Electrical Myths, Part 5: Let’s Talk Toads and Diodes

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There are lots of ways to tow a vehicle behind a motorhome, but flat towing is generally the most convenient and popular method.  For vehicles with amicable drivetrains, most of the process is fairly straightforward.  You pick out a base plate to match the vehicle, tow bar, braking system, and so on.  But then you have to figure out how to get the the car (your toad) to relay the signals from your motorhome–parking lights, tail lights, turn signals, and brake lights.  You’ll find various off-the-shelf wiring packages, some better than others.  This time, we’ll talk toads and diodes and why you may or may not need them.

The Myth

You may have heard this before: “If you don’t use diodes, you’ll back-feed your car’s (toad’s) electrical system and fry something.”  There are lots of variations of it, but when you try to get an explanation of the physics from the purveyor of that knowledge, the story generally falls flat.  You might get anecdotes of someone who didn’t, and had a problem with a computer.  You might get a reference to a so-called expert.  You might not get an explanation at all.

Let’s think about what’s actually going on.  Normally, your car has some switching mechanism to control power to a particular lighting circuit on the car.  On an older car, it might be pretty simple.  Power flows from the battery, through a fuse box, to a switch, and then directly to the lights.  Many decades ago, you might even find only one wire to the light itself, with it finding ground through the body of the car.  In those cases, the task of figuring things out it generally pretty easy.  On a more modern car, you might find a relay or a computer module controlling the lighting circuit of interest.

While there are a few exceptions, most vehicles switch the positive side of the circuit.  Depending on the age of the vehicle, that can be accomplished any of several ways.  But one thing remains constant: when the light is off, there’s a high-impedance path from power source to light source.  That can be a mechanical switch that’s open, or a semiconductor.  But your RV supplying power to the lamp doesn’t subject the light to any condition it wouldn’t normally see.  Power comes from your RV, flows through the bulb, and to the chassis ground that’s also connected to your RV.  That 12V (nominal) supply does feed the switching device, but current doesn’t flow because the switch is open.

What’s the Problem with Diodes?

There are at least four problems when you make use of diodes for your toad lighting: photometric compliance, reliability, flasher operation, and getting all of the marker lights to work.

Photometric Compliance and Brightness

First of all, diodes have a voltage drop when current flows through them, regardless of whether they’re in the circuit during toad or standalone use.  This means that your lights aren’t as bright as they should be, and very easily could fall below the minimum brightness levels required by law. (Federal law, at 49CFR571.108, spells out the requirements.)  How much?  We can calculate the drop in brightness with the following formula and a little algebra:

\(P = \frac{V^2}{R}\)

\(V\) is the voltage, \(R\) is the bulb’s resistance, which we don’t need to know, and \(P\) is the power consumed, which is proportional to brightness.  If we want to know the drop in power when we reduce voltage to the bulb by the diode’s forward voltage, we’re looking for the ratio of \(P_2/P_1\).  Substituting the right hand side of the first equation into that ratio and adding the subscripts, we end up with the following formula:

\(\frac{P_2}{P_1}=\frac{V_2^2}{V_1^2}\)

Notice that resistance cancels out, and the only thing we need to know to find the ratio is the two voltages.  Assuming we know you’re right at the test voltage (12.8V) at the light under normal circumstances, and a typical diode forward voltage of 0.5V, we can find the answer to our question:

\(\frac{P_2}{P_1}=\frac{V_2^2}{V_1^2}=\frac{(12.3)^2}{(12.8)^2}=92\%\)

In practice, given the long distance between the switchgear in the motorhome and the back of the car, along with multiple extra connections, it’s typically worse than that.  The voltage drop across the diode stays constant, even as the supply voltage to it drops–look at the case where we’re only getting 12.0V to the diode input:

\(\frac{P_2}{P_1}=\frac{V_2^2}{V_1^2}=\frac{(11.5)^2}{(12.8)^2}=80\%\)

That’s a big drop in brightness!  Think about the increased time in traffic, or perhaps in bad weather, for another driver to see your toad, or recognize that you’re braking.  There are other components of complying with the photometry requirements in the federal standard, but all are fundamentally based on a proper light source with certain light output characteristics, including both brightness and distribution.

Reliability

Of course, the diode still acts within the bounds of Ohm’s law, so it dissipates power on its own too:

\(P_{diode}=V_{drop} \cdot I\)

\(I\) is current through the diode, which is related to the power through the bulb.  While there are a number of bulb types, a common 3157 at 27W should see 2.1A.  Back to the previous formula, that means we have 1W of heat at the diode for every brake light bulb:

\(P_{diode}=V_{drop} \cdot I=(0.5V) \cdot (2.1A)=1.05W\)

Similarly, for every parking light (you have a minimum of 4 bulbs illuminated when towing), at 8W each, you have 1.25W through the diode.  That might not sound like a lot, but it adds up.  That power is what makes the diode hot, and given enough heat, it will fail.

You also have spade connectors on either side, which can introduce additional resistance and heat.  They’re also not nearly as reliable as the factory connectors, which are generally weather-sealed, and at the very least much better assembled than a field crimp for the spade terminal.

Flasher Speed and Monitoring Systems

This issue is diminishing in importance as an increasing number of vehicles use electronic flashers.   But many candidates for flat towing still use conventional thermal flashers for controlling the on-off flashing of the turn signals.  The change in current when you introduce the diode will change the flash speed–perhaps not enough that you notice it, but nevertheless slightly slowing the flash rate.  We’re not talking about the flash rate when being towed, but instead the flash rate when you’re operating the vehicle on its own.

