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Older pump from my Eberspacher D1LCC.

fuelfilters1_big.jpg pump03_big.jpg

D1L pump dismantled.


Douglas Greer had the same problem with his Webasto pump clicking but not pumping. He says Webasto fuel pumps cannot be taken apart. Using a bicycle pump solved the problem and saved him from buying a new one. Nice to see our Eberspacher pages even help Webasto owners!


If connector is fitted without depressing the retaining clip it is prevented from pushing fully home and does not make a connection, a fairly common fault.

Pump not clicking

People assume that because the pump is not clicking it is faulty and remove it, then most find it is not faulty.

When the heater is started the ECU makes a number of checks as described on the Faults page. If any of these pre-start checks fail the ECU aborts the startup and no fuel is pumped. Look at the Faults page first to eliminate those causes. Note also that pumping is delayed on modern heaters for up to a minute after switching on while the glowpin preheats.

If the heater passes the pre-start checks and the blower speeds indicate everything is proceeding as it would normally but the pump is not clicking go to the Pump Mechanical section below.


A fairly common problem on Airtronic D2 D4 D5 pumps is not pushing the connector fully home when re-connecting,  the locking clip has to be squeezed in while fitting.

If the nut is tight use two spanners on the inlet pipe otherwise the complete end may rotate relative to the body, breaking the seal. A clogged fuel filter can restrict the fuel flow causing non starting or flame-outs.

It may need a pointed implement dug sideways into the plastic just inside the filter to lever it out.

In most services the filter can be cleaned and does not need to be changed, we never replace ours.

If the filter clogs regularly remove it completely and fit a small external fuel filter. Eberspacher say the normal type with a paper filter can have problems with waxing up in cold weather, the Eberspacher version has a metal grid filter element. One user reported that removing the internal filter caused air leaks and a small fibre washer was fitted to eliminate this.

The nut at the other end holds a ball bearing and spring non return valve. Do not undo this unless needed as the spring is very easily lost. Also it can alter the fuel rate.

Badly neglected filter with its replacement.

More fuel filter pictures are on servicing pages.

Pump Mounting

Pump must not be mounted less than 15 degrees from horizontal as it can trap air bubbles.

One users pump was mounted horizontally by a professional installer, it failed mechanically after two seasons light use. It should be mounted between 15 and 35 degrees from horizontal with the outlet higher than the inlet. Eberspacher recommend mounting about 30 degrees to allow for vehicles parking on a slope or for boats to tilt from horizontal.

Reducing Pump Noise and Pump Ventilation

The clicking sound of the pump can be reduced by fitting one of the newer rubber clamps in place of the older type clamp as shown on buying advice pages.

Metal fuel pipe can transmit this noise throughout the boat though direct contact or via the fixing clips. Fit the pipe inside cheap 6 mm internal diameter hose and clip that.

Mount the pump on something solid and use rubber connections.

The new D2L and D4L pumps are designed to be quieter. We have no information on compatibility with Airtronic D2 D4 heaters.

Some users ask if the pumps can be mounted in foam or similar materials to reduce noise. This has been done successfully but under some fault conditions continuous power can be supplied to the pump and it can become very hot. If it was completely encased there would be no easy way to dissipate that heat so Eberspacher do not recommend it.


Pump Dismantling

Fully dismantle the pump only if necessary, be extremely careful as the ball bearing and spring which create a one way valve are very easily lost, both during dismantling and handling. Placing the pump in a cardboard box or a plastic bag may help catch springs. Even gripping a spring gently by the ends caused it to fly. After searching for it on 3 occasions during testing and photographing I now always grip them gently by the sides.

If the pump is adjustable mark the nut and record its position before undoing, the nut may not be fully tightened.

Ian Whittaker found fully screwing the fitting home interfered with the free movement of the ball and prevented the pump working. He suggested using an O ring to hold the shoulder away from the body.

When replacing the spring take care to insert it straight, preferably hold pump vertical and allow it to fall onto the ball bearing. When initially replacing mine with the pump at an angle I felt it catch slightly on the side of the bore so re-inserted it. This was almost certainly the cause of one pump spring found with the end bent over so it did not seat properly on the ball bearing.

After reassembly of any disassembled pump you need to do a fuel quantity test to check its calibration.

Pump Electrical

Eberspacher use a pulsed type pump. Once power is applied it operates a solenoid which produces a single spurt of fuel. The power has to be removed so the solenoid can return to its starting position before the next pumping stroke.

The pump power is pulsed on and off to operate, this is done by the Electronic Control Unit (ECU).

