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Motorhome tech advice: our experts answer your questions


Do you have a problem with your motorhome or campervan that you need help on? An issue that you can't resolve yourself? Fear not - the MMM team of technical experts are always on duty, available to tackle all your motorhoming problems, however small and niggling they may be.

You can read the latest technical questions from MMM readers and answers from the MMM experts below. If you need an answer to your technical question, we'd love to hear from you. Just email MMM's team of experts at mmmtech@warnersgroup.co.uk

You can buy digital issues of MMM magazine here

The MMM team of experts:

Tech Help Editor: Nick Fisher, Base vehicle expert
Peter Rosenthal, General advice
Clive Mott, Electrical expert
Mike Hill, Bodywork expert
Barry Norris, Technical & legal advice
Terry Acreman, Ex-Peugeot technician
Andy Harris, TV & 12V expert


Q Does my motorhome get an MOT extension?

Our motorhome (a 4,500kg Auto-Trail Frontier Delaware S) is due for its first MOT in June and, with the current lockdown, we feel it is unlikely that we can get the MOT and service done in time.

We are some way away from a garage that is able to manage a 4,500kg motorhome and our nearest Fiat dealer with suitable facilities is about 60 miles away (we had one within walking distance of our home, but that has recently lost its Fiat franchise).

I have seen that cars, vans, etc, plus goods vehicles and coaches will have a six-month period of grace but I have not seen anything to confirm that this will extend to motorhomes, especially those over 3.5 tonnes. Do you know if this period of grace will apply to motorhomes?

I was aware that cars, van and goods vehicles have been granted an automatic extension of MOT certificates for six months if the current certificate expires on or after 30 March 2020 and had noticed that there was little to nothing said regarding motorhomes.

I have done some digging and have found the following page on the VOSA website - gov.uk/guidance/coronavirus-covid-19-mots-for-cars-vans-and-motorcycles-due-from-30-march-2020

If you click on the first symbol for car, van and motorcycle MOT tests, the information is given for cars, motorcycles and light vans or other light vehicles. Clicking on this highlighted ‘other light vehicles’ will provide a list of all classes of vehicle up to Class 7 for MoT purposes; and motorhomes are listed in Class 4.

In fact, reading all of the various gov.uk site information it seems that the six-month MOT extension is applied to all vehicles that can be tested at a regular MOT test station.

However, HGVs that require testing at VOSA sites have been granted a three-month extension.

All websites make it clear that owners and drivers are responsible for ensuring that their vehicle remains in a roadworthy condition and that safety checks can still be made at garages, which are allowed to remain open as they are considered essential services.

Nick Fisher

Q How do I keep my two leisure batteries topped up on my 2004 Auto-Trail motorhome?

If your batteries are of the standard wet lead-acid type, the motorhome’s built-in charger should keep them topped up, but not much more. If you want a charger that will do more than that, by constantly monitoring them, adjusting the charge current accordingly and giving them the occasional high voltage/low current charge to keep them healthy, get a ‘smart’ charger. There are many options, but the 10A charger from CTEK is up there with the best.

Andy Harris


Q Why is my motorhome tripping the house fuses?

When I plug my motorhome into the house feed, it trips the main fusebox in the house, plus the one in the motorhome. All the 12V is working and I have changed the mains lead, but still the same. The motorhome is a 2016 Elddis Encore 254.

We’ve had this problem also. It is most likely that it is caused by earth leakage and not an overload.

The house will have a residual current device (RCD) circuit breaker and this measures the amps going up the live wire and the amps coming back down the neutral wire. If all is well, these two figures are identical.

A domestic RCD will detect if these two figures differ by any more than 0.03A (30 milliamperes) and, if so, switch off the house. You need to find where the small parts of the amp are escaping to earth.