Another feature your car’s manufacturer built in requires either rapidly flashing your turn signal (“hyperflash” in casual jargon) or turning the indicator on solid when current drops to a level that would indicate a failed bulb.  The sensitivity to this fault condition will vary, but you certainly push closer to the threshold when you introduce a diode.

Marker Light Operation

In most installations, using diodes, connections are made at the rear of the vehicle close to the lights of interest.  Usually overlooked are the side marker lights, and in particular the front side marker lights.  If you install a diode at the rear lamp housings, funneling power only to the rear, only the taillights get illuminated.  You might get lucky and have a vehicle that uses a taillight bulb to illuminate the rear side marker light, but you’re not getting the front side marker lamps illuminated.  They’re a must-have, both legally and for the basic safety of your combination vehicle.

Ok, I get they’re bad.  What’s the alternative?

This is where we get into the good news, with a little homework.  If you connect at the proper location, you need less stuff.  A typical set of diodes will run close to $50, so you can definitely save real money.  If your vehicle has combined stop and turn lights (i.e. the brake light and turn signal use the same bulb), you can simply pass the signals straight back to the bulbs, and meet the technical requirements for a towed vehicle.  But such an installation could be better, and we’ll get to that in a minute.  If your motorhome and towed vehicle both already have separate brake and turn signals, great!  Studies have shown that you’re 5% less likely to be rear-ended for that alone.

If only one of the two vehicles has separate brake and turn lamps, you’ll need to convert between the two configurations anyways, with or without any diodes.  But before you think that’s the case with your motorhome, check the trailer wiring–more than likely it’s a combined signal, even if the motorhome has separate lights.

Motorhome with Combined Brake and Turn Lights

As I mentioned, you can get away without any special equipment if your motorhome and toad have the same combined brake and turn lamps.  When you do it this way, you don’t illuminate the center high-mounted stop lamp (CHMSL).  That’s the center (or “third”) brake lamp that’s been shown to significantly reduce rear-end collisions, particularly when the vehicle is not equipped with separated brake and turn signals.  Yes, you’d be legal without that light as a towed vehicle, but what little extra effort it takes is worth the improvement in safety.

Whether your toad has combined or separate turn signals, start with one of these:

This little device has internal logic that looks for both left and right combined signals coming on at the same time, and when it detects that, it switches on a separate, dedicated brake light output.  If you have combined brake and turn lights, this brake output goes only to the CHMSL and NOT to the brake lights at each corner.  If you have separate brake and turn lights, you want this output connected to feed all of the brake lights.

It also “subtracts” the brake signal from the two other outputs, giving you a left turn only (without brake signal) and right turn only (without brake signal) outputs.  If you have combined brake and turn lights, do NOT use these outputs–you need the combined signals from the motorhome directly to your combined brake/turn lights on the toad.  For your vehicle with separate turn signals, these outputs are what go to your turn signals.  Done with diodes, you illuminate the lights only on the back.  Done without them, you light the entire left turn or right turn circuit, including the front turn signals, which means that you see your turn signals are working properly in your motorhome’s rearview mirror/camera.

The key in either of these situations is connecting to the circuit that feeds the bulbs, not some other control circuit.  In decades past, the turn signal switch, brake light switch, and headlight switch had all of the lighting current flowing through them, and you could make connection directly at those points.  Some vehicles now use those switches as logic-level switches, or even pass data between the switch and a module that actually switches power.  Diodes won’t protect you in these situations, but they’re still sending 12V power to the bulbs, same as your motorhome would supply.

Motorhome with Separate Brake and Turn Lights

In almost all cases, a motorhome with separate brake and turn lights will still have combined signals on the standard 7-pin RV trailer connector at the back bumper.  If it does, and your towed vehicle has combined signals as well, follow the configuration in the section above (including the 2 to 3 converter for making the CHMSL work).  If your towed vehicle has separate signals, you have a couple of options.  The best is to use the separate signals to feed the toad–you’ll eliminate the converter module.  You’ll get the signals right 100% of the time instead of most of the time–that converter is still guessing to some degree to figure out the information it doesn’t get directly.  It’s advisable to use relays on the motorhome to keep towed and towing signals isolated, in the event of a fault that causes a fuse to blow, replicating the separately-fused circuits that are (or should be) there already.

If you’re in the minority where you don’t have combined signals available on the RV, and you need combined signals on the towed vehicle, you’ll need a converter.  The most robust system for this purpose is the Jackalopee, but its size and capacity are serious overkill for most towed vehicles.  More common is a 3-to-2 converter, like one of these:

Your RV’s separate brake, left, and right turn signals come into the unit, with a left-and-stop and a right-and-stop signal leaving.  Those two outputs should go to the lights at the back corners, but also take the brake input directly from the motorhome to the CHMSL on the back of your vehicle.  That way, you get your towed vehicle signaling almost exactly as it would when you’re driving it on its own.

What’s the Myth’s Origin?

It’s hard to say, but my guess is that it originates with someone either not making connections at the right places or mis-wired trailer/RV plugs.  Maybe someone hooked things up themselves, not understanding the wiring diagram, and when it didn’t work sought a professional who fixed the problem and added diodes.  Maybe someone really goofed the wiring up, and their customer went somewhere else and got the story that the missing diodes were what caused the problem.  But I can say this:  I have personally gone through the process on quite a few makes and models, including a number where you “had to” use diodes.  Diodes didn’t make the difference between a good install and a bad one, either on paper or in practice.

As with all of the posts in this series, help us by filling in with the stories you’ve heard, and other possible explanations.

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