How to resolve 2005 Sequoia SR5 trailer towing wiring issues?

I recently installed a Curt model 13440 trailer hitch on my 2005 Sequoia SR5, and now I am ready to set up the wiring for connecting to a "common" type trailer lights system. But confusion galore!!?? There are lots of interesting yet rather puzzling clues as to the present setup. First of all, there was a Hopkins model # 310-0041-560 Rev. B 7/02 Hoppy Electronic Taillight Converter attached to the vehicle with the white "ground" wire attached to a grounding point on the body and the "tail/marker" (brown), "left turn" (yellow), and "brake" (red) original module wires bundled together and going up through a hole in the underbody ceiling. So the module appeared to be "permanently" wired into the vehicle's circuits, except for the "right turn" (green) wire coming out of the module and wrapped into a bundle and tied off with a zip tie and unconnected to anything. Because of earlier parasitic battery drain issues and concern that the module could be causing the battery drain, I cut the module wires going up to the hole in the "ceiling", and I installed a matched pair of 4-flat connectors for those three wires so that I could unplug the module from the vehicle's wiring. This also allowed me eventually to easily test the bundle of three wires going up through the hole for signals. It turns out that the taillight, left turn, and brake signals do actually appear as one might expect on those three wires. But the fact that the right turn (green) wire from the module is separated from the bundle of three and is not connected to anything is indeed very curious, not only in itself but also because it is very reminiscent of the Amazon.com display of, say, "Hopkins 48925 Tail Light Converter with 4 Wire Flat Extension"; two other Hopkins converters at Amazon.com are similar. If you take a look at that converter at Amazon.com, you will see that the three (presumed) "tail/marker" (brown), "left turn" (yellow), and "brake" (red) input wires are bundled together, but that the (presumed) "right turn" (green) input wire coming out of the module is wrapped into a coil by itself; this is similar to the Hopkins converter that was on my Sequoia. This suggests to me that the three bundled wires going up into the hole are probably going to a terminal block of sorts where the three signals are readily accessible near each other but that the green wire is supposed to be threaded separately way over, say, to the right taillight to connect with the right-turn signal circuit. But there is more evidence to examine. This demands a new paragraph.

First of all, the owner's manual says in the "Forward" on the very first page, "Please note that this manual applies to all models and explains all equipment, including options. Therefore you may find some explanations for equipment not installed on your vehicle." This would seem to explain why the fuse block containing fuses 11 (Towing tail 30A), 12 (Batt charge 30A), and 13 (Towing BRK 30A) is missing from my engine bay. Incidentally, these fuses seem to be for heavy-duty towing, which perhaps is associated only with a likely optional "towing package". By comparing layout patterns of fuses and (presumed) relay modules in the fuse boxes and the shapes and orientations of the fuse boxes (on page 461 of the owner's manual) and the fuze boxes I find under the hood and the locations shown in the "F RELAY LOCATIONS" section on page 20 of the wiring manual "2005 Sequoia EWD610U.pdf" from this website, it appears that the missing fuse box is what that page 20 labels "Engine Room R/B No.3". I would suppose that the note in the owner's manual about "equipment not installed" also applies to the wiring manual. This requires another paragraph about what seems to be _plausible_ assumptions and resulting inferences in the hopes of "easily" tracking down what circuitry my Sequoia actually has without laboriously tearing the vehicle apart and incurring all the risks associated with doing that.

