Diagnose Battery Light
 

I drove to work the other day, raining had wipers, lights and defroster on. It was losing power, wipers slow couldnt tell the lights it was light out. The way home it was losing power, and died as I was pulling in a parking lot. finally got home after getting the truck jump started a few times. Today I put a multimeter on the battery engine off 12.4 volts. When I started it went to 10.4 volts for a split sec, than back to 12.1. I heard choke on when idle settled down went to 14.4. Battery light is still on.

14.4 right after startup sounds good and makes me think alternator is OK. Should drop to around 13.5 or so after a bit. I'd have the battery load tested. That 12.4 before start is marginal and the drop to 10 when cranking sounds like a bad battery.

It cranks real strong, interior light brighter than my living room light. My neibor said it was like 10.4 for a split second. than went back up to 12.4. Choke was on, when the idle came down it went to 14.4. The battery is only a year old if that. But what should the battery voltage read when the truck is off?

Engine off battery should read
12.8 is a new battery
12.5 is a 3 year old battery
12.3 is a 5/6 year old battery

Battery light comes on when alternator voltage drops below battery voltage
Battery is only used to start the engine, after that ALL voltage comes from the alternator, yes above 14volts after start up but then it should stay above 13.5volts the rest of the time engine is running, NEVER below that

test truck wiring first
Key OFF
On the back of alternator is the B+ wire/stud, use alternators case as the ground and test voltage on that wire/stud, don't disconnect it, should be EXACTLY Battery voltage, 12.4 in your case
If its not then fusible link is bad

Then unplug 3 wire connector on alternator
Test yellow wire, should be EXACTLY Battery voltage, if not fuse is blown

Test green wire, should be 0 volts
Turn key on, test green wire again, should now be battery volts, can be 12.3 in you case, this is the battery light wire, FYI


If wiring tests OK, and battery light is coming on then alternator is failing

I didnt test these wires yet because i was pulling my fuel pressure regulator off today. I did pull the battery cable off and the truck stayed running.

But I did notice that the battery was not being held down. When this started happening is when i did a 75 mile trip on the highway.

Do NOT do this, you will blow out the voltage regulator
Back in the old days there were external voltage regulators that had relays, so no problems
The new internal regulators use transistors instead of relays, so you can fry them when you pull off battery cable and reconnect with alternator running

Its your alternator so do what you want, just a heads up

Yep, I'd go with Ron and expect the alternator is failing. I definitely misread your first post. Voltage should go right up to 14+ volts after start. Your's dropping to 10 then up to 12 then to 14 shows something isn't right. Should eventually settle around 13.5 when warmed a bit. I'd still have your battery load tested since 12.4 seems borderline.

The past two days I had the battery disconnected. I hooked up battery today and it wouldnt start. I got my multimeter and the battery was 11.5 volts. I jumped it, started smoking under the hood found out valve cover leak. Than I checked the battery again it was 13.3-13.8 volts So it looks like battery is charging but not holding the charge. Thank You RonD I will never take battery cable off when engine is running!!!

Self draining is usually how a battery ends its life

Car battery's have 6 cells and plates inside each cell that trade electrons to charge up and discharge
As these electrons travel back and forth between the plates they can build up in one spot on a plate, if they build up enough to connect 2 plates together then that cell will be active and start to pull voltage from the other cells, self draining

I tested the yellow wire it was 12.4 volts, same as battery. I tested the green wire it was 0 volts, turned key on it was 12.4. I didnt do the stud, but it is sure leaning towards a bad alternator.

This morning when I went out to start my car, it started because of new battery. But I looked at the battery gauge and oil gauge they were both just above zero. I saw my neibor he took me to work its not far thank god. I broke down and bought an ALTERNATOR. I put it in when I got home. It wasnt hard took the intake off, loosened the three bolts holding the alternator. Took belt off with a 3/8 breaker bar. Took plugs off and stud wire. Reverse steps to put back on.It was definatly bad you can here the bearings as you spun it around by hand. But now the truck jumps up and down more noticeable when stopped at a light.I already changed the IAC valve last year. What can this be!!!

?? "But now the truck jumps up and down.................."??

Yes, its like the truck is starving for air or fuel. But I filled the tank, and on the way home it didnt seem to do it. It seems to act up more in gear. I was at a light it was shaking real bad I put it in neutral and it settled down. When im stepping on the gas it doesnt do it. I changed the fuel filter 6 months ago, what else can I do!!!

Reads like its misfiring, check the vacuum hoses, maybe you pulled one off doing the alternator

This has been happening since I had a half a tank of gas. This Friday I filled the tank and it stopped shaking when I was stopped.

You may have a broken/cracked hose inside the gas tank, when fuel level drops too low air is being pulled in, but this is a long shot

I did have a leak in the fuel neck, but I replaced that hose. I am from N.J. and the last two days were like 60 degrees plus. No problems with the truck in the warm weather. Today was a little cooler at night coming home, had the defroster on and it was jumping a little bit not as bad. Shut the defroster off and no jumping at all. Do you think I could have got air in the fuel system taking the fuel pressure regulator off?

You sure it's not just the AC compressor cycling? It can engage and disengage and feel like a stutter and change the RPMs a bit.

At first it was really bad, jumping up and down, the longer i stayed stopped at a light the worse it would get. Now it does seem like the compressor. How do you stop that and how did it stop doing it real bad?

If it only does it when you are on a setting that runs the AC then that could be it. Use the defrost but pull the electrical connector at the compressor and see if the problem stops.

I think it is the compressor now. But before I wouldnt even have the defroster or anything on and it would jump up and down from bumper to bumper. The longer I stayed at a light the worse it would get. As soon as I started driving it stopped. Does it do that with the defroster because the R134 is low.

Low refrigerant will cause the compressor to cycle more frequently until the level gets too low and the compressor won't run at all. If you think it's low, you'd need to get a gauge on it to verify. If you don't have a gauge set then you should take it to someplace that does auto ac work. You can really mess things up if you don't know what you're doing when it come to AC.

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Ok Ill have to check the pressure.