The 999cc Fire 45S Uno works perfectly until I turn on the lights. As I turn on the side light the engine revs slow down. When the headlights are turned on the engine revs slow down more to the point that eventually the engine will stall. The engine always starts up again and will run OK when the lights are switched off. Other loading such as the heated rear window or engine coolant fan do not interupt the engine revs. I have cleaned the earths at the battery neg terminal and also on the inside inner wing near the headlight on the opposite side to the battery. The alternator has been tested and is OK. Any thoughts please?

might be a slightly loose / slipping alternator belt, this means the engine must work harder to produce the energy to generate power, or could be an earth problem, I would check belt tension and if that dosent do it, adjust the idle speed,,, my car also did this, i also found my car had a faulty vacuum advance unit

If the alternator belt was loose, it would just provide less resistance to the engine and not cause it to slow so much as it is. The problem then might be an occasional flat battery as the power gradually is not replenished over a few cycles.

@ramster: you have done the things I would have checked. The lights seem to be requiring a disproportionate amount of power. Perhaps as an experiment, try a length of wire from battery to the earth "hedgehog" near the headlight and see if the idle dips less. It is of course normal to have a dip in idle with things on like lights. Perhaps your idle is set too low?

Isn't it possible that the alternator bearing are worned and it is hard to turn? My uno produces the same error as yours except the "engine stall" part. But when i turned on the lights the idle speed falls to 700 from 850. I had to renew the starter motor bearings last week becouse they were worned and I think the alternator bearings are worned too.

well my belt was loose, not enough to squeal unless i hit a deep puddle,, but i found that the engine struggled especially when cold if the electrics were on ie lights heater brake light etc, it was if it was " hunting " for extra power, my battery was fully charged too ( its a good bosch battery), it also started cutting out when the brakes were applied as the lights were using power it was also flickering when indicators were on,,, I changed the belt and tightened it up now its absolutely fine,,, so before trying anything, Id say check your belt then work from there with a multimeter and clean earth points!!!

dtodd2112 said:

well my belt was loose, not enough to squeal unless i hit a deep puddle,, but i found that the engine struggled especially when cold if the electrics were on ie lights heater brake light etc, it was if it was " hunting " for extra power, my battery was fully charged too ( its a good bosch battery), it also started cutting out when the brakes were applied as the lights were using power it was also flickering when indicators were on,,, I changed the belt and tightened it up now its absolutely fine,,, so before trying anything, Id say check your belt then work from there with a multimeter and clean earth points!!!

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Fair enough. This is a symptom of old electrics and marginal ignition. If there is less/ little resistance provided by the alternator because of a slipping belt, the only way the electrical system can affect an engine is through poor ignition as a result of significant voltage drop. Most cars should run OK with the alternator disconnected. 12.5V is fine. 13.8V as provided by the alternator is better though!

Belt tension is pretty quick to check, so got to be worth a go.

well it could be worse, we could have a modern car that would be a nightmare with all its electrical complexitys :devil:
It hopefully shouldnt be serious, probably somthing stupidly simple, then youll kick yourself