Stupidest error message ever?

[bitofaclue]

For years I've been putting up with a "COOLANT SENSOR BAD" message on the Information Center display.  When it first appeared a few years ago I bought a new coolant (temperature) sensor, screwed it in... and the message stayed.  For a long time I suspected the computer, but it wasn't important enough to care about.  Now I've just replaced the computer (with a boneyard special) because it was reporting the throttle position sensor as bad (I'd swapped another TPS in and the error stayed) and it was running like crap.  Running nicely now and I decided to chase this annoyance down.

Hitting the book it quickly became apparent that I may have been looking at the wrong sensor all along - it's complaining about the coolant level sensor in the reservoir, which is connected to the information center, not the ignition controller.  Incidentally, this circuit appears to be a late edit in the factory manual, because it's sloppily-documented compared to all the other electrical stuff.  Anyway, I checked the float switch, and it's fine, and I topped off the reservoir and the message disappeared.  Maybe it comes back, maybe not.  I'm pretty sure I haven't been driving with a consistently low reservoir level for five years.  

So here's why this is stupid:

It's a switch.  Single pole, single throw.  It's either open or closed, corresponding with "level okay" or "level low" respectively.  There's no other state, and no way for the computer to know whether there's a defect in the circuit or the switch.  All it can legitimately report is [nothing] or "coolant level low".  How on earth can it presume to find a defect here?

[esi]

It could be getting resistance though the circuit.  The computer doesn't  recognize, so it sends a bad sensor  message.

[cLAY]

ZJ I assume?

It's the VIC module(as in the actual screen). It's been well documented on NAGCA, Mallcrawlin and thespeedfreaks
as a bad solder inside the module. Can be fixed if you can use a solder pen.

[bitofaclue]

It could be getting resistance though the circuit.  The computer doesn't  recognize, so it sends a bad sensor  message.

Good point - conceivable, but unlikely.  I actually design stuff like this, and normally we use a digital input to sense a switch closure, since it only has two states (open and closed, 1 and 0).  To sense something in between - a resistance range like the coolant temp or throttle position sensors output - you need an analog (A to D) input and completely different wiring.  But maybe they've done something tricky.  To find out I'll have to figure out how to get at the information center connector and see whether there's excess resistance between it and the reservoir switch.

[bitofaclue]

ZJ I assume?

Yup.

It's the VIC module(as in the actual screen). It's been well documented on NAGCA, Mallcrawlin and thespeedfreaks
as a bad solder inside the module. Can be fixed if you can use a solder pen.

Wow - thanks for the lead - I'll try to research this further.  And yeah, I can handle an iron, so if I can get the damn thing out of there I'll be able to fix it just fine.