'86 4Runner SR5
Every once in a while, when it gets rainy, my running lights (the yellow sides and the red rear), as well as the dash lights, come on. Its the same as if my headlights were on, but they're not, and the switch in the cab is definitely not. And the truck is not even running, so my battery drains.
I'm assuming its a short somewhere, especially since it seems to happen when it rains (I live in the NW so its a common occurrence), but I don't know where to start looking.
Any ideas? I'm not very familiar with the electronics (kinda new truck). Any help is greatly appreciated.
This happens spontaneously, sometimes in the middle of the night so I have teh pleasure of waking up to dead battery.

 

pop hood and see if any of the relays are wet under hood, then check the driver side kick panel for water, it has to be getting water in there somewhere.

 

I'd be replacing all the fuses and light relays.
Sounds like they're bad, and the moisture is causing them to short and turn on.

 

Yeah I don't see any water getting in, not visibly anyway.

 

So its probably like peow130 says, something's just bad and the moist air is enough to short that something. Sounds like a fun wild goose chase!!
Anyway, its still raining and I don't have a garage...

 

Start with the relays. Fuses are SUPPOSED to be "shorted."
A fuse can't be bad and cause the battery to drain.

I'm not sure if the parking lamps / tail lamps use a relay. If you stick your ear by or your finger on the relays and have someone turn the headlights on you should be able to hear a click on the headlight relay. Then listen / look for something similar when the switch changes from off to parking lamps. If you find such a relay I would take a water hose to the outside of the truck and underneath and see if you can get the failure to occur. If it does pull the relay out and replace it or swap it WITH AN IDENTICAL ONE (if you have one) on a different circuit.

Thats how I'd go about troubleshooting. It does sound like a snipe hunt and if you can get a circuit diagram and confirm there IS or IS NOT a relay it might save some time. If there is no relay then for sure check the wiring diagrams to see if they switch ground (or you can use a test light and unplug one of your lamps). If they are switching ground than you could have something somewhere that is almost grounded and moisture is enough for it to conduct. I suspect this would only make 1 of your corner lamps turn on though.

Hope it helps and good luck on the snipe hunt.

 

First of all, thanks jonnydclark for the detailed reply, and I've finally gotten time to start looking into this again. In the mean time, the problem has gotten a bit worse. Instead of being unable to turn off the running lights, yellow in front, red in back, and the dash lights, they will now just not turn on ever.
After looking at wiring diagrams and my FSM (from 88 instead of 87), I thought I'd tracked it down to the tail light relay. But I replaced it, and still no lights. And the dash light dimmer switch checks out also. Soooo, once again, I'm at a loss....
(bump)

 

It seems like there would be some sort of central fail point that might cause all three to fail at once?

 

A friend was recently dealing with something similar, this was my reply:

 

Thanks corax, but I haven't had those symptoms yet. Going to check anyway.
Right now after checking relay, I'm thinking it's probably the switch except I can't figure out how to get in there.
I fixed a brights problem in there, but where you twist the knob for parkers and then lights seems impossible to open. Anybody know any different?

 

i am having a similar problem with my 81. so far i traced it to the a couple leaks - one coming from the rubber gasket in the firewall (lines run to fuse box) and the other one still a mystery.

are you checking for leaks as its raining or just after the fact? i poured water over the front fender/hood area to simulate rain, thats how i found the first leak so far.

 

I'm not so sure its from rain anymore. I thought it was, but living in the NW in the winter, its raining all the time anyway.
I thought it might be the switch itself, but I just got finished replacing it with one from a junkyard ($45) that had tested out and it didn't fix. So once again I'm dead in the water.
All the bulbs are fine, fuses, relay, switch, all checks out. So..... balls

 

Oh, I'm back home in Colorado. Very dry here

 

Solved!
It was a irregular tail light relay. Turns out there is a an EFI relay that is in the place where I thought the tail light relay should be, and the tail light relay was attached remotely by some wires that plugged into the little board where the fuses (and some other relays) are.
So, it was exactly what I thought, I had just been looking at the long one for a looonnngg time. Anyways, the lights work now.