Blowing Fuse 27 (Signals/Backup lights) on a 97 850 Wagon

BobD · Post by **BobD** » 01 Sep 2008, 20:05

All,
I replaced the rear shocks in my 97 850 wagon this morning. After 236K miles the driver's side shock mount had broken and I figured it might be time to change them. Went out for a test drive and found I had no more clunking from the broken shock mount, but also no blinkers. The car is primarily driven by my daughter, so I don't know if this was a problem before or after I changed the shocks. My daughter believes the blinkers were working last night before I changed the shocks. So I got home, pulled the fuse for the blinkers/back up circuit and it was blown. I replaced it and as soon as I switched the ignition to "on" it blew again. At the time car was in park and signals were not activated. I turned on the Emergency Flasher and it worked fine with all bulbs flashing as they should. All other lights are working fine, the headlights have high and low beam, and the running lights are fine.

Trouble shooting so far
1) Load side of fuse reads almost dead short to battery ground (Less than 2 Ohms on my meter)
2) Pulled the load floor back out to ensure I had not pinched the harness going to the rear when I reassembled the shocks or the load floor. Everything looked good, no bare wires and no indications of the harness getting crushed.
3) Checked the full length of the rear harness going to the signals and back up lights. Again no bare spots or crushed areas.
4) Pulled the bulbs from the back up lights and turn signals in the front and rear. I found no bare wires leading to or in the front or rear connections. There was no corrosion causing lead to lead shorts and the load side of the fuse still read to ground with the bulbs out. Tried a new fuse and it blew as soon as I turned the ignition "on".

I've run out of time to play with it today so I'm asking for help. I am beginning to think this is a problem in either the PNP switch, the flasher relay, or the blinker switch. Has anyone heard of any of these components failing by shorting to gound? I think this would explain why the fuse blows as soon as the ignition switch is turned on. My belief is that if it was in the wiring going to the lights the fuse would blow only after the car is put in reverse or a signal is activated, and there would also be problems with the emergency flasher.

Any ideas from this brilliant crew that has identified and fixed most of the other problems I've had with this car in the year and a half I've owned it?

Thanks in advance.

Post by **Ozark Lee** » 01 Sep 2008, 21:57

What is curious is that your hazard lights work fine. Schematically the hazard lights are also the flasher for the turn signals but it is more or less a black box and I don't know what happens inside of it. Fuse 27 feeds it as does fuse 13. Fuse 27 also feeds the reverse lamps - how are they working? The reverse lamps may be the source of your problem.

The wiring harness on my son's sedan got chopped in half and a broken wires is a common occurrence on the wagons at the point where the tailgate hinges.

Your VOM will be your friend for this one. The electrical problems are tough to find but easy to fix once you find them.

...Lee

BobD · Post by **BobD** » 01 Sep 2008, 22:26

With the fuse blown both neither the turn signals nor the back up lights are working. I think the next step will be to see if the problem occurs when I disconnect the turn signal switch. If it's still a dead short at the fuse box then I think that should isolate it to the back up light circuit. Then I'll be probably be checking the PNP. From pulling the back end apart looking today I didn't think the harness for either the back up or turn signal circuits goes through the hinge, but I'll double check.

Post by **Ozark Lee** » 02 Sep 2008, 08:48

I didn't think the harness for either the back up or turn signal circuits goes through the hinge

You are right, I was thinking of the license plate light.

...Lee

BobD · Post by **BobD** » 02 Sep 2008, 19:04

Shoot. I was hoping you had given me a new lead to chase.

Post by **BEJinFbk** » 03 Sep 2008, 11:47

Got Schematics???

http://volvo.techno.org.pl/wiring-diagrams.html

BobD · Post by **BobD** » 03 Sep 2008, 19:24

BEJ,
Not this good. Thanks for the link. Most helpful.

The problem is in the wiring harness someplace between the fuse box and the PNP switch or the emergency flasher. I pulled both the flasher relay and the connector off the PNP and I still have the short. So neither of those components is shorting to ground and they are the first switches in the line. The signal switch isn't reading to ground so I think that's OK.

Has anyone had a fuse box develop an internal short on just one fuse? I've never seen that before, but this is the first Volvo electrical system I've ever delved into.

But now its just a matter of time and finding the wire that has chafed down to copper against a bolt or sheetmetal piece. Wish me luck. The VOM is definitely my best friend right now.

Post by **MadeInJapan** » 03 Sep 2008, 19:57

It is a known fact that the bundle of wires entering the fuse box opening (from the side/bottom towards the air box) is known to become corroded and/or chafed, so check this area out.

BobD · Post by **BobD** » 07 Sep 2008, 17:58

Is the internet great or what? A guy in Fairbanks gives a Polish site to a guy in VA, which let me finally locate the sucker. It was under the dash between the connector coming out of the fuse box and the flasher relay. I think the root cause was the wire harness rubbing against the knee bolster or the steering brace. It looks like the bolster had been removed previously. I took the lazy man's way out and just clipped the old wire and routed a new one. Hopefully I won't be chasing this gremlin's brothers anytime soon. If I have to go in, can anyone point me to a link that shows how the drivers side knee bolster comes out? Thanks fo all the help.

Post by **MadeInJapan** » 07 Sep 2008, 20:49

It's very simple...just a couple of screws from under and then pull downwards and out.