop reading. From our illustration, the voltage drop of the accessory ground during engine RUN and all circuits turned ON to maximize the electron current through the sheet metal ground, results in a voltage drop of 0.03 V. This confirms the sheet metal ground is good.

Once the engine is running and all circuits are turned ON this critical voltage drop test takes about 10 seconds to complete and confirms the critical accessory ground cable is performing properly.

What causes this voltage drop?
As electrons flow through a wire and bounce from atom to atom, a force similar to friction causes some electrical energy to be released. We can place the accessory ground cable between the test leads DMM and read the accumulated voltage consumed by the electrons bouncing along through the wire. We call this voltage reading a voltage drop and abbreviate the term voltage drop as ”Vd.”

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A reading that is too high is most likely due to corrosion where the wire connects to the vehicle at either end (-BATT terminal or G101). If corrosion is not the problem we have proposed that a circuit may be drawing too much electron current exceeding the current handling capability of the wire size used for the accessory ground.

Remember Ground Side Vds

  • 0.01 = 10 mV (a very good ground Vd reading for electrical circuits)
  • 0.05 = 50 mV (maximum ground side voltage drop for on-board computer ground)
  • 0.10 = 100 mv (considered by ASE to be a good electrical ground circuit) 0.20-0.30 mV (ground circuit beginning to experience corrosion)

In Figure G07V below the DMM red test lead is moved to sheet metal (2) to test the integrity of sheet metal (2) as a good ground circuit and the quality of the ground strap.

Fig. G07V Validating Accessory Ground Sheet Metal

The reading obtained in Figure G07V of 0.04 V confirms that sheet metal (2) is providing a good ground circuit for all electrical circuits connected to sheet metal (2). This measurement also confirms the ground strap provides an adequate path for ground electron current between sheet metal (1) and sheet metal (2). Notice that testing sheet metal (2) to provide a good ground does not require testing the sheet metal at G300. The red test lead can touch any point on sheet metal (2) and produce a valid Vd reading.

We also conclude that the ground strap is dropping 0.01 V because sheet metal (2) is reading 0.01 V higher than sheet metal (1).

About Ground Straps
The proper functioning of this electrical system is dependent on a good ground strap and the connections at either end. Whenever you see an original ground strap on a vehicle know that it plays a vital part in the proper operation of the vehicle’s electrical system. Below in Figure G08V the proper method for checking the quality of the ground strap confirms it is in good condition.

Fig. G08V good ground strap Vd


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