What Is a Digital Multimeter? (And Why RVers Need One)

How you use electricity in an RV differs from what you’re accustomed to at home. RV electrical systems aren’t complex, but troubleshooting them is nearly impossible unless you understand how to use a digital multimeter to test different aspects of your DC and AC power systems. 

We’ll cover what a digital multimeter is, its most common applications, and why it is such an important electrical troubleshooting tool for RVers.

What Is a Digital Multimeter?

As an electrical diagnostic tool, a digital multimeter measures and verifies the three factors of Ohm’s Law: voltage (volts), current (amps), and resistance (ohms) of an electronic device.

A digital multimeter (DMM) has probes, a clamp, or leads that insert into the multimeter’s input and then connect to the device being tested to take measurements. Electrical engineers and RV technicians use these devices daily when working on electrical systems to verify the state of a system or circuit for safety.

Nowadays, digital models have replaced analog meters by combining the functions of a voltmeter, ammeter, and ohmmeter. There are several types, but most RVers use a general-purpose handheld unit. 

Why RVers Need a Digital Multimeter

A digital multimeter is an important tool for any troubleshooting of your RV’s electrical system, such as: 

• Testing battery charge levels
• Testing electrical outlets
• Testing converter/inverter input and output
• Testing circuit breaker input/output
• Testing incoming AC power from a campground shore pedestal

Using a digital multimeter allows RVers to verify whether a system, circuit, or device is safe to work on before conducting other troubleshooting methods. It also gives you important data to reference when interacting with other RVers in forums or contacting an RV service specialist to troubleshoot an issue over the phone. 

How To Use a Digital Multimeter

Effectively using a digital multimeter requires an understanding of what DMM setting to use when testing certain RV devices. That’s why we’ve broken down the basics of volts, current, and amperage below and then laid out different multimeter use cases for RVers. 

However, let’s begin with the basic steps of using a digital multimeter: 

1. Check the digital multimeter and device under test for physical damage.
2. Connect the appropriate probes, leads, or clamp to the color-coded inputs of your digital multimeter.
3. Configure your digital multimeter to the desired setting: resistance, voltage, or current. (Reference the graphic above).
4. Verify your DMM’s functionality by testing it with a known voltage source.
5. Place the probes, leads, or clamps on the positive and negative terminals of the device under test to take a measurement.
6. Watch the digital multimeter’s display screen for safety warnings while working.

Understanding Digital Multimeter Readings

You need to understand your RV’s electrical system to effectively use a digital multimeter and accurately interpret its readings. This interpretation stage is where many RVers run into trouble. 

Knowing the approximate value you hope to achieve makes the meter much easier to use. For example, if you are reading AC voltage on an outlet, you would expect to see something around 120 volts, give or take 10% with “normal” operation. So, if you see 11.9, you may have your meter set to DC volts.

So, let’s offer some helpful definitions: 

Voltage (volts) = Electrical pressure between two points in a circuit 

Also known as electromotive force (EMF), voltage is the force that causes current to flow. An example is your RV battery. It stores “boxed voltage” (i.e. energy ready to be used, but without anywhere to go)

Current (amps) = Movement of free electrons in the same general direction along the wire

Current flow measures the number of electrons that flow past a given point in a given time period. Amperage measures the quantity of electrons that flow through a conductor (aka the wire). An example is the electrical service – 50 or 30 Amp – provided by a campground power pedestal or the power provided by a portable RV generator.

Resistance (ohms) = Opposition to electron movement

As resistance decreases, electron flow increases. As resistance increases, electron flow decreases. With more resistance, voltage drops. An example is the electrical wiring in your RV.

The “Magic Triangle”

If you know two of these values, you can find the third using the “Magic Triangle”:

To use the Triangle, you must understand the appropriate letter definitions and applicable equations below: 

Digital Multimeter Use Case Examples for RVers

The remainder of this guide explores how to use a digital multimeter for RV troubleshooting. Each potential test includes a description of why you might want to conduct that test and its steps. 

