Volvo lighting funnies - Ian L
Volve S40 (a friends car, not my V40) driving along the motorway at 70mph, notice that the instrument and headlights (at least) flicker or dim briefly in braking or using the indicators. Pulled into the next services without turning off the engine repeated test with car idling but problem, increase revs in neutral to 2500rpm (cruising speed in 5th) and problem reoccurs....lights dim (almost go out) briefly when applying brake sufficiently to illuminate brake lights or turning on indicators.

Turn off car, further problems for the next 120miles home.

Weird, is there some kind of central light management box that had its logic all screwed up and the restart effectively rebooted its PLC?

any ideas welcome


Ian L.
Re: Volvo lighting funnies - sam
fan belt slipping, or dodgy earth somewhere?
Re: Volvo lighting funnies - Alwyn

Extra load from brakelights is not being supported by either battery or alternator. Maybe an earth fault too, as Sam says.

Try adding load by switching on the heated rear window. Do lights still dim or only when brakes applied? If no dim, then it's not the alternator or battery.

Check volts across battery terminal under fault and no-fault conditons and with engine running and not running.
Re: Volvo lighting funnies - Ian L
Thanks, but I am not convinced....the fault is a definite on off flicker, any lights being activated cause this, but only on the first on cycle of the indicators or the brake lights turning on...then they are back to normal and dont change when the indicators or brake lights turn off. Heater/rw demister didnt affect. Also no further problems after restarting the car.
(with the same alternator and battery after all!).

A definite fault condition that went away without moving the car.

Difficult to proceed further since the fault hasnt repeated since even under identical circumstances.

I belive that some modern cars have a PLC driven master control box for the lights with all of the switches as inputs to this box. Can anyone confirm this?
Re: Volvo lighting funnies - Ian L
No flickering (not dimming) with engine speed, just when other lighting circuits are first activated...but not after the initial switch on. I think this eliminates aproblem with the output of the alternator?
Re: Volvo lighting funnies - Alwyn
Unless it is a "tired" regulator in the alternator.

"Extra load wanted over here" shouted the Volvo. "Yeah, next time",said the regulator.

It's nothing to do with your problem, but my Volvo had a fault on the "dim" headlight circuit, you know,the 40% headlight setting. It used to flicker as I was travelling along and was a real pest.

It annoyed other drivers, always flashing their lights at me. I could not find the relay which was causing the problem and was very glad when it broke altogether.
Re: Volvo lighting funnies - dan
My 480 had the flickering dash display problem, but then appears to have every electrical problem 'threatening any moment' anyway. Didn't seem to be related to other activity or the display brightness setting and has now gone away (as has my brakelights coming on of own accord when parked up problem - old thread).

I would expect the display to dim momentarily when you wap on the headlights or something. Should l not take this as normal then, and expect to replace my regulator?

Re: Volvo lighting funnies UPDATE! - Stargazer {P}
Well, its been a long time, so long in fact that Dave kindly dragged this out of the archive (have I really been around that long?)

As for the lighting problems on a friends S40, well the Volvo dealer could never reproduce the fault, so it was ignored while it slowly got worse, in fact with time it became clear that every electrical circuit was losing power briefly whenever the electrical load increased, this was made more evident when the radio cassette was replaced with a CD player which was much more sensitive to voltage irregularities.

Eventually on using any electrical item while driving (indicators, brakelights, rear window demister, windscreen washers/wipers etc would cause the lights to dim, all instruments to fall to zero, internal instrument lights to dim, radio to cut out briefly.

At this point decided to demonstrate to dealer who agreed that it was a real problem, they provided an equivalent car for two weeks and took the car in.

Turns out that a combination of the wiring loom and ignition switch was breaking down under increased load....but not all the time. The earthing was ok. With almost a complete new loom
and new ignition switch seems ok after 10 days. All fixed under warranty as the fault had been reported before it had expired (9 months ago)

Hope this is useful to somebody, thanks to Dynamic Dave for reviving the thread.



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