1998 F-150 Door Ajar Light Problems

Spray a shot of WD-40 in the door latch assembly on the door.
Specifically toward the bottom of the latch inside the door.

this happened to me several years ago on my f150. what i did was first spray wd-40 on the door latch mechanism. if that didn't work i took the door trim off and sprayed lithium grease on the inside handle and latch. no more problems

i also had this problem with a previous ford that i owned and was able to fix it by spraying wd40 into the door latch which is where the sensor is located, i guess they get grease on them and stop working . hopefully this will help you out.

I have the same problem and DAMN is it irritating! I've written to Ford and
got no real response. I thought it must just be me. Last week, though, I met
a guy with a 96 Ranger that has the same problem and that's what prompted me to
check the newsgroup. I'm going to write another letter and I'd like to get as
many people in on it as possible. I'd even consider talking to a lawyer if
enough people are willing to participate. Please get back to me on this and
maybe we can get something done about it. BTW-My Ranger is a 95 and I (also
the kid I met last week) also have a problem with the dome light and with the
headlights being adjusted to high. It looks like my brights are always on.
I've been pulled over for it before.

I have the same issue! I have come to realize though that it is caused by weakened door pins. If you open your doors and try to move them up and down, you will most likely notice some play or slop . It only takes a slight difference to make that dam light go on! Change your door pins and I'll lay money on it that your troubles will be history. :) 1A DGUYZ

I have been working on fords for years because that's what they need. My friend and I have 1999 ford f-150's and the door ajar light came on one day and we took the door apart. "What a wast of time that was". I found that if you open the door and close the latch a little so you can stick the straw from a WD-40 can up behind the latch. Spray up there where the latch switch sits and close the door a couple of times it solves the problem. /// Do this with the window up or you will have a mess on your window afterwords.///
See picture
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This solved the problem and made the switch move freely and happy like it was 10 years ago.
I hope this removes your frustration.

Thanks .
Tim can fix it.

The door latch switches mounted to the latches fail quite often. The key in switch also fails quite often. So much for Ford reliability. Anyway, replace the door switches and you should be ok. Both are fairly easy to replace. The latch switches are cylindrical in shape with a plunger that sticks out of one end. They corrode and stick. These are also an input for the key in warning chime. The key warning chime is inter-related but would have it's own symptoms.
1998 F150/250 Workshop Manual
1998 F150/250 Workshop Manual Switch —Door Open Warning Lamp Removal

1. Remove the front door latch; refer to Section 501-14A .

1. Twist and remove the door open warning lamp switch.

The key switch usually breaks or wears out and makes contact. It is on the lock cylinder. This would cause the chime to not come on, or stay on. This is less likely on the newer design of the 98 which is pictured.

There are some things about ford that **** -- this is one of them. WD40 first solution hopefully a quick fix. If that doesn't work try this...
http://www.offroadrangers.com/forum/showthread.php?t=19469

The newer fords have the switch built in the door latch assembly and over time they tend to stick, spraying wd-40 or equivelant on the latch assembly in the door has worked for me in the past. I do mechanic work and the only other option I know of is to replace the door latch assembly which is rather expensive.

take the fuse out and ignore it.