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Electrical fault finding on a motorhome


Words and photos by Clive Mott

Clive Mott is MMM's electrical guru and spent his working life engineering forklift truck battery technology. He has a gadget-laden A-class that makes the USS Enterprise seem basic. He's part of the team of experts answering motorhome and campervan technical questions in every issue of MMM - click here to buy digital issues of the magazine.


Modern motorhomes are becoming increasingly complicated with more safety and comfort systems being added. As always, the hidden complications are intended to make them easier to use. 

Nearly all motorhomes have two forms of electrical power, the first being from the 230V mains via a hook-up cable and the second from the motorhome’s leisure battery.

Be aware that both can be lethal, although death from 12V is rare; some awful burns can be caused if the battery is shorted out with a spanner, so great care is needed when working with these systems.

If you are unsure, then you should employ the services of a qualified automotive electrician.

Motorhome electrical faults on the mains

The 230V AC supply from the mains is something that you should always leave to the trained experts.

However, there are a few common problems associated with a lack of mains power that can be safely investigated, with a little care.

A motorhome will have an incoming RCCB (residual current circuit breaker), which is a switch that will automatically open if an earth fault is detected.

This works by comparing the amps flowing in the live with the amps flowing in the neutral; these should be identical. If they differ by more than 0.03A then some amps have escaped as earth leakage and the RCCB will switch off, opening both the live and the neutral connections coming into the motorhome.

The mains switch box (fusebox) will also have two or three over-current circuit breakers alongside the RCCB to protect individual mains circuits inside the motorhome.

Frequently, these are located in the back of a cupboard and can sometimes be quite easily knocked and switched off.

Typically one circuit breaker will cover the built-in battery charger, the mains part of the fridge (if it is a three-way gas/12V/230V fridge) and various other accessories.

Another circuit breaker will provide protection to the mains socket outlets inside the motorhome and possibly a third for an air-con system.

So, if your motorhome’s 230V sockets don’t work when you are hooked-up, but everything else does, then it is likely that the circuit breaker has been knocked off. If you have your electric heating on and put on a kettle, you are very likely to trip the breaker in the hook-up point. When these breakers are located behind a locked door it can be an embarrassment.

So, steering away from doing any live mains voltage tests (best left to qualified experts), what can you check after confirming that all the breakers are closed?

Plug a mains-powered piece of equipment into a socket to see if it works – a phone charger or even a toaster. If it doesn't, then borrow an alternative hook-up lead to check whether that is the culprit.

The most useful tool for electrical fault-finding is the digital multimeter. These are inexpensive and there are many to choose from.

This tool can be set to measure voltage (electrical pressure), current in amps (the electric stuff that flows through the wires) and electrical resistance, which is measured in ohms.

Checking for low-resistance is another name for checking for continuity, making sure that one place is joined to another.

A common fault is associated with the hook-up lead. These have a blue 16A plug at one end and a blue 16A socket on the other. In most cases the three individual wires at each end are secured by pinch screws. These screws can – and do – come loose, especially as they get shaken about while the motorhome is being driven on the road.

With the multimeter set to the ohms range, check between the L pin at one end and the L socket on the other end for continuity.

The electrical contacts of the socket are a long way down the hole, so a long test prod is required. Then check between N and N and finally between E and E in the same way.

They should all read 'zero' ohms.

It is good practice to check the tightness of screws regularly as preventative maintenance.

Also check that there is no physical damage to the cable sheathing along its length and, if there is, replace any damaged cable.

There tends to be very little technology in a motorhome associated with the mains installation other than the built-in charger. Some have a separate on/off switch and some have the mains input to them via a kettle-type plug and this is worth a check if the charger isn’t working.

Low-voltage systems in a motorhome or campervan

All but the very simplest 12V systems (sometimes found in smaller campervans) have a remote control panel and a separate, much larger, control box.

Typically, a single multicore cable with a plug in each end will link the two.

Sargent, in the UK, tends to fit the charger, the mains RCCB and individual circuit breakers, the relays (electronic switches) that control the various systems, and a control system to link it to the control panel, all in one box.

Schaudt, with its range of Elektroblocks, also combine the charger and the 12V switchery within one unit.

Other suppliers tend to keep the mains and 12V systems separate; CBE, BCA and others favour separate units and a separate charger.

All of these systems use rows of blade fuses and often some remotely located fuses for the individual 12V circuits. If you short-out a cigarette-lighter connector or do something else equally careless, the corresponding fuse will rupture (blow) and switch off that circuit.

It’s always good to check that a fuse is good with a continuity check. 

There is no excuse for not reading the manual supplied with your motorhome, this will detail the use of every fuse and its location.

There is frequently a fuse in the leisure battery compartment and these can suffer corrosion. Similarly check the battery connections are clean, bright and tight.

If no mains equipment is working at all and your battery is not being charged, then the RCCB, hook-up lead, breaker at the hook-up point and connections are all suspects.

Don’t forget that circuit breakers and RCCB switches are in the 'up' position to indicate they are on.

A typical lack of mains power is often associated with low-power campsite hook-ups, such as 6A or 10A.

