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Wiring issue


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16 replies to this topic

#1 Mikevan10

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Posted 03 October 2012 - 10:47 AM

Today's problem in my Subaru fleet is that the brake lights suddenly stopped working on my 1992 Legacy sedan. I spent a little while troubleshooting last night and got as far as determining that I have power to the brake light switch and that if I bypass (with a jumper at the connector plug) the brake light switch the lights still do not illuminate.

I do not have a Factory Service Manual for this car. Can anyone help me find one? I do have an FSM for my 1997 Legacy but the wire colors do not match so it is not much help.

My guess is that I have a bad connection at some intermediate connector plug but I do now know where any such plug(s) may be. The FSM, I believe, has a (quite difficult to follow) means to locate connectors and ground points. Or if anyone here is familiar with this series Legacy you may know where they can be found?

Any other suggestions?

Grateful as always,

Mike V.

#2 Mikevan10

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Posted 03 October 2012 - 03:14 PM

Looking at some diagrams I have for other cars, they reference a "Rear finisher" whic apparantly has light bulbs in it? Anyone know what that is?

#3 Fairtax4me

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Posted 03 October 2012 - 03:19 PM

Rear finisher is just the fascia on the trunk.
Check here for 1992 FSM.
http://www.main.expe...u_manual_scans/

Assume you've checked the fuse? :-P

#4 Mikevan10

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Posted 03 October 2012 - 04:00 PM

Thanks for the link to the manuals. A bit rough and not 100% complete but I think it will be a big help! Thanks again.

I don't think a fuse could be the problem since I verified that I have power to the brake light switch. Truthfully, I have not figured out which fuse protects this particular circuit. It is not marked as such on either of the fuse box covers.

Heading home to troubleshoot now. Will report back.

#5 Fairtax4me

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Posted 03 October 2012 - 11:03 PM

Is it just brake lights or is it tail lights too?

It looks like, (the scan is really grainy so it's hard to read) the tail and brake lamps all ground at the same point somewhere in the trunk. If none of the lights work it could be a bad ground. If it's just the brake lamps... did you check all the bulbs? :-p

#6 Cougar

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Posted 04 October 2012 - 01:19 AM

See if the middle brake light works ok. If that works then try cycling the emergency flashers switch a few times. I think the brake light circuit runs through it. Also see if the flashers work.

#7 Mikevan10

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Posted 04 October 2012 - 08:52 AM

Thanks for the comments guys!

First, to respond to your remarks:

The problem is only with the brake lights. All the other lights on the car work perfectly.

The bulbs are fine (proven as you will read below).

Last night's troubleshooting showed that the wiring from the brake light switch to the brake lights, the grounds and the bulbs themselves are AOK. I know this because if I connect "clean" 12 volts to the pin connected to the WB wire on the brake light switch connector plug (i.e. the output side of the switch), all three brake lights light up bright an beautiful!

Rechecking the input power to the brake light switch shows that it is low. Reads like 8 or 9 volts on my multimeter. Now, I would have thought that 8volts DC would give me at least a glow, but what I think is happening is that there is a bad connection somewhere in the power feed to the switch and as soon as I try to use the power to light the lights, the voltage drop across that bad connection is too much so I get essentially 0 volts at the bulbs and no light. Does this sound plausible? The problem I have with this theory is that, if correct, I would think I would still read around 12 volts on my meter when I check it since there is essentially no current flow until it gets loaded.

Anyway, I am trying to understand the power supply routing to the brake light switch. The FSM (Section 6-3, page 38) says the power comes to the switch (via a GW wire) from "FB-1, Fuse No. 12". I would have thought this meant FUSE BOX No. 1, but elsewhere (for other circuits) I see reference to, for examble, FB-20 and I am sure there aren't 20 fuse boxes in this thing! Can anyone please tell me exactly where I would find the correct fuse? I want to trace the power flow back to try to find the suspected bad connection.

Does this make sense? Any other ideas?

Thanks again gang.

Mike V.

#8 Fairtax4me

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Posted 04 October 2012 - 09:35 AM

I haven't figured out their labeling system yet, the FB and MB tags do seem to indicate which fuse box the power comes from, but no there certainly are not 20 of them. If you scroll to the main power distribution diagrams you will see the wiring coming out of the fuse boxes will have the same labels. You should be able to follow from there to determine which panel you need, but I'm guessing it will be the dash fuse panel.

Low voltage would mean low current flow (since they're directly related, along with resistance) so it would make sense that 9v could not deliver adequate current to power 5? bulbs.
I wouldn't expect there to be any connectors between the brake switch and fuse panel, except obviously the ones ON the switch and panel.

#9 Mikevan10

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Posted 04 October 2012 - 09:35 AM

OK. I have done some more homework. Studying the diagrams in the FSM, I think I have determined that the power routing to the brake light switch starts, of course, at the battery, enters the Main Fuse Box (M/B) which is under the hood at the Fusible Link (FL), then goes to Slow Blow Fuse No. 3 (SBF-3), then exits the Main Fuse Box via a Brown (BR) wire and goes to Fuse No. 12 in the "Fuse and Relay Box (F/B)" Now, I do not know but I would have thought F/B was the fuse box inside the passenger compartment, to the left of the driver's legs, but I do not remember this having any relays in it. Nor do I believe the fuses in that box are numbered. Any help here?

