FAQ
TL;DR: 20 A is the stated fuse ceiling for a constant-plus feed, and “connect each ground separately” for reliability [Elektroda, Zico63, #9316229; viper555, #6790733]. Follow the pin map below to avoid lamp errors. Why it matters: Correct 13-pin wiring keeps lights legal, prevents CAN faults, and protects up to 3.5 A per lamp circuit.
Quick Facts
• Pin 3 & 13 are both grounds; pin 3 serves lights, pin 13 powers caravan feeds [Elektroda, staszekel, post #6348607]
• Wire sizes: 0.75 mm² for lights, 1–4 mm² for grounds, 4 mm² if caravan fridge is used [Elektroda, staszekel, #6348607; viper555, #6790733].
• Fuse ratings: ≤20 A on constant +30, ≤15 A on ignition +15 [Elektroda, Zico63, post #9316229]
• Load limit: one 42 W stop circuit draws 3.5 A [Elektroda, Zico63, post #8520872]
• 13→7-pin adapters cost approx. €10–15 online (*eBay listing*).
What is the standard 13-pin trailer socket wiring diagram?
Pins 1-8 handle lighting: 1-left indicator, 2-rear fog, 3-ground (lights), 4-right indicator, 5-right side light, 6-brake, 7-left side light, 8-reverse. Pins 9-13 power caravans: 9-constant +30, 10-ignition +15, 11–12 spares, 13-ground (aux). Colours vary by market [Elektroda, staszekel, post #6348607]
Can I tie grounds on pins 3 and 13 together inside the socket?
Yes. For normal trailers, shorting pins 3 and 13 simplifies wiring and works safely [Elektroda, viper555, post #6790669] Keep them separate only when a caravan fridge or battery draws heavy current [Elektroda, viper555, post #6790733]
What wire cross-section should I choose for each pin?
Use 0.75 mm² for lamps, 1 mm² minimum for common ground, and 2.5–4 mm² when powering caravan appliances from pins 9–13 [Elektroda, staszekel, #6348607; viper555, #6790733].
How big should the fuses be for the permanent and ignition feeds?
Place a 20 A fuse on pin 9 (+30) and a 15 A fuse on pin 10 (+15) when switched by a relay [Elektroda, Zico63, post #9316229]
What does pin 2a (fog‐light return) do and must I wire it?
Pin 2a disables the car’s rear fog lamp when the trailer plug is inserted, preventing glare [Elektroda, esox53, post #21171273] If your socket lacks the internal switch, leave pin 2a open; the fog will stay on both vehicles.
How do I retrofit a 13-pin socket on a CAN-bus car like VW Passat B5?
Fit a dedicated towing module (Jaeger, Westfalia). It reads lamp signals, powers the socket through relays, and prevents bulb-out warnings [Elektroda, Zico63, post #8493762] Universal modules also work when wired per diagram and coded with VCDS.
Is there a quick way to convert to old 7-pin trailers?
Use a molded 13-to-7 pin adapter; it preserves pin 3 ground and maps lighting pins correctly. Cost is about €10–15 and avoids wiring two sockets that could overload circuits [Elektroda, Zico63, post #9138688]
Why do my trailer brake lights go out after one second?
A weak factory module can’t supply 3.5 A to two 21 W bulbs. Use the lamp wire only to trigger a relay, then feed the trailer stop circuit from pin 9 constant +30 [Elektroda, mgen, post #8520823]
How can I stop the car’s fog lamp when towing without a fancy module?
Install a socket with an internal microswitch: bridge the car’s fog feed (wire 2) through the switch to the trailer’s pin 2. When the plug inserts, the switch opens and the car fog turns off [Elektroda, borbet, post #16306220]
What’s a safe 3-step method to wire a new 13-pin socket?
- Crimp wires to pins using colour map above; slide gland. 2. Mount 20 A and 15 A fuses near battery on feeds 9 and 10. 3. Test each lamp with a multimeter and a 21 W bulb load before road use. Fault-free test prevents CAN errors [Elektroda, multiple posts].
Do wire colours always match the EU diagram?
No. German looms often swap brown/black positions lights, while Scandinavian looms may use grey for constant +30. Confirm each wire with a test lamp; mis-colour is a common failure cause [Elektroda, ericssoun3m, post #19729104]
Can I run two sockets at once for a light bar and trailer?
Avoid it. Parallel sockets double current and can dim all lamps or melt thin OEM cables, as one Fiat Doblo owner discovered [Elektroda, Zico63, post #9139050] Use one socket and a Y-split only if rewired with 2.5 mm² leads.
