17v at alternator, but 12.5 at battery. Red battery light is on. Need advice, supposed to be camping! (1 Viewer)

[Surfdc]

A month ago my battery light came on, and my scangauge was fluctuating between 13.0 and 18v.
After some testing, I replaced the alternator with a Denso from dealer and also replaced battery.

No change

Next I cleaned the 3 prong plug out of back of alt and that fixed for rest of February.

This afternoon, after packing car for camping, the red light came back and scangauge is fluctuating again.

Tested volts with car running:
12.59v at battery posts
17.0v at alt post and ground to block.

Pulled alt and took it to AutoZone and had it tested and it passed. So I'm assuming i didn't get a bad alternator and its not the voltage regulator.

All grounds intact and clean.

I'm heading to the dealer tomorrow morning 7am to get a new fusible link.

I've searched till my eyes crossed, So thanks in advance for any help so I can get to the campsite at least tomorrow sometime.

 

[White Stripe]

What does your alternator plug look like?

 

[Surfdc]

 

[Malleus]

There are only two sources of electricity in the truck and one is rated at 12V. The wiring can increase resistance measured at a given point, but it cannot generate electricity.

The problem is the voltage regulator; its job is to limit the output of the alternator. Without a properly functioning voltage regulator, the alternator will continue to generate higher voltages as the engine rpm increases, reaching its maximum rated output at its maximum rated rpm. It's essentially a generator; when it spins, electricity comes out.

Rather than relying on your scangauge, put a meter on the output terminals of the alternator and increase the engine rpm to 2500. This is near the maximum output rpm for the alternator.

There could have been a short somewhere in the harness which damaged the regulator; those diodes aren't bullet proof. You should always remove the negative (ground) cable from the battery when you remove or install the alternator; one of the leads to the alternator is hot directly from the battery through the fusible link.

The battery light on the dash may or may not be related to the alternator output. If there's an open in the circuit to the combination meter, the light will come on. This can happen if the regulator fails, or if there is a loose ground or broken wire.

Is the electrical system completely stock, or has it been modfied?

 

[Surfdc]

Modifications:
-Slee light harness
-Additional side turn signals and harness that piggybacks off original turn signal
-heavy dury ground kit. (Essentially big wires that replace the original 4 grounds).

 

[azusa]

Why don't you do voltage drop test, start with batteries positive post to alternator and starter positive Post, negative battery post to engine block. Engine running turn on fan at Max and headlights, put it on the voltage setting and it should be below 1 volt. This will isolate which side you have a problem with. With that kind of a voltage drop it's going to create a lot of heat, you can feel main cables and battery terminals for excessive heat.

 

[george_tlc]

Pretty likely you have lost FL Main 2 (one of the fusible links). The OBD power comes before this link (i.e. from the Alternator side). So, OBD would show high voltage while the battery gets no charge (hence the 12.5V at the battery end).

Buy 2 fusible links while at the dealer, one for the repair and one as a field spare...

You can measure voltage right at the FL Main 2, each end, and will likely see 12.5V on one side and 17V on the other. Of course, jostling the fusible link bundle may temporarily fix the problem.

cheers,
george.

 

[Surfdc]

Pretty likely you have lost FL Main 2 (one of the fusible links). The OBD power comes before this link (i.e. from the Alternator side). So, OBD would show high voltage while the battery gets no charge (hence the 12.5V at the battery end).

Buy 2 fusible links while at the dealer, one for the repair and one as a field spare...

You can measure voltage right at the FL Main 2, each end, and will likely see 12.5V on one side and 17V on the other. Of course, jostling the fusible link bundle may temporarily fix the problem.

cheers,
george.

George, you nailed it. I was Messing around with volt meter and found that everything before the FL was 17v and everything past was 12.5.
I replaced the alternator because the dealer warranty exchanged it and I picked up a new fusible link as well. Installed both and getting 14.6 volts everywhere I meter now.
It’s funny, I remember replacing the FL already, but I checked my records and it was 10 years!

 

[elijahfunk]

Good evening everyone,
had a similar issue today. Battery light was going on and off while driving voltage reached from scangauge 16.5v turned off the car to let everything cool down went to restart the car cranked freely no spark and no check engine light on ignition
Also lost power to headlights tail lights radio OBD
Any recommendations on fixes would be greatly appreciated

 

[elijahfunk]

Good evening everyone,
had a similar issue today. Battery light was going on and off while driving voltage reached from scangauge 16.5v turned off the car to let everything cool down went to restart the car cranked freely no spark and no check engine light on ignition
Also lost power to headlights tail lights radio OBD
Any recommendations on fixes would be greatly appreciated

Checked every fuse all fuses are good no power to previous mentioned affected areas

 

[george_tlc]

^ fusible links would be #1 suspect.

cheers,
george.

 

[Surfdc]

Well sure sounds like fusible link failure to me.
Next voltage regulator.
Next bad/dirty grounds
Next corroded or lose connection between battery terminals and alternator.
Street with your volt meter and see where the loss is.

 

[elijahfunk]

Well sure sounds like fusible link failure to me.
Next voltage regulator.
Next bad/dirty grounds
Next corroded or lose connection between battery terminals and alternator.
Street with your volt meter and see where the loss is.

OK, I’ll start with the feasible link, also the cable from the alternator looks a little burnt up

 

[elijahfunk]

Well sure sounds like fusible link failure to me.
Next voltage regulator.
Next bad/dirty grounds
Next corroded or lose connection between battery terminals and alternator.
Street with your volt meter and see where the loss is.

Replace fusible link still no power to affected areas

 

[george_tlc]

OBD light not coming on and engine can crank is typically/usually a fusible link issue. I'd be checking you replaced them properly and they are making good contact at the battery. Verify all battery connections and ground connections while you are at it.

Get out the multimeter and start verifying voltages at various points. Come back with more diagnostic info and we can move on from there.

cheers,
george.

 

[elijahfunk]

OBD light not coming on and engine can crank is typically/usually a fusible link issue. I'd be checking you replaced them properly and they are making good contact at the battery. Verify all battery connections and ground connections while you are at it.

Get out the multimeter and start verifying voltages at various points. Come back with more diagnostic info and we can move on from there.

cheers,
george.

Found this this is the cable that connects to the alternator,

 

[elijahfunk]

Found this this is the cable that connects to the alternator,

View attachment 3851043

Also, battery cables are connected in grounds look good confirmed connection

 

[elijahfunk]

Found this this is the cable that connects to the alternator,

View attachment 3851043

Update replacing that cable fixed everything