led third brake light relay

Not to beat a dead horse but I may have another explanation on this topic. Think about the 12v signal coming from the brake switch. Full 12v looking for a path to ground. 2 systems independant of each other are looking for 12v when the brake is pressed, and ground when it is not being pressed.

When you have LED's hooked up that 12v goes to the LEDs and resistors, and then to ground. The point of course is after the electrical work of lighting the LEDs up and passing through the resistors has been done there isnt a full amperage 12v signal going to ground. That equals a working circuit.

By adding the relay in parallel you have provided an additional ground. Now as described earlier in this thread the fuse blowing is likely due to the feedback of 12v backwards across the relay coil. Another option would have been to install 2 diode isolated relays in series (cut the brake switched wire and feed it via one of the relays), then the other relay would operate the ground signal to the cruise's violet wire.

One source switches both relays, but each circuit is now separated completely.

Basically the brake signal wire 12v switched comes through 2 diodes (bands facing towards the relays) to pin 86 of both relays, and of course pins 85 both go to ground. The car side of the brake switch wire you cut will go to pin 30 of relay #1. Pin 87a of relay 1 goes to ground, and pin 87 of relay goes to 12v (can be the 12v source at the switch or fuse panel)

On relay #2 for the cruise you wire pin 30 to the violet wire from the rostra cruise, and pin 87a to ground, or following their diagram.

This method is very elaborate and may contain unneccessary parts, but it is 100% certain that the 2 circuits will work as intended completely independant of each other.

I think most of this was covered, but I thought these comments may help someone else in the future understand a little better.

The rostra cruise is looking for a solid ground on the violet wire, and in fact does not even need to see 12v at all. Just the loss of that ground when the brake is applied.

This is how I wire many standard transmission vehicles with the tach wire hooked up and 2 signals (clutch and brake) disengaging the ground from the violet wire. All in automatic mode. This way, the cruise sees the RPM jump when the clutch is pushed in and turns off the throttle, and as a fail safe when the clutch is fully engaged and the clutch switch is switched, it then turns off the ground to the violet wire. Multiple added fail safes.

Nice one!

The other thing to consider here, is when you have the LED lights in place, the 12v signal is passing though a load, and a group of resistors. Consider that when the circuit is not energized (pedal not pressed) you do not have a full ground signal at the switch. The ground is also travelling through those same resistors. Grounds obviously do not "travel" but if you measured the resistance at the switch to a known ground it would be greater than true ground.

For example. You can find the ground wire after the LED lights and measure the voltage present when the pedal is being pushed, and you may very well see 12v, or slightly less. You have to remember it is at a milliamp rating by the time it reaches that grounding point. Still a short to ground but with no amperage behind it the circuit works correctly.

When a normal bulb is present you do not have a resistance to ground. You likely have full ground at the switch, or close to it. Whether or not that stays constant with a load applied is another matter, but it is something to be considered when talking about these types of issues.

That gets back to the question - what is the maximum ground impedance/resistance required for that sense circuit to operate. Or paraphrased for lamps, what is the minimum wattage required - 42W, 21W, 10W, 5W, 2W, 250mW etc as used by lamps and LEDs. For LEDs there is also the issue of their "zener-like" voltage drop. But IMO this whole issue was pathetic for reasons including that the unit should (probably) have pulled itself down in the first place, and given that it didn't, the info provided by the manufacture is misleading and IMO incorrect (ie, that wire "should not" contact ground, yet they short it direct to ground via the relay!), and IMHO it's a stupid solution anyhow (ie, a relay - why not a resistor or lamp?).

All valid points there oldspark.

In no way do my statements reflect the opinions or views of Rostra, but it has been my experience that when you become a rostra dealer, your technicians become field engineers for their products.

I will leave it at that since I have nothing further to say about this company.

Some very valid points here, I just wonder if racerjames is the new me? As an aside, some years ago I installed a Clifford alarm R/S in a Renault Clio. A couple of months later I hear the R/S isn't working. On examination the only thing I can find is that the owner has installed LED rear lights. Yes, not going to ground, reading about 3V, enough to inhabit the R/S through the brake light circuit. A parallel relay was required.

It's the Rostra website that gives the crap info. Why no one else has managed to have the info corrected or clarified beats me. Maybe "field engineers" aren't clever enough, or maybe they don't bother once they figure out the solution. Or maybe Rostra ignore everyone... From my reading, this has been an issue for several years. Though I can see arguments for both pull-up & pull-down "defaults", I cannot understand their IMO pathetic info and solution. Maybe they lack the English skills for a user-friendly else technical explanation, though it seems to me to be a lack of design skill. Incidentally, I haven't yet written to Rostra - a PC-crash issue killed that for a while. I may still do it, though it may also become yet another of my "mere observations". (Sometimes I don't hand out the rope - I just let fate take its course.)

howie ll wrote: I just wonder if racerjames is the new me?

I am but a mouse among men here!

Not to drag this thread out into a rostra bash fest, but i recently did a data cruise in an 05 F150. In the instructions, it clearly stated to run a VSS or equivalent (forget what they called it) out to the ECM on the passenger side of the firewall. Well with that wire hooked up the cruise would not engage. Remember I followed the directions perfectly. I call tech support and the guy gives me an attitude like I am supposed to know that the wire at the ECM is not needed and is creating noise the cruise can not understand.

Now how exactly would a field tech know that unless stated in the manual?

The unit ended up causing all kinds of issues in that truck, to which some of them could have been blamed on the engine tuner module located on the dash. Think bullydog or equivalent (not diesel). Something like it was preveting the computer from seeing cam sensors and a whole list of other things. The dealer removed both items at the same time so we don't know which was the real problem. Anyways I need to stop bashing Rostra..I get paid handsomely for installing and engineering their products!

Huh! All I ever got were about a dozen DEI T shirts and 2 Kenwood anoraks!

racerjames76 wrote: I am but a mouse among men here!

And I'm the rat! I now understand RacerJames' reply about becoming field engineers a bit better. (Dare I say "funny" how the chiefs blame the Indians? It's part of what I call Karpinism or "The Karpin Club", or maybe what Drucker, Covey, et al (self proclaimed "walkers") call "yes-men gone wrong"...) My "bashing" is directed at modern info systems and education. Crap begets crap. And subject to its exponential growth, the number that know the "correct" answers are ever diminishing. So far we are still surviving despite the overwhelming majority of bad info, but I wonder how long for... That techs & readers regurgitate incorrect reference material is to be expected. However with the dumbing down of so many skills, it's as if no one can "think" their way to something better. Regular readers will recognise my generic socio-fiscal comments above. How that applies to Rostra is IMO yet to be seen. I expect however either to be ignored or met with aggression, else simply not understood (IMO as with OP bbstacker) - unless they're golfing down their local Karpin Klub... LOL.