A few days ago I had a bad FICM. As a result one battery had boiled over. Take it in, dealer agrees, bad ficm, replaces ficm and boiled over battery but suggests to replace the other also. Wasn't sure if I was buying that both bTteries failed, take it to parts store for test, battery tests good, Alternator tests bad.

On the way there, buttery light comes one. I get the new alternator, install it, still have a bettery light. Take it back and get another battery and a replacement alternator. Install both, still have a battery light. Go back and clean all connections and grounds that I can get too. Battery light is still on.

Here's what has me stumped. I go out this am and check the batteries, 12.5v, turn the truck on, 13.5 volts at battery terminals, and 14v at alternator. After the truck warms up. Battery light comes on, but I'm showing 13.8v at alternator.

Come inside to research more on the forums, go back out, alternator shows 12.2v.

btw, both alternators tested good, off the truck, on the bench. There has to be so wing I'm missing. Why would the voltage drop after the truck warmed up? Why would the battery light stay on if alternator is putting out 13.8v?

Any help is greatly appreciated.

 

Here's a couple more tests you can do if you have a multimeter that reads AC Volts. Also inspect both sides the plug on the back of the alternator very closely. They sometimes overheat and make a poor or intermittent connection.

Checking Ripple Voltage
Ripple voltage or (AC voltage) can be measured by switching your DMM to AC and connecting the black lead to a good ground and the red lead to the "BAT" terminal on the back of the alternator, (not at the battery). A good alternator should measure less than .5 VAC with the engine running. A higher reading indicates damaged alternator diodes.

Alternator Leakage Current.To check alternator diode leakage, connect the multimeter in series with the alternator output terminal when the car is not running. Leakage current should be a couple of milliamps at most; more often, it will be on the order of 0.5 milliamps. Use care when disconnecting the alternator output wire; make sure the battery is disconnected first.

I'm a little confused on a couple things.
The FICM caused the battery to boil over? Is this a dual alternator truck or have you tried two different alternators? The dealer repaired it to start with but now you're working on it yourself?

 

A FICM requires a good voltage, but cannot fry a battery. only a bad battery or alternator can cause a fried battery.

I would always replace both batteries together, its not expensive and saves a lot of head-scratching

 

Rusty, here's what I came back with, ripple voltage is .3vac, and leakeage was 0. Is that last one still good?

On the leakage I disconnected both batteries, then alternator output wire, red probe to alternator, black to the wire.

Truck does not have dual alternators. When I got it back from the dealer, he said the ficm would boil over the battery, he said to replace it and the other. I was hoping to only replace one. After that is when I went to test alternators.

My question is, why when the truck is just started will it show higher volts from alternator, then when it is warm? Also, if the alternator is putting out 13.5+ volts will the battery light be on? Thanks for the help. Where to next?

 

Well, I thought for sure it was going to be a diode but they are good. The regulator sounds ok with the voltages you're seeing. The only thing I can think is wiring from the plug (one goes directly to the bat light). I'm a little fuzzy on how this works because the light is self powered and it takes a certain voltage to turn it off but it can be illuminated by over voltage also. Hopefully one of the other guys can be more helpful.

The battery light is pretty reliable. Did the original battery get load tested?

 

The guy that said the ficm boiled over the battery should not be within a mile of your truck.

Diodes can test good when cold and go south when warm. The term for that is self healing and is applicable to certain solid state devices.

 

I checked the connection at the back of the alternator, there is a slight arc mark on the ring. Cleaned it up and reconnected it. Battery light didn't come on right away, then it flickered, then came on steady.

First old battery replaced by ford, not sure if that was load tested.

Second battery tested good at auto zone, but replaced it from ford anyhow.

Both alternators are DL3643-18-10 alternators.

Called auto zone today to ask for the gold series alternator.

Not sure if its the alternator anymore or not. I also ready that a loose/tight belt could give the same readings. How do you know if its too tight or too loose? And is there a way to adjust the tensioner? I'll most likely get a new belt while I'm at the store.

 

I think it might be something in the wiring going to the regulator. I would do a voltage drop test from the battery to the "A" terminal (org/lt blue) wire, when the alternator isn't charging. It should be less than .5 V. I almost wonder if it could be something strange like the glow plug module robbing voltage from field coil, because I think it draws from that same circuit.

 

I suppose 2 bad alternators off the shelf is a possibility. I have gotten one on more than one occasion, but sure seems unlikely. The tensioner isn't adjustable that I'm aware of. 69cj is right of course about it working ok cold and failing when it warms up.

 

On a three terminal alternator, one lead goes directly to the dash light. When you connected that terminal, are you sure it went on properly? If no connection, that light would stay on.
It also sounds like the second wire going to the ignition to "excite" the alternator is working as you are getting charging.
I'd go back and check and clean that terminal connector.

 

I also though it was a simple circuit with the alt powering the light but we had a thread a good while back with diagrams and discussion. Can't find it now.
But the bat light circuit isn't that simple. It is powered (lit up) through the instrument panel and voltages from two other circuits are required to keep it off. I can't remember the details though. (Sorry brain ain't working good-I have the flu)

 

Alright, so assuming I did the volt drop test correctly, I read about 5v both ways.

So I would assume that my cables are bad and that there is corrosion inside the wire that I can't see?

 

Didn't see that last post.

I checked all the connections on the back of the alternator. It is a three prong connector, but only the outside slots have wires. The middle is empty. The alternator side is nice and shiny, stuck a thin pice of metal to scratch the inside of the prong on the connector. The positive lead off the alternator is cleaned up as well.

 

Went back, with a helper, to redo voltage drop test. Positive resistance was .244 but negative was still 5v. Will pull negative cables tomorrow to chase down that gremlin

Thanks everyone so far.

 

Also check your ground to the fuse panel under the dash. I wonder why you are getting these batt lights with good voltage at the batteries and from the alt charge post, so could be the power feed to that panel or it's ground. Have you recently done any work in this area recently to cause a change?