Still hoping for comments on my previous post too...

Thanks

#10 edrach

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Posted 04 October 2012 - 09:39 AM

and I am sure there aren't 20 fuse boxes in this thing!
Thanks again gang.

Mike V.

If your Legacy is like my Impreza, you have two fuse boxes in your car. One is under the dash on the driver's side just to the left of the steering column. The other is in the engine compartment up again the driver's side fender (has a bunch of relays as well as fuses under the cover). I hope that helps.

Also, if you have an owner's manual for the car it will tell you which fuse performs what function.

#11 grossgary

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Posted 04 October 2012 - 10:03 AM

Common issue is the wiring, where it goes into the trunk and "hinges" can, over time, wear through and crack the insulation and wiring. Have to pull the covering and yank on the wires - sometimes they "appear" fine visually but a light tug will separate the compromised wires. that's the most common issue besides bulbs, fuses, simple stuff like that.

#12 Mikevan10

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Posted 04 October 2012 - 10:38 AM

I think I have proven that the wiring from the brake light switch to the lights, the ground circuit and the bulbs themselves are all just fine as I described above. If I am missing something there PLEASE advise.

I think I should now have enough understanding now to troubleshoot the power supply wiring (which, according to my logic, is where the problem lies).

Good suggestion about checking the owners manual for a fuse box chart, Ed. I think I have that manual.

I think someone posted fuse box charts on here before too. So if anyone has one handy.........

Thanks all. Keep the ideas coming and I will keep y'all posted.

Mike V.

#13 Fairtax4me

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Posted 04 October 2012 - 12:17 PM

There is a fuse panel diagram in the FSM. It's kinda hard to read though.

#14 Mikevan10

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Posted 04 October 2012 - 12:41 PM

FTFM,

The only fuse box drawing that I found in the FSM (besides the schmtics of course) is on page 14 of section 6-3 and it just shows a layout with 21 fuses. They are not numbered or anything so there is no knowing for sure which fuse is "No. 12" from that drawing.

I'll check the owner's manual tonight.

Thanks again.

#15 Mikevan10

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Posted 05 October 2012 - 07:49 AM

OK, more troubleshooting last night....

Long story short, it appears that I have good clean 12 volts DC getting TO the fuse box inside the car (via the Brown wire), however that power is not getting to the 20 amp fuse (No. 12). So I tried, briefly, to pry the back of the fuse box off while it was still in the car dangling by the wire harnesses. No joy. So I unplugged all of the harnesses from the fuse box and removed it from the car. Once I had it out and on the bench I could see that there are eight "clips" holding the back cover of the fuse box in place plus what appears could be some plastic rivet like things that may be melted/welded during the manufacturing process. Bottom line being it does not look like the cover is meant to ever be removed. Now I am concerned that I may have done some damage to the circuits inside when I attmpted to pry it off. I did pry up one side of it maybe 1/8 inch. Comments?

Anyway, I measured the resistance between the fuse holder contacts and the input and output pins and both sides indicated OPEN circuits.

So, at this point I am hoping that I did not do any damage and that all other circuits in the fuse box are ok, but that the circuit associated with Fuse No. 12 is no good. In that case, I am thinking of just bypassing the fuse box and putting an in-line fuse in a jumper from the brown power feed wire over to the brake light switch. A complication is that Fuse No. 12 also appear to protect the horn circuit, although the 1992 Legacy FSM that is floating around the internet is missing a bunch of pages and the horn circuit seems to be one of them. So I don't know what is involved in that circuit - relay or whatever. Maybe not a concern as along as I know which wire leaving the fuse box is the one for the horn in which case I could just put another in-line fuse in another jumper from the brown power feed wire to the horn circuit wire. Make sense?

I'd appreciate any and all comments on this!

Thanks again,
Mike V.

#16 Fairtax4me

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Posted 05 October 2012 - 04:13 PM

Most fuse panels are assembled in such as way that they can not be opened and serviced. If there is high resistance in one circuit others may be affected as well. I'd go get another fuse panel from a junkyard car. Open the "damaged" one for gits and shiggles. :brow:

#17 Cougar

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Posted 06 October 2012 - 10:32 AM

All the Subaru models I am aware of have two fuse panels. The Main Panel is located under the hood. When you see tabs marked like MB-31 that means it connects to the Main Panel fuses. If you see tabs like FB-20 that refers to fuses in the dash fuse panel. There are relays in the fuse panel also.

You indeed did find a bad power connection when your meter showed a low voltage on the lead to the brake switch. It is so bad that is appears the load of your meter put on it caused the reading to be less than 12 volts.

My info for a different model year shows fuse 12 also supplies power to the horn relay via a RY colored wire. If you have that wire also you will need to add it to the new fuse line.

Edited by Cougar, 06 October 2012 - 11:19 AM.





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