2500 RPM and Alternator failing, why together?

[brw1994]

So this is the second time it's happened, I've been through bog holes and come out later on with an alternator failing to produce full charge and my car would idle between 2000-2500rpm. Both times it has happened the problems came together in pair.

First time, I flushed the alternator before I went to bed and woke up without a single problem. I assumed water must have found itself into something causing the 2500rpm but dried out over night.

However, about 2 months later the same thing happened, I flushed the alternator as much as I could and nothing happened. So out the alternator came and it was filthy, full of dried out mud. Pulled it apart to find a brush was stuck and all the contact points were cover in mud and silt.

After fixing everything and replacing it back in the car everything was fine, no high rpm and charging fine..

I want to know why is it I am getting both problems at once, what is the under charging from the alternator doing to cause a high idle? Or is this just a huge coincidence?

Thanks.

 

[DC runner]

So, I am gonna take a stab at this and totally call B.S. if anyone knows I am wrong. I will def. admit when I am spitballing.

This is my, educated car experienced opinion.

Your alternator has variable output and it is directly linked to engine RPM. The faster your engine is going, the more energy you are cranking out.

I know, for the most part, cars are barely able to maintain at idle. Some, depending on the load (accessories or drain like power plug) will actually lose power at idle. This is why we shove huge batteries in our vehicles. We understand that unless you drive for a bit after you start your car, you won't be able to replace the energy you lost in cranking the car. Cranking the car is by far the single largest energy expense and it takes it in a split second. Obviously, every car is going to be different, but this is why it is important you don't always do short, 1 mile, trips. You are never giving your car battery time to recharge.


So. Anyway. And this is my thinking, but could your cars electrical system be reading a low voltage output from your alternator and be trying to compensate by busting up the RPM to push more voltage out?

Again. I have NO IDEA if older cars are even this smart.... but this is me, at 1 am, just spit balling.

 

[brw1994]

Your logic behind your answer is spot on. You are correct in a way but wrong in another.

I found out today what actually happens is after going through a muddy bog hole and the alternator has a brush get jammed the rpm increases but from what I believe it's nothing to do with trying to increase charges but because the auto choke is linked to the feed line off the alternator to the battery and henceforth the battery is being charged and it goes down the line.

I knew what was wrong with the alternator just never understood why I was getting the rpm

 

[ToyoTech]

Stop driving in the mud. Its just bad. U get crap everywhere and can cause problems. The rpm issue could be anything and maybe not be related to the alternator acting up but more so driving in mud

Your idle rpm will not increase because the alternator is asking for it. The alternator is control how much it outputs internally without having to up te rpm.

 

[fourwd1]

:keyboard:I

Your idle rpm will not increase because the alternator is asking for it. The alternator is control how much it outputs internally without having to up te rpm.

Partially true. Correct the alt can not control engine rpms.
The alts internal regulator controls the voltage it puts out (around 14V),
but the amount of current it will supply depends on engine rpms.
At idle a 60A alt may only be able to supply 10A.

 

[Ryanroche]

Battery light and high idle

Hi guys.

I'm new to the 4Runner, being a recent new owner of a 1990 SR5.

Notice yesterday the battery light came on. Checked the battery. Low on water so I topped it up - the indicator was showing red. Doesn't have a green light - just clear now after I topped it up with demineralised water.

Drove around yesterday, no problems, however, by the afternoon it had developed a high idle - often between 2000-2500RPM.


Drove to work this morning with no problems other than the high idle - started fine.

I don't have many tools to do all the proper checks.

Is it my battery?
Is it the alternator?
Is it something else causing the high idle?

What can I do to find out?

Thanks in advance

 

[89FourRunner]

Stop driving in the mud. Its just bad. U get crap everywhere and can cause problems. The rpm issue could be anything and maybe not be related to the alternator acting up but more so driving in mud

Your idle rpm will not increase because the alternator is asking for it. The alternator is control how much it outputs internally without having to up te rpm.

x2

Mud bogging doesn't prove anything, is a game of chance - not skill - and completely destroys your vehicle. When wheeling - I would imagine if it is possible to go around mud - that would make you a more competent wheeler.

Of course - who am I to say such things.