I know this could be in the free manual but I'm struggling getting the old shop program to work on my Windows 10 PC.

Hoping someone is actually familiar enough to know the behavior.

The battery is a fairly large Interstate battery, which is probably a year old and seems strong. With the vehicle off, it sits at 12.x for voltage.

The other night the battery light came on in my s-type. It seemed to otherwise idle fine, but I decided to check a few things.

Turning off and turning the car back on made the battery light go away, but here is what I found with my multimeter. With the car idling and no accessories going, my voltage was around 13.8 volts. Once I started turning on accessories, blower fan, headlights, but at idle speed, it dropped to 12.4-12.8, and at a couple points it hit 12 flat or 11.x

I'm more familiar with these numbers being at or just above 14v without any accessories running and it dropping to mid-high 13s with accessories running while at idle.

I do know the alternator was replaced by the previous owner with a used/rebuilt one. But it's been in there for quite some time without a battery light - so i doubt that is the problem (in reference to the threads I found saying get your original rebuilt instead of using something different, because it causes the battery light to come on). I think if it was a problem with a different alternator - the battery light would of surfaced a long time ago.

So, after that long wind - anyone know how the voltage should behave on this 2000 4.0 s-type?

 

I think I'm going to replace it.

Fired up a bunch of accessories simulating what you would have on during winter. The car had already been running for a while.

At idle with blower motor on full, heated seats on, rear defrost and headlights on - the voltage dropped to low 11 and stayed there while idling. In my experience, that is not right.

Unless someone wanted to tell me what theirs reads for me on a 4.0...

Cheers

 

I dont have the same car as you but as the electrics are the same here is my data. when just started it charges at 14.4 volts after 5 mins the voltage drops to 13.5.

The battery reads 12.6 volts I also trickle charge the battery so that it is always fully charged because the maintance free batteries are never fully charged by our cars but only to aroud 80% capacity.

so I try recharging the battery first and test it by turning your lights on for 3 mins it has to be 12.6 volts plus.

 

The electrics are not the same.

The redesigned cars (2002.5MY-on) have a smart charging system not present on the earlier cars. It raises the voltage e.g. when cold. The early one does not.

The voltages seem rather low so I'd start by replacing the battery.

 

I agree with JagV8, I'd check/replace the battery first if need be. Then move on from there if you need to.

 

Sorry I did not know that the electronics had changed.