[ Signal diagnosis · 6 min read ]
Alternator D+ signal not reaching the Elektroblock
Your motorhome charges perfectly on 230 V mains but when driving on the road the auxiliary battery doesn't go up. The problem is almost always the alternator D+ signal not reaching the Elektroblock. Here's the 10-minute diagnosis.
What D+ actually is
D+ is a signal line coming from the alternator’s control circuit (or, on more modern alternators, from the engine ECU) that goes high (approximately 13-14 V) only when the engine is running and the alternator is producing output. It drops to 0 V when the engine stops.
The Elektroblock reads this signal to decide whether to energise the coupling relay and connect the starter battery with the auxiliary one. No D+, no coupling, no charging while driving.
10-minute diagnosis with a multimeter
Step 1: measure D+ at the alternator
Start the engine and let it idle. Open the engine bay. Locate the alternator and the D+ terminal (small spade or screw-on, usually a thin yellow or blue wire). Probe it in DC against a chassis ground point.
- Expected: 13.8-14.4 V.
- 0 V with engine running: alternator fault, not an EBL issue. Common on 10+ year old alternators. Garage job.
Step 2: measure D+ at the Elektroblock
Keep the engine running. Access the EBL’s Block 2 connector. Identify the D+ pin (typically yellow wire, specific pin position depends on EBL model — the manual shows it).
- Expected: 12-14 V (slightly lower than at the alternator due to wire resistance).
- 0 V while the alternator shows 13.8 V: wire is cut or 2 A fuse has blown.
Step 3: find the 2 A D+ fuse
Between the alternator and the Elektroblock, almost all motorhomes fit a 2 A fuse in the engine bay. Common locations:
- Fiat Ducato / Peugeot Boxer / Citroën Jumper: on the small fuse block on the right-hand side of the engine, inside a small black box.
- Mercedes Sprinter: in the main fuse box on the firewall.
- Ford Transit: on a dedicated fuse block under the driver’s seat on some campervan conversions.
If the fuse is blown, replace with a 2 A fast-blow fuse. If it blows again, there’s a short on the wire between fuse and EBL (abraded insulation touching chassis). Repair the wire first.
Common secondary causes
Modern “smart” alternators. On Euro 6 vehicles, the alternator may stop producing D+ to save fuel when the ECU decides. Workaround: a dedicated DC-DC battery-to-battery charger (Victron Orion, CTEK D250 etc.) replaces the EBL’s reliance on D+. The EBL still works but no longer handles the charging-while-driving channel.
Old alternator with worn brushes. The alternator produces 13-14 V intermittently. The EBL sees random D+ values and the coupling relay chatters. Audible symptom: metallic buzz from the EBL when engine idle changes.
Workshop repair
This fault is not inside the Elektroblock in most cases. It’s in the engine wiring, in the 2 A fuse, or in the alternator itself. There’s nothing to send for repair.
However, if measurement 2 shows that D+ reaches the EBL correctly and charging still doesn’t happen while driving, the fault is in the EBL’s logic that reads D+. In that case, bench diagnosis required and repair between €100 and €140.
Related guides
- Auxiliary battery not charging — complete charging-fault matrix.
- 70 A coupling relay — once D+ arrives, this is the next component in the chain.
- 12 V button not activating — if the LT panel is also acting up.
Read next
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Signal diagnosis
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Next step
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