How much voltage drop justifies building a headlight harness?

I have a 2009 Tacoma that has relay switched headlights. My best guess by looking at the headlight wiring is that it uses 20-ga wiring. There is a separate fuse and wire for each filament, but only one relay for high and low (fuses after relays).

I measured the voltage drop per the instructions on Daniel Stern's website. The truck was turned off, but both bulbs were installed. I'm not sure if that was a good idea (not having it running), but it was late and I figured the drop is probably going to be pretty much equal regardless.

The driver's headlight, both high and low beam, had about a .5V drop; .25V for ground, and .25V for positive.

The passenger headlight, with a much longer wiring run, had a drop of about .75V: .25V for ground, and .5V for positive (I think it was closer to .8V total).

The battery tested at 12.53V to start (which I started on the driver's side), and I believe it was clear down to about 12.1V when I was finished. So, it looks like I'm losing more than 5% of voltage--which as I understand translates to much higher light loss.

Currently I have a set of about year-old Osram 70/65W bulbs in the truck.

Would these numbers justify building a wire harness, or upgrading the factory wiring? I could probably run a heavy duty ground wire to the existing wires with practically no effort, but I don't know how much it would help. Solutions beyond that, of course, require more cost and effort.

The bulbs are rated for intensity at 12.8V, and for life at 14.0V.

I'm not sure what you mean by "tested at 12.53V to start" and "12.1V when [you were] finished", unless you mean that with the engine off and lights on, you're reading 12.53V right at the battery.

You may want to repeat the test with the headlights and engine on.

Going by the numbers, if you had zero voltage drop from the 12.53V at the battery, you're still missing a lot of output.


Vd = Design Voltage
Va = Applied Voltage
Bd = Design Intensity (B for 'brightness'; I didn't want to use I which could be confused with current)V
Ba = Actual Intensity

[(Vd/Va)^3.4)*Bd=Bm

12.53/12.8 = .979
.979^3.4 = .930
So, at 12.53V, you're at 93% of the original intensity.

Plug in 12.1V and it gets worse:
12.1/12.8 = .945
.945^3.4 = .826

So, you're sitting at about 83% of the intended intensity. What does that mean for that bulb? Going by the specs for the H4 Rallye 70/65 (2000/1350lm), your high beams will at most be 1860lm, and be as low as 1652lm. The low beams will be 1256lm best case; 1115lm worst case.

There's more to it than the loss in intensity, as with the loss in filament luminance comes a loss in beam focus and just the "punch" or throw of the beam.

Relays: You want them.

Yes, the 12.53V was across the battery with engine off. At the time the truck was in the garage and I didn't really want to get it out and start it up. It's tight quarters and my test leads are short. Things had to be precariously perched over the top of the engine, and the last thing I wanted was something to fall off and into the mechanical cooling fan. I also had to pull the bulbs out of the housing to get a good reading. I'll see if I can get some better leads and test that way. I was using a cheap meter (I have two, and they concurred), as my nice one has some battery issues right now.

I assumed that voltage drop would probably be about the same, regardless of the voltage being put out by the truck's system (resistance and so forth would be consistent). Is this a correct assumption?

ETA: So, the next question is, if I am building a harness, how much difference does 12 GA make over 14 GA, especially if using a separate wire per bulb?

With the charging system active, you'll still have voltage loss, but at least you'll start higher than 12.53V (one would assume). However, the best way to tell would be by doing the measurements that way.

Assuming your relays and sockets can handle 12 gauge wire, I'd say go for 12 even if 14 is already overkill (which I don't know if it is). This is just that part of me that says "I'm spending money, and I'm spending my TIME-- I might as well do it AWESOME."

Well, that makes sense. I think the fuse block will support 12 GA. I already have some headlight connectors (nice ones), but I don't know what gauge they support. Most likely they will do 12 GA.

iroc409 said:

Well, that makes sense. I think the fuse block will support 12 GA. I already have some headlight connectors (nice ones), but I don't know what gauge they support. Most likely they will do 12 GA.

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I did this on the cheap with my geo metro, adding relays with control wires to the existing selector arm's output. I just soldered the 12gauge wires to about 1" of the existing wiring. For my purposes, replacing the connector and wiring wasn't within budget. I suggest getting crimp-on blade terminals, soldering directly to relays or fuse holders is nerve-wracking with a butane torch.

AnAppleSnail said:

I did this on the cheap with my geo metro

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As long as you got the results you wanted, and as long as the relays and sockets are high quality, and your work is good (no failures at inopportune times, as if there could be an opportune time), then that probably is OK. Whenever I get around to doing it on the Previa, I'm going to go with the overkill method, because I'm too lazy to want to get up and work on something unless I can really "do it right". (Not a criticism of your method, of course!)

I have read that soldering in automotive applications can be problematic. Solder wicks up the wire, and tends to make it brittle. Over time, vibrations cause the wire to break. As I understand it, a *proper* crimp is better than solder, but solder is better than a bad crimp (for automotive applications).

