Lights dimming when heated screens/seats switched on...

[Classic Grad 98]

...means alternator trouble looming, doesn't it?
This was at the end of a 20 minute journey, by the way.

[Eighteeteewhy]

Aye probably...I guess you had everything on though, lights, heaters etc. That's a lot of strain. Could just be a bad earth. New or old car?

[davepoth]

Not necessarily, but probably. If you had everything on the car turned on (fan, wipers, heated screens, lights, foglights, radio, heated seats) in cold weather you'll be close to the maximum output of the alternator anyway. Get it checked see what it's kicking out amps wise.

[Eighteeteewhy]

Lol

[Classic Grad 98]

Yes, lots of gadgets and gizmos switched on, and the cold won't help the battery deliver the juice...
It's a 2003 Mondeo TDCI with 130,000 miles so I'd forgive the (presumably original) alternator if it's packing up. I'm looking for over 14v across the terminals with the engine running aren't I?
Tonight's drive was particularly gentle owing to conditions so maybe a day hooked up to the optimate and a long drive with gadgets off will cheer it up.

[WeirdNeville]

Not necessarily - those heaters are a big current draw and the battery would cold too, hurting its output. A bit of voltage drop is to be expected especially if you're running both at once.

i wouldn't freak out unless the car had other symptoms too.

[PumpkinSteve]

My car is brand new and if I use the electric windows while the heater is on the lights dim, not worried about it at all TBH.

[Classic Grad 98]

I'll just keep an eye on it. No other symptoms and the engine doesn't turn over any slower than before.

[jbi]

take it as an opportunity to upgrade to something with more amps

[Classic Grad 98]

There's an old shop down here that re-furbishes automotive rotating electrics. I go there whenever I need a starter or alternator. The oldish bloke running it always sells me a better quality, higher effort machine than i walked in with and every time he's a damn sight cheaper than a cheap parts stockist part. Doesn't mind me being in his workshop either and usually offers me a cigarette! Love places like that. Seems a rarity these days unfortunately.

[Nick1point9]

Your alternator is fine, I doubt there are many alternators that will supply sufficient current to keep lights, heated windscreen and seats etc going without a slight dip in battery voltage. I take it you were stationary, idling from the way you say at the end of a journey?

Try it again, but rev the engine to roughly a 1/3 of its usable rev range, should be fine then.

[jurbie]

My 2004 Mondeo does exactly the same thing and has done for as long as I can remember. I wouldn't worry about it.

[Classic Grad 98]

Encouraging, thank you. When I said 'at the end of a journey' what I meant was nearing the end. I was just trundling down a hill maybe a mile from home. Radio, heated seats, headlights, fogs, climate control etc all on. I switched both screens on and noticed quite a marked dimming. As you say though, it's an exceptionally high load situation. Good to know that other Mondeos are the same.

[E30M3SE]

Take it this is happening at engine idle speed...?

[Classic Grad 98]

Not quite, 1400 RPM perhaps...

[mudster]

Check the tension on the alternator drive belt just to make sure. As above though, wouldn't worry too much about the lights dimming slightly with everything switched on.

[Pothole]

I reckon every car I've ever driven has done this.

[jbi]

Classic Grad 98 said:

There's an old shop down here that re-furbishes automotive rotating electrics. I go there whenever I need a starter or alternator. The oldish bloke running it always sells me a better quality, higher effort machine than i walked in with and every time he's a damn sight cheaper than a cheap parts stockist part. Doesn't mind me being in his workshop either and usually offers me a cigarette! Love places like that. Seems a rarity these days unfortunately.

There's a place like that I know of as well...

"Come back in 20 mins and your alternator will be ready"

Place is hidden above a garage (no sign) up 3 flights of mucky stairs with old alternator parts lying all over the place.

Cheap as chips as well

[carmadgaz]

My Focus always dims slightly on turning the heated front screen on, Alternator was changed (as it packed up) and it still does it, guessing it's a high draw circuit.