Voltage Drop when Hot, Warm

Ok so this is weird and annoying.
Vehicle: 2008 Sequoia with 5.7L and Tow Package. 70 000 miles.

Problem: Voltage drops to below 13V when car warms up, or its Warm... (above 150F)
When car starts in the morning (fully Cold voltage goes to 13.9 V and stays like this until car warms up. than starts dropping to 13V and hovers around 13V (+- 0.2V).

If I start the car when its warm: Voltage starts at 13V and hovers at that voltage.
Serpentine Belt was replaced some time ago with new battery (probably 4000 miles ago). No squeals during start up and when its running.
So trying to figure out what gives.
No problem before Winter now when temperatures had stood up suddenly voltage is junk.
Any ideas ?

 

I assume you checking the voltage at the battery? Or from the inside from some other source? I am not overly sure if that would make a difference, but it could. But I really think your alternator is done. Take it out, and have it tested by advance auto or auto zone. For reference, I have a 4 cyl truck and my voltage goes from (when very cold) 14.4 volts to (very warm) 13.5 volts.

 

The ECU has a voltage sense wire that controls the alternator's output. If the vehicle is cold, then when you first start the engine you will have a slightly higher output from the alternator as it does it's initial recharge of the battery from starting your engine, as well as the common parasitic loads of a vehicle that has sat for a while. It does this because after sitting, your battery voltage is at 12.6 volts, or a tiny bit lower depending on the active loads.

This is where things get a little different: When the engine is hot, your battery still has a "surface charge" that can be as high as 13.5 volts, or even more. When you start the engine the ECU will not ramp up the alternator as much, hence the 13V that you see. As mentioned the true reading should be done at the battery terminals.

If you are concerned with alternator output at idle, then an easy test would be to start the truck up warm, connect a volt meter to the battery terminals, and watch the voltage gauge as you load the system by turning on all your lights, and the high beams, as well as your hazard flashers. If the voltage remains at 13V or higher, you should be fine. If it drops down to 12.8 or more AT THE BATTERY, then you may have a weak alternator.

 

Ok I understand ECU role but FSM says truck should be between 13.2V and 14.8V when 2000 RPMS and no load. Truck stays 13V and just bounces voltage by 0.2V up and down. Nothing in manual if that voltage is at cold or warm.
Truck definitely had higher voltage than this before. So I am trying to figure out if this is Alternator dying or something else (lose ground, lose cable)
Battery problem again ( realized battery is wet)

For comparison My Tacoma stays at 14V and above. regardless if its warm or cold.

Just dont want to swap $1000 alternator for nothing, especially alternator on that Truck is major pita to swap.
My reading comes from Scanguage. but planning today to deep dive and get reading straight from Alternator cables. Wondering if any relay would play role in this. Somebody on Tundra forum wrote they had the same issue and it turns out one of those massive fuses where a problem.

 

The prior question from myself and another still stand: Where did you measure the voltage at? That is a major factor there.

 

Please see Section 9.3.3 in the Car and Deep Cycle Battery FAQ on www.batteryfaq. org for a Charging Voltage Temperature Compensation Table.

 

I am having voltage issues as well

 

Hey Gear! Haven't seen you in a while! What kind of voltage issues are you having?

 

If you have a healthy alternator and battery, then look for voltage drops in the cabling, fuses, relays, etc. on both the hot and ground sides. I bet your battery is not being fully charged. To prevent sulfation you need to keep the battery fully charged with the battery manufacturer's recommended temperature compensated charging voltages.