By Mark Sansom

If you’re the owner of a Winterised Defender 110 or 90 Land Rover from the early 1990s you will have noticed they all have a Webasto pre-heater installed under the bonnet.

The parts manual shows it as assembly RRC7142, Heater Assembly.  To the civilian market it is called the Webasto DBW46 water heater.

The Webasto DBW46 is basically a small boiler that heats your engines coolant by pumping the coolant through a burner.  The coolant then circulates through the winterised cooling system of the landy.  The illustration below shows how the coolant flows round the vehicle.

Now the science bit; the water heater does just what it says on the tin, it’s an independent heater that warms the engine coolant without the main engine running.  The system is initiated by the use of a switch located on the dashboard.  Give it a turn to the right and a green light should show from the switch. This now pushes power to the heater unit which starts its combustion fan running and warms the glow plug. Seconds later the fuel pump starts clicking, which is it pushing fuel to the injector nozzle at the front of the heater unit. Inside the heater unit a piece of felt is drenched in diesel and as it passes over the glow plug it ignites the fuel with a “woof”. This clicking and burning continues for up to 20 seconds when the unit moves up to full power.  The “woof” becomes a “roar” and the heater starts warming the coolant and pumping it around the engine block into the heater matrix.  When the water reaches 50 degrees C the vehicle fan starts blowing the heat into the cab of the vehicle.  The “Webasto” then maintains the coolant temperature between 70 and 80 degrees.  If the temperature raises over 85 degrees it cuts out and shuts down via a purge cycle that shuts off the heater.

Most of the components on your Webasto are replaceable;














In addition to the above list there is a dosing pump which pumps fuel from the Land Rover fuel supply and comes out of the Land Rover fuel filter in the middle of the bonnet via a small pipe that loops forward to the Webasto installation. 

All of these parts are controlled by the Webasto brain the ECU.  This ECU shows its age with the “Made in W. Germany” label.  After all these years it still works.

 The Webasto has its own independent exhaust system, with its own silencer which bolts to the front left wing of the Land Rover.


The most important part of a Webasto to make it work is a small ring of flame proof felt which acts a as the catalyst for the diesel to burn.  Over time these degrade and in some cases they fail.  This item should be replaced with every service the vehicle has,  subject to usage.

Since they left the military most of these “Webasto Heaters” have not been serviced and many make assorted noises and various plumes of smoke issue forth from the neglected heater.  The “Webasto” should be serviced with the Land Rover as part of a routine service.  Indeed, it is likely that your “Webasto” has probably not been out the vehicle for the last 15 years.

Due to the age of the heaters, spare parts are becoming scarce, and expensive!  It is worth keeping an eye on auction sites such as Ebay where they occasionally come up.

To service your Webasto the first thing you have to do is remove it!  Each one is held on by three bolts.  Two behind the ECU into the reinforced wing lip.  And one in the wheel arch.

These are removable with either a 13mm or 10mm spanner depending on the last mechanics choice. Prior to this you must remove the bayonet coolant hoses, one on top and one underneath.  You will lose coolant, but only the amount from the Webasto.  You will also need to disconnect the fuel pipe from the dosing pump; again this is a bayonet clip so no diesel should drip.  As you withdraw the whole assembly you will need to disconnect the cable cluster from the ECU. With a bit of a wiggle and twist you will now be able to lift the whole unit out from the Land Rover.


 Once the unit is withdrawn the first thing to check is the felt in the burner.  To get to this you need a flat head screw driver and prise off the clip from either side of the end cover assembly.  This is the point that the diesel is blasted over the glow plug and the burning felt.


The end cover assembly has a seal round it, make sure that this isn’t lost or your Webasto will not work again.  The felt will be blackened, but as long as it is viable it can be retained.  If not it needs replacing with a new part.  This part can come in service kit, with new seals, but be aware it costs a small fortune.

This felt is still viable and does not need to be replaced at this stage.

Once the felt is checked it is then worth checking the ECU for water ingress.  It should be disconnected and with some Vaseline reattached to ensure good connections.

The glow plug is worth unscrewing and checking for integrity.  If it’s broken there is no repair for it, you’ll need a new one.  Remember to get the right one, as they do make them in 12v and 24v as well.

If you’re feeling adventurous you can take the whole unit to pieces, into all components and giving them a good clean, especially the sensor type items, as over time the coolant covers them in lime scale and general rubbish from the coolant system.  These parts can be easily removed, cleaned and replaced.  If one of these parts fail, then you are going to have to dig deep as they are very expensive now.

Each of the electrical items has an idiot proof pin system, which ultimately joins the ECU.

Once you’re happy that you have cleaned and inspected the unit, then it is time to reassemble the Webasto.  Remember to reconnect the wiring loom top the vehicle before you bolt it back into your Land Rover. Once it is bolted back into the vehicle, reconnect the coolant hoses, and the fuel line.

Make sure that you do not have any air bubbles in the fuel lines of the dosing pump or it will not be able to draw it through the system.  You will need to bleed the coolant through the Webasto, to ensure it is full.  To do this you need to loosen off the t-shaped valve with a 10mm spanner.


Usually air will hiss out of the pipe, until a stream of coolant flows out.  Tighten off the nut at this point. Remember to top up the coolant after you have bled the system.

If all has gone according to plan you should be able to turn the switch in the cab and it will fire into life.

Common faults with these old Webastos include;

  • Air pockets in the fuel line from the filter to the dosing pump.  I replaced my fuel hose as the original was full of something nasty that blocked the hose and prevented the fuel getting to the heater.
  • ECU failure; not a lot you can do about this apart from replace it at vast expense.
  • Various pumps and fans not working; again need replacement.
  • Any of the service items failing; I’m afraid you guessed it they need replacing.
  • Fuses blown or the relay stuck; all can be replaced.

This is just a layman's view of how the fix a Webasto, I have only learnt this from trial and error and I may not have got all the facts right, but I have now got a working Webasto. It proved invaluable in the snows of February, and it was a real luxury to get into a vehicle which was warm, demisted de-iced and started at a quick turn of a key.  Webasto Manuals are now available to download from the web. And if you want a copy of the AESPs drop me a line.


AESP 2320-D-122-524                   Winterised Cooling System Base Repairs

AESP 2320-D-122 302                   Winterised Cooling System

Webasto User Manual bbw46 / dbw46