With the multimeter set to volts DC, you can check the actual battery voltage by connecting the meter directly across the battery.

You can also check the voltage where it arrives at its destination, such as some interior lights.

But remember, this will be a slightly lower reading than the voltage at the battery due to the voltage drop in the wiring. This will be especially so with halogen lamps but much less so with LED lights.

The process of fault-finding using a digital multimeter is best done with the meter’s negative test lead left connected to battery negative, so extend this if required.

Then work along the circuit with the positive test lead checking the voltage at:

  • The battery
  • After the fuse
  • After any connectors
  • At the switch
  • At the bulb, as one example

Test that negative connections to appliances are actually negative by measuring the voltage at the appliance with the meter positive lead – the reading should be close to zero volts.

If the voltage on the negative connection to an appliance is close to 12 volts then there is a break in the negative/chassis connection, which will need sorting.

Common electrical faults in a motorhome

Electric step does not retract

Electrically powered retracting steps have a small plunger-operated switch built into the mechanism, which switches off the motor when the step is fully retracted.

When in their retracted position, the steps are in-line with the tyres and, consequently, dirt and road debris is thrown up at them when travelling. This often results in the switches getting full of water and grime and, so, eventually they fail and, hence, the steps fail to retract. 

In many cases a bit of work to access the switch and remove it pays off, as they can frequently be dismantled, cleaned and put back into service, or replaced.

Alternatively, the step circuit may not be getting its supply indicating that the engine is running and will only operate manually.

Leisure battery does not charge when driving and fridge stays running on gas

In most cases a pair of relays, the split-charging and the fridge relay, need to be energised to enable the leisure battery to charge when driving and thus to switch the fridge to be powered by electric not gas.

The coils of these relays will connect between the battery negative (chassis) and the D+ terminal on the vehicle alternator. When the engine is running, the alternator will be charging and about 14V will be present at the D+ terminal. You can check this with a multimeter set to volts.

One set of relay contacts will close and connect the leisure battery and starter battery positives together so that the leisure battery is also charged.

The other set of relay contacts will close and provide a 14V feed to the fridge to cause it to switch over to 12V mode (for automatic fridges).

In some cases, these two relays and associated fuses are under the bonnet. The relays have their coil-negative connection connected to the bodywork and frequently this is done by the relay fixing screws.

So checking for sound connections to the relays and measuring the voltages at them is paramount.

Others incorporate the relays and fuses within the main control box; however the logic is the same. Some more complex Sargent systems are different and diagnosing these and any with battery-to-battery chargers are best left to the experts.

Solar panel not charging

This is where the use of a multimeter set to volts is invaluable. First, find the solar voltage regulator, often a separate unit but sometimes its function is incorporated into the main electrical control box.

Without any 230V hook-up connected and with some sunshine, measure the input voltage to the solar regulator, that is the pair of wires connecting it to the solar panel.

Expect to see a reading that is at least the same as the battery volts.

If the previously measured battery voltage is around 14V or a tad above and the control panel in the motorhome indicates zero charging amps, then expect to see even more volts from the panel as the regulator will have switched off because the battery is charged.

This is correct and not a fault. Most modern solar regulators and mains chargers charge the battery up to a peak voltage then reduce it to a maintenance level.

The typical maintenance voltage will be 13.5V to 13.8V. Peak voltages will be around 14.4V for wet lead-acid batteries, 14.7V for AGM lead-acid batteries and ideally 14.6V for the new lithium ferrous (LiFePO4) batteries.

Starter battery goes flat in a couple of weeks

If your motorhome's starter battery keeps going flat, you need to check if there is any continuous battery discharge when everything is supposed to be turned off.

Again, use the multimeter set to the amps range and move the test leads to the amps position.

Then disconnect the starter battery negative lead from the battery.

Attach one test lead to the battery negative terminal and the other to the removed cable. The meter will indicate any residual current flow.

Anything more than 0.03A is excessive. Items such as aftermarket alarms and reversing camera systems are frequently the culprit.

Remove fuses one at a time to see which of the circuits is causing the battery drain.

The same arrangement also applies to the leisure battery.

Always return the test leads back to the volts positions before you put it away; the meter presents a very low resistance to the test leads when in the amps positions so will effectively provide a short circuit fault if you use the meter to read volts.

Safely working with batteries

When carrying out any electrical work, the mains hook-up lead should always be disconnected, as should the negative connection from the leisure battery and safely secured away.

The reason is simple. The battery negative connection is connected to the vehicle chassis/bodywork so if you are using, for example, a spanner to loosen a clamp bolt and the spanner also touches any adjacent metalwork you will not get a massive inrush of amps. Do this with the positive clamp first and the spanner can instantly glow cherry red and spark excessively.

Then to compound this mistake, the spanner flash can ignite the gases from the battery that has just come off charge, blowing off the cell tops! If this happens, acid from the battery can be sprayed. That’s why it’s always important to remove the negative first.

If in doubt ask for help from an expert or use a qualified professional.

Electricity is like the wind, you cannot see it but you sure know when it hits you...


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