Pelican Parts Forums - fuse 18 blowing if headlights on...

This will take some 'spanin.

I've had my 83 911 SC cabriolet back from an engine rebuild for a month now and I've put about 2000 miles and everything has been fantastic. Last Sunday I went for a 5 hour 175 mile drive through western MA and southern VT and NH and it was great. I didn't drive all week since it has been rainy here in the Boston area. So yesterday I decided it was time to do some of the projects I've had on my todo list for a year since the weather was iffy.

First up was installing a JWest headlight relay kit (https://www.jwestengineering.com/911-Headlight-Relay-Kit-74-89_p_14.html but purchased from Pelican). This was in preparation of changing my stock headlamps for Hella H4s now that I'm doing more nighttime driving.

So, I dove in to do the install and disconnected the battery ground as one should dol. I didn't take photos of the "before" state - stupid me, I usually do that but this seemed pretty straightforward so I skipped that step. And this is where it "gets interesting". About 8 years ago I left my headlights on and drained the battery, so, being an enterprising Porsche owner, I did some research and installed a relay and buzzer to alert me if I turn the car off with the headlights off. However, when my "blown airbag saga" started last April (2024, read my thread) I disconnected the buzzer for some reason I don't recall now.

Now, when I went to install the headlight relay, I saw relays attached to the bottom front of the fusebox and simply thought "thats the old buzzer relay". So I traced the blue and green wires from that relay to the fuse panel and disconnected them and pulled them out from behind the fuse panel. Then as I was re-reading the JWest instructions, I realized that this "relay" was actually a JWest headlight relay kit that I had installed 8 years ago and forgot about.

So this job was going to be "easy" since it was already done. I snaked the blue and green wires back unto the top terminals but enough time had passed (I got a call and had some water) that I didn't remember which terminals they connected to but the instructions were clear and I must have had them installed as per the instructions. SO I hooked them up to the terminals described and moved on to the H4s.

The Hella H4 kit was an easy install. I moved on to task #3, replacing the rear turn signal lenses since mine were destroyed sitting in the hot summer sun all last summer while the car was "in the shop". That job was straightforward too.

I then hooked up the battery ground to test the lights. All of the indicators and brake lights worked fine. I then tested the headlamps, The ignition was off and the headlamps did not come on when I turned on the switch. I thought that was odd since if that's how they worked, I wouldn't have drained my battery years ago leaving the lights on. I turned the key to accessory and now the headlights turned on. Ok, I thought, all is good, reset the clock time and pack it up.

This morning I was planning to go out for another long drive on some twisty roads. But when I started the car it ran like crap and I noticed my clock was off by hours - turns out it wasn't running. That seemed odd but I recalled the "fuse 18" issue that folks here have dealt with many times. Fuse 18, as you may know, is always hot and powers the clock, interior lights, glove box light, and for those of use with 81-83 SCs, the OSX relay. "Ah-ha I thought, let's check that fuse" and sure enough, it was blown. I replaced the fuse and the car started up and sounded great so off I went. I always drive with my low beams on during the day and turned them on as I backed out of the garage. Things were going well when about 20 minutes later, the car started running horribly and I noticed my clock was not keeping time. I pulled over to find another blown #18 fuse. I replaced it thinking that it was highly unusual so I started heading home. 20 minutes later it blew again. This time I pulled into a little roadside stand and had lunch then replaced the fuse. While I ate I was pondering the problem and what I'd done.

1) the JWest relay kit was already installed and although I had disconnected the blue and green wires, re-installing them shouldn't be a problem.
2) the taillights were working fine and also was a simple task, likely not an issue
3) the Hella H4s were also an easy install and shouldn't be a problem but they do draw more current

But it is odd that my headlight don't come on unless the key is on - is this normal? It is not how I remember the lights working.

So, I decided to drive home with the headlights off to see if there was a correlation. I drove for over an hour and didn't have any issues or a blown #18 fuse.

So here I am trying to figure out how to diagnose and fix this. I don't have any more white fuses so I ordered some. Just for diagnostics, I'll go out for another hour long drive with no headlights and if I have no problems, turn the lights on and see if fuse 18 blows again.

But there should be no connection between the headlights, relays, and fuse 18 so I just don't get it. I don't think I spontaneously have a short on one of the fuse 18 circuits given that it was fine when I parked last Sunday and now not fine after doing the 3 projects. Any suggestions on how to diagnose? I have a multimeter and other tools.