Something to keep in mind:

The voltage at the OBD port is roughly .2-.4 volts LESS than what is seen at the battery terminals.

Also the PCM only reports the FICM voltages in increments of .5.


So, you might see 12.4 volts at the OBD port and only see 12.0 reported for FICM Low.

With the FICM wiring being a marginally shorter run than all the way into the cabin etc, it's entirely possible to have higher voltage reported at the FICM than at the port. And techinally both will show slightly less voltage than a reading taken directly at the Postive terminal of the battery.

I would visually look over all 3 FICM plugs, corrosion of the FICM relay and corrosion of the PCM plugs. Also, visually check the harness as it runs over and around the valve cover and valve cover/ intake bolts.

Josh

 

Another thing to consider is that the SG2 is slow in reporting so you might be getting old readings at times when they're farthest apart, if that makes sense. I know on AE the voltages on my truck change fairly fast and very often compared to SG2.

 

I've read through this thread twice and I'm still puzzled. The original question was "how can FLP drop to 12.5v and FMP stay at 48v ?" First off FLP is FICM logic power (not low). Logic power is internally regulated and cleaned up in the logic side of the FICM. As long as it's above 12v (maybe even 5 or 6v, I don't know it's operating voltage) everything is kosher. FLP is also independent from FMP which is the output of the power side of FICM, the current supply for that side of the FICM is represented by the FVP PID.

The orange wait to start spring light on the dash doesn't represent when the glow plugs are on. It's simply a "wait to start" indicator and even at 60 something degrees the glow plugs will often be on for several seconds after the engine starts. This is a big load and significantly drops system voltage. Ford specifies the voltage can drop as low as 9.5v and not be a problem, although it could indicate a bad alt or weak batteries if it happens in warmer temps. This is consistent with what you are describing on the 10.5v after start.

Measuring voltage at the lower alt output lug isn't necessarily a good test. Unlike the upper unit which is internally regulated, the lower one while also internally regulated is controlled by the PCM. It ramps up when the upper unit is struggling in an attempt to maintain system voltage and kinda loafs along when not needed.

I explained in an earlier post how the measured voltage can be different at different points in a circuit. If there is a constant and significant difference ( say 1v or more) between bat voltage and the PID for FVP maybe there's is an issue. I'd check my ScanGauge codes for both to start with. If they're ok your back to the voltage drop test.

 

Both alternators are PCM controlled in the dual alt set-up. Even though they are designated as primary and Secondary, the PCM will switch back forth between each one and sometimes choosing both to operate as a team.

Hence why you cannot just remove the lower alternator and drive along *****-nilly, according to the Q91 Operations PDF there is cutting and splicing needed to make sure the upper alternator remains charging at all times.

Josh

 

Also just to discuss splicing battery cables. Things may have changed but when we were taught electrical "theory" they made it clear it was just that. Theory. No one had witnessed an electron traveling through a circuit, but examples could be demonstrated to support the various laws governing the behavior of an electron. One of these ideas is that electrons in a DC circuit travel near the surface of the conductor. In an AC circuit current flows easily through the interior of the conductors. That's one of the reasons why a home is wired with mostly solid copper conductors and vehicle charging systems are wired with braided wire. The braided conductor has much more surface area and can therefore carry a higher amp load. When you splice a braided wire it makes a choke point with significantly less surface area even if the splice is installed perfectly. A properly installed and insulated splice in a battery cable can last a long time IF the vehicle always starts normally, but if the starter fails and causes a huge amperage draw or there are engine problems with extended cranking, the splice will be the first place to overheat and fail. I would not recommend splicing a battery cable unless it's a temporary repair.

 

The WSM (Workshop Manual) states that the PCM monitors both generators to determine the output of each unit. It also states that the PCM controls the lower generator by turning it off when the glow plug system is commanded on. As soon as the glow plug system stops cycling, the PCM powers up the lower generator (to avoid damage to the glow plugs from excessive voltage).

It continues by stating that the generators operate independently of each other and that "The control is through the internal voltage regulators".

Based on the above, it seems to me that only the lower one is controlled and that the control is only on/off.

