MOT rules - dipped beam switching permanently linked to sidelights allowed?

[wasabi]

Long story short - very elderly Audi A3 8L, something kept blowing fuses all this week on dipped beam on both sides and light controller fuse too. Suspect control circuitry and not something near bulbs, given disconnecting one side still blows several fuses. Scary.

Gave up trying to find fault and instead have bodged up a mini circuit completely independent for these 2 lights - and it works. Only problem is my little switch is currently under the bonnet. So long term have two choices. One is 'do it properly' and wire the thing through the bulkhead and trace it back to the proper control dial. Downside of this is it might be that very dial blowing the fuses, and is a PITA to do the run. Or (lazy hat on) simply fire a relay into the sidelight circuit and use it to switch the dipped beam on. It will work, but will it keep MOT station happy?

[peterb]

Ask the MoT station first. They are testing against the construction and use regulations, so if you are complying with those, you will be OK.

Best place to ask would probably be a DVLA testing station, and the question is "do the side lights and headlights have to be switched independentlty?" If my understanding of the problem is correct.

[filbert]

When I did them, side lights had to be independent to headlights.

[herulach]

I'd guess its not a hard requirement, my c30 had no sidelights from the factory (since it didn't have discrete drls it just used the dipped beam with no rear lights) However, I know there are rules about switches having to work if they're fitted. Best to check before hand I'd guess.

[DanceswithUnix]

My first thought is that you have a wiring loom short.

A mechanic I used to use told me he rewired VW/Audi wiring looms on something like a monthly basis, and he had quite a small garage so I was quite surprised it was that often.

[wasabi]

My first thought is that you have a wiring loom short.

A mechanic I used to use told me he rewired VW/Audi wiring looms on something like a monthly basis, and he had quite a small garage so I was quite surprised it was that often.

My first thought too after some quick process-of-elimination diagnostics. To be fair she is 16 years old - have had it 6 months and this is the first fault. Exactly where the the issue is - plus lack of daylight hours and it being freezing cold makes me unwilling to dig deeper till next summer. One gotcha I have if I take it to a garage is that the headlight cluster is a cheapy aftermarket affair with HIDs. They're going to look at it and want to replace with factory spec. even if I say look, have disconnected HIDs individually and the fault still happens within minutes on the other side. It is always going to be on their mind. Mine too still if I'm being honest.

Also I'm a cheapskate Safe workaround that doesn't leave me plunged into darkness is all I care about as the end user.

[herulach]

My first thought too after some quick process-of-elimination diagnostics. To be fair she is 16 years old - have had it 6 months and this is the first fault. Exactly where the the issue is - plus lack of daylight hours and it being freezing cold makes me unwilling to dig deeper till next summer. One gotcha I have if I take it to a garage is that the headlight cluster is a cheapy aftermarket affair with HIDs. They're going to look at it and want to replace with factory spec. even if I say look, have disconnected HIDs individually and the fault still happens within minutes on the other side. It is always going to be on their mind. Mine too still if I'm being honest.

Also I'm a cheapskate Safe workaround that doesn't leave me plunged into darkness is all I care about as the end user.

Whether switch fails is a bit moot then as the HIDs definitely will if you don't have all the auto level/wash gubbins

[wasabi]

Whether switch fails is a bit moot then as the HIDs definitely will if you don't have all the auto level/wash gubbins

Have run HIDs for years on cars and bikes, and never had the gubbins and never had an MOT fail either. Internet myth really. It wouldn't pass construction and use, but that is a different test.

[herulach]

Have run HIDs for years on cars and bikes, and never had the gubbins and never had an MOT fail either. Internet myth really. It wouldn't pass construction and use, but that is a different test.

They changed the rules last year to make it an explicit failure if it's absent auto level/wash. I suppose there's every possibility they'll still pass, but it is a failure

[Butcher]

I'd guess its not a hard requirement, my c30 had no sidelights from the factory (since it didn't have discrete drls it just used the dipped beam with no rear lights) However, I know there are rules about switches having to work if they're fitted. Best to check before hand I'd guess.

Your C30 did have sidelights as they are a legal requirement. You seem to have DRLs and sidelights confused though. Sidelights are low power lights required for using a car at night and must have both front rear switched together. DRLs are for use in the daytime and don't require rear lighting.

[wasabi]

Reviving old thread.

Part of sidelight relay idea motivation was A lazyness, and B, I couldn't find how to get from engine bay to inside. Now finally found the grommet so have in the meantime been running in dirty bodge mode. Battery - 25A fuse - switch run into the cabin - splitter - two 15A fuses to two lights. Not good and I don't like the 25A fuse single point of failure although not had any issues.

I'm trying to remove as many single-point-of-failure items as possible from the circuit. Am I right in thinking the following would be best short of paying Audi more than the car is worth to fix it properly?

Assuming 2 completely independent circuits apart from switch.

Battery - inline mini blade fuse - one half of DPST rotary switch (rotary to avoid accidentally knocking) - light

OR

Add two relays back into switching side of the circuit but same principle as above. But it adds twice the number of fuses into the whole affair and I'm tempted to keep it simple. However losing the relays means I won't be able to go back to the original Audi switch should I want to for some reason.

Sourcing suitable DPST rotary switch is proving a little troublesome. Think I might end up with one designed for 240v circuits as they're nice and chunky. Something like this

[Zak33]

25 A fuse x 12 volts = 300 watts

so unless you have monster power bulbs, 25a is ... plenty!

[peterb]

25 A fuse x 12 volts = 300 watts

so unless you have monster power bulbs, 25a is ... plenty!

The purpose of the fuse is to protect the wiring feeding the device, so the rating of the fuse should be chosen with that and the load in mind.

A filament lamp has a very low resistance when cold, so the rating of the fuse has to be sufficiently high to cope with the switch on surge - which is of a very short duration.

But assuming your sidelights are 5W each that is 500mA of current, so a 5A fuse will be more than enough.

[wasabi]

25 A fuse x 12 volts = 300 watts

so unless you have monster power bulbs, 25a is ... plenty!

My concern is current setup's wiring safety reduces road-lighting safety. 25A could theoretically blow and take down both sides, even though I'd expect one of the 15A jobs to blow first. Might order a slow blow one as temp fix till weather heats up a bit and I get time to tinker.

Ideally looking as long term fix to take a single fuse blowing both sides out of the list of possible occurrences. i.e. use of a DPST switch, if it does what I think it does....