Mondeo misfire & electrics

Alternator. If your battery is hot it's certainly being overcharged. JB

Damn, beat me to it.

so after fitting the new battery and connections are now tight , does the symptoms/fault still happen?

Yes, the symptoms I described happened on the way home tonight, after changing the battery this morning.. After re-reading my post I realize that I did not make that clear.

Cheers

JB,

I understand that a hot battery indicates overcharging but how would it explain the rest of the symptoms?

Thanks (and thanks for such a quick response).

JM

Most of the cars electronics are designed for 12VDC nominal supply voltage. If the regulator fails in your alternator, the output voltage may swing as high as 50-75V! If a few diodes fail in the rectifier you'll get some nasty AC ripple on the DC rails and the sensitive electronics *really* do not like that at all.

JB

If the cars system is designed for "12v" (i.e. 11v to 14v when charging), and all of a sudden you start putting 18v, 20v, 30v into it, it'll get upset.

I work in electronics, and the term we use is "it'll let the Magic Smoke [0] out".

Pete.

[0] Magic Smoke. The material that is inside electronic components that enables them to work. If it goes wrong, the smoke escapes, and it stops working.

ok, using a multimeter you need to check the alternator output. with the engine running and all eletrics off you should get a reading of about

13.5-14 volts(appox) now turn on headlights, main beam,heated rear window and check voltage, again it should drop but stabilize at or around 12.7 volts(appox). keep watching as the fault as you say happens after 10 mins the voltage by now should be the same12.7(appox) good luck

Thanks, I'll let you know how it goes.

I see. So when it cools down, the Magic Smoke seeps back in to allow it to work for another few minutes?

Nah, Magic smoke vanishing is the end of that component and you haven't got there yet (although if your alternator is kicking out the wrong power then it could happen soon). You could have a heat related problem in the alternator. There is a bit of electronics going on in there to turn a fairly high AC voltage into a minimal DC one.

I've come across a computer that wouldn't start up on a cold day but when a tech bod wandered over and restarted it was fine. There was a fracture in the main board that completed the circuit once the board warmed up.

It could be that there's something similar in your alternator that means that it supplies the right voltage until it gets warm and a component disconnects and starts making it supply a partial AC or a higher voltage.

It could also be a problem on the ECU but that is harder to check for. I'd test the alternator first.

Warwick

Checked voltage with engine running, 12v. Revved engine, no change. Turned on lights & heated screen, voltage started to drop, at approx

11v the tacho needle dropped off and the seat belt light came on.

So I'm guessing that it was charging a bit but is now not charging at all.

Alternator now on garage floor awaiting replacement.

Thanks

John

Only if there's no battery connected. The internal resistance of a good battery will hold a fully turned on alternator down to about 17 volts maximum - less if the battery is discharged. Many electronics in a car have regulated supplies since the voltage from the battery varies so much from starting through to running, and most common regulators won't stand 75 volts without being fried.

It would be poor design if they do given the power rail is pretty dirty at the best of times. Again, most critical electronics include reverse connection protection in the form of a series diode. This will also remove any wrong polarity 'ripple'.

57 miles, several stps & starts, with replacement alternator and all looks ok. Thanks

John