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Disclaimer: I have very little electrical knowledge.

I have wired electronics (radar detectors, dash cameras, GPS units, etc.) to about a dozen vehicles in the past, but have ran into a strange issue with my 2016 Toyota Tacoma (which, I'm told, uses Can Bus).

I've found a switched 12V source (sensor on brake fluid reservoir); verified using a multimeter. 12v when the truck is running, 0 when it's off. I've spliced in a 12V accessory, and at this point the voltage dropped to 5V. If I disconnect my accessory's ground, the voltage goes back up to 12V.

I have tried numerous grounding locations for my accessory - does not seem to make a difference.

This appears to be the only easily-accessible, switched 12V source where I'm looking (engine bay), and I'd rather not run cables through the firewall to an in-cabin source.

What do I need to do in order to have my accessory run off this 12V source?

I'm guessing I could wire my accessory to the truck's battery via a relay, and use the switched 12V to control the relay, but I've never done that, so it's just a guess. I would prefer to avoid such complications, if possible.

pacoverflow
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Yuriy Babenko
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2 Answers2

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I think the comment reveals the problem: you've tapped a sensor wire on the brake fluid reservoir. Could it be that the 12V supply is routed through a red lamp on the dashboard that lights up when the sensor sees low level and connects the wire to ground? If that's the case, it would measure as 12V on the multimeter, via the un-lit bulb (low resistance). Your accessory draws some current (to ground) via the bulb and the voltage drops to 5V. Maybe the bulb glows dimly on the dashboard. The multimeter does not draw enough current (almost none at all) to light the bulb or to drop any voltage across it.

I think you'll need to find another place to tap off a switched 12V supply.

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Did more research on this and the consensus seems to be that Tacoma's electronics detect the additional current (of the accessory), and drop the voltage automatically as some sort of safety/precaution mechanism. No alerts/lights/errors as a result of this, though.

I ended up using an add-a-circuit fuse tap to tap a switched fuse in the engine's fuse block.

Yuriy Babenko
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