Low voltage is probably the most common Eberspacher fault but very little information is given elsewhere on solving the problem.

The most obvious causes are due to low battery voltage or a battery in poor condition, these are very simple to cure.

If the battery is not the culprit the fault can be very inconsistent and perplexing.

Simply turning on something like a light can be the difference between working and failing when the voltage is marginal.

Typically an Eberspacher might fail to start; often on a cold dark evening; then starts ok next day.

It always works perfectly when taken to a servicing agent.

Understanding why will save hours of cursing and kicking the Eberspacher in the frustrated hope it will suddenly start working!

Although this page is based on 12v  Eberspacher D1LC compact, D2 and D4 heaters the information is applicable to all Eberspacher air and water models with a low voltage cut off. Check the quoted cut-off voltage for your model in your manual.

In a typical installation the battery powers various items; such as lighting, radio, tv, inverter as well as the heater.

The battery is usually charged from the engine but may be from a separate battery charger.

Two wires from the battery go to the Eberspacher. In some installations the chassis is used as the return wire (also referred to as negative, ground or 0v).

To the majority of non-electronic trained people everything is simple, i.e;

  The battery gives out a constant 12v supply which only changes when the battery runs flat.

  The voltage at the far end of the wires must be the same as at the battery terminals.

If only this was true.

The output is anything but a constant 12v.

The voltage changes depending upon how much charge remains in the battery; fully charged and rested usually about 12.6 volts.

As the battery discharges the voltage drops.

The voltage changes with the total current drawn from it; more current drops the voltage.

The voltage changes with temperature; as the temperature drops so does the voltage.

Charging the battery raises the voltage considerably; it also warms the battery.

When the charging stops the voltage drops; it then drops more over a couple of hours or so.

Low temperature also reduces the capacity of the battery.

Add in the effects of aging and deterioration of the battery with use and you can see why low voltage causes intermittent and unpredictable problems.

Wire is not lossless; all cable heats up slightly as it passes current. Each electrical connection also has losses.

The losses in the wire and connections reduce the voltage reaching the heater.

It is very easy to make a mistake and to fit a cable that is too thin without realising there is a problem.

For example the D2 takes about 8 amps maximum, wiring tables show 1mm2 cable is rated up to about 14 amps.

If you did use 1mm2 cable it would safely carry the current but the voltage drop would easily become excessive.

Heavier cables are normally required to reduce the total voltage drop.

When the voltage at the Eberspacher drops to 10.5 volts lasting for 20 seconds it shuts down. (10.2 volts for some models)

During starting the glow plug takes about 8 amps for a D2 or D4,  the older models take a lot more, about 20 amps.

This is when the voltage drop will be worst and most low voltage problems occur.

The usual symptom is the heater fan runs for about 20 to 45 seconds during starting, then the unit shuts down.

As the battery discharges lower voltage can cut it off when running. In each case it will usually make another attempt to start.

Also a nearly flat battery can cause failure during the pre start checks.

Eberspacher quote all electrical measurements can vary by 10%, however the voltage cutoffs are set quite accurately.

A short - term get you going solution can be to start the engine or connect the charger so the battery voltage is increased; then switch on the heater.  Another help is to switch off items taking high currents from the battery.

The permanent solutions are one or more of the following:

If the battery is in poor condition replace it. Now is a good time to decide if you need a larger capacity battery, especially if you have installed a fair number of the latest electronic gizmos.

Change your battery charging routine so the battery is re-charged before it gets too low.

Check all power connections are clean and secure. Check there is no corrosion between the mating surfaces.

Check the cables for physical damage. All connections must be of a good professional standard; no twisted unclamped wires.

Remember a bad connection in the return wire has just as much effect as one in the supply wire.

Check the fuse and fuse holder are in good condition.  The barrel type fuse holder fitted on older models such as the D1LC compact is very unreliable, replace with a good quality blade type fuse holder. The fuse should be changed to 30 amps for older heaters which take 20 amps glow pin current such as the D1, D3, D5,  D*LC  D*LCC and similar age water heaters.

Reroute the wires to shorten the length and / or replace them with thicker wires.

Eberspacher recommend 4mm2 wires for up to 2.5m individual wire length; ie 2.5m positive wire and 2.5m return wire.

Increase the cable size to 6mm2 up to 4m, and 10mm2 or more for longer runs. The length calculation should include cables from the battery terminal to the connection point. Treat these as minimum sizes, the thicker the cable is the better it will be for starting.

Older models take about 20 amps during starting so thicker cables than these will be needed.

If the wire supplying the Eberspacher also carries current for other devices and is not very high current rated, consider running a new cable from the battery switch; or if no switch is fitted, from the battery terminals. Remember to include a fuse.

