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.
|Want more ? Enjoy a free subscription to Motor Age magazine to get the latest news in service repair. Click here to start you subscription today.|
- 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.
|Fig. G06V Validating accessory ground sheet metal|