Heated steering wheel mod - success!

@Emmantik, the steering wheel turned out great! I completely get why you want to keep it. I would definitely consider swapping in the proper OEM Toyota clockspring, just because you don't know what the current draw is by that heating element. I'm not an EE by trade, but I have tried to teach myself some basics while doing this so that I'm not doing what this guy does (watch the first 60 seconds):



Those traces for the steering wheel buttons are maybe 24AWG or 26AWG. The higher the AWG the smaller the wire diameter. (See attached photo from the web) If there is no thermal controller or way to regulate the element, it can just keep heating up. As heat of the conducting material increases, so does resistance within the wire, which leads to more heat, and so on. Whether it is a heated wheel, seat, or oven element, the principle is the same: a high resistance wire generates heat.

Based upon the current draw of the OEM wheel and the 10A MAX rating of the circuit, the smallest size wire one can safely use with enough "buffer" is 16AWG. At least with the OEM heated clockspring if it does cook a wire, it will be in just the circuit for the heated wheel, not in the same cabling as the airbag. I thought long and hard about using the regular clockspring to save cost, and the risk was too great for me knowing both myself and family members would be behind the wheel of my 4Runner at some point. I'm a firm believer in that if you give somebody truthful, well laid out information, they come to the right answer for themselves and their situation.

 

Glad the explanation was helpful. Hopefully I didn't go too into the weeds on that one. There's a lot of "assuming" with that or any aftermarket clock spring.

 

this killed my idea of possibly using the cth heated wheel with oe parts [clockspring, switch, etc]

 

@Old Red Hi Old Red! I am just trying to wrap my head around this!

So from what I understand, the main concern(s) I have is;

1) The wires I'm using is not sufficient enough to handle the load of the heating pads?
2) The traces/circuits inside the non-heated clockspring ribbon (specifically the ones used for heating pads) are not sufficient to handle the load?

Is there anything else I am missing or I'm not thinking about?

Thanks!

 

@Old Red

So I checked the specs for the heating pads I ordered and installed. They are 36W.

If I used this conversion tool online using 36W at 12 V DC, it gives me 3amps rounded to 2 decimal places, so let's say 4amps.

https://www.thecalculatorsite.com/conversions/common/watts-amps.php

I used this calculator to determine the wire size required at 4amps with a length of 2'(0.6096m) from steering wheel to fuse box and it gives me gauge 21, let's bump it up to 20 gauge. (return run on the circuit taken into consideration in the calculator)

https://www.fabhabs.com/dc-cable-sizing-calculator

Based on this info, as long as the wires used for this mod is at least 21-20 gauge, and the traces in the non-heated clockspring ribbon for (heating specifically) can handle 3-4amps, then I should be in the safe zone?

Would love to know your input on this!

Thanks!

 

***Please forgive any grammatical mistakes, I’m writing this up between shifts on little sleep****

Ok, this is good info to do this exercise with! Once again, full disclaimer: I'm not an EE by trade, so if there is someone here that wants to check me on these calculations, please do. I'd love to learn if I am incorrect here.

I'm going to do this step by step so if it seems “lecture like” it’s is so very not the intent. I just want folks to be able to follow along if they have never done this before. Also, doing it this way so if there's an error I'd like it pointed out.

The key point here is consistency on wire gauge throughout the entire circuit length.


Heating Pads:

36W = ~3A draw; that is what the pads would pull at a clean 12V from the source. Again, the 3A assumes no loss of voltage over the length of the circuit, which is never the case, so let’s use 4A for this. We are also going to ignore any losses due to resistance from connectors or other components as it is probably negligible for this exercise.

Circuit Length:

The length of the circuit for calculation of wire gauge needs to include the wire from the power source (fuse block) all the way to the device (heating element). This gauge needs to be consistent throughout the entire circuit length, and includes the following segments:

• Wire from fuse block to clockspring =2ft
• Ribbon Cable Wire inside of clock spring =2ft*
• Leads from clockspring to heating element =0.5f

Your Total Circuit Length One Way = 4.5ft or 1.37m

(Calculations below)


Ribbon cable
Winding diameter ~2.5"
Winding revolutions ~3.5"

C=2π*r so 2π *1.25" = 7.85" length per winding *3.5x revolutions ~ 24"

Voltage Drop

Now we need to calculate voltage drop from the source to the device. Acceptable drop is anywhere from 2 to 3%. Remember, if supply voltage decreases, the device will draw more current to operate which in turn increases power loss, and produces heat. Power (P) in Watts = Voltage (V) in Volts * Current (I) in Amps. In this case, we want the thinnest wire which should have the most voltage drop (due to highest resistance) to be the heating element itself.

