Yet another heated seat post! (With a twist?)

Ok I got my Taco about 2 months ago, 2000 V6 Xtra Can with low miles. I've been saving all my electrical add-ons to do at once.

I'm a low voltage/automation tech and NEED my wire work to be perfect. It's like a compulsion. I'm not happy with a fuse tap and some rando splices. I've got an accessory fuse block for my add-ons and was planning on relays for everything, until I started looking at the heated seat kit I have.

10a total draw for both seats on high. That's a decent current to pull. There are no switched circuits in the engine bay fuse box I'm willing to mess with, so my plan was to run constant power directly from the battery to the accessory fuse block, then cleanly tap a switched circuit (probably cig lighter) in the interior to power the switches and trigger the relays for the heated seats (repeat for the front light bar, rear work lights etc). The thing I can't figure out is how the heck these seats heaters are wired up! It's unlike anything I encounter at work and so I can't wrap my head around the way they've done it. There's only one power wire, with an online fuse. They say to tap the radio circuit (a 7.5 fused circuit for a 10a draw) and all the power seems to run through the switch, which as far as I know, is bad practice. I was of the opinion that you keep high current out of switches (and the dash) by using relays.

Wiring diagram is below and I can't find any examples online of a JD1914 relay being used this way. Can anyone explain this wiring to me and suggest a way I can rewire to feed it directly from the fuse block.

Bonus question, as I'm relatively new to auto wiring (although ost of it is the same as my work). Is powering the switch from a switched circuit acceptable, even though the power for the device comes from a constant power line? I'm assuming that if the switch is on the on position and the power is cut, it will revert to it's unpowered position (Normally Open (NO) switch) triggering the relay to cut power to the accessory. Any chance of a trickle current or battery drain?

 

If a mechanical relay is connected to the battery but the switch that controls it is only hot with ignition on then the relay will only close with ignition on. Electronic relays can have leakage current though, it depends on the relay. I just ordered a time delay relay that consumes ~4.5 mA when off so it will be fed from an ignition hot only source to prevent battery drain.
I have a fuse/relay panel in the engine bay to organize power sourced there as well as another fuse block in the cab(under center console) that I’ve split the buss bar in so that half of the fuses can be always hot and half get fed by a relay that triggers with the ignition. For this relay I swapped one of the 12V sockets for a USB charger/12v meter combo and use the other set of socket terminals to trip that relay so I don’t need fuse taps for either always hot or ignition hot power in the cab. A fuse tap alone is only good for 10A anyway and was insufficient to supply the ignition hot side of the fuse panel for anything more than switch power.
I have aftermarket seat heaters as well and will look into whether the controle modules have any parasitic drain. If they do then I’ll move them to the ignition hot side of the in cab panel as well. Right now just the roller switches that control them are.

 

Hey thanks for the great info. I like your idea of yet another relay to heat up a section of the fuse block. My plan originally was 2 in the engine bay, one constant, one switched, so I can do the switched one with a relay, meaning I can still pull power right from the battery but it'll still be ACC (switched) power.

I talked to a car audio install friend and he just said "naw just tack it onto the lighter circuit, it'll be fine" despite the fact that I'm pretty sure it's 16ga wire which is rated at 3.8ft @ 15a. Dodgy.

 

The two halves of the interior fuse panel are fed by 40A fuses on #10 wire. There’s also a ground buss under the console with a #10 ground wire to the battery. There are two great sources here that I use all the time to learn and share info on as well as the threads on specific accessory mods.
https://www.tacomaworld.com/threads/diy-build-and-install-a-bussmann-rtmr-fuse-relay-block.399454/
https://www.tacomaworld.com/threads/show-off-your-aux-fuse-panels.308677/
There’s no end to the info that can be found on this forum given the variety of mods being done both for off road rigs and pavement pounders like me.

 

Thanks, I has scanned through those and bookmarked them earlier, I'll go through them in depth now.

A friend is just explaining to me that in the circuit diagram I posted, it looks like they're using the relay as a logic gate. Low power setting = one heating pad is on, Hi power setting = two heating pads are on.

 

I wanted a fuse block in the cab to eliminate as many additional wires as possible. With empty slots in both the in cab and engine bay oem fuse panels it would have been nice to use those instead but in reality it’s much simpler and better access to use auxiliary panels you can customize to your needs.

