Vehicle: 2008 Ford F250, 4WD, Super Duty, V8-6.4L DSL Turbo, Automatic Transmission

Mileage: 168,994

Problem: The shop got a Ford F250 in because the radio didn’t work, the PRNDL gear indicator light was out and there was no instrument panel operation unless the engine was revved to about 1500 rpm.

Details: Thinking that it could be a charging system problem, the technician tested it first. The charging system was working perfectly. Next, he connected a scan tool but could not find any diagnostic trouble codes in any of the modules.

It was time to call ALLDATA Tech-Assist. After a little research, this is what the Tech-Assist consultant came up with. It seems that Ford uses diodes (NOTE: They look like a fuse) to prevent voltage spikes that occur when a device using a wire coil and magnet stop spinning. An A/C clutch can act like an ignition coil. When it’s spinning and quickly stops, it can create a large voltage discharge (field collapse). The diode won’t allow voltage to back feed into the A/C relay and damage it.

 

Confirmed Repair: In this case, the A/C diode was in the wrong location in the fuse block. It was in the One Touch Integrated Start (OTIS) slot. Ford diesels don’t use OTIS (see the diagram). The technician put the diode in the correct spot and checked to be sure the other diodes were in their correct locations.

It’s always a good idea to verify the fuse block configuration when dealing with odd electrical problems on Fords or any type of vehicle for that matter. Problem Solved!