Long story short....my gauge cluster wiring is so crappy its 50-50 whether the power to the battery light (which then goes to the alternator as an exciter wire) works. Nothing else in the cluster is being used anymore (lots of standalone gauges) so would just as well remove it.
To do this I need to wire a resistor into the exciter wire to replace the 12v 1.5a bulb in the cluster that currently lights up when exciter voltage is going to the alternator.
I have a 100ohm 1watt resistor here. Will that be sufficient/correct for the job?
btw, I have a voltmeter in the dash, so not worried about having the battery warning light functional...just want to bypass it entirely.
kb58
Dork
11/6/15 4:37 p.m.
"Probably" I'd go for a 2W, but give it a try before permanently installing it to see how hot it gets. If it says cool but it doesn't charge well, lower it to maybe 68 ohms (standard value).
kb58 wrote:
"Probably" I'd go for a 2W, but give it a try before permanently installing it to see how hot it gets. If it says cool but it doesn't charge well, lower it to maybe 68 ohms (standard value).
It only needs to work for a moment to excite the circuit and get the alternator charging. After that pretty sure it's not doing anything..
1W was the largest I found at Radio shack (the rest were 1/2 or 1/4w)....
Bosch alternators will generally self excite when you rev the engine. If you are going to the trouble of adding a resistor, why not just mount a bulb socket?
Also, the resistor doesn't flow any power at all once the alternator starts charging.
I think the bulb socket is the easiest way... plus if you drop the fanbelt, the light will act as a warning about overheating
I would do a bulb so you retain the dummy light feature.
I'd retain the bulb for already mentioned reasons. A resistor also does not behave the same as a light bulb.
Kenny_McCormic wrote:
I'd retain the bulb for already mentioned reasons. A resistor also does not behave the same as a light bulb.
Perhaps, but BMW in later e30s put a resistor bypass to do the job of the bulb when (when) it burns out, so the charging system will still excite.
So for this purpose, BMW thought a resistor worked fine.
Sadly my car is an early one before they fixed that issue, so mine just has the bulb and no resistor bypass....hence why I want to make one :)
I'd copy what BMW did then. Or just check all your trouble lights at key on before starting.
Kenny_McCormic wrote:
I'd copy what BMW did then. Or just check all your trouble lights at key on before starting.
The latter is what I currently do. If they don't come on I have to reach under the dash and jiggle the connector, lol.
The plan is to copy what BMW did. The only thing I was tryign to determine in this thread was what size resistor to use to replicate the bulb (I suck when it comes to electrical calculations, etc).
It's actually fairly difficult to calculate without knowing some more info, easiest to just figure out what they used. I can tell you that a 100 ohm resistor is going to kicking out a good 2 watts of heat with the system in a bulb lit condition with a burned out bulb, I'd guess BMW used a much higher value 1/2 watt unit. Something like 500 ohms by my math, though without knowing how much current the alternator needs to excite, we can't know if that will work (edit: it just won't start a fire).
You can do all of this with ohm's law I=V/R, and to save the algebra, V=IxR, R=V/I. I=current, V=volts, R=ohms. And amps x volts=watts
I don't think it matters a great deal. Check the resistance across, say, a 194 bulb, and use that as your value. Do not concern yourself with heat, or current flow, because the only time current is flowing is when the light is on. The light bulb is grounded through the alternator when its not charging, lighting the bulb. As soon as the alternator fires up, it sends 12 volts back up that wire, so you have 12 volts on each side of the bulb, creating a voltage difference of zero, hence no current flow, no lit bulb.
In reply to Streetwiseguy:
I'm no electrical engineer, but I feel like "may start a fire if key is left on" is a poor design.
so why the hate against a tiny bulb?
In reply to Kenny_McCormic:
Then don't use the resistor.
