The D+ label has a 5 next to it. I think it's for the 5A fuse, the other 3 fuses also are labelled like that.you would think so as its marked dplus
I’ve checked the fuses and I’ve disconnected the box .I then false fed the red/orange lead with 12v which made the step auto retract which it has not done as it should I’ve ordered a new box hope it worksThe D+ label has a 5 next to it. I think it's for the 5A fuse, the other 3 fuses also are labelled like that.
Having said that, if the relays are triggered by the D+signal, that will switch the power contacts, which will switch on voltage when the engine is running - for the fridge or split charge relay, for example.
If it doesn't switch on and you think it should, worth checking that D+fuse.
Relays are soldered and yes the habitation does not get cut off as it should on starting engineProbably it's one of the relays that has failed. It might be the same relay that cuts off all the habitation circuits when the engine starts. Is that happening as it should?
Probably the relay will just unplug to replace it. If it does the habitation circuits as well as the step, you'll need a 5-terminal relay, not the more common 4-terminal type. They look very similar, the 4 terminals are exactly the same, but there's an extra one on the 5-terminal type.
If you unplug the relay you can test it by feeding 12V to the coil terminals, which should make it click. Then test that the power contacts switch when it clicks, using the ohms range of the meter, or continuity test if the meter has that option.
New box installed step now auto retracting on starting engine also the habitation 12v switches off as it shouldAs GeoffnDee says, if you can get to the soldered relay terminals, power should be reaching one of the terminals (the COM one). It should also be reaching one of the other terminals (the NC one). It should flip to the other terminal (the NO one) when the D+ signal triggers the relay as the engine starts.
If no power at all, maybe the relay is OK and there's a problem further back with the power feed.