Most classic Hymers have a Truma 3002 - the heater with the brown front panel. One of the commonest problems is that it works perfectly for years, then suddenly stops! When you push and turn the knob, you should get that familiar tic tic sound - if you don't, then it's usually an ignition problem. Two things happen when you push and turn the knob - turning it operates a small switch that switches on the ignition unit - that makes the tic tic sound, and pushing it opens the gas valve to the pilot light. Once the pilot light is lit, the thermocouple inside the burner operates, and allows the gas to keep flowing when you release the knob. If you release the knob too soon, then the thermocouple doesn't have time to heat up and the pilot light goes out., The basic operation is just like lighting the cooker.
Once the pilot light is lit, if you continue to turn the knob, the main burner will ignite with a familiar whoosh. The point on the knob dial that this happens depends on the ambient temperature. If you open the knob fully to a high setting, then you will usually get both the pilot and the main burner igniting together.
There are 2 problems that can happen at this stage - either no ignition at all - ie no tic tic sound, or the pilot lights, but the main burner doesn't.
No ignition
The way the ignition works is that at the lower left hand side of the fire, when you take the cover off, is a small plastic box about the size of a cigarette packet. This is the ignition unit. Usually it has a single or double AA battery in it, but not always. Some vans don't have the battery - the box is supplied with 12v from the van supply. Either way, you need to make sure that the box either has a good battery, with clean battery connections, or a good 12v supply. The battery supplies power to the ignition unit which transforms it into a pulse of electricity that produces a spark at the electrode in the burner - just light the spark of a cigarette lighter.
At the back of the ignition unit, there should be a single wire coming out of the middle of the back. Make sure the knob is in the off position and pull this wire off. If you now turn the knob on as if to light the fire. The box should tick - if it doesn't then the box is suspect. The wire is called the HT lead - it carries the pulse of power for the spark. It gets worn and brittle with age, and its insulation breaks down. Sometimes it breaks completely, other times it sparks to the metal chassis - sometimes you hear a muted tic tic, but either way, the fire won't light. So first you need to know you have a working ignition unit - make sure the battery is new, and the connections are good. Be careful - if the unit tics to your fingers you will get a shock, not fatal, but no fun either. Always make sure the knob is OFF whenever you go near the HT lead with your fingers. With no HT wire connected, the box should audibly tic when you switch on the knob. If it does, then you know you have a working ignition unit, and the problem is with the HT wire.
The other two wires from the ignition unit go to the rotary switch that is located at the bottom of the long shaft of the control knob. As the knob is rotated from the zero/off position to the on position, you should hear a gentle click. This is the switch that completes the circuit of the two wires that go to the ignition unit, and it should start ticking (with no HT wire connected). This doesn't often fail - but as there are 3 possible points of failure - you have to be able to eliminate each one. If you get no audible tic tic from the ignition unit, then it is definitely suspect, but you also have to eliminate the (rare) possibility of a faulty switch. You can do this by pulling off the switch wires and touching the two pins together with the tip of a screwdriver - this completes the circuit and the unit should tic.
Non ignition is usually always the battery, or dirty battery connections, or dirty connections on the ignition unit. But if you have a good tick from the ignition unit (with no HT wire connected) then it's likely to be a damaged or old HT lead, and unless it's sparking to chassis in the section before it goes down into the underside of the fire, then the only solution is to take the fire out, exposing the underside where you can see the rest of the HT wire leading to the burner. Usually, if you are sure the ignition unit is working and ticking nicely, you will find that the wire is either broken, or corroded, or the insulating sleeve is broken and it is sparking to the chassis. It is possible to repair if you know what you are doing, but usually it's best to replace the whole HT wire with a new one from Truma spares. Taking the fire out isn't actually that complicated for a confident DIY'er with a decent tool box, but can only be recommended with a whole bucket full of warnings about gas safety - you will have to be totally confident about breaking the gas supply, and then leak testing on reconnection, and you have to be quite sure that you have reconnected the chimney pipe, with its silicone gasket ring, perfectly, as well - otherwise you risk deadly fumes leaking into the van. So basically for most people, at this stage, you need a qualified Truma guy, unless you are sure it's the ignition unit, in which case this can be replaced DIY with no safety issues.
But it's nearly always the battery, or a faulty 12v supply, or dirty connections around the box - but if it happens to you, you should be able to narrow it down. It's embarrassing and expensive to pay a professional, when all you needed was a new AA battery, and to clean a few connections.
No main burner ignition
The other common fault is that the pilot light lights, but the main burner doesn't. The Truma heaters are unusual in that the pilot light is big enough to provide a reasonable amount of heat - it is basically a low setting. On a fully working heater, as the thermostat cuts in and out as the ambient temperature rises and falls, the heater changes from low setting (pilot only) to high setting (main burner). You will hear the familiar whoosh, as it does so.
The commonest reason for the main burner not to light is a sticking gas valve. If you take off the front of the fire, you will see that the main control knob is connected to the main gas valve by a long control rod. The bottom end of this control rod locates in a recess in the top of the shaft of the main valve, allowing it to be both pushed in, and rotated. The shaft is spring loaded, allowing it to spring back when pushed in. What happens is that after many years, it gets full of dirt and dust, and gets stiff. A common cause of the main burner not lighting, is that this shaft is not springing back fully, and is not allowing the main burner gas valve to operate. The remedy is to clean and lubricate it as best you can, and work it up and down so it presses in and returns fully without sticking. You can use WD40 or silicone spray - anything that works really, but preferably something that doesn't attract more dirt. Common sense is the best ingredient.
Over many years of operation you may notice that when you light the fire, the knob often springs back quite slowly after you release it - this is a sure sign that it is dirty and needs a clean.
Another reason for the main burner not operating is a faulty thermostat. This is the copper tube thing at the bottom right. This is connected to the main gas valve by a thin copper tube. As the temperature rises and falls, liquid inside the tube expands and contracts and operates the valve. It is a sealed system and not serviceable. The copper thermostat and the gas valve have to be replaced as a complete unit, and is basically a professional job.
