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