Sunday, 10 July 2011

Toyota Corolla tow-bar

Last week I re-discovered the "joys" of car repair whilst fitting my new tow-bar and its electrics. Despite the relative youth of my 2004 registered car, the usual hatefulness of seized fastenings reared its head and I spent an afternoon drilling out the two bolts I sheared off removing the air-filter box from the engine bay. (A hint for Toyota Corolla owners doing that... work inside the wheel arch and thoroughly wire-brush the road-dirt off, then oil these bolts from the underside before trying to remove them!

Anyway - I was having a bad day, and would not have even needed to remove the air-filter box had I not rather enthusiastically attempted to remove the 100A main alternator fuse with a pair of pliers - damaging it in the process.

It turns out that whilst all the other (admittedly smaller) rectangular fuses in this car just pull out - and are stiff, (so you use pliers), that the main fuse (A male straight-legged PAL type which is specific to Japanese vehicles), is BOLTED from within the fuse-box. You have to unbolt the fuse-box, split it in two - then unbolt the fuse before removing it. These fuses are quite hard to find (especially on a Sunday), and for now I've got an 80A replacement tiding me over as an interim fix until the 100A fuse comes through the post.

Car electrics are an interesting game, and despite being an electrical engineer (ok - perhaps this is WHY it took so long), fitting the tow-bar wiring was a time-consuming process. The tow-bar I bought online, with a £30+ 13-pin Euro style wiring "kit" actually only came with the hitch socket and a rather poorly thought out wiring assembly consisting of 12 wires threaded through a PVC sheath. None of the (legally required) bypass / bulb-failure notification relays were included, and I was left to improvise my own wiring to the battery.

I bought a neat little bypass relay from the local car-parts place, which performs the required functions of stopping the trailer lighting board "hiding" a blown indicator bulb on the car, and allowing you to identify a bulb failure on the trailer when towing. (It beeps when indicating if the bulbs are Ok, and is silent when you don't have a trailer attached, or its indicator bulb is blown).

It turns out that splicing into the ungenerously short wiring looms of the car is very time-consuming if you (like me) have a partly irrational hatred of the ubiquitous "ScotchLok" connectors usually used for this kind of job.

Whilst in general, I like IDC connectors, I have never liked to see these in car wiring. It is difficult to imagine how the insulation piercing contact part can be sized correctly for whatever arbitrary cable your car might be wired with. Usually, IDC connectors are designed to work with a particular type of wire, and I think Scotchlok are popular for their ease of use rather than their technical excellence.

Rather than use Scotchlok connectors and risk "damaging" my car wiring loom, I opted to take the more drastic approach of cutting the existing wiring and soldering in my new branches, sealing up the joints using adhesive lined heat-shrink tubing. This produces a much neater modification than when using bulky Scotchlok connectors, but it does take a lot more time and effort.

Being rather pedantic, I ended up creating a new wiring loom for the tow-bar electrics to tap into, using a 20 pin mating connector carrying all the necessary lighting and power connections. This should allow me to unplug and change the towing electrics if that ever became necessary.

I found that by cutting one support on the back of the C-pillar trim panel, I was able to create enough space to locate the towing relay behind that and away from the usual boot-area installation commonly seen when tow-bars are fitted. The relay beeps at me to inform of its operation, so I don't really want to bury it behind a trim panel deep inside the boot of the car underneath all my tat where I won't hear it! (I installed the towing electrics on the passenger side of the RHD vehicle. The central locking transponder module occupies the corresponding area underneath the driver's side C-pillar.

Wiring up to the vehicle's power supply was the big job.. rather than route straight to the battery, I opted to find somewhere to connect to in the dash-board fuse-box wiring. To get the new wires past the bulkhead and into the engine bay I would have had to dismantle a significant amount of the dashboard anyway, so I figured it would not be a big deal.

Removing the dashboard is a 5-spanners job according to the Haynes manual I bought _after_ having completed the task. (I wanted to check up on the wiring diagrams for my model year, and could only find the face-lift 2004- version online.) To remove the top half of the dashboard (arguably the easy half of the job), you have to:

  • Remove the air-vents
  • Unscrew the top half of the dashboard (screws under the air-vent)
  • Remove the instrument cluster from the dashboard
  • Remove the air-conditioning controls from the center console
  • Remove the car stereo (and remaining air-vents it is attached to)
  • Disconnect the sensor on the dashboard (I think this is a sensor relating to the air-conditioning system, but am not sure)
  • Disconnect the passenger airbag, and remove the large bolt which fixes it to the structural member running inside the dashboard
  • Remove the A-Pillar trims
  • Finally, unclip and remove the top of the dashboard (taking precautions to store it carefully, and upright - as there is an airbag still attached to it!)

This gives you access to the fuse board wiring, and I opted to dismantle the main feed connector to it and solder on an additional tail to the crimp-tag inside, taking it out to my new in-line fuses to power the tow-electrics. This avoids me having to route wires through the bulkhead (which is already pretty busy with wires), play with the engine-bay fuse box or add additional ugly wires directly to the battery.

Soldering to the main fuse-board feed connector was a little challenging, and required a powerful and hot gas soldering iron. The wiring feeding the fuse board is by necessity, quite large - and it and the connector it crimps to have both a lot of thermal mass and an effective thermal path to conduct heat through and away from where you're trying to solder. I certainly wouldn't recommend this method to anyone.. I'm "special" sometimes, so chose a rather obtuse (and potentially risky to achieve) way to connect my new wiring.

I added a new relay to power the switched live supply in the trailer socket, and took the coil feed for the relay from a splice off the cigarette lighter socket wiring, which was conveniently available. I also took the opportunity to gut my TomTom Satnav charger and wire the circuit board directly to the cigarette lighter wiring loom - thus freeing the socket again for other things, such as my phone charger. (My Satnav sits on a custom made bracket to keep it from obscuring my precious windscreen visibility, and the wiring is routed through the driver's side air vent where the Satanv bracket is mounted).

No comments:

Post a Comment