OK, so then I have two more questions. I recently picked up parts to update my auxiliary fuse block from a cheapie to a nice Bussmann waterproof relay & fuse center (10 bussed fuses, 5 ISO 280 micro relays). I am using one relay for an ignition hot circuit, and two or three for some other lighting. The relays are tiny, but are rated at 35A. Should I look to getting new standard Bosch-style relays for the headlights, or is it OK to use two of these micro relays I already have?

The only reason I can think against this is the slightly lower current handling (and small contacts) of the relays and the reliability. Additionally, with separate relays I could mount them next to the bulb, eliminating some length of wiring. The pros of the micro relays would obviously be less clutter in the engine bay, and less additional parts that I would have to purchase since I already have them. I'd just need a headlight plug to make the triggering circuit, and some 12 GA wire to connect everything.

Tyco has discontinued production of these style relays, so I currently have Song Chuans for them. I understand these were OEM for GM for many years, so I might be able to get better ones from a GM dealer, though I haven't had any issues with the Song Chuans I've used on my truck so far (they have been there for about two years, and are in non-critical applications).

Another question, which I am pretty sure I know the answer to, is what about updating the factory wires to the headlights? I have been through the factory wiring manual, and it seems that if one could procure the proper terminals, one could bypass the factory 20 GA wire in favor of bigger wires (I would imagine no more than 14 GA for these terminals). I've already checked, and Toyota doesn't list a connector part number, only the fuse/relay box as a whole. I don't think the headlight harness has a modular plug in to the fuse block--I think the wires go directly. It probably wouldn't eliminate quite as much voltage drop as a completely separate harness, but it would use mostly factory parts, and generally in my experience OEM is the most reliable.

I wouldn't stake my life on potentially-undersized, off-brand relays.

In my car the wires to the Rightside Headlights are twice as long as the wires to the Left side headlights. So if I doubled the wiring thickness to the Rightside headlights, they would be as bright as the Left side headlights.

That's why they're staying as they are - I can't see any difference between them.

If you're going to be really honest with yourself about whether any modification is making a visible improvement, then only modify one side - leave one side original.

When I bought bluetint bulbs which claimed to improve visibility, I put only one in.

Sure, it was a sexy blue colour, but so noticeably darker that I took it out and threw it away.

Scheinwerfermann said:

I wouldn't stake my life on potentially-undersized, off-brand relays.

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That's a good point, although the ones I have are rated for 35A. The connectors are just tiny! How about Delphi AC-Delco? Being an OEM, I would think they are decent.

My brain is scattered this morning.

I have found this, which is about the same: ACDELCO Part # D1786C {#19116057}. I have found another model that may or may not handle higher current, but is a 87/87a. It would be nice to find one with a dual 87 terminal, but even the GM dealer has little information on config & specs. I can't find current ratings, so far. They are used as fuel pump relays, A/C compressor relays, lights, etc--so they *should* be enough.

It seems like Delco's are made all over: Japan, Korea, Canada, China. I've seen Denso, Omron & Siemens so far as brands.

I don't trust "ratings" from companies without a reputation. Who ever heard of Song Chuan? Anybody can stamp any "rating" numbers they want on a relay or in a catalog. If the relay doesn't live up to the "rating", where's their incentive to care? I don't see that they have one!

Reputable relays can be bought from Bosch/Tyco, Siemens, Omron, Potter & Brumsfeld and maybe Delphi, and probably a few others. I don't understand the fascination with the micro form factor, though; seems to me you are already aware that smaller relays = smaller contacts. Why not just use the bigger (hardly oversized!) "mini" relays that are about 25mm cubical? It seems to me you might be making the search for relays harder than it needs to be. Of course the dealership isn't going to have information on relay configuration, they're just going to have part numbers and (probably inflated) prices. Appropriate relays in good quality can be bought easily and inexpensively from the usual suspects (Susquehanna, Stern, etc.)


(I say "maybe" Delphi because keep in mind, ACDelco is a major aftermarket brand of parts, most of which are not the same as the OE parts installed in GM vehicles. And without wanting to drag this thread into a debate over the merits of various car brands, I can't help noticing you decided to buy a Toyota rather than a GM product. I would probably make the same choice myself, because of my experience with GM vs. Toyota quality. That logic might apply to GM component parts, too.)

I have similar reservations about the ratings, of pretty much any relay. I generally try to run well below the maximum rating on any relay--maybe just because it makes me feel better.

The reason for the micro relay is the packaging. I looked at great lengths to what options I had for adding circuits and relays to my vechile, including pulling used parts from another car. I chose the Bussmann RMTR 15303 series because it offered the most compact, all-in-one solution I could find.

