How to Modify a 110 Defender 12s STC50324 Tow bar electric Kit to fit a Discovery 1.

[John_P]

Why you may ask.

1. http://www.mailorder4x4.com/acatalog/Christmas_Special_Offers_.html had them on offer at 37.50 +VAT retail is 155.50 + VAT
2. Have needed one for the last 10 years have been running from a cobbled together socket wired to a cigarette lighter plug running through the centre of the vehicle

I ordered a few parts including the STC50324 mid morning via Internet, an hour or so later I received telephone call back, one of the other parts was out of stock what would I like to do, I opted for another part.
9am following day parts arrived well packaged up big thumbs up for service will use them again.

What do I get in the Kit
See attachment 01 STC 50324 Kit.jpg

1. Mounting Bracket
2. Voltage sensing Relay
3. Rubber Seal
4. 12S Socket
5. Bag of nuts, bolts, rivets and relay mounting brackets.
6. Bag of tie wraps and heat shrink tubing
7. 2 Fused Relays
8. Harness
9. Box and instructions for defender

What tools and accessories do I need.
See attachment 02 Tools.jpg

1. Heat shrink tubing (optional but preferred)
2. Fuses 2 * 5amp 1 * 10 amp.
3. 2 * bullet connectors 3 * spade connectors. and Crimping tool
4. Soldering Iron and solder
5. Screwdrivers (small and large)
6. Side cutters, pliers
7. Touch up paint
8. Drill and bits 3mm 5mm.
9. Some short lengths of cable
10. 3 * inline fuse holders
11. Heat Gun
12. Insulation tape
13. Knife
14. Rivet Gun
15. Multimeter.

A word on car electrical connections
Use the correct cable size (or larger don't go overboard be sensible) for the job.

Fuses are always good but useless if the correct size isn't fitted.

Don't be tempted to use scotch locks ever! they break the insulation and let the damp in the wire corrodes over time and you get a bad connection.

Use a soldered joint rather than a inline crimp connector, it's stronger, won't pull apart and wont degrade over time. It will also be smaller and if insulated with heat shrink tube or amalgam tape will be water tight (within reason).
Same applies for inline devices like fuse holders if it won't need to be removed don't use a spade or bullet connector just solder it and insulate properly if it ever need replacing just cut it out and solder in another.

If possible solder crimped on terminals after fitting it will help prevent joint degradation.

Properly prepared soldered joints always take longer but always last longer and are more reliable.

Planning
Before I started I laid the harness along the side of the car and reckoned I could make it fit without to much hassle.
The harness is really designed for under seat mounted battery of the defenders but then my disco is shorter than a 110 and with creative routing I could make it fit.

I decided I would mount the relays under the bonnet you could mount them under the chubby box and just extend the 2 wires to the battery this is how I would do it if I was to do it again.

Because I opted to mount on the RHS wing in the engine bay I needed to modify the harness

On with the Job

The defender 12S harness uses the plug and socket (See attachment 03 Mod connectors.jpg) to splice into the reverse light circuit at the gearbox switch (it is used as a ignition switched supply to operate the relays only when the ignition key is turned it does not use the switching function in any way) this is the same connector the disco 1 has so if you mounted the relays under the chubby box the harness would reach the switch without modification you then would only need to extend and route the 2 battery connections it would also increase the amount of harness you have to play with under the vehicle.

See attachment 04 remove insulation.jpg
I removed the insulation tape and cut of the connection to the reverse light switch as I intend to pick up an ignition feed from terminal #1 on the fascia fuse board.

See attachment 05 solder on new wires.jpg
I soldered on a length or red wire insulated with 2 layers of heat shrink tubing, extended the battery +ve wire by approximately 30cm (soldered and heat shrink tubing) and re taped up the harness. I used the piece of flexy conduit removed from the reverse light connectors to protect the extended battery +ve

See attachment 06 mod complete.jpg

Disconnect the battery (ignition on pos 11 off disconnect battery within 15 seconds or to hell with it and stuff the siren with rags until the battery dies)

I made a mounting bracket for the relay mountings but you could just drill, paint hole with touch up paint to prevent corrosion and rivet to the wing.
See attachment 07 relay bracket.jpg

I use a rivnut and secured the bracket to the wing in an existing hole and clipped on the relays.
See attachment 08 bracket in place 01.jpg and 09 bracket in place 01.jpg

I tried to pass the red wire into the main Vehicle harness through the bulkhead and into the foot well but gave up and pierced a hole through the grommet and passed the wire through

See attachment 10 wire routing.jpg
Foot well side trim panel is removed for better access you could mount the relays here above the other relays using OEM part AMR5697 relay bracket £1.80 + VAT it would gain you another 10-15cm in harness length but require passing a larger bunch of wires back through the bulkhead

Securing the wire to the existing harness and up to the fuse boards.
See attachment 11 fascia fuse board.jpg
Terminal #1 is switched live when the ignition is turned ON so this is the connection we want.