Setting Your DMM to DC Voltage

Testing battery charge and/or output

Why

• To check the battery charge level
• To check if the battery is good or needs to be replaced
• To check if solar panels are successfully charging the battery

How

1. Place the positive probe on the battery’s positive terminal
2. Place the negative probe on the battery’s negative terminal
3. Fully charged lead acid should register 12.4-12.8 VDC
4. When lead acid charge falls below 11.6VDC, 12V electrical devices begin to fail.
5. Lithium-ion batteries can reach up to 14.1VDC.
6. Lithium iron phosphate (LiFePO4) batteries can reach up to 14.6 VDC

Testing converter output

Why

• To check if the converter is good/bad
• 12V devices aren’t working when connected to shore power (in other words, your RV’s electrical system isn’t effectively converting 120VAC to 12VDC for devices that require it)

How

1. Disconnect battery cables from the coach battery.
2. Plug your RV into a reliable AC power source.
3. Set your volt meter to the Volts DC setting.
4. Place the meter’s probes on the disconnected battery cables. Red probe to the positive battery wire. Black probe to the negative battery wire. Ensure good connections to the cables.
5. Voltage should read between 13.6 and 14.4 volts DC if the converter functions correctly.

If the output voltage reads 0.0 volts DC or the battery isn’t charging:

• If equipped, ensure the battery disconnect switch is in the normal use position.
• Check for an open inline fuse in the battery wire circuit.
• Check for loose wiring connections.

Testing that the tow vehicle is charging the RV battery through a 7-pin connector

Why

• Ensure the tow vehicle is charging batteries while connected
• If a dead RV battery doesn’t recharge when towing

How

1. Start your tow vehicle
2. Place the negative probe on the ground charge lead in the tow vehicle’s plug
3. Place the positive probe on the battery power lead
4. Verify voltage to the plug (if no voltage, check the fuses under your vehicle’s hood)
5. If voltage, plug the 7-pin connector into your tow vehicle.
6. Place the negative probe on your battery’s negative terminal
7. Place the positive probe on your battery’s positive terminal
8. Voltage should read at or above 12.8VDC. If lower, further troubleshooting is needed.

Here’s a 7-pin diagram for reference: 

Other potential use cases where you’ll need to set your DMM to DC voltage include testing inverter input, solar panel and charge controller output, and roof vent fan input.

Setting Your DMM to AC Voltage 

Testing interior or exterior electrical outlets

Why

• To check the operation of GFCI and non-GFCI outlets
• To confirm/refute if a given AC RV appliance may need replacement (i.e., your microwave stops working, you can verify power to the outlet it’s plugged into to check if the outlet may be an issue before deciding to remove and replace the microwave)

How

1. RV must be connected to an AC power source (generator/shore power)
2. Place negative probe in outlet ground (bottom half-moon shaped)
3. Place positive probe in hot line (smaller of the two vertical slots)

A good outlet should read 120VAC (+/- 10%). The National Electrical Code (NEC) states that 114 volts is the lowest acceptable operating voltage.

Testing incoming AC voltage to an inverter

Why

• To ensure lack of AC voltage isn’t an issue causing your inverter to malfunction

How

Some RV inverters are equipped with a dropout setting if the incoming AC voltage is lower than the programmed threshold (80 volts AC is standard on many units.)

To test:

1. Set DMM to the volts AC setting.
2. Connect meter probes to AC neutral and lead wires. Red probe to lead wire. Black probe to neutral wire.
3. Ensure voltage is present and above your inverter’s volts AC dropout level.

Testing shore power output

Why

• To confirm reliable shore power before plugging in your surge protector and RV
• To eliminate a shore power issue as a possible cause of the lack of AC appliance function inside your RV

How For 30 Amp

Outlet leads (i.e., on the pedestal outlet):

• Upper = Ground
• Bottom left = Hot
• Bottom right = Neutral

1. Place negative probe in ground terminal
2. Place positive probe in hot terminal
3. Measurement should be 120VAC (+/- 10%)
4. Ensure the breaker for the 30 amp outlet is On before testing
5. Probes can be swapped without impacting the effectiveness of this test

How For 50 Amp

Pedestal outlet leads: 

• Upper = Ground
• Bottom = Neutral
• Left = One Leg of 120VAC
• Right = Second Lef of 120VAC

1. Place one probe in the ground and the second probe in the right lead. It should measure 120VAC (+/- 10%).
2. Leave one probe in the ground and place the second probe in the left lead. It should measure 120VAC (+/- 10%).

