High idle and the alternator (1 Viewer)

[devo]

I have a high idle, @ 1000 rpm on my 1997. I’ve read a ton of post about this and some of the reasons for the high idle and I am looking into it.
One thing that has been mentioned is the alternator. The reading suggest that if the voltage regulator is bad it can effect the idle speed.
Would someone please explain to me how the voltage regulator affects idle speed? Thanks

 

[Malleus]

I can't explain that, but I can offer this:

The ECM controls engine idle speed through the Idle Air Control (IAC) valve. AFAIK, there is no voltage input to the ECM that depends on the alternator output voltage, that is related to the IAC signal. If it exists, this is what you need to look for.

One input that is counterintuitive (at least it was to me, at first) is that the engine coolant temperature is an input to the A/C control circuit in the ECM logic. If your cooling system is faulty, at idle (for example, if your fan clutch doesn't work, or work properly), your engine temperature at idle may exceed the allowable setpoint and if your A/C is on, the ECM will turn it off, until your engine temperature drops below the fault setpoint. This is germane to your problem because the ECM uses the A/C on switch to set the engine idle speed.

All of this is connected, again, to the IAC. That thing is a source of constant frustration when it doesn't work right.

 

[devo]

Thanks @Malleus
I’ve been dealing with high idle for a few months. It was hanging out @ 2000! So with the help of mud and my FSM I changed some parts related to the idle. You could say I threw some money at it but I’ve seen some changes. The list is long!
Both 2 wire temp sensors, my IACV benched checked but I changed it anyway. Both O2 sensors, EGR valve. I pulled my TB and cleaned it and tried to adjust a new TPS but failed miserably! I even swapped the ECU (ECM) and nothing changed. My wire harness near the EGR valve looks fine. So after all of that my 1FZ-FE with 305K miles runs super smooth. No issues starting or performance. It just the pesky idle hanging @1000 and dropping when warm to @800. I never had a rough Idle. I do need to check my timing. All this trouble shooting brought to the alternator question.
I don’t know the ECM power source circuit?

 

[Malleus]

The ECM power source circuit check in the problem matrix starts on page DI-114 and is a catch all of the connections to the ECM. It's Toyota's way of saying "if you checked all the usual suspects, look in the basement and the attic, because it's got to be in there somewhere". That would be my absolute last resort. You'll need a multimeter, a pair of needle probes to go with it, and a weekend. And beer.