1987 - 1996 F150 & Larger F-Series Trucks1987 - 1996 Ford F-150, F-250, F-350 and larger pickups - including the 1997 heavy-duty F250/F350+ trucks
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New Guy with New (used) truck. Picked up a nice 1990 ford f150 4x4 4.9 litre with 5 speed. Just a little help with the map lights that have 2 switches. Do they only work with the key on. Courtesy light seems to work fine. Bulbs ohm check good.
Here's the map lamp schematic for a 88 which is probably the same for your 90.
The Dome lamp bulb and the two map light bulbs get 12 volts from the same
source which is Fuse #8 and is hot at all times.
The route that the 12 volts gets to the dome light is as follows.
Either thru the right hand or left hand door jam switch or the headlight
switch when it's rotated all the way counter clockwise.
The Map light witches get 12 volts to one side of them at all times via a
Light Green/Yellow wire from Fuse #8. When the switch is turned on,
the 12 volts goes thru the switch to one side of the bulb. The other side of
each bulb is tied to ground so the bulb lights up.
If you remove the black plastic trim piece that covers the interior rocker panel
and holds the carpet edge down you should see the Light Green/Yellow wire and
measure 12 volts. Or you might have to remove the rear plastic trim panel to
get a look at the wire as it goes up the back side of the cab up to the map lamp
On my vehicle, the wire eventually broke at that area because there was not
enough slack in the wire.
To make it simple, yes...the key needs to be on to turn the map lights on. Not true for the dome light, as it should turn on when you either open the door, or rotate the headlight switch. Also, the keys need to be on to turn the bed light on as well.
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