I got a set of LED headlights to replace the old halogen bulbs in my '84 Allegro. They got great reviews, though some users said they had to swap the low and high beam wires. It's a standard 4 light system with 4x6 bulbs. Or so it would seem. I couldn't get the new lights to work properly on the low beam setting. The low beam bulbs will only come on with the high beams. And when they do, they come on as high beams. If I switch the wire on the new bulb, I can get the low beam setting to come on (just one row of LEDs light up). But under no circumstances to the bulbs light up when I switch to low beams.
It would seem to be a simple fix - just swap the wires. But it isn't. I can't figure out how they have this thing wired up. It should be simple. 3 prongs on the low beam bulb:
Ground (black wire)
High beam + (red wire)
Low beam+ (yellow wire)
The high beam is 2 prongs, presumably ground and hot.
When I pulled the wiring out, I did not see what I expected. The red and yellow are spliced to the high beam plug. I had expected to see the red and black running there. In testing the low beam plug, it seems like the polarity is reversed on the low beam terminal. With the high beams activated, I get +12V on when I test from the red to the black and a -12V when I test yellow to black. That would allow the normal headlight to work, but not the LEDs.
Just for fun, I plugged them into my '64 Impala's 4-light system. They work beautifully and without having to swap wires around. So there's something goofy in the motorhome wiring. The lights have always been really dim, even for old halogens. Not sure if that's related.
It would seem to be a simple fix - just swap the wires. But it isn't. I can't figure out how they have this thing wired up. It should be simple. 3 prongs on the low beam bulb:
Ground (black wire)
High beam + (red wire)
Low beam+ (yellow wire)
The high beam is 2 prongs, presumably ground and hot.
When I pulled the wiring out, I did not see what I expected. The red and yellow are spliced to the high beam plug. I had expected to see the red and black running there. In testing the low beam plug, it seems like the polarity is reversed on the low beam terminal. With the high beams activated, I get +12V on when I test from the red to the black and a -12V when I test yellow to black. That would allow the normal headlight to work, but not the LEDs.
Just for fun, I plugged them into my '64 Impala's 4-light system. They work beautifully and without having to swap wires around. So there's something goofy in the motorhome wiring. The lights have always been really dim, even for old halogens. Not sure if that's related.
The light they throw is so much brighter. There's a big tree in the way in the first pic but you get the idea. Again, right side is still the stock halogens.
MrWizard wrote:
nice.. you got it all figured out
let us know how bright they are
They are crazy bright. As I said they're DOT approved so hopefully they don't blind people if adjusted properly. It's up on ramps now so I can't tell what other drivers would see. Low beams light up the path in front of it like day time. High beams - nutty. And I've only done the left side.
If the low beams are too much I'll snip the wire to the bottom lamp.
wa8yxm wrote:
Incandescant headlights are polarity insensitive. So it is possible they reversed the wires.. That's all I can say.
Your voltmeter is your friend.
Yeah that's all I can figure. With a 3-wire plug I only had 27 combinations to try. One of them worked. I snapped a pic for when I do the other side.
I finished the left side. The right side is the old halogens. Once it gets dark I'm going to do a side-by-side comparison. Yes it's dirty - it's been sitting in that spot under the trees since June.
I got greedy and tapped into the low beam circuit for the bottom lamp. Normally that one is not lit up when low beam is selected. But...since these lamps are both high and low, I figured 2 low beam lights on each side can't hurt. They draw a lot less juice than the halogens so it should be fine. Low beam just lights that top row of LEDs and the one in the middle. High beam lights them all up. They're DOT approved, too.
I got greedy and tapped into the low beam circuit for the bottom lamp. Normally that one is not lit up when low beam is selected. But...since these lamps are both high and low, I figured 2 low beam lights on each side can't hurt. They draw a lot less juice than the halogens so it should be fine. Low beam just lights that top row of LEDs and the one in the middle. High beam lights them all up. They're DOT approved, too.
Okay I figured it out.
Thanks to all for your suggestions, but rgatijnet1's idea of checking the '64 wiring did it. Yeah, I should have thought of that. To my surprise, the '64 wiring had the same negative voltage/polarity thing going on. Using the '64's pin-out as a clue, I was able to figure out the right combination of wires on the motorhome. It would have been nice if Tiffin stuck to GM's wire color scheme. But no, that would have been too easy. What should be the low beam wire (yellow) is actually the ground. I got the low beams to work on the low beam setting, but they'd go out when I put on the high beams. I moved the wires around again and now it's all working. I'm not going to pretend I know exactly why, but I've invested a good chunk of my Sunday trying to figure out what should have been a 15-minute job.
Thanks again!
Thanks to all for your suggestions, but rgatijnet1's idea of checking the '64 wiring did it. Yeah, I should have thought of that. To my surprise, the '64 wiring had the same negative voltage/polarity thing going on. Using the '64's pin-out as a clue, I was able to figure out the right combination of wires on the motorhome. It would have been nice if Tiffin stuck to GM's wire color scheme. But no, that would have been too easy. What should be the low beam wire (yellow) is actually the ground. I got the low beams to work on the low beam setting, but they'd go out when I put on the high beams. I moved the wires around again and now it's all working. I'm not going to pretend I know exactly why, but I've invested a good chunk of my Sunday trying to figure out what should have been a 15-minute job.
Thanks again!