 

Ok, based on this page /diagram of how this relay works, I still can't wrap my head around what they're doing with this...

https://m.delcity.net/documents/automotive_relay_diagram.jsp

With the High Power switch setting connecting to both pin 86 and 87, it can't function like a relay normally would.

 

Mine have just two wires that split to the seat and backrest pads with a switch wheel that connects to different resistors so mines a voltage controlled module. I urge you to inspect your harness for deficiencies since mine changed wire sizes after the fuse holder. Verify your connector ratings as well. This should be SOP for adding aftermarket equipment to our rigs.

 

Don’t over think it. The coil is an electromagnet and when the small current flows through the coil it closes the high current relay contacts. On 5-pin relays the switch is changed from connecting the normally closed contacts (30-87A) to the normally open ones(30-87). 4-pin relays don’t have pin 87A.

 

It takes time to internalize knowledge and have it finally register. Just keep rereading and looking at the pictures, you’ll get it for sure. I do this for every mod I do until I can visualize every aspect. Doing by rote works but I want to know how to fix it if it breaks. The hardest yet was the switching for front/rear anytime cameras, that one made my head hurt far too long.

 

Most mechanical relays are non polarized, it doesn’t matter which way the current flows, you can have them switch or be switched by connecting to 12V or ground. The coil pins are 85 and 86. You switch can supply 12v to either one of these and if the other pin is grounded the 30-87 connection closes. Likewise you can connect 12V directly to either 85or 86 and when your switch connects the other to ground the relay will again connect close 30-87. Polarity on 30-87 makes no difference either.

 

I

In their example with 86 connected to ground and 85 connected to an accessory think of it like that accessory being your cigarette lighter that supplies the trigger current to close the switch and 30 is connected to the battery with 87 connected to your aux fuse block.

 

Definitely! Take a look at this amazing ground junction that was hiding under some tape:

 

I totally get that, and the example given there. What I don't get is the way it's wired in my diagram above )zoomed view attached) that reflects the way it's wired in the seat heater kit I have. They have 86 & 87 connected to the High Power switch, 87a and 85 connected to ground and 30 going out to one of the heater pads.

 

You’re right, their schematic you drew is confusing. I’d have to know more about all 3 components(switch, connector, relay) to make sense of it since I have to bludgeon my way through this stuff myself. Having 86 and 87 supplied from the same source just means that the switch both triggers the relay and supplies its power when yellow is hot, like your friend said to heat the second pad but it’s confusing why the relay is needed to begin with if the switch can handle the power already.

 

Redrew your circuit. Something is wrong. 86 and 87 shouldn't be jumped together otherwise Heater 1 can never be turned on... it's also a floating load, a no-no.

 

Ok, I solved it! What a wild ride... Yep the diagram is screwy. It doesn't help that the pins are reversed on their switch. I pulled out a power supply and my multimeter and went to town.

The long and short of it is that it's actually a clever little circuit that changes the flow of power from being in series for the low setting, to being in parallel for the high setting.

In low, power flows into the yellow wire, which hits the relay, triggering it to switch over to output pin 87. The thing is the power from the yellow wire is connected to 86 (to power the relay coil and activate it) and also to 87. So when the relay is activated, that power is connected from pin 87 to pin 30, flowing down the white wire to the first seat heating pad. It flows from there down the blue wire to the junction near the pad connectors, then up into the second pad, and through it to ground (blk). That put the two pads in series. My resistance measurements were about 9ohms each pad, to total resist is 18ohms and current is 0.75 amps (assuming 13.6v).

So when you switch to high power, the yellow wire goes dead, the relay coil is unpowered and switches off, connecting pin 30 to pin 87a (ground). So now the white wire is providing a path to ground, the reverse of what is was before! With this new path to ground, power is flowing down the blue wire (skipping the relay) to the junction near the heating pad connectors and splitting off to each pad. As each pad has its own path to ground (one has the white wire one has the normal ground) they're in parallel and total resistance drops to 4.5ohms, so amps are 3.0, assuming 13.6v.

Interesting use of a relay. I've never seen that before. Sadly it means I can't keep the higher amperage wires out of the switch, but at least I understand now.

 

Amazing, thanks! Makes more sense now. I just posted about sorting it out. You're saying it's done incorrectly? Is it dangerous?

 

I just think you drew it wrong, now reading your latest post

 

I reviewed your diagram and it seems the same, the only difference is that I had hi/lo reversed...