Sunday, 10 August 2008

Fitting a Volvo XC90 Towbar

Here are my top five tips to fitting a non-detachable towbar to an XC90. All the cheaper variants appear to be the same design.
  1. You will need a 19mm open-ended spanner. The chassis hanger on the exhaust side uses three bolts - the front-most bolt is in a slot that will trap a ring spanner as you tighten the bolt. Elsewhere a ring spanner combined with ratchet 19mm spanner works well.
  2. Take care with the age of the car - the wiring changed in April 2004 - the earlier model uses a cheaper vehicle specific wiring kit (approx. £40) whereas later models require a kit costing £120.
  3. Expect the towbar fitting to take about 2 hours. It is a simple design - the crossbar fits inside the rear bumper and is secured with 10 bolts, but some of these are tricky to get to - you will need two people for about an hour whilst tightening the chassis bolts. Tighten the crossbar bolts on the left-hand side first (next to the exhaust) - these are more difficult to get to.
  4. If you take the right-hand floor support out to fit a vehicle specific wiring kit leave it out whilst fitting the towbar. Access to the chassis bolts on the inside is easier.
  5. Use the Volvo electric fitting instructions, available from the Volvo owners website. These tell you how to remove the internal trim without breaking it, and how to disassemble and reassemble the lower boot components to fit the wiring.
I wasn't sure whether to go for 7 or 13 pin electrics. I went for 13 pin electrics which will become the European standard in 2009 at the expense of being incompatible with existing twin 7 pin electrics. There are adapters available on eBay. Oh, and one last thing - wear glasses when tightening the chassis bolts next to the exhaust - all manner of stuff falls into your eyes otherwise!

Saturday, 9 August 2008


If you haven't happened across the Instructables website I'd highly recommend it. It is a website devoted to communicating how to do things.

A large variety of topics are covered, but I was especially interested in a couple of descriptions as I have been looking at the issues around creating a display out of seven-segment digits. I bought a whole load off ebay a while back and am looking at creating a display comprising 10 rows of 8 characters.

The maxim chip MAX6951 uses a technique called Charlieplexing. It trades complexity of the driving logic for number of connections. I'd come across the chip (and the technique) before without knowing how it came about. Anyway, if you're interested an explanation can be found on instructables here.

I haven't found an easy way of providing a display with so many characters. Multiplexing breaks down as a technique past about 8 characters, and this in turn means the chip count goes up.