The ECU sends pulses for a  D2 or D4 heater on the green/red wire, faster pulses give more fuel, slower less fuel.

The brown wire of the pump is connected to the battery negative supply, the joint is sometimes hidden inside the wiring loom.

The pump is not polarity sensitive so connections can be either way round.

Unlike older heaters the pump on modern models does not start pulsing immediately the heater is switched on, the Eberspacher does some checks and preheats for between 15 and 60 seconds before starting pumping.

Note multimeters are not designed to measure pulsed voltages, readings will be meaningless and can be different for different makes of meter.

A steady non pulsed voltage on the pump will not pump fuel, but it can get hot. In that case check the pump is ok as below. Check for a short circuit on the pump wire to +12v, otherwise replace ECU. If measuring this steady voltage the pump has to be connected otherwise you can get incorrect readings.

The resistance of an Eberspacher D1LCC, D2, D4 or D5 pump should measure 10 ± 0.5 ohms (24v 36 ± 1.8 ohms)

A 24v pump tested on a 12v heater receives 1/4 of the pulse power, it passed the start up checks but did not click or pump.

Check the connections for corrosion and the wires for mechanical damage.

[For the technically inclined only - Airtronic D2 and D4 active pulse measured on oscilloscope about 40ms constant width, variable pulse rate]

Testing the pump with a 12v (or 24v) battery

WARNING Failure to disconnect the pump wiring before doing this test can damage the ECU.

Connecting the battery to the loom instead of the pump by mistake destroys the ECU.

Connect a wire from one pump terminal to the battery.

Connect the second wire from the other pump terminal and dab it on the other battery terminal which will give a single pulse of fuel.

The wire has to be disconnected and reconnected for each pulse.

A small spark is usually visible as this is done together with a loud click from the pump solenoid.

If it does not click the pump may have either an electrical fault, check its resistance as above, or a mechanical fault.

If it clicks but does not deliver fuel see Pump Mechanical section.

Weak clicks may be caused by electrical or mechanical faults. A 24v pump on a 12v heater could also cause this symptom.

After a sticking pump was removed and the fault cleared one user wanted to check if it was working before going fifty miles back to the boat. The number of pulses here are only very rough initial estimates, at best only confirming the pump may work.

D2 roughly 50 pulses per ml, D1LC roughly 60 pulses per ml, D1LCC roughly 65 pulses per ml.

Measured number of pulses per start attempt

D2  first start attempt  186 pulses,  second start attempt  181 pulses    (2 pulses per second)

D4  first start attempt  265 pulses,  second start attempt  145 pulses    (2 pulses per second)

D2 and D4 count can vary by up to about 2 pulses.

D1LCC  257.

D4L manual quotes 200 strokes for 5.5 to 6.0 ml and 316 to 350 strokes per minute on full.

A fuel volume test will still be needed if the pump has been disassembled.

Pump Mechanical

Pump problems usually happen if the heater is not used regularly and may be caused by gummy fuel deposits inside the pump.

Eberspacher say regardless of the season, run the heater every month for a minimum period of 15 minutes which will ensure that the fuel in the pump and lines to the heater does not gel and solidify. We run ours only once or twice during summer, we are using kerosene which might gel less.

Pumps use the fuel as the lubricant. Mounting the pump horizontally or below the minimum 15 degree angle can cause air bubbles to be trapped which can either block fuel flow or prevent lubrication to parts of the pump resulting in mechanical damage and pump jamming.

The pump clicks are quite loud and are louder with no fuel than when full of fuel.

Pump is clicking but no fuel is pumped during bench testing

Check the filter in the base of the pump is clean.

Dismantle the pump making sure you don't lose the spring and ball bearing.

Clean the ball bearing and the bottom of the tube where it makes contact, this is a one way valve so dirt can stop it working.

Check the spring is clean and looks in good condition. Clean the tube containing the spring.

If these tests do not solve the problem try immersing the bottom fitting completely in the fuel while testing to eliminate the possibility of an air leak in the connector or lower piping.

Pump does not click during bench testing

Read paragraph at top of page before deciding it may have mechanically jammed.

Sometimes it can be freed by banging with a lump of wood or a spanner.

Hardened fuel deposits are the usual cause. Fill the fuel filter cavity with acetone and leave to soften . I had one pump that had still not freed after about an hour using acetone. It was sealed in a plastic bag for about 3 days. There was still a small amount of acetone in the filter, the fumes softened the deposits enough to free it when retested.

One reason for not working in cold weather could be fuel waxing or water freezing inside the pump.