The test can be done with the first probe placed in neutral and should yield the same results on a good outlet.

Testing shore power for reverse polarity

Why

• Check a new shore power source for safety before plugging in a surge protector and your RV.
• We still recommend using a surge protector, but this test can help you avoid damaging a surge protector if the power at the pedestal has been wired incorrectly.

How For 30 Amp

1. Place one probe in the ground (upper outlet opening)
2. Place the other probe in the neutral (lower right opening)
3. Reading should be 0.0VAC
4. A reading other than 0.0 indicates reverse polarity and is a sign you should NOT plug into that outlet

How For 50 Amp

1. Place one probe in the ground (upper outlet opening)
2. Place the second probe in the neutral (bottom outlet opening)
3. Reading should be 0.0VAC
4. A reading other than 0.0 indicates reverse polarity and is a sign you should NOT plug into that outlet

Setting DMM to Continuity

Testing whether your shore power cord has gone bad

Why

• To check a shore cord is good before placing it into use
• To ensure it hasn’t been damaged while your RV has been in storage
• To eliminate it as a potential issue when troubleshooting the lack of AC power in your RV (after ensuring power from the pedestal measures as expected)

How

• Ensure both ends of the shore power cord are disconnected
• Test each leg for continuity (ground, hot, and neutral)
• Test ground by placing one probe in the ground on the cord’s female end and the other probe on the ground lead on the male end.
• Repeat for hot and neutral (identifying hot insert/hot lead on female/male ends and doing the same for neutral on female/male ends)
• You can also do this test using the ohms setting on your DMM 
• All ohm readings should be less than 1.0.

Setting DMM to Resistance

Testing the heating element for a water heater

Why

• Your water heater doesn’t heat water on the electric option
• It only heats using propane

How

1. Turn off your RV’s 120VAC electrical system
2. Place positive and negative probes on the metal plates underneath the screw heads securing the heating element
3. Reading should fall between 9.6 and 10 ohms, depending on the element’s total wattage
4. Readings that fall more than 10% out of this range call for a heating element replacement. 
5. A trained RV technician should be contacted for this replacement.

Testing the heating element for a refrigerator

Why

• Your refrigerator isn’t cooling on AC or DC power
• It only cools when running on propane

How

1. Find the manufacturer’s specification for acceptable resistance (ohms) measurement in your owner’s manual (based on refrigerator make/model)
2. Disconnect refrigerator from RV’s 120VAC electrical system
3. Disconnect the heating element from the refrigerator control board
4. Place probes on the heating element leads
5. Measure ohms reading
6. A reading within 10% of the manual’s suggested resistance measurement is considered acceptable

Other examples include testing a refrigerator thermistor or a simple DC switch. If you’re ever confused or uncertain about working on electrical components in your RV, the safest option is to schedule a service appointment with an RV technician at your local Camping World. Remember that electrical shock can be lethal.

---

Here are a few more helpful electrical troubleshooting resources. Remember that you can always schedule a diagnostic service with our team of certified RV technicians to identify the issue and resolve it quickly so you can get back to enjoying your RV lifestyle. 

• Troubleshooting RV Electrical Systems
• Troubleshooting RV Converters and RV Inverters
• How To Troubleshoot Your RV Electrical Outlets

What questions do you have about digital multimeters for RV use? Let us know in the comments below.

The information provided in this blog is for general informational purposes only and not intended to take the place of professional service providers. While we strive to provide accurate and up-to-date information, we make no representations or warranties of any kind, about the completeness, accuracy, reliability, suitability, or availability with respect to the information, products, services, or advice contained on the blog for any purpose. Any reliance you place on such information is therefore strictly at your own risk. In no event will we be liable for any loss or damage including without limitation, indirect or consequential loss or damage, or any loss or damage whatsoever arising from loss of data or profits arising out of, or in connection with, the use of this blog.