How to fit a decoder to a loco with a 6-pin NEM 651 socket
Swap the blanking plug in a 'DCC ready' 6-pin loco for a plug-in decoder, line up pin 1 correctly and test it safely on the programming track.
- Time
- 30 minutes
- Difficulty
- Easy
- Cost
- Moderate cost
- Steps
- 8
- Checked
What you’ll need
- A 6-pin (NEM 651) plug-in decoder suited to the loco's motor current
- Small screwdrivers Search Amazon for precision screwdriver set
- Tweezers Search Amazon for fine tip tweezers
- Double-sided foam tape Search Amazon for double sided foam tape
- Kapton or insulating tape for exposed metal Search Amazon for kapton tape
- Magnifier or head lamp Search Amazon for magnifying head lamp
As an Amazon Associate we earn from qualifying purchases. The Amazon links go to search results, so you can compare what’s available.
Before you start
- Always disconnect power before working on the loco (ESU).
- Do not force the plug: bent pins can short and damage the decoder.
Step by step
-
Step 1: Check the loco runs well on DC
ESU says only a loco that runs smoothly on analogue is worth converting – a decoder won't fix poor running. Clean wheels and pickups and fix any mechanical faults first.
Next: step 2 -
Step 2: Remove the body
Follow the loco maker's instructions; clips and screws vary. Work on a soft cloth with power disconnected.
Next: step 3 -
Step 3: Find the socket and pin 1
The 6-pin socket is a single row of six (NEM 651). Look for the pin 1 mark on the socket or circuit board – usually *, +, a dot or '1'. Pin 1 is Motor + (orange) and the decoder plug's pin 1 is the side with the orange/red wire (NMRA S-9.1.1.1, ESU).
Next: step 4 -
Step 4: Remove and keep the blanking plug
Pull the dummy plug out straight and put it somewhere safe – you need it if you ever run the loco on DC again.
Next: step 5 -
Step 5: Plug the decoder in
Check the pins are straight, line up pin 1 with the marked corner and press in gently. ESU warns not to judge by which way the wires face – the pin 1 mark is the only reliable guide.
Next: step 6 -
Step 6: Secure and insulate
Make sure no decoder component touches metal. Fix the decoder with a small piece of double-sided tape. Don't wrap the decoder in insulating tape – ESU says it can overheat; insulate the loco's metal parts instead.
Next: step 7 -
Step 7: Test before refitting the body
Place the loco on the programming track and read CV1 (it should be 3 on a new decoder) or CV8. Then try it on the main at address 3. Check direction and lights.
Next: step 8 -
Step 8: Refit the body
Make sure no wires are pinched by the body or caught in the gears, then refit.
Tips
- The 6-pin connector is rated 0.5 A per pin continuous (0.75 A peak) – it is meant for small, non-sound installations (NMRA S-9.1.1.1).
- If direction is wrong after fitting, you can fix it with CV29 bit 0 rather than turning the plug round.
When to get expert help
If the loco has no socket and needs hard-wiring, or the motor is not isolated from the chassis, a local model shop or DCC installer can fit it for you.
Questions people ask
Lights work but in the wrong direction?
Check pin 1 alignment first. If correct, the loco may be wired differently – see CV29 bit 0 or the decoder's function mapping.
Can I fit a 6-pin decoder in an 8-pin socket?
Not directly. Use a decoder with the right plug or an adapter harness from the decoder maker.
Sources
Checked 18 October 2026. We wrote this guide in our own words, using:
- NMRA S-9.1.1.1 Six and Eight Pin Decoder Interface (nmra.org)
- NMRA S-9.1.1 Decoder Interfaces (nmra.org)
- ESU LokPilot 5 instruction manual, Edition 7 (distributor copy) (intermountain-railway.com)
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Printed from dcctrainautomation.uk/guides/fit-a-decoder-to-a-6-pin-nem651-loco/ · Checked 18 October 2026 · General information only. Follow the maker’s instructions for your own equipment, and keep mains wiring inside properly made, enclosed power supplies.