Thanks to 'Clyde Wanderer' for supplying the servicing photos and info. The servicing photographs were originally posted in Yachting & Boating forum and given to us by the original poster. They were taken during re-assembly and some details are missing but will be very helpful. Thanks also to Irvine Brown for providing additional photographs.
As these units are very old our advice would be if working well it is better to leave them alone. Unlike the modern versions these do not have a glowpin screen so regular servicing may not be so important. Any disturbance of corroded connections increases the risk of failure after servicing. There is also the possibility of shearing one of the fixing screws if it has seized solid. On the other hand if the heater has problems you should service it. One simple cause of problems can be the rotor holes are blocked by carbon. These heaters are obsolete and no longer supported by Eberspacher. Many D3L heater faults will be beyond economic repair, replace with an Airtronic D2 or D4.
Click on images for better quality
The filter in the fuel pump should also be checked during the service. Details are on the D2 and D1LCC service pages.
Initial disassembly and reassembly details are not included here, they should be straightforward.
The black rubber breather pipe prevents condensation from forming inside the rubber contact cover. Inspect for damage.
Fuel/air mixture regulator is factory set and should not be adjusted without an exhaust gas analyser. CO2 figures are not quoted in the manual.
Flame sensor temperature switch on the left. Overheat reset switch middle. This end of the breather pipe fits into the rubber housing around the glow pin.
Red reset button on the overheat trip switch can be pressed through rubber bung on outside of the case. Clean any corrosion from the sensor connections. Check the condition of the sensor retaining clips.
Remove the five fixing bolts, a forum article says it is difficult to remove them without first removing the motor circuit board. Separate this unit from the burner and motor units.
This is the fan for the combustion air. Unscrew the rotor fan.
Disassembled unit. Re-assembly order: Main housing, thick washer, rotor, thin washer, finally the rotor fan which is shown upside down.
Location of the thick washer.
Clean the rotor. It is important clean the holes, use something like a small drill bit to poke the holes clear. The thin washer should be between the rotor fan and the rotor.
Check the fuel pipe outlet is clear. Manual says fuel is supplied to a rotating open fuel distributor on the blower shaft, ie the rotor in the previous picture.
Fit two replacement fibre rings. Fit the thin washer before fitting the rotor fan.
Always fit a new gasket, make sure it matches with the extra hole opposite the fuel pipe. Check condition of the O ring. Refit rotor hand tight only, if done up too tight you may have trouble unscrewing it next time.
Fitting new wick in the combustion chamber. New wicks are listed in the Melorautoelectrical Eberspacher 2012 Price List at over £60. At that price they will only be replaced when badly damaged.
Position of the rubber ring seal. This will match up with the hole shown upper right on the photo showing fitting two replacement fibre rings.
Refit the combustion air blower with the rotor fan facing downwards so it fits into the burner. Ensure the green gasket lines up with the bolt holes and that the rubber ring seal is ok. Based on our experience with later heaters I expect the red gasket can be reused if in good condition.
The cutout fits round the exhaust port.
Rubber drive coupling. This will connect to pegs on the motor drive shaft (not shown) when the blower unit is refitted.
Blower unit. Check the motor circuit board (not shown) for loose or corroded connections.
Servicing D3L Heater
There is a 4TT (4 amp very slow blow) fuse for the motor in the ECU base. There usually is a 16 Amp ceramic fuse in the power lead. If replacing 16 amp fuse with a non ceramic blade type increase rating to 30 amps as the thermal characteristics are different.
Faulty ECU which was fitted on a heater with an 81 82 83 label.
This 91 92 93 label D3L sold on Ebay has a different ECU.
The CD4011BE chip centre right looks heat damaged in this photo but it could just be residue from an old sticky label.
The D3L glowpin is electrically insulated from the body nut and outer case so when testing connect test probes as shown. New glowpins fit D1L and D3L and have 3 washers. The D1Ls I bought did not have the fibre and large metal washers above the black test probe fitted so they may not be needed. Failures of these older generation glowpin types are fairly common, usually failing open circuit. 24v and 12v D3Ls use the same glowpin. Replacements are available.
Glowpin and two views of the standard connector. The physical construction of this glowpin has caused confusion to some users so a simple description may help. The central threaded rod forms one connection and is joined to the bottom of the heating coil. It passes through the glowpin body inside a ceramic tube which insulates it from the top hat shaped part that forms the other electrical connection. That top hat part passes inside more ceramic insulation so it does not make contact with the large nut and threaded body. Its end is welded to the upper part of the coil.
There are several glowpin types fitted in Eberspacher air heaters so if replacing ensure the correct one is bought. Unlike modern Eberspacher heater models the 24v D3L uses the same glowpin as the 12v D3L.
Check glowpin for damage. These glowpins from two heaters are still working but the coils on the one on the right have bent into an S shape. There are localised deposits on that side of the heating coil so it is showing signs of aging.
Additional information from Ken W. There is a small collar washer on which the rotor sits. If done up too tight the rotor easily crushes or distorts this and most likely will be out of line with the shaft. An alternative for this tiny fragile washer might be to cut out a circular segment from a small spring and slide it down the shaft into the washer groove.