The first check is to just connect your hook-up lead to the house, but not the motorhome, lay the lead (not the plug) in any puddles you may have! If this trips the house then it’s the lead. If not, read on…

Check number two starts with turning all the mains circuit breakers in your motorhome off. Then connect the hook-up lead. If this causes the house to trip then it is likely to be water ingress into the motorhome hook-up connector and immediate wiring.

The third thing to check starts with closing just one of your motorhome circuit breakers.

If this causes the house to trip then you need to investigate further those services served by that circuit breaker. Your manual will tell you what breakers protect which services.

If not, then things get more laborious. Open the circuit breaker above and close the next circuit breaker to see if it causes the house to trip.

Continue this until you have tested all of the motorhome’s circuit breakers.

Here are some of the items that can cause such earth leakage:

Water ingress via a side locker getting into the electrics. In our case, the water got into the space heating insulation and caused the leakage. A good dry out with a fan heater on an extension lead and then a night running on gas dried out the heater and I fitted a new weather seal to the culprit locker door.

Via the fridge vents. Water can get into the electrics at the back of the fridge. Also, there can be a failure of the mains element in the fridge.

Microwaves are not really designed for a mobile application so bits can come loose and fall off inside. Unplug it to be sure it is not that.

4 Faulty battery charger.

The guessing can go on for quite a while but if you have got this far and not found the problem then you need to enlist the services of a properly qualified motorhome electrician who will investigate further.

Clive Mott


Q Why is the ignition clicking on my Mercedes-based Rapido?

I have 2006 Rapido 997M A-class on a 2.7-litre Mercedes, bought from new. Very occasionally during the last 15 or so months, when the ignition key is turned on there is a light ‘click’ but no other sound.

I then switch off and turn the key again and, on this occasion, it starts immediately. I never have to try a third time.

This may not happen again for two or three months. The vehicle has never given any other problem in 14 years except for this mystery.

I have cleaned the battery terminals, but it still happens.


I have limited experience with Mercedes vehicles, but an audible click and no start can usually be narrowed down to one of two items.

If the click seems to be coming from the dashboard, this tends to be due to the vehicle not being happy that there is enough power available in the battery to allow the starter to operate. This can be caused by the obvious culprit, the battery, or more probably (because it only ever happens on the first attempt), a poor earth between the chassis and the engine.

The battery terminals are just the start of the process. The battery is earthed near the battery, but additional earths are required between the chassis and the engine or gearbox to complete the circuit to the ECU and starter motor. Sometimes, a poor earth can improve once some current has passed through it and warmed it up a little (poor earths generate heat).

If the click is coming from the engine bay and from the starter motor in particular, the problem is likely to be a sticking starter solenoid. Again, the initial attempt to start may have the effect of freeing it up enough to work. If the engine ECU detects a significant electrical resistance it will cut the power to the starter motor.

I have never seen a vehicle yet that can communicate this problem to the driver via a warning on the dashboard but, if a diagnostic device is plugged in, it may be able to show a code that would be described as ‘start permission denied’.

The starter is controlled by the main ECU and then by a relay. It is not uncommon for relays to malfunction.

I think it best to advise that the vehicle is checked by someone who has experience with Mercedes Sprinters because, while the problem seems manageable, it could result in you being stranded somewhere in the future.

Nick Fisher


Q Can I still use my motorhome to tow an A-frame and car when I reach 70 years of age?

Just a query regarding the old chestnut: licences after 70. My current driving licence, which was issued in August 1976, is the old variety. It expires in May on my seventieth birthday. On it, I am entitled to drive groups A and E only (no mention of C1, etc).

I drive a Warwick Duo Peugeot Boxer 3-litre campervan and tow a small VW Fox on a braked A-frame, the ’van being plated at 3,500kg. Will my new driving licence still permit me to drive this combination at 70 or will I need to get C1 added?

Will I require a medical, etc, or will I get away with just a straightforward renewal? I do not intend to drive anything above the 3,500kg in the future. Looking forward to your advice.


