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Rev Drop with Lights on

Darren Groves

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So when I turn my lights on there is a significant drop in revs, approx 150rpm is that normal? Makes setting a happy idle speed for both day & night time troublesome.


I have a 45amp Lucas alternator, battery volts are fairly constant 14v whether at idle or higher revs, very small drop when lights are switched on so alternator & battery seem to cope with it OK. Lights have standard H4 halogen bulbs.


Any thoughts?

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Hi Darren, The load or power required to drive any charging plant will affect the idle

once the intial re charge has topped the battery back up the load drops and has less effect on idle


In the end of the day the charge energy comes from the fuel tank , has less effect on bigger engines

with a little more Hp at idle and alternators can produce more charge at idle so consume more energy

than the old dynamo




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Yep, this is what you'd expect.


The effective difference between an alternator and a dynamo is that the former can react to the demands on the electrical system by increasing its output. The headlights take a lot of juice and there's no such thing as a free lunch so for the altanator to generate the extra juice it has to increase it's load on the engine. In a modern car this all gets fed back to the ECU to automatically increase the fueling to keep the correct idle. In ours idle is basically 'how much the throttle is open by default' so you have the same fueling but an increased load => lower RPM, just like going up a hill without moving your right foot.


Start by making sure you're choke is set up correctly, so the first part of the choke's movement pushes the tickover cam and increases tickover - without opening the choke.  You can now do the following:

1. Increase the tickover a little for the winter months when you'll be doing more night driving.

2. When gets dark in the summer give it a little bit of choke.  I'm not talking cole morning starting choke, just that first bit to press the tickover cam and give you those few extra RPM at idle.

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