I realize that the WSM has been shown to have some mistakes. I wonder if this section (414-00-2) is accurate?

If it is, it would seem to me that it is possible that one of the alternators might have a bad voltage regulator.

 

I agree Josh, I was shooting for a short and simple explanation of why checking voltage at the output lug isn't a good test (to determine if the alt is good or bad) . Guess I over did it.

Do you see an issue with the op's truck?

 

I would tend to think that, as most dual alternator systems run 100%, it's up to each alternator to keep it's own voltage in check.

But looking over this:

https://www.fleet.ford.com/truckbbas/non-html/Q91.pdf

Seems to say otherwise.

Josh

 

My original question has changed into " why does flp drop to less than 13 volts when everything I've read and Ed at ficm repair say that 12.5 or less will kill a ficm."

could the voltage regulator in the flp side of the ficm be getting weak once it gets hot?


Am I being a worry wart? Hell yeah, but I want to avoid the truck being down unexpectedly or having a larger repair cost because of something that could have been prevented.

 

FICM main power or FMP is where we look for the 48v output from the amp side of the FICM.

FICM vehicle power or FVP is the power supplied to the amp that makes the 48v output. It is important that this voltage be present in sufficient quantity for the amp to do it's job. I believe this figure is what you are thinking of causing FICM damage. The amp on this side of the FICM is designed to output 48v regardless of it's input volts. If input drops far enough, long enough it will overheat and burn itself up. The input voltage is directly dependent on battery and charging system power available and the wiring connecting the bat volts to the FVP.

FICM logic power or FLP is the power supply that operates the logic side of the FICM. While it is important for this voltage to remain high enough (10.5v is a good number) that the silicone "chips" operate without errors, it isn't a huge concern for FICM damage. The vast majority of damaged FICMs have problems on the amp side of the unit, not this (logic) side.

Unlike the FMP, the max voltage you will measure on FLP and FVP will never be more than the power available in the batter/charging system. Ford says 9.5v is the minimum acceptable value after cold start while the glow plug circuit is powered up. I don't think anyone actually sees 9.5v unless there is a charging system problem, I mentioned 10.5v above as a more acceptable number for a system in good condition and often this number will stay even higher, in the 12v range. It will also vary on the same truck as the ambient temp drops. A truck that holds 12+v at 80° ambient may only hold 10v at 0° ambient due to longer glow plug "on time" and reduced battery power.

Check TSB 08-26-3 for details on checking voltages.

 

Thanks for that post, Rusty. That even made since to me, but alas, I cannot rep you.

 

Great explanation of the difference between FMP, FLP & FVP!

 

thanks for the explanation! Like I said, I'm a worry wart on preventative stuff, I need my truck to run every day.

as long as the flp is ok fluctuating to 12.5v, I won't worry about it.

thanks to everyone! I really appreciate the willingness to guide those still learning the complexities of these trucks.

 

Hey all,

I'm currently running my 4th single alternator on my 2005 Excursion (6.0) I did upgrade to the 140AMP VRS the 110AMP stock unit.. however, today when I went to the store, my battery light came on again and stayed on for about 10 mins, then went out. This is the same symptom that I've had with the other 3 110amp alternators... does anyone have any suggestions? I know the glow plug system draws an enormous amount of AMPS on start-up, but this is getting really crazy. I have two brand new motorcraft 850 CCA batteries, and on my automatic charger they top off within a few mins. WTH? Help, this problem is really hitting us in the pocket book. We love our truck, (we have a 26' boat we tow with her) but dang, I'm about to rip my hair out.

Any suggestions on what test points I should use with my multimeter to find this problem, or any reassurance that this is simply just a start-up issue with the glow plugs?

 

Is the plug and harness at the back of the alternator in good shape? Might want to look at it closely, a good connection here is important. Unfortunately, the rebuilt units from the chain stores don't have a good track record on a 6.0 powered trucks. While they might work ok on a gasoline powered sedan, the load on them is pretty high with the diesel.

After going thru a couple of alternators myself, I took one to a local shop for repairs. After returning it the first time they fixed it by installing some different "heavy duty" parts and it's been working correctly for a couple of years now. Like yours the indication was an intermittent BAT light on the dash.