late 04 6.0 2 year old batteries, dc power alt (185 I think) Ed's ficm with 40 hp tune. 3 weeks ago started flipping thru the scangage checking readings. noticed the flp was bouncing 12.5 to 13.5. turned on the voltage bounced from 13.9 to 13.5. Called Ed, he suggested to load test the batteries and maybe the relay was getting weak. picked up a new relay, not much change. batteries loaded tested ok, although the tech didn't disconnect them, I ASSumed he knew what he was doing. Makes more sense to test them separated from each other. Haven't pulled the alternator to have that tested it seems like it puts out enough.
basically what I want to know is why the flp drops to 12.5. the fmp hasn't dropped below 48.5 since I started watching. It seems to happen at highway speeds when the eot is higher. Around town when eot is 194 the flp stays 13.5 or higher.
I assume the scangage voltage is reading alternator output. why would the alternator output be higher than what the fmp by as much as 1 - 1.5 volts? at times the fmp is 12.5v and the vlt is 13.5 or higher. the flp dosen't go lower than 48.5, even then .
Sounds like it's time for a voltage drop test. Several good videos on youtube. I don't kno if the ScanGauge displays what the ECM tells it or if it reads the voltage from the pin in the OBDII connector. Either way, it's not uncommon for system voltage displayed to be as much as half a volt different from what you'll read at the batteries, and that can be different from what you read at the output lug on the alternator. This has to do with the wire length and diameter, where the power and loads are connected and connection resistance.because of these variables, the various circuits will commonly read a little different depending on where you take the measurement.
The FICM is designed to take the voltage that is supplied to it and output 48v. It works pretty good if input is 12.5 or so and up. When input voltage drops it must work harder to reach the 48v output but it will try, but the lower the input the more heat it makes. That's why charging system problems are so hard on it.
Clearly the voltage drops your seeing are too high. I'd do the voltage drop tests, clean up anything that doesn't measure out right and check again. Somthing a simple as a loose connection coud be the issue. Many times the problem turns out to be on the ground side of the circuit.
I ran a 1 or 2 /0 from the alternator terminal to the positive with a 200 amp in line fuse
As I've recently learned, that helps, but it's simply not enough to maintain the proper voltage for both the truck and FICM.
From my own experience:
-The Texas heat had taken it's toll on my batteries and they needed to be topped off with some distilled water. Both were low on water, a few of the plates were exposed and some of them were barely covered. Since topping off both batteries, I've noticed MUCH better performance overall.
Topping off my batteries helped the electrical system on my truck run much better and kept my FICM voltage between 13.5 and 14.0 volts, but one thing was missing: the Cable upgrade
-I went to O'reilly auto parts to purchase what I would need to perform the 2/0 battery cable upgrade. I had 10 feet of battery cable ready to check out but before I could hand him my card, another associate stopped me and asked what I was going to do. I replied that I was going to rewire my truck since the cables and connectors suck on my 6.0 sucked. He went to the back and brought this back: Super Start® 04357 - Battery Cable | O'Reilly Auto Parts
-The long and short of it is that the cable needs to be lengthened ever so much (about 1.5 to 2 inches) to fully reach the driverside battery properly, so you'll need two butt connectors and some extra cable.
-The best approach is to also to NOT replace the cable all the way down to the starter. (It's already 2/0) You're better off using some bolt cutters to cut the existing cable and use a butt connector to splice the two cables together.
With the aforementioned cable upgrade:
-My 140 amp alternator has no problems keeping my batteries charged
-FICM Voltage rarely drops below 13.5 and is able to maintain a more consistent 14 volts
-Even with the AC and lights on, my voltage rarely drops below 13.5-14 volts once the glow plugs are off.
top alternator was putting out 14.5. lower alternator 13.5. both batteries where at 14.1-2
scangage was at 13.7ish (still bouncing around). ficm (also bouncing) from 12.5 to 13.7.
While driving the truck and it's getting up to operating temp, the FLP and VLT stay close. Usually within .2 volts and they stay around 14 volts. once its hot, they get further apart. after 15ish minutes of being at operating temp the FLP will bounce down to 12.5v.
The FICM Low Power never matches the battery voltage?
it does briefly, while it's warming up (driving). after it gets close to operating temps, they may flucuate to the same voltage. I question the scangage because it will occisionally be that the flp is HIGHER than voltage, by maybe up to .5v. don't know how that can be.