Headlights reducing idle RPM significantly

[HKforte]

I've tried doing some research regarding this, but so far I've had no luck.

I have a 1st Generation 3.0 4Runner. When I turn on my head lights, the vehicle's idle RPM significantly slows down. I'm sure I have an electrical problem, but I don't know where to begin troubleshooting. Could it be the alternator? Any ideas?

Thanks.

 

[fourwd1]

It could be the alt is going bad, but it is also somewhat normal.

The headlights create a significant drain, so the alt has to work harder, so it loads down the engine a little.

It would be especially noticeable when the engine is at idle, because the alt output at low rpms is minimal.

 

[HKforte]

Thanks for the response. Looks like my alternator may have gone bad. What's funny is that I've had this issue with this vehicle for the past 4-5 years. I would have expected the alternator to stop functioning by now, but it still works.

 

[helusay]

quickest way to check your alternator is to start the truck and disconnect your battery, if it runs the alternator is still working, but since your lights are dimming yours may be on the way out.
-good luck

 

[fourwd1]

BTW, if it is the alt it's probably the brushes, which you can replace yourself.

 

[HKforte]

The RPM does get slower when the headlights are turned on. The RPM speeds up again when the headlights are turned off. When I disconnect the positive terminal from the battery, the engine shuts off. Also, I have an 80A fuse in my fuse panel which is completely smashed but looks like the circuit is still closed, since the wire within the fuse is not broken.

I believe I should swap out both the 80A fuse and the alternator.

 

[Kmvreter]

Correct. And do it before you ruin your battery (if it already isn't) due to incorrect, low charging.

 

[Daveed]

Brushes were $3.50. New (reman) alt was $179! Easy choice for me cause only the brushes were worn out.

 

[FISH HUNTER]

Also, iin addition to the brushes, nside the alt are 4 (I think) diodes that control the voltage output (like a regulator). As these diodes go bad or break, one at a time, the voltage/amps start to drop and the battery starts to get less and less of a charge, this will kill your battery as it really never gets a full charge and is always in a catch-up mode in getting charged, this will kill the batteries life. If your local auto store can lend you a battery/alt tester (like a small Milton), you can check for the voltage drop and also do a load test on your battery.

You can try to turn your lheadlights or highbeam, and rev up the engine, do the light get a lot brighter when you rev up the engine?....if so, that is another sign of a week battery and a poor chargeing alt.

 

[helusay]

Just out of curiosity, what is your voltage meter on your dash reading while you are at idle? Mine is around 15 maybe 16.

 

[fourwd1]

The dash gauges are very inaccurate, use it for a reference only (ie, it's normally 'here', and today it's higher/lower).
Mine reads 17V, but that's not the real voltage.

The alternator has a "diode trio", and they can go bad a number of ways. I had a case where the battery would go dead over night. Hot shot and it was fine all day. Next morning dead or nearly. Troubleshooting showed there was a noticeable current drain with all fuses and wires pulled. One of the diodes had a low resistance short to ground, not dead short, but enough to pull 1.5 Amps.