If the simpler solutions do not work you can try more drastic methods.

Blow through with a bicycle pump or a compressor. Start with low pressure.

Another solution I have seen mentioned on forums is drastic but if the pump is crackered you have little to lose, however we have not tried it ourselves so it's your decision. Remove the fuel filter which is located in the pump body and the rubber connecting pipes before heating the pump in an oven at 200 degrees C, gas mark 6, for about 10 minutes. Test it again without fuel while it is still hot, take care not to burn yourself, try banging it at the same time if it is not clicking.

Any alternative suggestions are welcome.

Please give us feedback on these and other solutions, we learn from feedback.

Skip in Tennessee USA had success using the compressor. You could also try pulsing the pump at the same time. He also found his fuel lines were gummed up and blocked so he attached a small clear gas line to the hard plastic fuel line, filled it with acetone (alternative possibly carb cleaner) and let it sit for about an hour. The fumes and fluid softened the blockage before he applied low air pressure to force the fluid through the blockage.

Lost pump spring and ball bearing or small O ring

Thanks to Mike Coats who was missing the smallest O ring and found an ebay source - Sealforce 01382489043   £1.99 for 10. He measured the O ring slot as 2.71 mm and 1.15 mm wide so assumes the O ring will be 1 mm wide. The bore it fits in is 4 mm. Sealforce sent him a selection of different sizes and the 2.7mm fitted best.

If you have dismantled the pump and lost the spring / ball bearing you can sometimes buy them on Ebay.

If these are not available contact us for seller details.

If you are tempted to try any old spring / ball the following may help prevent wasted time. In the old Eberspacher D1L pump shown above and in a D2 pump the ball bearings measured 4.44 mm diameter. The springs which were very weak, 10 mm long, 3.15 mm diameter and held the ball against the fuel outlet to act as a one way valve. The D1L spring was placed in the outlet pipe and compressed against some kitchen scales until they just about touched the pipe. The contact point was difficult to judge and my best guess was roughly 35 grams.

Stronger springs will reduce or stop the fuel flow as will not having a spring fitted.

Original pump spring and replacement spring test.

Arrow shows compression distance for 35 gm pressure.

Solenoid pumps - How they work

Eberspacher fuel pumps are sealed so investigation of what is inside would not be easy. If you have a faulty pump please donate it. Thanks to Andrew Leonard for donating a faulty D2 pump which we hope to cut open for photographs for the page. There is a good explanation of a solenoid pump at www.kcsaab.com/aquamist/website/cp/sys1/techsys1/pump.html.

There are some obvious differences from Eberspacher versions but this does explain the basic principles of solenoid pumps.

Alternative pumps / Pump adjustments

Eberspacher fuel pumps are manufactured to match the specified fuel volume for the specified heater.

If the correct pump is not available some can be adjusted by turning the small nuts on the outlet.

Loosening the nut reduces the pressure on the ball valve, increasing the amount of fuel.

The main problem is the parts are brass which is a soft metal, very often they are too tight to move and only the spanner turns rounding off the flats making adjustment impossible.

Only one of the two D1LCC pumps I have was loose enough for some fuel volume adjustment tests on a D1LCC heater. Fuel quantity with that heater and pump combination should be between 3.4 to 4.65 cc.

Test results

Nut fully tightened 4.0cc, exactly middle of desired range for this heater.

Largest of the two brass nuts undone by 3 flats, ie 1/2 a turn  Fuel pumped 4.5cc

Nut undone by 6 flats, ie 1 complete turn  Fuel pumped 6.9cc

Nut undone by 9 flats, ie 1 1/2 turns  Fuel pumped 7.8cc

Nut was becoming too loose at that point and fuel was starting leaking so no further adjustment tried. Be very careful, the ball and spring are very easily lost if the parts separate.

A quick repeat check showed results differed with the same adjustment, the loose nuts would have needed some ptfe tape to tighten and seal the joint. Air may also have been introduced during adjustments.

I strongly recommend doing multiple fuel volume checks after completing the adjustment, it can take several start attempts to remove air in the system which alters the pumped fuel volume.

Increasing the pressure on the ball valve should decrease the fuel volume, I have not tried it, I guess the adjustment to be limited for reliable operation of the pump.

I need to repeat these tests using a D2 pump, I did try tweaking one a few years ago and the results did not show a big change but I was doing it to a working heater and time was too short to check.

Please let me know if you are successful when you do any adjustments and send the results, it’s how I gain more expertise.

Be very careful before buying pumps if they do not specify the heater type or voltage.

