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Ground Circuits: Part 11

Measuring ground side voltage
Thursday, July 19, 2018 - 07:00
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What if the ground strap is missing?
Sometimes a technician may forget to reinstall the ground strap that was removed during the repair process. Since the vehicle seems to work okay technician may think the ground strap wasn’t very important in the first place. Big mistake! In Figure G09V the ground strap is missing.

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Fig. G09V Ground strap is missing

If Sheet Metal (2) is not a good 0.0xV ground
The missing ground strap has caused a 3.19V drop between sheet metal (1) and sheet metal (2). As we saw in a previous illustration a good ground voltage on sheet metal (2) would be about 0.04V. This high ground voltage at G300 will adversely affect the operation of lamp circuit #2. The lamp would be a little dim. If the lamp was a DC motor the DC motor would be running a little slow and cause the DC motor two draw a higher current. If the lamp was a solenoid it would be very sluggish and may not be able to do it’s job. If the lamp was a relay and the ground was the ground for the relay coil the relay may or may not operate. If the ground was for the circuit connected to the relay contacts the circuit would perform poorly (lamp dim, DC motor slow RPM, solenoid sluggish).

Since we have a significant voltage drop difference of 3.19V it indicates sheet metal (2) is providing a poor ground for G300 through some other circuit(s) on the vehicle. The remedy to fix his vehicle would be to inspect and clean the ground strap. Then confirm repair with a good voltage drop.

If the ground strap is missing
If the DMM reading is approximately 14 V it indicates that sheet metal (2) is not grounded and the voltage on sheet metal (2) is B+. The lamp would be out. All circuits connected to sheet metal (2) would not function. Since the engine is running, the B+ is the same as the charging voltage produced by the generator. Sheet metal (2) depends entirely on the ground strap to be connected to the ground circuit.

If you encounter a vehicle with a missing ground strap because you can see the bolt holes in the sheet metal where a ground strap would be connected, perform the voltage drop measurement as illustrated in Figure G09V.

If the reading is 0.0x V the missing ground strap has not affected circuit operation of all circuits connected to sheet metal (2). However, I have seen vehicles that seem to have no electrical problems that had a history of ball joint failures because of a missing ground strap. The ball joints were providing the ground circuit for some sheet metal (2).

I have also seen vehicles with the floor shift between the two front seats create sparks as the driver shifted gears. The complaint is: “When I shift gears I see sparks coming from the gear shift lever.” A missing ground strap on the vehicle was the reason.

In conclusion
If you know this vehicle is supposed to have a ground strap between two pieces of sheet metal and it’s missing, I suggest you install a ground strap even if it seems to make no difference in a vehicle’s electrical performance. It may prevent problems from developing in the vehicle’s electrical system.
Be careful if you add your own ground strap because you might re-channel ground electron current through engine bearings or the water jacket of the engine which could cause mechanical failures down the road.

More on ground voltage measurements next time.

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