Mixing LED and incandescent

[rosso75]

I'm wiring a trailer. I have an odd assortment of lights left over from other projects that I want to use rather than buying all new stuff, but it's a mix of LED and incandescent. I've seen posts on the eTrailer.com website saying it's not a good idea to mix the two, as the power draw is different. However, I've also seen posts on the same site saying it's ok to use LED brake lights because they're on their own circuit, and not mixed with something else like marker lights. Seems reasonable, but then it occurred to me that as soon as I plug the trailer into my 1996 truck, which obviously was made without LED lights, I'd have the LED trailer lights on the same circuit as the trucks incandescent lights, so the "don't mix them" logic seems to go out the window as I'm sure there's thousands of people who tow a trailer with LED's with a vehicle that has incandescent.

So I'm seeking wisdom. Enlighten me on using LED lights with incandescent. Any thoughts appreciated.

[GTS225]

Not sure where the "don't mix" crowd is getting thier info, or what info they're getting, but it makes not one bit of difference. Yes, the current draw is different between LED's and incandescent, but either way, they are connected in parallel, so the wires share the current, and the wires don't care either way.
Where you start getting problems is replacing your turn/stop lamps with LED's,without swapping in an LED-specific T/S flasher.
And BTW; you are interconnecting LED's and incandescents in the running lights, if you're running a mix. Side marker lamps are hooked into the same circuit as your tail lamps.
Just make sure you have the polarity right on the LED's.

Roger

[Tom&Shelly]

Yes. Agree with what Roger says.

Tom

[QueticoBill]

I would expect the only time it makes a difference or causes a problem is if the circuit is controlled by other than a dry contact, such as the flasher noted. There are other solid state switch devices that might cause a problem, but I don't know how frequently they are used in automotive controls. Dimmers using SCRs, triacs, IGBTs, or similar can be finicky with mixed loads.

[rosso75]

Y'all are awesome. Thanks for the info

[working on it]

I would expect the only time it makes a difference or causes a problem is if the circuit is controlled by other than a dry contact, such as the flasher noted. There are other solid state switch devices that might cause a problem, but I don't know how frequently they are used in automotive controls. Dimmers using SCRs, triacs, IGBTs, or similar can be finicky with mixed loads.

This may be an entirely different subject than the LED/incandescent topic, but how about when your tow vehicle uses PWM (pulse width modulation) signals to power just about everything, including the lights, via the LCM (definition varies: Light Control Module, or Light Check Module). And, there's the AHM (German: Anhanger/Trailer Module) that seemingly is a must-have for towing hook-ups. People on the https://xoutpost.com/bmw-sav-forums/x5-e53-forum/ forum I frequent, are always having lighting and trailering problems due to these modules (BMW used more modules than anyone, even back in 2001).

The previous owner (next-door neighbor) of my '01 BMW X5 E53 3.0i "project car" (my final one, after 58 years of tinkering automotively) bought/reconstructed/renewed/rebuilt just about every system on the vehicle, for 3.5 years, specifically in preparation for trading it to me, for my '66 Chevelle drag race car (that had been sitting, unused, in my garage for 13.5 years, after I quit racing). Among other mods he'd made, to get it ready for me, was adding an aftermarket hitch and a strange homemade 4-way trailer wiring harness. It has three relays wired into it, which I don't know the reason for, but it seems to work; he tested it pulling his race-bike to races, pulling a 4.5'x 10' utility trailer (also traded, for my 18' car-hauler), and it worked.He's an electrical engineer (so of course it should), but he moved away and has gone incommunicado, so I can't probe his reasons for the relays.

3 relay problem.JPG (47 KiB) Viewed 3338 times

So, if I want to install

• 1) a brake controller,
• 2) a 4-way to 7-way harness, and
• 3) perhaps brighter LED lights

on my squareback trailer, should I trust the wiring as it is now? and use the ETBC7 harness

4-way to 7-way plus brake controller wiring.JPG (71.49 KiB) Viewed 3338 times

to convert it to 7-way with a brake controller, rip out the homemade 4-way harness and start over with a new harness

new 4-way converter.JPG (58.59 KiB) Viewed 3338 times

or buy a used factory BMW AHM trailer module off of E-bay, and go from there? Factory trailer wiring harnesses are unobtainium, and get quickly snatched-up from scrapped BMW's.

As it stands now, I'm reluctant to start afresh and throw money and parts at my BMW just to get a 7-way & brake controller installed, just to go camping (very rarely, now), but I'd like to. The X5 is rated to tow 5000 lbs (6000 lbs for V8's), so my 2225 lb trailer isn't even half of that, and the large 4-wheel disc brakes are really good, so I see no problem towing legally in Texas/Oklahoma (4500 lbs in Tx/3000 lbs in Ok), so I could if I wanted to. But, I should probably leave well enough alone, and tow with my '98 GMC 1500 or '04 Chevy 2500HD.

I still would like to know what the three relays do, anyhow.

[RJ Howell]

I'm wiring a trailer. I have an odd assortment of lights left over from other projects that I want to use rather than buying all new stuff, but it's a mix of LED and incandescent. I've seen posts on the eTrailer.com website saying it's not a good idea to mix the two, as the power draw is different. However, I've also seen posts on the same site saying it's ok to use LED brake lights because they're on their own circuit, and not mixed with something else like marker lights. Seems reasonable, but then it occurred to me that as soon as I plug the trailer into my 1996 truck, which obviously was made without LED lights, I'd have the LED trailer lights on the same circuit as the trucks incandescent lights, so the "don't mix them" logic seems to go out the window as I'm sure there's thousands of people who tow a trailer with LED's with a vehicle that has incandescent.

So I'm seeking wisdom. Enlighten me on using LED lights with incandescent. Any thoughts appreciated.

Issue I've had in 'mixing' is the rapid flashing of the led signal (turn) light. This is compensated with a resistor. I bought mine off Amazon.
https://www.amazon.com/iBrightstar-50W-6ohm-Load-Resistors/dp/B07HF47RV9/ref=asc_df_B07HF47RV9/?tag=hyprod-20&linkCode=df0&hvadid=241918478831&hvpos=&hvnetw=g&hvrand=10474712003386571967&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9002292&hvtargid=pla-638650005047&psc=1&mcid=cb38482fb46a3091a5470761f30e847e&gclid=CjwKCAiAkp6tBhB5EiwANTCx1PW151US6kXwZYHfGZl8wjsfgAD4SmkS48pOvF_HxdDpUvjyRlNzAxoC5-gQAvD_BwE

[bdosborn]

Our 2020 Ford Transit monitors the current level at the turn/brake signals in order to alert you if one is burned out. Ford uses incandescent bulbs for all the exterior lights (really Ford? Common!) and replacing them with LED can lower the current draw enough to cause a burnt out bulb signal to come up on the dash. I replaced the reverse lights with LED and so far no issues but haven't had the guts to dip into any of the other lights. The vice versa is true, tapping the trailer lights directly into the turn/brake signals can cause a current level error.

Bruce