What does this mean...

[weq92f]

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While under full load and full texas heat...

If I measure 13.3v at battery + and - posts, then measure 13.7v at battery + and alternator ground post, what accounts for the difference?



Thank You!

-klb

[The Rocknrod]

Resistance?

[SunSoaked]

Originally Posted by The Rocknrod

Resistance?

[weq92f]

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I'm asking the question because I really don't understand what is happening.

Based on your responses, I'm going to assume then that the difference is simply the higher voltage at the alt.

What I am not following properly is exactly how my measurement can change by simply moving the ground point from the battery to the alt? This must be Electrical 101 then, yes? Are we not in the Electrical section of the Forum?

-klb

[The Rocknrod]

Resistance - https://science.jrank.org/pages/2335...esistance.html
And it's probably not a straight shot from the alt post to the battery so that adds to some loss.
Ohh and where the alternator "reads" the voltage makes a difference, where the (usually white) wire is attached in the system. If it reads it close to the alt then at distances farther away like the battery may be a lower voltage as well.

[weq92f]

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OK thanks, I understand voltage drop and the sensing of the multi-wire ( original ) alternator and how the internal regulator works. BTW, my alt in this case is a single wire unit.

What I'm asking about is why I see a different voltage on my meter while leaving the positive probe on the positive battery terminal at the battery and then touching the negative probe to two different places: 1) negative post on the battery ( 13.3v ) and 2) the ground post on the alternator ( 13.7v ).

That seems counter intuitive to me.

I'm not touching the alternator output post at all, not directly. I'm only using the ground post on the alt to complete the circuit as far as the DMM is concerned.

I'm showing my ignorance here,

-klb

[Andy4639]

This is basic electrical stuff.

Read through this page it will help you understand.

http://www.madelectrical.com/

The alternator puts out what it needes to keep the truck running. It doesn't put out the max voltage if it did your battery would explode. The system draw determines how much it sends to the battery to keep up with demand of the system.
Take your meter and put it on a hot wire on the fuse block inside the truck and ground it some place and see what it's reading with all your stuff on. This will give you a good reading of what your alternator is doing and if it's less than 13 volt's at idle you need some work done to the electrical system.

[Wrenchbender Ret]

Try grounding the wire to the frame of the alternator. It shouldhave the same reading as with the ground on the battery. If not you have some resistance in the bat. cable ton the engine or the alternator mounting to the engine. If it is different from the terminal you must have resistance in the ground term. to the alternator case.
George

[truckster]

The answer is electrical resistance - the voltage is being produced at the alternator, which is where you're reading 13.7 volts. When you measure at the battery + terminal and the alternator ground, you're shortening the route the current has to travel. When you're measuring at both battery terminals, the current has to travel farther. Depending on where your battery ground is, the current has to travel through the alternator bracket, possibly through the block, through a lug and into a copper wire, through another lug and into the battery post. All of those add some resistance. If any of those points are dirty or corroded they'll add more resistance.