Please tell me if I'm just crazy, or if I need to continue to troubleshoot looking for an issue. Long story short: 2000 SR5 4x4 5VZFE with 180,000 on the odometer, I've owned it for the past 2500 miles and only recently noticed the issues described below. Over the past few months I've installed lift/wheels/tires, roof rack, plugs & wires, fuel filter, oil & filter, and the rig averages 16MPG combined driving as it sits. All data provided by ScanGauge II, for reference. This rig never runs above 198-200* in the hot Texas summer, and voltage out of the alternator is solid at 14.1v. Within the past 250 miles or so, with squeaking old belts (startup only), my temp will climb to 205-210* and the voltage output drops to 13.8-13.9v simultaneously. I assumed this was due to belt slippage, so I replaced all 3 belts (alternator belt was shredded!) and took her for a drive with the same results. Can poorly tensioned belts cause this? They don't squeak/squeal and "feel" tight based on what I've read, although I do not have a belt tension gauge. Scangauge shows no codes. The coolant and oil both look fine, no loss of coolant that I have noticed. Radiator is probably original and will be replaced at some point in the near future, I've been compiling parts to replace everything that I touch whenever I change the timing belt. What am I missing? Where else should I be looking? Flush cooling system? Troubleshoot thermostat? I'd like to troubleshoot as much as possible prior to randomly replacing parts, any help/information is greatly appreciated.

Sounds like belts to me. There's a thread on here about replacing the belts and adjusting the tension. It was resurrected not all that long ago, so it shouldn't be hard to find.

My bad, it's actually a YotaTech page (referenced in a recent thread)

How to: Change Drive Belts 3.4 L 5VZ-FE - YotaTech Forums

x2 on adjusting the tension on the belts. The belt will stretch over time, especially when brand new. So if it seemed tight enough when you put it on it may not be now.

I appreciate both of you taking the time to post. I followed that install thread and it sure made life easy for me! I'll check tension and readjust as soon as I can.

I don't buy it, you would have to have a very loose belt for the voltage to drop. Under a heavy (electrical)load the alternator pulley could slip some, but not just casually. Also the water pump is run off the timing belt, the fan of course is run by the accessory belts, two of them, so not likely to slip.

Of course if all your belts are loose, pay me no mind.

If your having a problem with your charging system then it can only be 4 things

1. Belt
2. Alternator
3. Battery
4. Wires and wire connections

As the voltage regulator is inside the alternator my bet is the bearing in the alternator or the windings melted a little or both.
Also make sure you clean all your grounds as well for good measure.

I'm trying to figure out why the two seem to occur at the same time, intermittently at times. What temperature is considered "hot"? Am I wrong in thinking 205* is abnormal? If it weren't for my ScanGauge I wouldn't have noticed anything.

Im super confused here. 13.8-13.9v is perfectly fine voltage. When cold higher idle could cause the little up in voltage. But that is way more than enough to give your 12v battery a good charge and run all your accessories. We are talking a .2v to .3v drop.

And in no way is a loose belt going to cause this drop. If the belt slips and alt slips you will see a drastic drop in voltage.

Yeah, I just noticed in OP's post that he was only talking about a slight drop in voltage. I originally thought I read a more drastic drop.

My Alt. belt is slipping at the moment and it's noticeable on the Voltmeter. from 13.9/14.1, the belt will squeal and voltage drops to 11.8/12.2 range.

I'll ignore the slight voltage drop for the time being and focus on the temperature. At what point is "too hot"? I did some searching and read that the temps I'm seeing are considered normal (by some people, not all) and that operating temps can climb in the winter, which confuses me. I love this T4R, although it's a 2nd vehicle for me, I don't want to jeopardize the longevity that these are known for.

212* is optimal engine temp for almost any vehicle. Too hot is 230* and above which can cause damage. Anything over 220* is time for concern. Under 200* and your heater sucks!!

Is your ac compressor kicking on when this happens? It could potentially account for the increase in both the electrical load and coolant temperature.

Possibly, but it's not something that I've ever noticed previously, and I think I'd see that during our 100*+ summer days. I'll definitely keep an eye on that...

Logged another 100 miles of various driving, no change in symptoms, temps never exceed 210*. Maybe I'm just totally crazy...thanks to all who have chimed in.