Ground Voltage and the DMM
Vd (Voltage drops) on the ground side of a circuit are very small. Using a DMM (Digital MultiMeter) allows measurements as fine as 0.01 volt which is 10 mV when using the 20 V range on a DMM. The 20V range simply indicates the highest voltage the DMM can indicate without automatically selecting (autoranging) to the next higher range if the measured voltage exceeds 20 V.
Which DMM do you have?
Some DMMs are manually range selected. That means you have to manually dial the voltage, amp (current) or resistance range you want to use. Some DMM’s are autoranging which means it automatically selects the proper voltage, amp or resistance range once the circuit is being measured. The autoranging feature requires 2-3 seconds to change the range which makes a series of quick measurements a little longer to accomplish if you are in a rush.
The first thing to know about your DMM is the voltage ranges available. Some DMMs have 2V-20V-200V ranges which is fine for 14 V systems. DMM’s that have 3V-30V-300V voltage ranges are better for 24V electrical systems because 24 V fits in the 30 V range. DMMs that have 4V-40V-400V voltage ranges are best, especially for hybrid vehicles with high-voltage battery packs that can be over 300 V so you need the 400 V range. Look in your owner’s manual to learn what voltage ranges are available with your DMM. Some also have a 1,000V DC range.
You can read ground side voltage drops regardless of the upper voltage range of the DMM. In other words, 0.10 is still 100 mV regardless if your DMM’s upper limit is 20 V, 30 V or 40 V.
- 0.01 = 10 mV (a very good ground voltage drop)
- 0.05 = 50 mV (maximum ground side voltage drop for an on-board computer)
- 0.10 = 100 mV (SAE considers an acceptable good ground voltage drop)
The second thing to know about your autoranging DMM is what voltage range does the DMM go to when first turned ON to DCV. Some autoranging DMM’s begin on the millivolts range 00.0 which isn’t a very useful range. The mV range may be unstable and fluctuate too much on most DMM’s to be usable.
A DMM may begin on the 2 V, 3 V or 4 V range depending on the DMM. This low voltage range is identified as .000 in the display. On this range the DMMs indicates-
- .001 = 1 mV (a really good ground Vd)
- .010 = 10 mV (a very good ground Vd)
- .050 = 50 mV (maximum ground Vd for an on-board computer ground)
- .100 = 100 mV (SAE considers an acceptable good ground voltage drop)
For measuring critical ground side voltages, the 2 V, 3 V or 4 V range is ideal. However if you accidentally probe a 12 V circuit the DMM will blink and automatically move to the 20 V, 30 V, or 40 V range which is the next highest range. The display may briefly indicate 0.00 then abruptly change to indicate the higher voltage. The decimal point moves to the right one digit. This can be confusing and demands you pay close attention to the DMM reading as the decimal point changes position. Most DMM’s change the voltage range in 2 to 3 seconds which can slow down voltage measurements when you want to go faster tracing voltage through a circuit.
Avoid DMM confusion – Cancel autoranging
I recommend in all my electrical classes that students using an autoranging DMM Press the RANGE Button the number of times it takes for the readout to first indicate 0.00.
This will lock the DMM on the 20 V, 30 V or 40 V range whichever your DMM offers. This will eliminate the DMM changing ranges and keep the decimal point from moving.
Ground side Vds (voltage drops) will look like this:
- 0.01 = 10 mV (a pretty good ground Vd reading)
- 0.05 = 50 mV (maximum ground voltage drop for on-board computer ground)
- 0.10 = 100 mv (considered by ASE to be a good ground circuit)
- 0.20-0.30 mV (ground circuit beginning to experience corrosion)
Power down the vehicle, disassemble the ground connection, clean all metal surfaces and reassemble the ground connection. Turn the circuit back ON and verify a good ground voltage drop.
What to do if cleaning the Ground Connection doesn’t help
The ground wire Vd (voltage drop) remains greater than 0.10 and you have just cleaned and reassembled the ground connection - DO THIS.
1. Inspect the ground wire looking for damaged wire. Replace damaged wire.
NOTE: If replacing a ground wire make sure the replacement wire is the same diameter as the original ground wire or larger diameter. Never use a smaller diameter wire. The diameter of the ground wire is determined by the level of electron current that must pass through it. In an effort to reduce overall vehicle weight, the smallest possible wire is used for the ground circuit wire. This presents no problem as long as the electron current through the ground wire does not become excessive.
2. Measure the electron current flowing through the ground wire with a current clamp. Determine why the circuit is drawing too much electron current. If the circuit has a DC motor expect low RPM due to defective motor bearings or restrictions to airflow.
According to Ohm’s Law for “Vds” (Voltage drops)
E = I x R
E (Volts) = Amps x Resistance in ohms
E (Voltage drop) = Amps through the wire X Ohms of Resistance of the wire
If E (voltage), so remains constant and the AMPS INCREASE and (wire) Resistance stays the same - the ground wire Vd will increase. Notice what happens as current increases.
- E = 1.0 A x .05 ohm = 0.05 V (excellent ground Vd with normal electron current)
- E = 2.0 A x .05 ohm = 0.10 V (acceptable ground Vd as electron current increases)
- E = 5.0 A x .05 ohm = 0.25 V (ground Vd starts to increase due to higher amps)
- E = 9.5 A x .05 ohm = 0.48 V (bad ground Vd due to high amps - not bad wire)
As electron current increases through the same amount of resistance in the ground wire (resistance stays the same), the Vd of the ground circuit increases.
Solution: Find the cause of higher than normal electron current. Hint: DC Motor (blower motor) running at lower RPM (motor dragging) causing higher electron current.
Measure Accessory Ground Vd (Voltage drop)
In Figure G06V the engine is running. The generator is the voltage source which means the generator is providing the electron current to operate all electrical circuits. During this time the battery is recharging and is “off-line.” The -BATT terminal is used for the DMM’s 0.00 V ground point. We have confirmed there is no voltage drop between -BATT (B-) and -GEN (B-). The -BATT terminal is easier to access than -GEN on this vehicle.
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.”
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.
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.
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.
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.
