Doctor Diesel

Shocking revelations

I feel that my recent experience with my Mk5 Golf 4Motion could be of interest to your readers.

On switching on one morning, my multi-function display advised that my off-side stop bulb had failed. I wasn’t sure whether this constituted an offence, bearing in mind that my car has a central stop light warning, but regardless I went ahead to change the bulb.

Working on the rear lamp units was certainly a lot easier than my experience with the front lamp units and the offending blackened bulb was soon revealed. There were two apparently standard tungsten bulbs plus a curious small central bulb which was obviously for the indicator.

As I recall, this was rated at six watts instead of the more normal 21 watts. It was then that I became aware that the two main bulbs were not dual (5w/21w) filaments (that one used to encounter Doc) but were both single filament bulbs rated at 21 watts!

I replaced the unit, but was left wondering how on earth the rear end stop lights could possibly function with such an odd bulb configuration. I subsequently contacted my local VW service department and the magic word CanBus emerged!

Apparently my Mk5 heralded the introduction of this new computerised wiring system which is now universal, with the possible exception of LED systems. I have yet to encounter anyone who has ever heard of CanBus.

I have long felt that Diesel Car Magazine should devote more time to technical details, i.e. tyres and transmission systems, as opposed to focusing on new car reviews, and I feel strongly that CanBus should be at the top of the list!

One other thing: you quite often dwell on the fact that diesel cars take a long time to warm up. I have never felt that my car was slow in pushing out some heat and it was only recently that I became aware that it was fitted with an auxiliary electric heater. Is this unusual? (Not according to Tom Jones! Doc.) I was certainly never advised that this was an option, but it would rate very highly on my list of desirable optional extras. Keep up the good work.

Ron Montgomery

Very interesting subject indeed, Ron. Thanks for the feedback regarding your thoughts on the relative space given in Diesel Car to technical features and things like new car reviews. Ian Robertson will obviously be interested in your opinions on this.

We do have a wide spectrum of readers and we have to try to meet quite varying interests. CanBus is a rather complex technology and not everyone wants to know what goes on under the bonnet and beneath the carpet!

CanBus is, as you discovered, a computerised control system for car electrics that essentially uses two control wires, Can high and Can low, to communicate with all car control systems, using coded signals. It saves a huge amount of weight and cost compared with conventional wiring, where an individual set of wires was run to each unit, like an indicator unit, or an electric window.

With CanBus the ‘high’ and ‘low’ relate to the signal speeds involved, Can ‘high’ being used for vital control systems like engine control units, ABS, and safety systems, ‘low’ being used for lighting and instrumentation. But enough for just now – some of our readers are already dozing off Ron.

Thanks for raising the subject though, which we’ll be revisiting one way or another in a future issue. Regarding electric pre-heaters, these are a standard item on many Volkswagen Group and other cars.

They used to be diesel powered on early cars, like diesel Tourans, but there were a few problems with the hot exhaust gases setting light to dry grass in camp grounds, and leaving mystified owners wondering why their trainers were apparently melting when they stood alongside the car!


webJetta1Used car buyer

I have a question regarding diesel particulate filter (DPF) issues with diesel cars. (What again? Doc.) I understand that they have only a 75 to 120,000 mile life and need to be replaced with a likely bill of anything up to £1,500, which of course needs to be taken into consideration when buying, against the petrol powered equivalent.

I wanted to ask if DPF problems are more associated with a particular make of car, as I have a £10k budget and am looking for a family/luxury car from 2008 onwards. I have considered an Audi A4 or Mercedes-Benz C-Class, but I’m looking at Mondeos as well.

My alternative is to buy a petrol car and get it converted to LPG, which seems to be the sensible option. May I have your opinions and advice?


Of course it depends very much on how long you intend to keep the car Chan, and thus whether you are likely to be hit by DPF replacement costs. There are no hard rules about DPF life and replacement frequency, which can depend on the manufacturer, how you drive, how regularly you have it serviced, and many other factors.

For all the negatives that we keep hearing about, people and fleets are still happily buying diesels for the lower running costs, and the fact that used diesels are always in demand and fetch better prices, particularly at higher mileages. Your typical motoring mix and how you drive are probably more important than the manufacturer, although some do seem to have got DPFs sorted better than others.

As long as you have some regular runs of ten miles plus, you should not encounter DPF problems. Mercedes-Benz did have quite a few DPF problems four to five years back, which I presume have now been sorted, and so did Ford with 2.0 TDCi Mondeos around that time.

From that point of view, I would favour the Audi A4, although you won’t get a 2008 car for £10K unless it has big miles. I suggest taking a look at the Honda Accord and Toyota Avensis, which both have good owner reports, are quality cars, and represent good value, particularly the latter.

You should be able to find a very nice low miles 2009 Avensis 2.0 or 2.2 D-4D for your money. I can’t really advise you on LPG conversion of petrol engines, but would suggest you steer clear.

Just as an afterthought, if you could stretch your budget just a bit, and performance is not too much of an issue, Motorpoint have a batch of 2012 1.6 TDI BlueMotion Technology DSG automatic Volkswagen Jettas, with around 16k miles on the clock, at £11,699 just now.

This latest Jetta (from 2011) is nearly as spacious as a Passat, has plenty of rear room and a huge boot. These Jettas are an absolute steal at this price, have nearly two years warranty still in force, and would last you for years. Very economical too! Just thought I’d mention it!




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