Eberspachers also cut off if the voltage rises above 16 volts for 20 seconds. This voltage can be reached if the battery is charged at a high rate when fully charged. The solution is to switch the charger off or to reduce the charge rate to trickle charge.

Typical power wiring to Eberspacher, fuses etc are not shown.

Eberspacher D2 wiring loom connector.

The insulation prevents easy testmeter access to the pins.

Positive red wire on pin 1, Negative brown wire on pin 10.

The voltage needs to be measured when the glow plug is on during starting which is when you usually need to be away from the heater to turn it on with the control switch.

The problem can be so intermittent by the time you have sorted out the testmeter and got access to the terminals, the problem has gone and it starts perfectly.

Also D2 connectors have sealant around the wires which makes it difficult to test the end of the cable while the unit is connected.

Overcoming these problems is easy if you make a simple wiring modification so the voltage can be easily measured.

The modification consists of permanently connecting two wires to the Eberspacher power wires.

Optionally you can fit a permanent voltmeter to monitor both the battery and Eberspacher voltages.

The obvious place to make the connections would be to the short length of wire between the loom and loom connector.

Personally I don't like making connections here as the wiring generally experiences maximum mechanical stress close to the connectors. I prefer to make the connection further up the cable and to hide the joint inside the loom sheath which gives a more professional appearance, there are similar joints hidden higher up in the loom.

The difference in voltage reading between these two points is negligible and the joint has better mechanical protection.

Eberspacher connector.

Similar insulation on the D2 connector pins.

Open up the loom about 5 to 10 cm from the connector, it may be necessary to carefully cut a slit in the sheath.

Ease out a couple of inches or so of the two thicker wires, the positive supply is red, the negative supply is brown.

Join wires onto both of these. Make sure you can identify the positive wire, I used red for positive but any colour will be ok.

My method is to remove some insulation, taking great care not to cut the wire strands and to solder the new wires onto them.

After insulating each joint the wires can be pushed back inside the loom sheath.

Alternatively joints could be made with crimp connectors or any other method that gives a reliable permanent connection.

Don't just twist the wires together, although it may work for a while the wires will oxidise, any subsequent movement of the joint causes intermittent connections.

The new wires can exit the sheath at that point or be routed inside the loom itself. I use an electricians wiremans tape or a piece of fairly stiff wire such as the earthing conductor removed from a scrap piece of 2.5mm twin and earth mains cable to thread through the loom. I temporarily fix the wires onto it with insulating tape and pull them gently through the loom.

The new wires will only be carrying very low currents for measuring, about 10mA maximum, so can be thin and can be quite long, say up to about 10m if required without affecting the voltage measurement.

The end of the new wires can be terminated on a screw terminal block in an area that is convenient for attaching a multimeter when needed.

However a far better method is to fit a permanent voltmeter. With some additional wiring and a switch the same voltmeter can read both the battery voltage and the Eberspacher voltage, making it doubly useful.

Eberspacher D1LC Compact wiring loom.

Eberspacher D1LC Compact loom connector.

Positive red wire on pin 5, Negative brown wire on pin 11.

The Eberspacher will shut down if the voltage drops below 10.5 volts or rises above about 16 volts for 20 seconds.

I would be concerned if the measured voltage at the Eberspacher regularly dropped below about 11 volts, it may still operate ok but it indicates there is a potential low voltage problem. The maximum tolerable difference between the battery and Eberspacher voltage readings should be 0.5 volts and ideally less than half that amount.

The voltmeter is also useful for checking power is connected when absolutely nothing happens if  the heater is switched on.

Since I installed a meter on the D1LCC fitted in my van it proved its worth when I had a problem starting the heater. The meter immediately identified the voltage was below normal, and I was able to quickly localise the intermittent fault to the two pin main power connector which I cut out and replaced.

The Eberspacher voltage will only be needed very occasionally so the voltmeter's main use can be to monitor the battery.

It would be nice if there was a simple set of voltage readings that could tell you how discharged your battery is.

I spent a lot of time trying to make sense of published voltage tables. 

The fully charged voltage is roughly 12.6 or 12.7 volts for batteries that have been rested, gel type batteries usually slightly higher.

One manufacturer quotes the following voltages for batteries with no charge or discharge for 6 hours, ie fully rested before test.

100% charged 12.7v,  80% charged 12.5v,  60% charged 12.2v,  40% charged 11.9v,  20% charged 11.6v,  fully discharged 11.4v

Every battery manufacturer quotes different figures for batteries during discharge and there are such wide variations between them it is impossible to make a decision.

Eventually I found some data for one particular make of battery that could help to explain some of these variations.

