RPM drop when I turn on the headlights.

[ninjacool1]

I have an old toyota with a 2e engine. I have gotten the alternator and battery checked and they are working fine. What could be the issue?

Topic

[george gonzalez]

That is perfectly normal.

[valde]

It’s an old carburetor engine. It doesn’t have ECU to keep rpms constant so rpms will change depending on load. If rpms drop too low then you need to adjust idle up.

[ninjacool1]

I see, thank you. BTW how much should the idle rpm be set to? I have it on 1000 currently.

[EricTheCarGuy]

I didn’t see where you listed the year of the vehicle but that’s OK. Electrical loads like headlights (which are a big electrical load) cause a mechanical load on the engine. The harder your alternator works, the harder the engine has to work to spin it. Therefore when you turn the headlights on, your engine has to work harder and the idle drops as a result. Most engines have an idle up feature to keep an even idle, but it really depends on the make and model how this is handled. As for what the idle should be, depends on the vehicle. It’s usually listed on the under hood sticker. Most times in the 700-800rpm range from what I’ve seen. Be mindful however as there is often a set procedure you need to follow when setting base idle. Just turning a screw is often not the way to go.

Good luck and keep us posted.

[Author]

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