Do not simply compare the kW ratings of pumps, they can be designed for different pulse rates.

The center pump in the picture on the left is a Webasto 9012868C or 1320292A and is advertised as suitable for most Webasto petrol models from 2 to 5 kw so you might think it would work on similar rated Eberspacher diesel heaters, not so. When fitted to an Airtronic D2 it pumped 3 times as much fuel as the correct pump, definitely unsuitable.

Photo Tony McNicholl.

Fuel volume test results for various heater and pump model combinations.

During manufacture pumps are set to deliver the optimum amount of fuel for a particular heater model.

This means pumps from one model will not be suitable for other models without adjustment.

We have tested alternative pumps on our heaters. No attempt was made to try adjusting pumps before testing.  I would appreciate if anyone is able to provide test results for any other combinations of heater with incorrect pump, (user tests).

Test Heater Model

Test Heater fuel spec

Pump Model on test

Pumped fuel volume

Test Result

Correct pump tests

Airtronic D2

3.7 to 4.4 ml

Airtronic D2/D4   pump 1

3.7 ml


Airtronic D2

3.7 to 4.4 ml

Airtronic D2/D4   pump 2

4 ml



3.4 to 4.65 ml

D1LCC  pump 1 vertical

4 ml



3.4 to 4.65 ml

D1LCC  pump 1 sloping

about 3.9 ml


Wrong pump tests

Airtronic D2

3.7 to 4.4 ml

D1LCC  pump 1

3 ml

Too low,  fail

Airtronic D2

3.7 to 4.4 ml

Webasto 9012868C

14 ml

Very high, fail

Airtronic D2

3.7 to 4.4 ml


2.6 ml

Too low,  fail

Airtronic D2

3.7 to 4.4 ml


8 ml

Very high, fail

Airtronic D2

3.7 to 4.4 ml

Hydronic 10

9 ml

Very high, fail


3.4 to 4.65 ml

Airtronic D2/D4   pump 2

5.6 ml

Too high, fail


3.4 to 4.65 ml

Airtronic D2/D4 (User)

5.3 ml

Too high, fail

Airtronic D2 12v

3.7 to 4.4 ml

Hydronic 10  24v,

(wrong voltage pump test, should be 12v)

No pumping


The D1LCC user test was from a D1LCC a user bought on Ebay which came supplied with a wrong D2 pump. That resulted in a condition where the Eberspacher overheated and shut down after 4 minutes only when used on the high heat setting with the fuel can in the van. On lower settings or with the fuel can on the ground it ran continuously. The heater was just at the point of failure with the pump delivering nearly 15% extra fuel than the allowable maximum.

A second user has confirmed their heater also overheated and shut down with excess fuel.

Another user reported a D1LC fitted with a D2 pump smoked excessively and flamed out, ie too much fuel.


Airtronic D2 and D4 pump.

pump04_big.jpg webastofuelpumps_big.jpg pump06_big.jpg

Three types of Webasto pumps.

They are not interchangeable with Eberspacher pumps.

Fortunately Webasto pumps are differently shaped to Eberspacher pumps so are fairly easy to identify.

Fuel filter

Pump types

Fuel pumps come in 12v and 24v versions as well as several different supply rates including 1 - 3 kW and 3.1 - 5 kW.

Pumps are adjusted during manufacture to deliver the correct amount of fuel for each model so pumps from a different model are theoretically not interchangeable. However by carefully altering the ball valve spring tension sometimes pumps can be adjusted enough to match other models. One method is to adjust the small nuts, make sure they are secure so cannot undo with vibration.

Any change will require several fuel volume tests. The adjustment range is limited. Do not expect a 3.1 - 5 kW pump to adjust to match lower heat pump versions.

The airtronic D2 and D4 use the same pump.

Note these pumps need the power to be pulsed on and off to work.

Pumps are not polarity sensitive so electrical connections can be either way round.

The fuel inlet is at the end furthest from the electrical terminals for all pumps before the D2L and D4L.

Airtronic D2L and D4L pumps have the terminals at the fuel input end, typical of Eberspachers lack of consistency with the new heaters.

Pumps use the fuel as lubricant for the moving parts.

Testing and disassembly details are on this page. We also include some test results using incorrect pumps.


Unscrew the 17 mm nut on the fuel pump inlet.


Airtronic D2 / D4 pump dismantled.

The output end of the pump body contains a small black seal at the bottom of the outlet which is not shown here.


One version of Airtronic type pump connector.

Left      Retaining clip in locked position.

Right    Press in while fitting or releasing.

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