This is the calculator I used with 4.5ft in the circuit one way, 4A, varying the gauge of the wire to see what voltage drop would be across the full length of the circuit.

https://www.rapidtables.com/calc/wire/voltage-drop-calculator.html

In summary, based on wire gauge, the associated drop is:

26AWG = 12.22% for a 1.47V drop

24AWG = 7.68% for a 0.92V drop

22AWG = 4.83% for a 0.58V drop

20AWG = 3.03% for a 0.36V drop

18AWG = 1.91% for a 0.23V drop

16AWG = 1.20% for a 0.14V drop

14AWG = 0.76% for a 0.1V drop

So if you take P = V*I you can see how the current draw increases to keep the heating element functioning as an effect of voltage drop. So to calculate current (I), we rearrange so that I = P/V. Note that this calculation is based on the above AWG voltage drops to illustrate current increase based upon draw of the element with that gauge wire.

Current draw based on AWG for the TOTAL 4.5ft circuit:

26AWG = 3.42A

24AWG = 3.26A

22AWG = 3.15A

20AWG = 3.09A

18AWG = 3.05A

16AWG = 3.03A

14AWG = 3.02A

So we see that the pads draw more than what their calculated current draw would be based upon the wire size. To essentially get no voltage drop or loss, and have the pads draw no more than 3A ever, we would need size 8-10AWG which is overkill.

Now, let’s specifically look at the clockspring ribbon cable circuit and the amperage that will be going through it to power the heating element. As I mentioned, it looks like the steering wheel button traces are 24-26AWG size. We “estimated” the clockspring ribbon cable length to be about 2ft per ribbon cable. So, with a draw of 4A, let’s see what the voltage drop and resulting current will be through 24-26AWG wire. Remember, 2-3% is ideal and “acceptable” losses.

Voltage based on AWG for the Ribbon cable ONLY:

26AWG = 5.43% for a 0.65V drop

25AWG = 4.30% for a 0.52V drop

24AWG = 3.41% for a 0.41V drop

So as we can see, even with the 2ft circuit length within the clockspring button traces, we are over the acceptable voltage drop. Let’s see what the amp draw through 24-26AWG would be with the actual voltage drop.

Current draw of 36watt element based on AWG for the Ribbon cable length ONLY:

26AWG = 3.17A

24AWG = 3.11A

Now that we have the actual current being drawn through the 2ft length of the spiral cable, let’s see what that size wire is rated to handle.

26AWG wire is rated to handle a max of 1.3A at 60C

24AWG wire is rated to handle a max of 2.1A at 60C

22AWG wire is rated to handle a max of 3.0A at 60C

20AWG wire is rated to handle a max of 5.0A at 60C

This table for AWG is all over the internet.

Link to table of AWG to Max A allowed for temps: https://en.wikipedia.org/wiki/American_wire_gauge


Let's use your calculator from above for consistency, and I converted 4.5ft into 1.37m for this calculator or ~1.4m

https://www.fabhabs.com/dc-cable-sizing-calculator

Based on the calculator and the desire to be at 3A draw for the 36watt element, the minimum wire gauge to safely have for this circuit is 20AWG, which it sounds like you have to and from the clockspring. However, you need to have minimum 20AWG throughout the entire circuit which is impossible with the factory non-heated clock spring.

Based on this, as I have said a few times, the factory non-heated clock spring is not equipped to handle the power draw of a heating element of any kind. Based on my calculations (presuming they are correct, double check me) you are not in the safe zone, you are in the danger zone. You might get away with it without any consequence, or you might not. The deciding factor is time.

The heating element is drawing a little over 2x what that ribbon cable gauge wire can handle, and because there is no control of power to the element except on or off via switch, this is a constant draw without any short protection. Is this going to cause it to go “Poof” immediately? Unlikely, but over time this will cause degradation of the insulation of the ribbon cable and make a short more possible. Any short to the airbag circuit will cause it to deploy. If/when this happens is anyones guess, but not having the correct gauge wire to supply power increases the likelihood of it occurring (I'm not a statistician either, so I can't give you the odds).

 

@Old Red

Do you have any pics of the ribbon traces from a heated clockspring? Would be good to see the difference in trace sizes between heated clockspring ribbon vs. non-heated clockspring ribbon.

 

You can go deep into the weeds performing a worst case analysis of electrical systems like this. Somewhere there must be an analysis and recommendation for the current-carrying capabilities of these exact flat-conductor type cables. But generally you are trying to keep the temperature rise below a certain level based on its operating temperature in an environment which is wound up in an enclosed container in a hot vehicle. This is to keep the insulating material from softening and degrading over its service lifetime, with all of the mechanical movement applied.