The one I picked has 5 relays and 10 bussed fuses in a watertight box (which is very small). I looked at things like the very common (in Tacoma circles) Blue Sea fuse block, and some of Hella's automotive offerings. I also reviewed a lot of lesser choices. The Hellas were the next choice, but I'd have to run a separate relay and fuse block (which do mount to each other), but would also have to a separate buss bar to the system. All the extra "separate" stuff adds complexity and cost. It also adds significantly to the physical footprint.

This package puts everything in one spot, which should make installing it a lot cleaner. It also greatly simplifies mounting. I can mount it basically where my existing block is, with very little modification. I would have to fabricate a new bracket and a "tray" of sorts to hold all the other stuff horizontally. On a very minor point, this also uses the same fuses that the truck uses (and the car).

This package isn't just for adding a headlight harness, otherwise I probably would have chosen something different. Honestly, I wasn't thinking of building a harness at the time but I hadn't done a voltage check at that point either. I figured the loss would be negligible in my vehicle--and apparently it isn't.

I am incorrectly using ACDelco and GM interchangably--unless the parts that come directly from the parts counter are AC Delco. The part number in brackets I listed (19116057) is a an actual GM part number. A dealer is expensive, but just as with Toyota parts there are a number of online GM dealers that will sell the genuine article for a reasonable price.

From looking around, the pictures I saw of stock available (other than even more no-name stuff) is Omron, Siemens, and Delphi. I've heard they used to be made (old part numbers) by Denso--so that may be an option as well. Tyco has discontinued their line of ISO 280 micros (I did contact them, and there are no replacement part numbers).

So, if Omron and Siemens make good relays, is there an issue using this form factor with their product? GM has been using it for about 20 years, and it looks like they still are. While I agree with you on overall quality, of all the issues I've had with vehicles, relays have only bit me twice--on a Toyota. It was a cooling fan relay on a 1990 turbo Celica All-Trac. The dealer replaced the part with a used part, which also failed. Shockingly, when a new one was installed the problem went away.

If it truly is the wrong part for the job (or quality parts are unavailable), then either the right answer is to add a couple of mini relays as you suggest, or not build it at all rather than do it improperly.

Certainly, parts are more slightly more scarce, which is why I hesitated over getting this version. However, the rest of the package was too much to ignore.

Seems like a pretty easy choice to me. What has been proven to work and recommended by the experts vs. what might/should work based on your best guesses.

I'd stick w/ the full size relays from a reputable manufacturer. Digikey, Avnet, and others are good sources for the appropriate relays as well, you're not limited to auto parts stores.

Diesel_Bomber said:

Seems like a pretty easy choice to me. What has been proven to work and recommended by the experts vs. what might/should work based on your best guesses.

I'd stick w/ the full size relays from a reputable manufacturer. Digikey, Avnet, and others are good sources for the appropriate relays as well, you're not limited to auto parts stores.

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It's not a complete guess. GM has been using this setup in a wide range of their vehicles for about 20 years in just about every circuit in the car from the data I see. The parts are available to at least some extent from non-auto-parts sources (Digikey, Waytek, Del City, etc). Bussmann creates much larger versions (far more fuses and relays) intended for use in transportation systems such as buses and so forth (according to their documentation). CanAm is also using them in their three wheeler as well.

But, perhaps it was a bad choice. As you say, I'm not an expert. Were I to find the same thing in mini relays, I would have been all over it instead.

iroc409 said:

GM has been using this setup in a wide range of their vehicles for about 20 years in just about every circuit in the car

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OK, but I don't hear anyone arguing (and if they did I would disagree with them) that GM's headlamp circuits are adequate.

Remember, relays don't have to be all in the same location...

Scheinwerfermann said:

OK, but I don't hear anyone arguing (and if they did I would disagree with them) that GM's headlamp circuits are adequate.

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Heh... apparently neither are my Toyota's circuits, with mini relays--but I keed...

OK, it's not a good idea to use them, so I won't (I have use for all 5 of them elsewhere). I already had a spot picked out near the driver's headlight where a pair of relays could go.

If I go with a pair of relays out in the open, I would really like to go with the weatherproof skirted style (with plug). I will dig around some (maybe I just need to spend a few minutes on TE's webste). But, do you know of anywhere I can get a skirted dual-87 relay? So far all I have found are Hella and no-name units, and neither were dual-87. Scheinwerfermann, I believe I've read on here you've had bad luck with Hella relays. Maybe you know where I can get something better?

Alternatively, maybe there is a two-relay water resistant box like Hella's 4-relay unit?

Nothing wrong with Hella relays, I have used them for over 20 years now without a failure in multiple lighting projects in numerous vehicles. I believe even susquehanna motorsports still uses them in their high quality harnesses the make?

I used a lot of Hella relays over the decades myself, but I've seen quality/reliability dropping pretty fast and seriously over the last five years or so. My experience is anecdotal, of course, but it's been enough to push me away from Hella's relays; I just don't have the failure problems when I use other brands.