Hi everyone, new on here so not sure if I am doing this right, I have a dethleff tag axle motorhome on a 3.0ltr ducato, I have just had a bulb failure warning light come on, couldn't see a bulb out, so had a look the next day and think it is the top rear side/marker light, have checked the fuses and they are ok had a look at the bulb and that looked ok but wasn't working, replaced the bulb and now all the side lights have gone off, took the bulb out and lights back on, have power to the top problem light when checked with a multi meter so don't know what else to do any help appreciated.

Hi,
what age..?
what type and rating was the bulb that blew.. what did you try top replace it with..??

:idea: so - if you put the old "dead" bulb back - in what happens..??

It's a 2008 registered 2009
Think it was a festoon type bulb not sure what wattage as it didn't say but was original German bulb, replaced with a 10w from my bulb/fuse kit, light came on for a split second then went off and took other side lights off, other lights came back on once bulb removed, threw original bulb away and when I took the other bulb out of the passenger side to try that one it broke on the end so couldn't try it.warning light still on but that's probably because I don't have any bulbs in the top marker lights now.

possibility,
there is a problem with the power consumption of the bulb you have installed

do you have any spec for what was fitted from the factory..?

No nothing in the fiat handbook (that covers the cab side etc) the handbook for the actual body only covers internal lights, can't see anything that relates to the outside marker lights, will have to have another look

Found a page in fiat handbook that shows different types of bulb, the one we have should be 12v 10w which is what we tried, will have to get under the van and see if any problems with the wires.

The side lights are supplied via the body computer and there are no fuses as they are 'auto resettable' within it. It appears that when you insert your new bulb you cause the body computer to shut the side light circuits down. The fact that with the bulb removed the remaining circuits come back point to a problem with the bulb, wiring or holder. you would get the problem you describe with a high resistance connection at the holder.
Incidentally Fiat leave a control signal for 'lights on' which the motorhome converter uses to activate the amber side markers and in some instances front/rear high level markers. This buffers the circuits from the body computer and failures of these does not bring up warnings. There is sometime a combination of this e.g. on my last Rapido the rear high markers were fed from the rear side lights but the side amber and front white markers were fed via a Rapido installed relay that was activated by the Fiat 'light on signal'.

What happens if you remove the rear sidelight bulb on that side and replace the high marker?

Haven't tried that will have a go, on another point we are touring Europe for 6 months and if I can't get it sorted before we come back will the warning light cause any further problems.

Mydethleff said:

Haven't tried that will have a go, on another point we are touring Europe for 6 months and if I can't get it sorted before we come back will the warning light cause any further problems.

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The warning light being on will not cause any problems and should reset as soon as the fault is cleared. It must be a fairly simple local fault easily fixed by a auto electrician.
As I understand it with the sidemarker bulb out the warning light is out and all other lamps work?? Also inserting the marker lamp bulb causes all the lights to shut down?? If so you could rewire the high level marker into the amber side marker circuit which is buffered from the lamp fail circuitry. As a last resort connect the marker lamp into the habitation battery circuit via a 10A fuse and a switch. Just remember to switch it on when you switch the lights on.

If it were me my first step would be to remove the rear sidelight bulb on that side and replace the high marker to confirm that there is no problem to the side marker lamp wiring. Then remove the marker lamp again and replace the rear side light lamp and when happy with that try the marker lamp again. I suspect the combination of the rear side light and high marker lamp in combination is suggesting to the lamp fail circuit that there is a resistance outside the expected value.

Have tried removing the top marker light bulb and then putting a bulb in top side light, still the same, when I put the bulb back in the top side light all the tail lights go

out and as soon as I remove the bulb they come back on, I think it's probably an earth problem, just got to try and trace the wires back when I am somewhere drier, as an answer the warning light stays on no matter what I do (probably because it's not fixed)

Hi, I had a problem with the rear top marker and side light , after hours trying to trace fault which ended up was in the body computer fuse I cut the lives and rewired to the relay side of the marker lights and all is well.
My local dealer service man said they used this fix on a regular basis.

Areyou sure its the correct wattage festoon bulbs go from about 7w to 28w so the bulb you're replacing it with may be overloading the circuit.

corcai said:

Areyou sure its the correct wattage festoon bulbs go from about 7w to 28w so the bulb you're replacing it with may be overloading the circuit.

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That was my suspicion..and the bulbs keep falling apart..so maybe they are giving high resistance