relay to break ground

OK have tied into another brake wire that has resting ground and 5 volts when depressed. This works if I put the secure idle main ground and the secure idle ground brake cable to this. If i try to use chassis ground on the main sec idle ground it just cycles iff and on doesnt disengage.I measure current and it's sitting at 140 milli amps through this circuit. Does anyone see any issues with running this amount of current through the brake switch when sec idle is engaged.

140mA should not be an issue. Most automotive switches would handle at least relay coil currents if not higher however steering wheel & similar electronic control systems are exceptions. FYI... ... and subject to my bleary eyed morning confusion... We have been assuming a common +12V switched system - ie, the switched/bulb side of the brake switch is normally GND and goes to +12V when the switch is closed (brakes on). However your another brake wire goes to +5V suggests the brake lights are not directly switched but go thru some logic circuit (BCM or other module). (I have read that certain F150s etc have "low voltage" systems but I haven't investigated further.) And in that case I suspect using a relay will not work... For systems that require a GND or not GND that may be ok but some may require the not GND to be higher than +5V. EG - connecting a relay coil between +12V and that line means the coil will still have at least 7V across it - in fact usually ~14.2V-5V = 7.2V with the engine running - and that's enough to keep most 12V relays on. The same can apply to logic circuits where perhaps the "not GND" needs to be higher than half the hot voltage - ie, above +6V to +7.2V etc. And connecting a 12V circuit like a +12V connected relay coil could damage the OEM BCM/module circuitry if it doesn't have clamps (diodes) to its +V rail. Yes - it's all very circuit dependent. Using diode isolation might prevent damage and get the additional circuitry to work but the direction of the diodes (& need for a pull-up resistor) depends on the circuitry. EG - if it's a GND switching brake switch insert a diode between the switch and OEM module, and a diode between the switch and your added circuit - diode lines towards the switch (eg, IN4004 etc diodes). Maybe a pull-up resistor to +12V between the diode & your added circuit if it requires +12V when not GND but that would not apply to a relay coil. Also FYI - conventional brake light circuits that use LEDs will not be at GND when off. Hence why many suggest adding relays (and one of the Rostra solutions means shorting +12V to GND!) tho my suggestion is to merely add a resistor low enough to pull down the sensing input, else a small bulb.

True LED brake lights tend to sit on about 2.5 volts. A bane for earlier Clifford remote starts, they wouldn't work because they thought the brakes were applied, always needed a relay or blocking system.

Hi, May want to read up on Ford's recommendations on wiring into brake switch circuits....page 4 on the following link: https://www.fleet.ford.com/truckbbas/topics/2015/2015_Electrical_Wiring_v1-0.pdf Mark

thanks everyone for all the great info. think we got it figured out and working fine.

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.

Interesting comments Peter, I have to break a NC brake switch on my car to actuate the remote start. About 10% of the time the dash display reads engine malfunction, clears as soon as I press the foot brake with the engine running. And yes my autobox is electronically controlled with a low voltage brake light feed to the CEM

Hmmm - I wonder if it's a borderline reading, ie temperature effects the voltage? That's why sometimes I like fully buffered addons... FETs etc only load a few nA. And then you get into the high or low sensing (ie, N-ch or P-ch FET for switching) or variable threshold or analog sensing... And then you end up with the universal PICAXE 08M2 solution - a $3 08M2 plus peripheral circuitry and programming else jumper selection for options... Ah yes, one day...