Great info Mark.
The p4 center columns' blurb seems to explain where good output signals can be tapped.
p25 has a circuit/schematic relevant to the p4 info. The switch's RHS circuit C278 is what I described above re diode isolation however I referred to a N.O. GND whereas C278 is N.C. (fine with pull-ups & electronics sensing but not for relay coils - unless normally activated (with Secure Idle activated) is ok...
The LHS switch side being N.O. from +12V could be perfect except that the sinking (grounding) current/resistance is unknown...
I do appreciate Ford publishing such info. IMO it is reasonable to expect people to add stuff that require
typical automotive signals and that documentation deals with that info. Of course for trucks that seems to be common - if only the same were down for passenger etc vehicles for all makes.
Howard - caution! Albeit more of a caution for others...
A grounded LED may sit at 2.5V but "12V" LED lights are often series strings of LEDs and hence sit at multiples of whatever LED voltage. EG,12V white LEDs are usually in strings of 3 and hence maybe 3x 3.5V = 10.5V at rated current. That voltage can be appreciably lower at lower currents, eg, 9.0V @ 1uA.
However the main point
Howard makes is that a LED will be
relatively far from GND = 0V in comparison to a bulb. Even a single red LED will probably be at least 1.4V and up to ~2.2V.
Such low voltages can be considered the same as GND by many sensing circuits.
And where relays are used in the pseudo- or phantom-ground mode as described in earlier replies, the LED(s) are unlikely to pass sufficient current to ground the relay coil.
Again, a parallel resistor can solve both above, however for a relay coil that resistor needs to be low Ohmage - hence why bulbs are often used even if 3W instead of original 21W etc.
And BEWARE - bulbs or low Ohmage resistors are fine when LEDs have replaced the original bulbs, but don't add either to other systems unless you know the bulb or resistor current can be handled.
But there may be systems that won't tolerate any of the above. It's possible that full +12V else LED 1.4V or 10.5V voltages are expected and anything else is considered to be a fault.
Hence on p4 of
Ween's link -
"
Splicing into the stop lamp switch on vehicles with Electronically Controlled Transmissions
can interfere with the proper functioning of PCM, speed control, and anti-lock brake electronic modules.".
BTW - do you guys get blank stares when you mention "leds"? Lately I've been having to point out that
Ell Eee Dees aka L.E.D.s are leds.
For those that refuse to accept the acronym I ask when they last flew QANTAS or NASA or dealt with NATO or LASERs or JPEGs.