In April 2011 I bought a dedicated voltmeter, described on ebay as Blue LCD Digital Voltmeter 20v, for under £4 including pp.

No separate power is needed, it takes power from the supply being measured. Working range: 7-20V DC.  Power drawn:  7-10mA.

This is only suitable for 12v heaters, extended range versions are also available which can be fitted on 24v systems.

Overall outer bezel dimensions:78 x 42 mm. Cutout dimensions:72 x 39mm

The switch should be double pole double throw (DPDT), with a center off position.

Maplin stock two types, 10A toggle JK30H at £3.49 and sub miniature FH05F at £2.49

If you buy from ebay select one with center off, ie "on off on"  and not "on - on", as both types are listed.

Mount the voltmeter and switch in a convenient position, if the voltmeter is close to the switch its wires will not need extending.

Run two wires from the battery, here I used red for the battery positive and black for the negative connection.

The wires can be thin but remember to include a fuse close to the battery, 1 amp rating is a reasonable choice.

Extend the two wires that you joined in the Eberspacher loom, here they were red for the positive and brown negative.

Your wires do not have to use the same colours but make sure you can identify which is the positive wire.

Solder the wires to the switch as shown. Your connections should be mechanically secure on the switch terminals before you solder them, ie feed the wire through the hole in the terminal or wrap it around the terminal.

My connections in the photos were just temporary connections to show where the wires connect so I only tacked them in place.

I used a toggle switch from my spares box which might be slightly different from the one you get but the connection details will be the same. The center off position allows the meter to be disconnected when not in use.

However the current consumption is very low, a 100 AH battery would power the voltmeter for more than a year if there were no other losses or battery deterioration.

The blue back-lit display looks good.

The rear of the voltmeter is not enclosed.

Joint with good sound mechanical contact before soldering.

Joint soldered, ready for insulating before placing inside the loom.

Switch with wires temporarily tacked on to show connections.

Upper red black pair to battery 12v.

Central red black pair meter connections.

Lower red brown pair to Eberspacher.

Side view showing one of the retaining clips.

Measuring battery voltage.

Measuring voltage at the Eberspacher.

The vertical axis shows battery voltage.

The horizontal axis shows percentage of full charge.

The 4 plots are for discharge currents of 5, 10, 20 and 30 Amps

with a 100AH leisure type battery.

What does the graph tell us about this battery?

If the 100AH battery is discharged at 5 Amps the voltage when it is fully discharged is 11.5 Volts.

If the battery is discharged at 30 Amps there will still be 70% charge remaining when the voltage is 11.5 Volts.

By the time the battery is fully discharged at 30 Amps the voltage will have dropped to 9.5 volts.

ie there is a difference of 2 volts between the 5 and 30 Amp discharge rates when the battery is fully discharged.

Your battery will probably have different voltages to these, they just illustrate the problem.

Hence you cannot just measure the voltage to decide how discharged your battery is.

What the voltmeter does is to give you clues.

If for example last week you measured the voltage before starting the engine and there was only just sufficient power to start the engine and this week the temperature and battery use are similar you would know not to let the voltage drop below what you measured.

Similarly you could measure the battery charge state at various times and temperatures with a hydrometer and correlate the measurements with the voltmeter readings.

The more you use the voltmeter the more familiar you will become with it and will soon be making good judgments.

Finally for those who are interested a little bit of theory.

For copper wire the voltage drop along it is given by the formula   E = 0.0167 * I * L / S

where 

E = Volt drop Volts         I  = Current Amps       L = Total length of go and return wires Meters   

S = Cross sectional area in square mm

So for 2m of cable (1m go + 1m return) at 8 amp loading using 2mm2 cable the volt drop would be

E = 0.0167 x 8 x 2 ÷ 2 volts      = 0.13 volts

Doubling the cable length would double the voltage drop.

Doubling the wire cross sectional area would halve the voltage drop.

Note that these figures do not include voltage drops across fuses, plug and socket connections etc.

This will not tell you how big the voltage drop is; only that it is the reason for the problem.

Before doing too much check for the obvious. If nothing is happening - no lights on the control and no fan running, the fault is more likely to be no power, i.e. switched off, fuse blown, wire disconnected. If lights are on but not starting, is the temperature control set too low? Next question; is the battery flatter than you think it is? If a battery voltmeter is fitted use it. Are the lights dim, do they dim more than normal when you switch anything else on?

If that has not located the problem and you suspect it is low voltage, turn off anything taking high power from the battery; charge the battery as you would normally, wait 15 - 30 minutes for the battery to partially charge before trying the Eberspacher. If it then starts up ok there is a very good chance it is the low voltage cutting off the heater. If the same happens on other occasions it will confirm the diagnosis.