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Old 09-13-2021, 04:19 PM   #1
tuppy
 
Drives: 2008 Yaris
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Tail fuse keeps blowing.

Hello. I'm looking for help troubleshooting a fuse that keeps blowing on a 2008 Yaris. It's due for inspection, and while looking it over I discovered that the license plate light was out. A new bulb didn't work, so the fuse was the next place I checked. Sure enough it was blown, but when replaced the new fuse blew as soon as the headlights were switched on.

I took out the rear running light bulbs, which I believe are also on the same fuse, and with those 3 bulbs out a new fuse blew right away again. I've taken the tail light assemblies out and there isn't any obvious damage or burns on the wiring/harnesses immediately visible.

I'm not sure if I should suspect faulty wiring somewhere shorting out, or the control stalk on the steering column. I don't know if it matters, but the car has daytime running lights (though I think it's just the front headlights that are always on).

Troubleshooting an electrical problem like this isn't something I've really done before, but would like to if someone explains it to me.

Everything else I've checked is working. Reverse lights, turn signals, break lights. The car beeps with the lights on and door open, the dash lights and dimmer work. The dome light and light inside the back work.
Edit: I just realized that the parking lights aren't working (turning the headlight control up one click). I think this is the same fuse, but I'm not sure if it's the same bulbs as the front turn signals, which do work. Just wondering if I need to take out the headlight assemblies and inspect there too.

Thanks in advance for any help.

Last edited by tuppy; 09-13-2021 at 04:37 PM.
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Old 09-13-2021, 09:13 PM   #2
CrankyOldMan
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You're right, that fuse feeds all of the side marker (in the HB), license plate and parking lights. You've got it isolated to the harness between the bulb sockets and the switch in the stalk--otherwise it would still blow with bulbs inserted (short after bulbs) or before the switch was turned on (short between fuse in the junction block and the switch).

I'd recommend doing ground continuity checks on all the wires in that circuit. That at least tells you which wire is shorted. Do you have a copy of the EWD?

The next level of troubleshooting the short will require either physically inspecting the suspect wires in the harness or getting a professional tool that can detect the short in the line at the point where it crosses. I.e. PowerProbe tools. A new wire harness from the dealer might be cheaper, if you're comfortable doing the work of installing it yourself.

The good news is that the wires go along the driver's side of the car under the sill plates. Pretty easy to get to that part, just pull them up. The bad news is that the harness is all wrapped up in electrical tape and stuff. I've got pictures if you want to see the mess of unraveling one.
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Old 09-13-2021, 09:27 PM   #3
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If you don't have a copy of the EWD, you can isolate it to the front or rear wires by unplugging just two connectors. On the beige fuse/junction box, the upper left connector is from the floor panel/interior of the car, the one immediately to the right is from the engine bay. If it's the engine bay, that's a big mess to pull the whole dash apart to get at the wires. If it's the floor pan, that's mostly just trim around the driver's side door(s).
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Old 09-13-2021, 11:18 PM   #4
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Quote:
Originally Posted by CrankyOldMan View Post
If you don't have a copy of the EWD, you can isolate it to the front or rear wires by unplugging just two connectors. On the beige fuse/junction box, the upper left connector is from the floor panel/interior of the car, the one immediately to the right is from the engine bay. If it's the engine bay, that's a big mess to pull the whole dash apart to get at the wires. If it's the floor pan, that's mostly just trim around the driver's side door(s).
Hi, thanks for taking the time to reply. I don't have a copy of the EWD, which I assume is the electrical wiring diagram.

Am I correct in my understanding that what I can try is to disconnect the front harness connector from the fuse box, put a good fuse in, and turn the lights on again. If it blows, the short is likely in the rear harness, and vice versa if it happens with the rear disconnected. Is it also possible it blows with both the front and rear harnesses disconnected, and is thus a problem with the controls at the steering wheel?

Could it also still be a problem with the bulbs/sockets in the front headlight assemblies? I haven't taken those out to look at them, and didn't know the front parking lights were on the same fuse earlier.
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Old 09-14-2021, 09:22 AM   #5
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Always happy to help when I can! Yes, Toyota calls it the Electrical Wiring Diagram (EWD) in the repair manuals.

And yes, that's the idea for isolating the short to one of the two harnesses. If it still blows after that, it's between the switch and the interior junction/fuse block.

Definitely check the running lights up front as well. The HB is a real pain because of how close the headlights are to the rest of the engine bay components but the sedan has a little more clearance between them. If you have a HB, the fender running lights are on that circuit as well.
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Old 09-14-2021, 01:52 PM   #6
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It is a hatchback. Do you have a digital version of the EWD, or know where I could find one?

It isn't immediately obvious to me looking at the under dash fuse box which connections are from the front and rear wiring harnesses for this fuse, and I don't think it wise to start disconnecting things randomly.
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Old 09-14-2021, 03:12 PM   #7
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As long as you're not breaking the connectors you're fine. ;) There's a black retaining lever thing that holds them in place and helps to pull them out.

Here's the layout of the junction block:
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Old 09-14-2021, 03:59 PM   #8
tuppy
 
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Quote:
Originally Posted by CrankyOldMan View Post
Here's the layout of the junction block:
Hi, thanks again for your help. I found a complete copy of the 2007 Yaris wiring diagram online and after studying it a bit was able to work out which connectors on the junction box I needed.

After disconnecting 4A and 4B and inserting a new fuse, I could turn the headlight switch on without blowing the fuse (though the headlights didn't work of course). Then I tried with just 4A connected without issue, and again with just 4B connected, which left me very confused. So I connected both again and the fuse still didn't blow. At this point I didn't have the bulbs in the taillight or license plate sockets, so I reinserted those, and they lit up just fine.

Before messing with the connectors I was looking at the taillight wiring again while talking about the problem with my father, and I noticed the wires going from the reverse/turn bulb assembly to the running light socket had all been pinched, and one had the tiniest bit of copper showing, no more than 1 or 2 millimeters. It didn't occur to me at first that this could be a short, because those wires are only a few inches long and run around foam padding on the taillight housing. But then I realized the taillight housing presses the foam against the metal body of the car.

Everything seems to be working now, and I was extra careful to keep those wires free when I put the taillight housings back on, with a generous amount of electrical tape over the tiny nick in the coating.
We had replaced one of those bulbs a month ago or so, and must have got the wires caught in between the body and the housing when we reinstalled it.
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Old 09-14-2021, 05:04 PM   #9
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Good catch! It's always frustrating when the problem mysteriously disappears and can't be replicated. I'm glad it was a simple fix too.
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