A careful look at the wiring diagram of section "Trailer Towing" beginning on page 112 of the wiring manual shows two parts of interest, namely "T10 Towing Converter Relay" and "T11 Trailer Socket". Page 47 in section "G ELECTRICAL WIRING ROUTING" shows that "T11 Trailer Socket" appears to be the "socket" (under the rear bumper) that the trailer's plug plugs into. Notice that on the top of page 113, the "*1" next to a wire indicates relevance to a 7-pin socket, and "*2" indicates relevance to a 4-pin socket. Initially, it might seem that, if the vehicle does not have a (presumably) optional tow package installed, the vehicle would then have wiring for the 4-pin socket. However, I suppose there could be an option that the vehicle, upon leaving the factory, had no wiring for any sort of towing. But note that there is a 30-amp fuse "21 Towing 30 A: Trailer lights (stop lights, turn signal lights, tail lights)" installed in my Sequoia. So this suggests that the Sequoia does indeed have towing lights circuitry for a 4-pin towing lights socket. By comparing the inputs and outputs of the "T10 Towing Converter Relay" with those of the Hopkins converter, it appears that the stop-signal input (presumably "STIN") gets fed simultaneously into the left-turn ("LTOT") and right-turn ("RTOT") output signal lines so that both turn signal lights light up at the same time to indicate braking. Presumably, that is done via a relay so that there is no direct electrical connection (such as via diodes) from "STIN" to each of "LTOT" and "RTOT". In other words, in the case of a 4-pin "T11 Trailer Socket" per the Toyota specs, the "T10 Towing Converter Relay" would seem to be necessarily present, and so no Hopkins converter would be necessary. (Although the relay in T10 probably keeps the output lines electrically isolated from the input signal lines, I wonder whether the typical Hopkins converter directly feeds the brake signal directly into each of the right-turn and left-turn output lines, each via a diode to "mostly" keep the right-turn and left-turn output lines isolated from each other. I plan to comment more about this in a moment. I say "mostly" because maybe a likely tiny "reverse-direction" current in the diodes may essentially mean the right-turn and left-turn lines are not completely isolated.)

I suppose that some of the Sequoia towing-oriented circuitry may be burned out or substantially deteriorated or the towing circuitry was never present in the first place even for a 4-pin socket, in which case a Hopkins converter may be appropriate.

So here are the crucial questions. What actual (thus presumably 4-pin-based) towing-oriented circuitry is present, if any, and where would a Hopkins converter be connected in the Sequoia wiring if needed? What can be __confirmed__ and what is __likely__ before or without taking the paneling apart to directly confirm what's available for towing circuitry? Second, how does one get access to the relevant circuitry for wiring purposes? I did once remove the rear door paneling to eventually implement an ad hoc alternative for the door latch mechanism; and I did once delicately take the paneling off of the left rear door to diagnose a power window failure to operate (both up and down); I also disconnected and reconnected a two-conductor connector likely for the power window motor(s), but I never found the source of the problem for the power window, and, after reinstalling the paneling, the window motor(s) operated normally both up and down, leading me to think that what I did probably reestablished a ground connection for the motor(s) so that both upward and downward functionality was restored simultaneously and that the two conductors in the connector were one for upward and the other for downward operation. (This reminds me that I need to get a special plastic (nylon?) spatula to help with removing Sequoia paneling with less risk of damage.)

More about the Hopkins converter:

Using an analog multimeter, I tried testing the Hopkins converter essentially as it was originally connected electrically to the Sequoia. I think I got the taillight signal at the output end of the Hokins converter, but I don't recall for sure; I should do that test again and edit this to clarify this comment. Anyway, I also tested the left-turn signal output from the Hopkins converter and got __no__ signal. My first impression was that I definitely needed to replace the Hopkins converter with a new one. So I selected for consideration the "Hopkins 48915 60 Tail Light Converter", Amazon's "top pick" (& an "Amazon's Choice") converter that Amazon says fits my vehicle. So I studied many of the Customer Reviews comments for this converter. A review titled "Choose a different Hopkins adapter" gave the converter a three-star (out of 5) rating and had a lot to say, some of it bit cryptic and unclear. In particular, there was this comment:

"The instructions should make it more explicit that you cannot test your connections incrementally. Connect white to ground and brown to running/parking, and the brown output on a 4-wire, or the running light on the LED tester you should buy as an add-on purchase, will have +12v. But sending +12v on any of the signal lines (green = right turn, yellow = left turn, red = brake) will not give a signal on the corresponding trailer line or lines unless the other lines are at ground potential, as would be the case if they are connected to an unlit bulb. The module must be sending a microcurrent back through the unlit bulbs, not enough to make them illuminate. That is not necessary, and is probably to allow less expensive manufacturing, but if your towing vehicle doesn't have bulbs in place you have bigger problems. Use the brown input or a tester to make sure you've tapped into the correct vehicle wire, or wait until all the wires are connected to test and troubleshoot."

I have a couple of comments about this. First of all, the phrase "unless the other lines are at ground potential" seems a bit ambiguous and confusing, especially in regard to what is meant by "other lines". What about the taillight line which would not be at ground potential during nighttime driving? I'm guessing he is referring to the three lines he calls "signal lines".

