I need help - Electrical

[xjdefx]

My truck is loosing voltage when I turn on the headlights. I took the battery to be tested and it checked out fine. Then I started the truck, removed the positive lead of the battery and it jumped up in voltage to around 14.01v. The alternator looks new and that jump in voltage makes me think it's fine. So, million dollar question, if the battery checks out fine, and the alternator performs well, is there anything else I should be looking at?

*Note I moved the ground as a test and it had no effect on the outcome. I need to figure this out, since I can't drive at night with this condition. Without running lights or fogs or wipers or any accessory the battery will hold charge at roughly 12.70 volts, basically enough not die.

Help please.

 

[British Slave]

What do you mean by "losing voltage"? That wasn't much of a check on the alternator, take it to a shop and have it load tested. It should produce at least 14 volts all the time. That 12.7 ain't good enough.

 

[dyeager535]

You do realize you may have just toasted your alternator, right? Removing the battery cable while it's running is a horrible idea.

Get a voltmeter, and test the output off the back of the alternator, the single red wire connected to the stud. Measure it with the truck idling, no accessories, then turn everything on and measure again. Anything less than about 13V with everything running is a real problem, I'd say if it's under about 13.5 with everything running it's a weak alternator, or your idle is too low.

If you don't have a voltmeter, go buy one. They can be had for very cheap, and are invaluable. You'll need it to see if alternator voltage is much above what your stock gauge is reading at the same time.

 

[bp71k5]

Get a voltmeter, and test the output off the back of the alternator, the single red wire connected to the stud.

Do you mean the big one that connects directly to the battery? I'm curious if there'd be a difference between measuring at the alternator post vs at the battery post?

 

[dyeager535]

Just minimizes the chance of error based on wiring...connections, corrosion, etc.

In a perfect world you'd be able to measure voltage everywhere on the vehicle and have it be exactly what's coming out of the alternator, but reality means that you can see 10% (more or less) voltage loss to your interior because of the wiring. This is exactly the reason you use relays on heavy draw items, and why that mod works on the headlights.

 

[xjdefx]

Yeah I have a voltmeter, I don't think I did any damage to the alternator, it acted the same, and I've done it before on others. I will indeed check the stud though. Anything is possible. Thanks!

 

[ChrisHall1985]

Yeah I have a voltmeter, I don't think I did any damage to the alternator, it acted the same, and I've done it before on others. I will indeed check the stud though. Anything is possible. Thanks!

MY 87 did the same thing it ended up being one cell in the battary and it tested fine a hundred times but i have also had a bad Reg in the alt do the same thing and you did not hurt the alt it a good way to test it but it can still be reg when the battary pulls juice when its hooked up

 

[xjdefx]

I figure what I'll do is have the alt tested and if it shows good, I'll go ahead and buy a new battery. Then if it still does it, I'll know to look elsewhere, athough I don't know where.