Compression Ignition Addict
Join Date: Oct 2013
Location: Torrance, California
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I can't deduce that these two maladies are related. Perhaps they are, but I would diagnose them separately.
With the relay completely removed there is no power applied to the blk/blu wire (which is the dome light circuit). Remember, Ford dome light circuits apply B+ to the bulb, and the other side of the bulb is to chassis ground. So somehow B+ is being introduced into the blk/blu wire. My guess is an old alarm system that had dome light supervision, or the underdash fuse box has a bad trace or internal corrosion. Another possibility would be one of the over head dome / map assemblies: There is B+ power at all times for the map lights. (lt grn / yel wire). If this were to get shorted to the blu/blk internally in one of the fixtures, the dome light circuit would be powered. There is a separate fuse for the map / underhood / glove box lights. Pull it too, and see if the dome light problem stays / goes away.
The Blower: There is a relay in the under hood fuse center. This relay supplies power to one side of the blower motor. The other side is grounded through the fan switch, via the resistor network to get the different speeds. Identify this relay. Try running the truck without the relay installed, and evaluate what the blower does without it. Also, if you can catch the blower acting up, touch / listen to the relay and see if it is chattering. If = yes, my guess is the coil of the relay is not being grounded properly. I don't know where the under hood grounds are. The power for the relay is supplied through a fuse in the under dash fuse box, which comes from the IGN switch. A jankey IGN switch contact (there are 5 sets of contacts, only one of which affects the running of the engine) could perhaps cause the blower problem, but couldn't be related to the dome lights with the relay removed.
There's some ideas.
Last edited by DaveP.; 04-28-2018 at 08:32 AM.