The clock spring flat cable is not stranded and can almost be considered similar to a PC board trace. Here’s some info on PC board trace width considerations, which for 5A work out to be 110 mil. If the width of the conductors in question could be measured that would give you some more confidence that there is enough copper to carry the desired 4A safely.

https://resources.altium.com/p/pcb-trace-width-vs-current-table-high-voltage-design

@Old Red explained it well. It’s best to be very conservative and aim for a 20 gauge equivalent to handle 5A.

 

No, I don't have any internal pictures of that one. It is currently installed in the 4Runner for the heated wheel. If I somehow destroy it on another project, I will open it up and take some pictures for documentation. My rule is that if the part is busted I'll break it more in the name of science, but if it is new I won't sacrifice it.

 

Thanks @ElectroBoy. I'll look at those PC board trace tables, as that is a much better representation for those ribbon cables. 110mil comes out to about 3mm for trace width if I did my conversion correctly. The factory traces looked to be about 1-1.25mm for the buttons. I can put a set of semi-decent calipers on the broken clockspring ribbon cable this evening to confirm. You might know better than me in this regard, but doesn't the amount of copper (i.e thickness) in the trace matter as well as the width? I generally don't know, hence my question.

 

PC boards are built from stacked layers where the copper foil is specified in ounces of copper per square foot, a holdover from the early days of PCB manufacturing. Multilayer board usually have 0.5 oz (or less) on internal layers and 1 oz on surface layers or maybe even 2 oz for high current capacity power traces. These flex cables I’m guessing use 1oz copper which would be about a 1.4 mil thick trace, about 0.04 mm. (Might be hard for you to measure, but maybe could guesstimate). The table was based on 1oz traces.

 

@Old Red

My questions regarding the heated clockspring;

1) The correct part number for heated option clockspring is 84307-35040, which includes the sensor?
2) Are the plugs/connectors the same for heated/non-heated clockspring?

Thanks!

 

1)The clock spring with the heated circuit is 84307-35040 as you mentioned. That will include the angle sensor. Confirmed by Old Red, and why its 4-500 bucks
2)The plugs are the same on both, for the airbag and buttons. The clock spring with the heated element will include an extra plug.(A plug that the heated wheel will plug into).

 

Great! Thanks!

 

Well I got the clock spring delivered first. Wheel itself and switch are on national backorder as I'm being told by dealers from east to west coast.
If anyone wants reference images, here they are. This is the part that is called out in Old Red's retrofit guide, includes the steering angle sensor. I can take more from other angles if needed.

 

Sorry guys, got distracted over the weekend.

@Emmantik, Yes to 1) & 2) as @medium.rare.pepe stated.

@ElectroBoy, I took a set of calipers to the ribbon cable, and here is what I got for measurements. I will say that eyeballing these traces I overestimated the size, as they were much smaller with the calipers. Take the width measurements with +/- 0.005. Thickness was easier to measure (See attached photos).

Both traces had a thickness of .002" or ~ 0.05mm, which correlates with what you said about 1oz. copper. The air bag traces had a width of 0.054" or ~1.37mm. Button traces had a width of 0.025" or ~0.64mm.

 

So it seems the traces for the button controls at 25 mils would equate to carrying 1.5 - 2 A, according to that linked guideline table. And the airbag traces at 54 mils would handle 3 A.
These guidelines are for continuous current through a conductor in a multilayer circuit board, limiting the temperature rise to 10°C. For these two applications (which are not continuous usage) that seems like a pretty conservative design, since a flat single conductor ribbon cable in air would most likely be able to dissipate the heat even better than a buried trace in a CCA.

 

This is great! Thanks for posting the info @medium.rare.pepe and @Old Red !!

@Old Red Just to clarify, the pics you posted on the measurements for the ribbon traces are from a non-heated clockspring, correct?

 

Does the switch harness plug into the clockspring? Just wanted to clarify as my only point of reference is the Cartrimhome install video. Just wondering if they use the same clockspring as OEM or their own aftermarket type that has a different way of connecting the pushbutton switch.

 

Toyota came through. After being told it might take months, I got the wheel and switch in less than 10 days. Looks like the heating controller is built into the wheel.

 

I wonder how hard it would be to get the stitching switched to red to match the rest of the interior.

 

Got the cartrimhome wheel delivered today. Mostly got it just to see how well its made and no plans to install it, so if anyone wants it, hit me up.
Took a picture of the circuit controlling the heating element if anyone is interested.