1976 ‘Group A’ licence relates to present day categories of B, BE, C1, C1E (with restriction code 107) plus a few other less relevant categories. Further details can be viewed on gov.uk

As your motorhome is rated at 3,500kg then you only need a category B licence to drive it. Assuming you make the simple application to renew, without submitting form D4, you will receive a photo card licence showing categories that include categories B and BE, but not C1 or C1E.

If you wish to retain a C1 licence to enable you to drive a vehicle over 3,500kg then you need to submit form D4 as completed by your doctor. The application can be undertaken online, but renewal of the C1 and C1E licences can only be done by post.

Regarding so-called ‘grandfather rights’, even with taking the simple renewal process and so dropping your C1 and C1E rights, you do still retain your BE entitlement.

People who passed their driving test from January 1997 have a category B licence, but no entitlement to category BE, except by passing a further test.

With a category BE you are entitled to drive a vehicle with a trailer where the combined weight is over 3,500kg. If you had just a B category the maximum trailer weight you could drive with your 3,500kg motorhome would be 750kg.

If sending your current licence to DVLA at the time of application, as required by a postal application, do take a photocopy of it as the DVLA has been known to renew licences without grandfather rights. Similarly, when you receive your new licence check that it shows the correct entitlements.

Barry Norris


Q What is the correct charger setting for lithium batteries?

I’ve fitted three Polinovel 100Ah lithium batteries, with a 160A battery management system to my motorhome, which has a Dometic 25A charger. The charger has three settings. Currently, I’ve left it in the setting for gel batteries, which allows a maximum 14.4V charge.

Setting 3 allows a max of 14.7V for AGM type I believe, and I’m wondering if I might safely use this setting to give a fuller charge when on hook-up. The battery specification says they can be charged to 14.6V (+/- 0.2V). This would seem to indicate that a 14.7V maximum charge could be tolerated.

300Ah of lithium is a lot of battery. You are correct in your assessment of the charging parameters. As you said, 14.7V is the top charging voltage for an AGM battery before it reverts to a maintenance voltage, typically about 13.8V. And 14.4V for a wet battery top voltage.

LiFePO4 (lithium ferrous phosphate) has a recommended top voltage of 14.6V, although many sellers of lithium technology allow a wide enough margin to use the AGM settings on a modern charger and thus achieve the maximum charge.

Don’t forget that you very likely have three charging systems that each individually need to be configured for the appropriate voltage: the mains-powered charger, the solar regulator and a B2B (battery-to-battery) charger if you have one. With 300Ah of lithium I can see that a B2B could be attractive. We also have a modest lithium setup in our motorhome and the settings used are AGM.

Clive Mott

Charging at the gel setting of 14.7V ‘true’ voltage would be within the quoted specification. It is certainly worth verifying the accuracy of the voltage actually delivered to ensure it is no higher than this in practice considering all inputs to the battery from solar and battery to battery chargers.

As the battery state of charge increases, its voltage also rises and a charge ‘cut off’ should be set that is below that of the BMS (battery management system) on the battery. In our case it is 14.6V.

Charging to a voltage beyond 14.4V will give very little increase in overall storage capacity due to the sharp increase in voltage at the end of charging.

Perhaps between 1% and 3% maximum. However, reducing the voltage range in use, both avoiding very low depth of discharge and very high state of charge by 0.2V, wll significantly extend the cycle life of the battery beyond the warranty level (after 2,000 cycles the battery has 80% of original capacity).

Some users may see 30-50% longer total life by charging to 14.4V.

The critical factor to protecting the batteries’ working life is to ensure the charge sources work to cut charge before the battery BMS takes over at a battery voltage of 14.6V to protect the battery from over charge voltage.

The BMS is a last resort safety device. It is not to be used as a charge controller device, as this can lead to sudden disconnection of power and potential damage to alternators.

This is why we recommend a battery-to-battery DC charger is always used if the alternator charge is needed to top-up lithium batteries.

