When I first start my car the lights intermittently go dim before coming back to full brightness. This effects the headlights and interior lights and I also noticed last night that the fan speed drops if the blowers are on full.

It does this on and off for about half a mile from a cold start then sorts itself out, the exception being a few nights ago when I covered a full quarter of a mile thinking "my lights are shocking" before the suddenly came on at full brightness

The other day at work I stuck a multimeter across the battery with the engine running and it was reading a solid 14.4V but intermittently dropped to 12.4V (at the same time as the lights dipped). While the voltage dipped there was a squeal from somewhere in the engine bay. I tested it for about 40 seconds or so and it dipped about half a dozen times, each time accompanied by a squeal.

The car is booked in for Wednesday but I always like to go with a bit of knowledge if at all possible so what is the likely cause of this? My first thought was maybe a slipping belt?

Failing secondary air injection pump maybe?

Failing secondary air injection pump maybe?

I have a CEL on for that funnily enough. Why would that cause the lights to dim though?

I'm thinking if it's faulty and taking too much power then it could cause lower power elsewhere (as there's only a finite amount coming out of your alternator). The SAI system is the only thing I can think of which runs "intermittently" when cold other than maybe the AC compressor cutting in and out as temperatures fluctuate between < 3 (i.e. too cold and it will be disabled to avoid ice buildup) and 3+

Put the fuse for the SAI or disconnect it and see if it makea any differance,

Might be worth checking fuse box on top of battery aswell as i've heard they can cause trouble and burn themselves out

Doubt it's the SAI as it only operates for about 90 secs at most. Alternator or it's belt would my guess.

My car does this when I go stationary. I thought it was because of a dynamo setup, when the wheels run, you have more power. When they don't run you have a drop in power to conserve amps?

My car does this when I go stationary. I thought it was because of a dynamo setup, when the wheels run, you have more power. When they don't run you have a drop in power to conserve amps?

1/ Cars now don't have dynamos, they have alternators.

2/ Alternators are powered by a belt from the crankshaft so have nothing to do with the wheels. If the engine is turning the alternator is generating power.

Why not try the simple things first, slipping fan belt maybe? And what sounds like a battery starting to show its age.

I would try increasing the tension on the fan belt first to see what that does.

My car does this when I go stationary. I thought it was because of a dynamo setup, when the wheels run, you have more power. When they don't run you have a drop in power to conserve amps?

If you mean a single slight dimming of the headlights as the car returns to idle then mine has always done that too. This is a different problem though with an intermittent flashing and much larger drop in brightness.

Why not try the simple things first, slipping fan belt maybe? And what sounds like a battery starting to show its age.

I would try increasing the tension on the fan belt first to see what that does.

I'm sure that would sort it (and by sure I mean hope it is that simple!) but wouldn't have a clue where to start.

It is the sort of job that I would have a go at armed with Mr Haynes but with it being dark when I leave for work, dark when I get home and christmas taking all weekend I think I'm going to have to pay to get it sorted for me on this occasion :(

Edited 20 December, 201114 yr by lordrobs

My first thought is a failing battery, or next a dodgy alternator / voltage regulator. This assumes that all the cabling is good and that none of the crimps are started to fail / melted fuse box.

I'm sure that would sort it (and by sure I mean hope it is that simple!) but wouldn't have a clue where to start.

It is the sort of job that I would have a go at armed with Mr Haynes but with it being dark when I leave for work, dark when I get home and christmas taking all weekend I think I'm going to have to pay to get it sorted for me on this occasion :(

It's not a hard job (or at least hasn't been on any of my other cars, but I have never replaced the fan belt on the octavia) the garage will do it and it should only cost a few quid. It's the only thing that would squeal/screach with the electrics that I can think of. Plus I would see about getting a new battery, does it start ok first thing of a morning?

Yeah it turns over fine and starts having turned over a couple of times. I think the battery is the original though so it could well be time to replace it.

Just to wrap the thread up I've just been told it needs a new alternator

Just to wrap the thread up I've just been told it needs a new alternator

...and just to unwrap the thread again... despite the new alternator it is still doing it :'(

DOes the alternator have a regulator pack on it? Was it swapped between the two alternators? I would be *very* annoyed at having to shell out to replace the alternator as it appears to not be the root cause of the problem.

Tensioner failure for the alternator belt?

DOes the alternator have a regulator pack on it? Was it swapped between the two alternators? I would be *very* annoyed at having to shell out to replace the alternator as it appears to not be the root cause of the problem.

Tensioner failure for the alternator belt?

Not too sure. It is back in on the 3rd so we'll see what the diagnosis is then.

I took this video of the issue for the garage to have a look at but it shows the voltage drop that corresponds to the headlights dipping. http://youtu.be/AgBkMsrr6qA

It wasn't too bad this morning (maybe because it was a mild 9'C?) but you can still see the issue.

Please excuse my rattly engine, rubbish camera and multimeter with a dodgy LCD display (my good one is at work!)

It sounds like there is a bit of an "clonk" at the same time as the voltage drops.

A slightly off the cuff idea - could you disable your electric windows (pull out the fuses for them) and see if the problem still occurs. I know that if I try and close a window which is already closed then that can makes the lights dim a fraction. I am wondering if one of the windows is trying to close for some strange reason.....

Edited 22 December, 201114 yr by mbames

Yeah there is a noise at the same time.

I'll give the window thing a try tonight. I know what you mean though about the dipping headlights when you pull close on an already closed window. This seems to be even dimmer than that though.

Yeah there is a noise at the same time.

I'll give the window thing a try tonight. I know what you mean though about the dipping headlights when you pull close on an already closed window. This seems to be even dimmer than that though.

Maybe all the windows are trying to close at the same time.... 4 x the current draw = dimmer lights

Well despite the best efforts of the garage and around 10 hours of diagnostics spread over 4 days (the issue only occurs from cold so there is a big down time in between trying to sort it) the garage though that the issue was resolved. As soon as I fired the car up it started playing up again! I think they are more frustrated with it than I am!

I'm going to try unplugging the secondary air pump myself and see if that helps but TBH I'm at a loss with it!

do they dim when you turn the steering wheel aswell? my fabia had a problem like that ages ago i would put up a window and they would stay dimmed for ages same with steering wheel it was fixed when the curtosy module and the switches for the windows where changed

Nope just on and off a random from cold for a couple of minutes :(

no problems with the wiring loom? battery's fine? do the dash lights dim at the same time?

Loom has been checked over as has every terminal, fuse, relay and anything else the garage could think of. Replacement battery was tried too, same problem.

Yes when the lights dip the dash lights also dip. Basically something is causing it to intermittently lose 2V across the battery :wonder:

There is a video in post 17 that shows the issue that I took one morning.

Edited 6 January, 201214 yr by lordrobs