Wiring up tow electrics w/o Volvo module (guide)
 

hi all,

So seems to be popular opinion of many that you must use the Volvo module, and only the Volvo module for tow electronics. Whilst it does have some nice perks, and is plug and play requiring no real skill/effort, its also quite expensive. When many like me have older vehicles, its hard to justify the expenditure. I thought id make a guide to help anyone else out who may want to save a few quid. This method cost me about £20.

This was on a 2005 v70 D5 facelift, on 7pin. I did reverse too which goes nowhere. If you wanted to add relays for 12V feeds i will point out where you can do this. Easy when battery is in boot. This process is the same for literally any car which isn't factory standard LED.

You MUST NOT splice the loom straight into the vehicle wiring, it isn't designed to take the extra current draw of added lighting, and will play havoc with the on board monitoring of lamps etc. Your car won't like it. You should use a bypass relay unit to tap into the existing wiring, without actually overloading the circuits or your car realising its there. It is essentially a relay box with its own power feed which uses the existing lines to switch its own outputs from its own power.

Bits required:
bypass relay unit (with or without reverse depending on 7/13pin config)
snaplock connectors
Some single core cable (~20A)
Selection of crimps, and a crimping tool
Side cutters
Multimeter
Socket set

1) fit your towbar of choice: with tow socket of choice, then route the cable up through the gland by the battery and up into the NS rear corner of the vehicle where your rear fusebox is.

2) establish which colour loom wires do what: As there wasn't any guides around i ended up looking at the light clusters and tracing which colours went into the loom, then using a pin sharp multimeter probe, pierced said cables one at a time to discover the functions. They are on a piece of paper in the pictures, though maybe different on your vehicle. Please please check, its very important not to **** this step up!! DO NOT PIERCE THE TWISTED PAIRS - they are canbus signal lines, you will quickly break modules if you mess with these.

3) power to the relay: IF YOU AREN'T CONFIDENT HERE, PLEASE DISCONNECT THE BATTERY NEUTRAL FIRST BEFORE THIS STEP. Its very easy to accidentally short the battery out to anything metal on the chassis. I didn't bother, but i play with electric daily. Remove fuse from fuse holder before fitting. I ended up taking a permanent live straight off the battery as it was next door. This was with a crimped on m8 ring, with an inline fuse hole right by it. Never do something like this unfused or should something go wrong, a fire could easily start. For the ground, i probed around and found a nice m8 bolt i could crimp onto again, located inside the NS left compartment base. Should be able to see the two black lines in the pictures - one for the relay, and one direct for the tow electrics side. A good ground is very important for reliable lighting.

4) wire vehicle loom to relay: one by one you can use the snaplocks to 'splice' each wire to the relevant relay input. Follow the instructions which come with your relay. You simply put the existing wire in one channel, put the relay input wire in the other half, then push the metal tab in to bridge them together, folding the plastic tab over after to minimise risk of shorts. The left indicator, left sidelight, brake, fog, reverse are on the split going to the light cluster in the NS compartment. The right indicator and right sidelight are in the thicker loom, which needs to be unwrapped to access.

5) wire the relay outlets: you are ready to wire in your tow socket lead into the relay outlets. Again go one at a time, taking notice of which colour goes to which outlet. On my kit the tow socket ground doesn't pass through the relay, so i bonded it to the m8 point.

6) cable tie things into something vaguely tidy. I stuck mine to the side of the rear fuse board. It could be tidier if you spent a bit more time, but i personally don't care.

7) put fuse in the relay fuse holder and test. Have a beer and enjoy.


If you were going 13pin, you would have the reverse attached to the relay as well. The bottom left m6 electrical point on the rear fuseboard is switched live, so could easily put a relay in to feed fridge etc from battery, previously used ground point, and this switched live to activate it. I won't do a guide, there are plenty out there, and you can get cheap relay kits on ebay with all the bits you would need.