Join Date: Nov 2010
Location: South Fayette/Pittsburgh PA
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The upper alternator is a 140 amp and the lower is a 120 amp if they are both factory and not aftermarket add-ons from the body builders.
Both alternators should provide maximum amperage at 2000 RPM. The internal voltage regulators determine how much output they provide based on the charge of the batteries.
Now honestly the charging system may not be seeing a fault. I know for a fact all ambulances are not created equal and their electronics vary. The workshop manual says if there is no battery light and no charging system DTCs then everything is working normally. 13.8v is not out of the realm of normal for a 6.0 even at 1600 RPMs. Hell sometimes the electronic readout for the system voltage is wrong. Make sure your testing it with an actual voltmeter. Also make sure your batteries are up to snuff. Weak batteries will obviously pull more of a load out of the system as they try and recover.
If you want you could try disconnecting each alt and testing the output with an amp clamp. I do not believe both run full capacity at all times, they only charge as much as needed. Running as many accessories as possible during testing will give the best results as the alts will be working at full or near full capacities.
Also one more thing, you need a FICM. All injector low circuit codes are a good indication of a failing DC/DC converter inside the FICM.
Ford Senior Master Tech
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