Simple procedures for identifying electrical wiring problems will help keep appliances and accessories running smoothly
There is nothing more frustrating to a motorhome owner than having the 12-volt DC equipment such as headlights, turn signals and clearance lights fail when you want to be on the road again. It is equally problematic when hooked up at a campground and the furnace, coach lights or DSI water heater doesn’t function. Often, a common source for this device or light failure is a faulty ground connection. Low-voltage motorhome wiring usually relies on connections to the vehicle’s metal framework to provide the path from the battery to complete the circuit.One clue the problem may be related to a ground connection failure is when the light bulb doesn’t light but tests OK with an ohmmeter and 12-volt DC power is present in the socket. This is a sign that a to-ground connection may be faulty. These ground faults occur naturally at points of connection because any time two different metals come in contact with each other in the presence of moisture, corrosion occurs, increasing the resistance of the connection until there is virtually no connection at all. Some environments we love to visit with our motorhomes become a contributing factor to corrosion in DC electrical connections. Parking by the ocean, lakes, ponds and streams provides a nonstop source of water vapor that condenses on metal framework in the motorhome. This condensation on chassis ground connections provides the moisture that acts as an electrolyte resulting in corrosion of the connection. A multimeter (volt-ohm-amp meter) is the best test tool to easily check for faulty ground connections.
A motorhome may often have two circuits to control power to one device: one light-duty wire through a switch powering a relay and the relay in turn controlling heavy-duty wires to power the load. Examples include the slideout motors in the coach and the starter motor on the chassis engine.
All direct-current circuits are fairly simple and conform to the simple diagram or pattern shown in photo 6. The same steps are followed to troubleshoot any light, device, motor or appliance. To visualize connections to a variety of loads merely substitute the light bulb shown in the diagram with whatever light, appliance, relay, motor or device is failing. The colors used for ground wires in 12-volt DC electrical systems will typically be white, black or green.
Using electron flow notation the actual moving electrons in a DC circuit flow from the negative post (smaller post) of the battery where a surplus of electrons are found along the vehicle frame and then through a ground wire to the load (light bulb, fan, appliance or electric device). Then, they pass through an on/off switch or relay along the positive wire to a fuse on the fuse block, and then to the positive post on the battery where a shortage of electrons exist.
Fortunately both the chassis and the coach DC wiring share a common ground point, the motorhome chassis frame and anything metal physically connected or wired to it. The chassis battery and the coach batteries are both connected to the metal framework.
Troubleshooting for Poor or Lost Grounds
Checking for the most obvious is often overlooked when something fails. Ask these questions as you begin your DC troubleshooting.
Has only one light or appliance failed? If not, then the battery cables or battery discharge is the cause.
- Is the device’s switch or main switch turned on? If no, make sure the device is turned on or battery disconnects are set to power on.
- If it is a plug-in device, is it plugged in properly? If no, push in the power (cigarette lighter) plug or check for a built-in fuse that failed.
- Is the fuse OK or is the circuit breaker on? (Not tripped) If no, change the fuse or reset the breaker.
- Do other similar lights or devices that share the same fuse still work? If no, a branch wire, main fuse or fusible link failed.
- Do I know or can I find out where the switches, fuses, relays and wires are located? If no, find a diagram or check where the wires run.
A multimeter (volt/ohm/amp meter) is an essential tool for troubleshooting DC circuits. Along with the meter, a long wire with alligator clips on both ends to use for testing the ground connections is needed. One piece of 14-gauge stranded wire like the one shown with the meter in photo 3 will handle a 2-amp load up to 42 feet, so it is of sufficient size to use with the meter for checking lost ground connections.
Use the voltmeter feature to verify that the battery or bank of two 6-volt batteries is charged and tests at 12-plus volts. Check the battery negative (-) post-to-cable connection by using the ohmmeter function of the meter (on the X1 or times one scale) with one lead on the battery post and the other on the cable connector; the reading should be very close to zero ohms. If near zero ohms, the next test would be the cable-to-chassis ground connection.
Place the alligator clip on the test wire on the negative post and the other clip on the other meter probe, then follow the heavy negative battery cable-to-ground connection on the chassis. (An example of a cable-to-frame connection is shown in photo 5. The second probe is touched on a bare part of the frame to test the resistance from the battery post to the frame. Here, readings should be 1 to 3 ohms because of the cable and test wire resistance. Readings higher than that require attention by cleaning the posts and reconnecting the cable connections. With the battery voltage and the basic battery-to-frame ground connection verified as OK the troubleshooting steps move to whatever light or device failed. Connect the alligator-clipped test wire to the negative probe on the meter and reset the meter for testing DC volts on a scale near 12 volts. Connect the other clip end of the test wire to the battery negative post. Make sure the circuit is turned on; for example, let’s say it is the running light switch because one of the running lights has failed. With a firm ground connection from the meter to the negative post, you can now begin checking for 12-volt DC readings at all connection points along the circuit.
The running light bulb is removed from the socket. The red positive test probe is touched to the connection points and
a 12-volt DC reading was achieved indicating a ground connection failure for our example. It is quickly verified by connecting one alligator clip to the battery negative post or shiny metal on the frame and touching the base of the light socket with the other clip after reinstalling the light bulb. The bulb should light.
If a 12-volt DC reading was not found at the light socket, the next test would be at the switch. If a 12-volt DC reading is not achieved at the switch (when turned on), move to the next step.
Test the circuit fuse. If you have a 12-volt DC reading on one side and not the other, the fuse has failed. If a 12-volt DC reading is not present on both sides, trace the fuse
block supply wire back to its source connection to find the fault, working your way along the supply wiring right back to the positive post on the battery, if necessary.
This same set of steps, in order, would be used to test any light or device failure. Once the failure point is discovered make the repair to fix the problem and eliminate the probability of similar failures in the future. Clean and reattach the connections and replace any damaged section of wiring.
Preventive MeasuresNow, let’s look at some things that can be done to reduce the risk of poor ground connections.
- At least annually, clean all connection cables with a wire brush to ensure a firm metal-to-metal mechanical connection at all battery joints as shown in photo 9.
- Clean the battery posts with a wire brush as well as shown in photo 10.
- Use an anti-corrosion pad at all battery cable connection points as shown in photo 11.
- Use an anti-corrosion coating such as the one shown in photo 12. Spray on all exposed battery and ground connections following the instructionson the can and renew the spray every 90 days.
- Should you find yourself cleaning and repairing to-chassis ground connections, use anti-corrosion spray there also, covering an area about 1.5 inches in diameter around the point of connection.
The above maintenance steps will help keep your rig well-grounded, allowing you to enjoy your travels.
Battery BasicsA charged battery works to provide electrical power because the chemical actions inside the batteries cause the anode (negative plates) to collect an excess of electrons and cause the cathode (positive plates) to be short of electrons. When the circuit is connected (turned on), the current flows to restore balance on the plates. When this balance point is nearly reached (about 10.5 volts), a 12-volt battery is said to be discharged and will no longer provide sufficient current to the circuit to power device loads. When a battery is recharged, the electrons are pushed back to create an imbalance and the voltage is again a nominal 12 volts.