Second, the idea that a microcurrent needs to go back through the unlit bulbs suggests that the microcurrent is current on the high-resistance, very-low-current, very-many-turns primary windings of a relay in the converter module. If that suggestion is true, that suggests (but is not a guarantee) that the Hopkins converter module is designed excellently for reliability and durability.

Third, the somewhat cryptic comment about "the brown output on a 4-wire" seems to agree with the Hopkins "taillight" output signal test I think I did but don't recall for sure and probably should redo just to be sure I did it and get the result I think I got earlier.

So from this Customer Review comment, it seems that maybe the old Hopkins converter that was attached to my Sequoia may be perfectly good after all and that all I may need to do is to get that green right-turn signal wire connected to the right-turn signal circuit in my Sequoia's right rear taillight. Does anyone know whether this is what is _supposed_ to be done (as designed by Hopkins)? In any case, I would like to be fully and easily informed about all the details of the Sequoia's design and the Sequoia's current status.

Concerning the "microcurrent", there is yet a second interesting, informative Amazon Customer Review for this Hopkins converter I've been considering from Amazon; in this case, the review is titled "with a pretty standard small trailer" and gives the inverter a three-star rating:

"Installation was fairly straightforward and the converter works, but I am concerned about long-term reliability. I used this on a 1988 Mazda B2200, with a pretty standard small trailer. When I put on one of the turn signals, the light that is supposed to blink does blink, but the one that isn't supposed to blink does so slightly. While it's still obvious that the turn signal is on, it's not ideal and may confuse some people behind me. It also concerns me that there is a possible design flaw which may cause it to stop working at some point in the future."

While the slightly lit "ghost" blinker could conceivably be due to the slight "reverse" current through diodes in a pair-of-diodes design that I suggested earlier and while it seems the pair-of-diodes design I conjectured would not prevent operation of one turn signal if the other of the turn-signal wires was not connected, the first review above seems certain that the ground connection via turn-signal light bulbs of both turn-signal lines is necessary for operation and testing of the turn-signal output from the Hopkins converter so that the first review seems to rule out the pair-of-diodes design I suggested. If the ghost lighting is due to an actual relay in the converter module as I suggested above, it seems likely that Hopkins gave the reliability of that relay a lot of attention in the design.


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On May 29, 2024, I bought a 2005 Sequoia SR5 "as is" (at about 247,193 on the odometer) but seemingly in very excellent running condition. It's been a challenge discovering all the things and resources I need to efficiently and economically put the vehicle into best condition for perhaps a life lasting up to 1,000,000 miles or more. With no evidence of a timing belt change, I had a timing belt kit installed. Then with the coolant level not staying at specs and with the shop recommendation for a more than $700 radiator replacement, I installed an aftermarket radiator plus part of a gallon of Toyota Super Long Life coolant both from Amazon for a total of $120. That was a surprisingly easy job after Customer Review comments clued me into a combinatorial "trick" that was needed. With a parasitic drain on the battery, I bought an owner's manual and did much exploration by removing the most suspect fuses one by one but eventually had a repair shop do a more thorough exam for circuits that could be causing the parasitic drain. The aftermarket radio and the power seat circuits were identified. I adjusted the driver's seat for the best "permanent" setting and then pulled the fuse for the power seats and the fuse "33 Rad. No. 1 20A", of which this last circuit had a 0.02A parasitic drain on the battery, but it seems the parasitic drain from the power-seats circuitry was substantially larger, maybe more like 0.1A.

A major use I have for this Sequoia is as a camping vehicle. So I removed all the rear seats and placed a memory foam mattress, a 0°F-rated sleeping bag, and several totes of camping gear in the cargo area and made a mostly summer-long, sometimes quite adventurous "maiden voyage" to the Eastern Sierra this last summer (2024).

 

Can we get a TL;DR version? My 2005 Sequoia has a 7 pin connector on the rear bumper.

 

Thanks for your comment. I'll try doing that. The long version helped me think through all the many issues and related questions. Some things I included simply because I thought at least some folks would like that kind of stuff as background. So yeah, some things clearly are not directly relevant (at least in a technical sense) and can be left out. The logic and details and likely inferences seem a bit longish and maybe could be condensed a bit, but I wonder how "digestible" the condensed technical detail and its "logic" would be. I suppose that varies from person to person. So I guess I'll need to also think about what technical detail is not likely to be so important. I'm open to suggestions. Maybe I can separate the matter into non-Hopkins matters and Hopkins matters somehow as separate posts.