Rowland King, Quality Source (suppliers of Polinovel batteries)

Q Can I fit a retracting step to my motorhome?

Is it possible to fit a retracting step to the habitation door on my 2015 Elddis Majestic 140?

Although your vehicle already has quite a low entrance step as it’s cut into the side moulding, it should be possible to add a step – assuming there is nothing located beneath the step tread (check first with a mirror before you do anything else).

So long as the step cut-out has a solid base, it should be possible to bolt a step directly through the step floor using bolts and large spreader washers. If the step isn’t that stout, you might need to have some brackets fabricated to brace it against the chassis rail.

The step needs to be strong as it has to support the weight of a human – assume a 20 stone man running over the step and you’ll appreciate that the forces involved are quite large.

I’d only look at single-step units – a double will be too low – and there are about 50 different models on the market. Reimo stocks a wide selection so that’s a good place to start (see reimo.com).

A manual Thule step will cost around £110 plus fitting, while electric ones are more costly to buy and dearer to fit as they need additional 12V wiring including a switch in the interior panel and relays.

With any electric unit, make sure the fitter wires it to retract with the ignition on and fits a warning buzzer to alert you when it is still extended. 

Peter Rosenthal


Q Why does the step of my motorhome not retract automatically?

I have an Elddis Prestige 175 (a 2016 model). Please could you advise where, oh where, I can find the relay for the Thule single step. It is not retracting when I start the engine.

We have searched high and low in the user manual, but cannot find it.


Most people believe that when a step does not retract that the problem is the relay. However, in many cases it can be simpler than that with a blown fuse or a broken connection being the culprit.

Not all installers follow totally the diagram in the Thule manual for the single step. The retract signal voltage in the Thule (Omnistore) diagram shows a connection from the vehicle key switch to the relay to cause it to energise and cause the step to retract.

In many cases an alternative voltage source is used for this signal.

The voltage supply that causes your three-way fridge (on automatic versions) to switch to 12V mode when driving is frequently derived from the D+ terminal on the vehicle alternator. This voltage is often shared with the electric step and also another relay to charge the motorhome’s leisure battery when driving (called split charging).

I assume that the step retracts if the manual switch is operated. If not, then the problem is more likely to be the retract limit switch, which is built into the step mechanism itself.

If this manual function is working then check to see if the fridge switches to 12V when the engine is running. If not, then a look at the connections to the alternator D+ terminal and the in-line fuse.

In many cases the fridge, step and split-charge relays can be found under the bonnet or close to the starter battery and it has been known for the relay fixing screw to also double as its electrical connection to chassis/negative. A little bit of corrosion at this point will give the problems you report. Sometimes the relay can be found close to the manual switch behind the panelling.

Another way to find the relay is to follow the two wires from the step motor. One wire will go to the manual switch and the other wire to the relay terminal 30. If in doubt use a professional.

Clive Mott


Q Can I modify the seat mounts so the cab seats in my motorhome can be swivelled and reclined more easily?

I have a Hymer ML-T 580 motorhome, a Mercedes Sprinter base vehicle, 2020 reg. It's sitting in the barn, brand new, due to lockdown! While familiarising ourselves with the vehicle we discovered that the cab seats are too close to the doors and cannot easily be reclined and swivelled in the reverse position due to the armrests fouling the door linings. Is there a modification that could be fitted to the seat bases when parked to move the seats inwards? This would need to be between the swivel and base mount.

You need to speak with the manufacturer.

I would expect there will not be any product out there that would do such a side movement, unless you can find a disability manufacturer who would adapt one for you. 

As you probably know, seats and seatbelts are protected by law on fitment and testing, so slight adaptations are not something that should be undertaken by just anyone.

I have to say that there are many models we have in our workshops for repairs that are very tight when swivelling into their reverse positions.

It just needs to be undertaken in a certain way to make for full turns.

Mike Hill



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