What type of resistor do I need?

not

monitored

Failure

It may not be that simple. Light bulbs have a much lower resistance when they are off than they do when they get hot. Depending upon the method of detecting bulb failures, the circuit may rely on the off resistance (which is what I suspect) or it may detect the high current flowing thru the wire when it's on (I doubt it). At any rate, you will need rather large (physically) resistors to dissipate the power (like

10W to be safe). 33 Ohms would be close to the on resistance of a bulb. When the bulb is off, it's likely to be about 1-3 Ohms.

I would be inclined to leave the bulbs wired into their circuits and paint them black.

To possibly clarify something...I know that on the vehicle, the 55W fog lights (optional), if not fitted, were replaced with a 1000 ohm (don't know the wattage rating, etc.) resistor to override the bulb failure system. I'm trying to apply the same concept, but I don't know the appropriately rating of resistor needed for a 5W bulb. Obviously I need just enough current bridged to make the module think a bulb is in place.

And when you do this, some weinerhead will tell you not to crosspost :)

I was going to do the calculation, as Rich did elsewhere in the thread. But if you already know that a 1000 Ohm resistor will fool the foglamp failure detector, I would strongly suggest that you start with that value. If it doesn't work, go to a smaller value until you find one that does work.

For power, use this formula:

P=V^2/r, where V is 12 V, and r is 1000 Ohms.

So that is P=144/1000 = 0.144.

So the first thing you should do is try a 1/4 Watt, 1000 Ohm resistor. This is a common type which can be found at Radio Shack (if they have Radio Shacks where you live).

If that doesn't work, try lower values of resistance.

Note: If you go below 680 Ohms, you better switch to 1/2 Watt resistors, and if you go below 300 Ohms, switch to 1 Watt resistors, and if you go below 180 Ohms, switch to 2 Watt, and if you go below 82 Ohms, switch to 4 Watt.

With a 4 Watt rating you could probably go all the way down to 43 Ohms. Based on what you have said so far, I doubt very much you'll need to go below that.

Good luck!

--Mac

[snip...snip...]

And please learn to cross-post instead of multi-post. That should be possible even with MS Outlook Distress.

Optimally, if you're not sure exactly which would be the best group, set the "follow-ups" to your best guess and CROSS post (many newsgroups in the same newsgroups line) to the three or four groups that you think might be germane.

Next best, leave follow-up blank and just CROSS post to the multiple groups. That way, at least there's only one conversation thread. It will still show up in all of the groups but good news readers will know to show the postings only in the first group that the reader gets to.

Worst is to MULTI post the same question to several groups. None of the multiple threads are joined. People will reply in one group when the question is already answered in another group. And you'll have to go back and re-read all of the threads separately.

--
Rich Webb   Norfolk, VA

Thanks Mac, This is where I was thinking my steps would lead to. I tried the 1K 1/2W 5% Tolerance Resistors. I suppose it's possible the contact was not seated firmly enough, but I will play with it a little more before stepping down to the 680 Ohm. Hopefully that might be the one.

Another thought: What difference does it make that the BFM is monitoring a

5W load as opposed to a 55W load like the fogs (Does that mean it would require a different value of resistance for the BFM)?

Note: The BFM for the rear lights is seperate from the BFM monitoring the front.

-John

Sounds like the bulb detector is using the lamp's presence as a pull-up or pull down, when unpowered, to detect continuity vs open or blown fuse.

1K should work in the second case also, regardless of power, if so.

RL

more

not

Do you know how the BFM works? On an old Volvo I had, it sensed the difference between currents flowing in pairs of bulbs of the same wattage on the offside and nearside of the car. When both bulbs were not present, the BFM did not indicate a fault.

Regards Ian

True enough. It's almost always better to be familiar enough with the newsgroup to know that the post is on-topic and post to that single group.

Of course, if a weinerhead does complain that crossposting is worse than multiposting, there's always that other handy usenet tradition of adding the w.h. to the "improve my signal-to-noise ratio" file... ;-)

[Traditionally, follow-ups to this posting should either point out grammatical and spelling errors or note that it's not on topic.]

--
Rich Webb   Norfolk, VA

Well, I just don't know how the BFM works, so it's hard to say. If I had to design a detector, I would probably just apply a small current to the circuit, and then set a threshold voltage above which was a fault, and below which was a good bulb.

But I think that would only work when the lights are off. You say this system only works when the lights are on. So, maybe it works by measuring the current when 12 Volts is applied to the bulb.

Or maybe it compares the two sides. Did you read Ian's post about his old Volvo?

Anyway, good luck!

--Mac

I've tried many of the combinations suggested by everyone, but I'm afraid I've been unable to fool the BFM with a resistor(s). I was, however, able to wire and conceal a small 'dim' 5W bulb into the circuit...looks and works fine. Thanks for everyone's expertise!