Testing an alternator.

[one4fun]

I had my alternator bench tested, passed all three. When it is on the vehicle, it will charge the battery correctly, then all of the sudden the turn signals will not blink with the headlights on, the power windows go sloooooooow and the windshield wipers barely move. When I notice this happening, I put a multimeter in the battery and find that there is only about 12.4v when the engine is running, yet when the problem is not present, is getting about 14.2 at the battery. In have gone thru my grounds and they all seem good. I wanted to try a multimeter at the alternator and then start the engine to see what the output is. My thought is this would help track down the source of the problem. Am I thinking correctly on this? If the alternator is putting out 14+ volts but the power at the battery is only 12.2, I can look for bad wiring between those two points.

So, my alternator has the two flat prong connectors for the internal regulator and one lug on the back which i believe is the wire that goes to the battery. If i disconnect the single wire from the back and connect the positive lead of my test meter to that post and the ground lead is connected to the negative battery post, will this tell me if the alternator is putting out the correct voltage?

 

[MadOgre]

No don't do that. Touch that back power post to read it but leave the wire in place that wire is what supplies your vehicle with power. It is the positive power out put of the alternator.

The voltage regulator which is inside the alternator senses when the voltage drops below a pre set amount and then kicks the alternator in to produce enough power to keep the voltage at that pre set amount usually 14.25 volts.

So if your alternator is not able to sense that you require more power it will not produce it. In other words all the grounds and positive leads need to be in good working order. It is possible that your wire cables are corroded inside there jackets. If you see green where the strands enter the jacket it could be that your wires are corroded.

Another good spot to clean is all the wire leads on the starter as this is where your alternator power is fed to the battery.

Doesn't hurt to clean the connections on the little junction box on the firewall.

I use WD40 on my connections as it helps to prevent corrosion and penetrates into the braids well.

 

[350runner]

X2 this^^^

Sent from the dust in front of you!

 

[one4fun]

Thank you. I will be diving into this on Saturday and will report back with my findings.

 

[89Suburban]

It could also be a fusible link in the alternator output line getting ready to fail. And on my truck the alternator feed wire runs right to the junction block on the firewall, not to the starter. I guess it is different for different years.

 

[MadOgre]

Yes and from there it goes to the starter and connects with the battery cable. This is the power feed to the battery. Often times that connection on the starter gets all gummed up and just needs a good cleaning.

 

[one4fun]

It ended up being the wire from the back of the alternator that was bad. It had been resting against the manifold/heads which was burning the wire. replaced the wire, wrapped it and secured it away from any hot surfaces.

It's now business as usual.

 

[MadOgre]

Right on!