This is in my friend's 2000 F350 diesel. What is it and what is it for?

 

It is an Auxiliary Powertrain Control Module, aka APCM.

It is to control engine speed for the operation of a PTO, aka Power Take Off, to drive vocational equipment like a hydraulic pump made by Chelsea or Muncie.

 

Some ambulances used them in E-Series applications of that era if I recall correctly.

 

Why would it be on a crew cab F350 that doesn't appear to have a PTO on the back of the transfer case?

 

That they did. But not that exact particular APCM shown in the photo.

The Ambulance package APCM didn't have as many buttons, but it was provisioned in the same size case.

Quite a few airporter transit busses based on the E-Series of the turn of the century were also equipped with APCMs, which had even less buttons.

 

Because there isn't one single 1999-2023 Super Duty that has a "PTO on the back of the transfer case."

The NP-271 / NP-273 transfer cases were never built with PTO capability, even with Super Duties that were factory equipped with PTO provisions.

Instead, the PTO access for '99 up Super Duties is on the side of the transmission.

When an APCM was fitted to an E-Series ambulance or airport para transit bus, the purpose served was as a throttle kicker, to elevate the idle of the 7.3L diesel to a level sufficient to charge the batteries.

The stock alternators on the 7.3L, even with the ambulance package, do not produce their nominal output amp rating at idle. The alternators produce their rated amperage at 6,000 alternator RPM, and they are fitted with roughly 3 to 1 pulleys. which means the engine needs to be at 2,000 rpm for the alternator to produce peak output.

The normal idle speed of 650 rpm does not spin the alternator fast enough to replenish the current consumed by the electrical loads of an ambulance parked at an emergency call, trying to save the life of the patient in back who needs the immediate care of both EMTs before getting underway. The APCM serves as a throttle kicker to elevate the diesel engine rpm, so that the alternator rpm will be high enough to keep up with the electrical loads consumed by all the lights and medical gadgets.

You'll notice a button on the APCM in Post #1 that says "Charge Protect". That is a setting where the APCM automatically adjusts the engine idle to sustain the system voltage to the limit of the voltage regulator, regardless of the amperage demand of current loads. The voltage set point in this application is 14.4 volts, but since the main PCM is protected by two diodes, the PCM sees about a two tenths voltage drop through the diodes. Since the APCM sees the same voltage as the PCM, the voltage level shown on the screen of the APCM with Charge Protect enabled will usually be 14.2 volts.

There are some enabling conditions for Charge Protect to work. The vehicle must be in Park if an automatic, neutral if manual. The Parking Brake must be engaged. The parking brake switch must also work, as that is how the APCM knows that the parking brake is engaged. It isn't enough to just depress the parking brake foot pedal. If the plunger to the switch hangs, then that has to be fixed before the Charge Protect feature will work.

Finally, within the enthusiast community some 24 years ago, adding a Ford APCM after the fact of production was a popular mod, lo these many years ago. So the presence of an APCM doesn't necessarily mean that the truck was originally equipped with one. To determine if the truck actually came with PTO provision, look on the side of the transmission for a 6 bolt plate.

 

Thank you very much for the detailed explanation. My buddy had no idea what this is for nor had I ever heard of such a thing. Again, thank you.

 

That's kinda cool. Could that have be used to run other accessories? Seems like it's just a more advanced up-fitter switch.