See attachment 12 ignition feed.jpg
Inline fuse is added 5amp fuse and a spade terminal crimped and soldered and heat shrinked.

See attachment 13 Harness 01, 02 and 03.jpg
I tie wrapped the harness to the existing brackets (used for brake pipes on some models)
and to the top of the outrigger passing a tie through a hole round the harness and back through the hole.

The harness then followed the top of the chassis passed the sediment unit and splits yellow and green across the rear cross member with the existing 12N harness and rear light harness and up into the boot.

 

[John_P]

Re: How to Modify a 110 Defender 12s STC50324 Tow bar electric Kit to fit a Discovery

See attachment 14 Boot connections.jpg
I've removed the rear trim to take this photo but you should have enough access from the light access panel (if you remove the seat on 7 seater)

See attachment 15 Fused connections.jpg
Make up 2 fused connectors (10 amp) bullet on one end spade on the other crimped soldered heat shrink .
I've labelled them with indelible marker pen so that I always know which wires go where as both fuse holders are the same colour

Yellow 12S harness wire via 10amp fused link to Reverse light Green Brown vehicle harness wire.
Probably should have used a smaller fuse as it should only be driving 2 * 21 watts the main fuse (F14 Fascia fuse box) on this circuit is 10amp so a 7.5 amp should be ample and would blow before the main fuse if needed.
Green 12S Harness wire via 10amp fused link to Purple (always powered) vehicle harness wire.

Some may say you don't need to put these fuses in but I'd rather have them and not need them than wish I had fitted them later especially on the purple wire that is always live.

See attachment 16 Trim inplace.jpg
You see you do just have enough room to access these wires without removing all the trim.

Fit the socket mounting bracket in a suitable place near the tow ball
Wire up the socket as follows
See attachment 17 12S wiring Diagram.jpg
I only had a small amount of slack (10cm) to my chosen mounting point on the RHS if you route the cable differently or want to mount the socket on the LHS then you may have to extend the harness.
I meant to add the Black earth wire and terminate it at an earth point in the rear but forgot so will have to do it latter.

Connect the 12S harness to the battery (Brown to positive Black to negative) the supplied lugs should slip over the bolts once the nuts are removed.
Reconnect the battery and silence that alarm.

Use a multi meter to check the correct operation of the socket.

See attachment 18 All Done.jpg
sockets fitted and protected by the rear steps.


So what would I do differently
I would probably try mounting the relays under the chubby box only reasons I didn't was that I had no suitable wire at hand to extend the battery connections, did not fancy removing the chubby box, add that earth to the socket and put an inline fuse just after the battery connection I know there is only about 75cm of un fused wire but I'm a belt & braces man.

All in all a straight forward job but then I knew what I was going to do before I started and how it could be achieved

 

[John_P]

Re: How to Modify a 110 Defender 12s STC50324 Tow bar electric Kit to fit a Discovery

Did you spot the error?

When I made up the fused links to join the two harnesses I put the labels on wrong and had the rear lights and permanent power crossed.

At least I could reach in remove the label and swap the wires round

John

 

[John_P]

Re: How to Modify a 110 Defender 12s STC50324 Tow bar electric Kit to fit a Discovery

It was getting late when I posted above and skipped a few photos and some info as it was getting late.

Here's the complete wiring diagram for the circuit.

Circuit operation

12S socket
Pin 1 Reverse lights +ve 12 volts only when ignition position II and reverse gear selected.
The vehicle is protected by a 10 amp fuse, fascia fuse box, fuse 14.
The load on the circuit will nominally be 2 * 21 watt lamps, 7.5 amp inline fuse fitted between the socket and vehicle harness in the rear load space, in the event of a fault in the 12S wiring harness or attached trailer it should always blow the 7.5 amp fuse first leaving the vehicle lights working.

Pin 2 and 6, +ve 12 volts via separate fused relays. The relays switching circuit are connected directly to the battery positive and load to the respective Blue and Red wires to the 12S socket.The relays coil is connected to the battery negative and the output terminal 4 of the voltage sensitive switch. They only energise when the ignition is turned to position II and the voltage to terminal 3 (ignition switched live) of the voltage sensitive switch rises above the required threshold (engine running battery charging) the output terminal 4 goes low enabling both relays.Harness and trailer are protected by 15 amp fuse blue wire pin 2 and 10 amp fuse red wire pin 6, both fuses located on the respective relay.

Pin 4 Constant +ve 12 volts supply. Fed via Satellite Fuse Box 2, fuse 6, 20 amp and fused in the rear load space 10 amp inline fuse.

Pin 3 -ve 12 volts "earth".

John