The power door locks in my 1998 Lincoln Navigator suddenly quit working recently and have not been able to figure out what the problem is.

Door locks are totally inoperable, lock/unlock do not work at all, driver's door, passenger door, and key fob all quit working at the same time. All were working just fine up to this point.

Things I've checked so far:
-30amp fuse under hood tests good, 12V on both sides of the fuse
-G200, ground tested good at the passenger kick panel, no corrosion, tight connection
-Driver's side door lock switch, I have ground present at the switch
-Lock relay block, had a hard time interpreting the wiring diagram on how this circuit works, but I have 12V power present at each black/white wire coming from the 30amp fuse that feeds each of the 3 relays at the relay block. I also have ground at the black wire coming from G200 tested at the lock relay and the unlock relay, both have ground present.

I unplugged all of the 3 relays at the relay block and applied power and ground manually to the heavier gauge wires feeding the door lock actuators and I am able to lock and unlock the doors that way. So I assume that the wiring going from the relays to the actuators is good.

Not sure where to go next. It doesn't make sense why everything failed all at once. Seems odd that all 3 relays would fail together at the exact same time, I doubt that could be it. My guess is that the wiring from the door lock switch to the relays is the issue...but why would that make both switches to fail along with the key fob? Any ideas? I appreciate any help you guys might suggest!

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Can we assume there are no lights lit up on the dash, for say an open door, rear hatch or rear glass?

 

No other lights, nothing else unusual, just a sudden failure of the power locks, no lock/unlock on any of the door switches nor on the keyfob. The windows switches work fine, windows go up and down on all the switches.

 

Does anyone know how the door lock is supposed to operate electrically? I have a wiring diagram for it, but my God, I've never seen such a simple concept overcomplicated to this extent like on this vehicle. I spent a few hours studying it and I really can't figure out how this circuit is supposed to work, the diagram is impossible to understand.

 

Unfortunately, with this design, the door lock switches tie into the GEM, and in turn the GEM controls the door lock actuators, both by commands from the door switches or the FOB. Since you have power at the lock switches, I think you're looking at an issue with the GEM itself. There has always been an issue with windshield leaking into the GEM and causing issues after extended periods of rain. Was there a lot of rain in your area prior to when the door locks stopped working?

 

The diagram I have must be incomplete, I did not see the GEM portion of how it ties into the switches. I studied the relays on the lock relay block and it looks like pin 85 and 86 are the power and ground that make relay switch terminals and send power and ground to the actuators (I believe, though I might be wrong). Pin 86 on each of the 3 fuses appears to go directly to the 30 amp fuse which is hot at all times. I do have 12V power on that pin 86...

But the weird thing is, when I press the door lock switch, and check pin 85 and 86, both become ground. It really makes no sense why the power side (pin 86) becomes ground only when I press the door switch, when it is supposed to be power hot at all times directly from the switch.

 

I figured out what the problem was. The biggest hint I got while testing was actually in that last message I posted. The power wire from the fuse was shorting to a ground somewhere, but I guess it wasn't enough to blow the 30A fuse.

I ended up running a new power wire with an inline fuse and cut the 6 power wires going to the relays and spliced it all together to make a new power from the battery to the relays. Problem solved!