Headlights went out @ 80mph in the DARK !

[Type911]

Driving home last night on the mountain hwy. that leads to my house my headlights went out. It was quite dark, fortunately they went out right near a roadside pulloff. I managed to limp home with the glow of the flashers, fortunately my house was only a few more miles up the road.
Checked my fused headlamp relay when I got home, sure enough the 15a fuse had blown. I run 80/100w bulbs and had only had the brights on for about 30 seconds when the lights quit. What would cause this ? Typically I do not run the brights much, however the mech. that installed the relay a few years back said I could go to higher wattage if so desired. I have run this wattage set up in all my cars for years and never had a problem, even on cars without a relay.

Any input is appreciated, thanks.

Chris
82 911sc

[RoninLB]

Chris..if your running REAL 100w high beams check out what the current draw is..I run 130/100 real watts which means serious electrics..and each bulb should have its own fuse.............Ron

[Wil Ferch]

Hey:
Wait a minute...are you running ONE ...15 A fuse for both hi-beams??
If so, what's the issue? Very clearly, amps = watts/volts.
100w / 12 volts = 8.3 Amps.
So....two lights through one fuse is 16.6 Amps on a 15 amp fuse.
Even if you'd thought you're running at 13.2 operating volts, this is MUCH too tight between load and protection...normally the load should not exceed 70% of the fuse rating...and the wire size should be concervativly rated, too.
I guess you'd blow a fuse ! No real mystery here.

---Wil Ferch

[Early_S_Man]

Chris,

Even though the nominal Voltage is 14 Volts (13.8 Volts to 14.2 Volts, per the factory service manual) instead of 13.2 Volts ... the principle is the same, about 7.14 Amps per headlamp high beam bulb, so for one fuse, 20 A or 25 A would be a better choice.

[Type911]

Thanks for the replies. This all makes perfect sense.
Since I did not install the relay, I just assumed everything was fine., too bad the p-tech did not think through this.

So, if I install a 25a fuse will that solve the problem ?

-Chris

[Wil Ferch]

Chris, Chris...
It seems you're looking for that quick, pat answer...it'll get you into trouble again.
Yes...25 A fuse is better, and correct, as far as it goes. Now, go back to my original post. How stout is your wiring leading from the battery ( I presume, as the source) to this single fuse, and from this fuse to the relay kit?
Is it 12ga ? ... 14 ga..... ? Or ( heaven forbid) 20 or 22 gauge ?
I'm really trying not to sound rude...but if you're answer is "I don't know"..., then, how can anyone on this list be expected to say only a fuse change will be OK? See my point?
Slow down, don't look for "quick" answers...and solve this ( or any other issue ) by looking at the "system"...not as a single component fix. Please don't read me wrong if this sounds harsh, as that is not the intent. I just see too much of this stuff when well-intentioned guys get into trouble
--- Wil Ferch

[Charley Dog]

Just to be clear, if you're suggesting ONLY replacing your 15 amp fuse with a larger one, you should think twice. The 15 amp fuse is specified on that circuit for a reason. The fuse is specifically designed to be the weakest link in the chain so that it will fail first and unspectacularly on overload. Removing this safety valve will result in your learning what the NEXT weakest link in the chain is, which of course could result in fire or expensive component failure.

[Early_S_Man]

Charley ... you don't seem to understand ... NOBODY in their right mind runs a fuse loaded at 95% of the rating!!!

And, this isn't an OEM fusing application, and nothing exits on paper as a 'specification!'

[Type911]

Wil,
I appreciate your input, not looking for a "quick-pat answer"
just trying to understand this. Your post has me thinking, I will check on wiring etc.. tonite and proceed with caution making sure it is done right. Thank you

The P-tech that installed this obviously did not take the whole picture into account, as there is only 1 relay for both headlights,
plus after he wired this, I no longer have my "flash" mode for the headlights unless the headlamp switch is on. He is a nice guy and all, but this kind of crap is not what I pay him to do.

thanks again for the replies

[Wil Ferch]

Chris and others:
Thank you for not misunderstanding my intent...
Well now...it is not a problem to have a single fuse on the main power feed to the relay kit. Making this 25 amp with 14-12 ga wire is probably OK.
The relay kit, in turn, should have :
- one output lead to the "input" side of the high beams...and another output lead to the input side of the low beams ( input...means input to original fuse box). White original wire lead is typically for high beams, yellow feeder for low, as I recall. Power entry to the original fuse box is on "top"....the load coming from the fuse box to the headlights is connected to the bottom. At the fuse box itself...the power will split to two fuses ( 2 for hi....2 for low)..so that each headlight has its own individual fuse for both hi and low. Stock, these are 8 amp fuses..I believe the fuse body color for 8A is white. So you would see 4 fuses for the hi/lo, 2 headlight system at this point. I can fax a wiring diagram I kept ( doesn't everyone after they do mods?)...once I completed the Marcus Sucro relay kit I installed, that I got from him.
--Wil Ferch

[wckrause]

Is it possible that the 15Amp fuse that blew is on the coil side of the relay, not on the load side?

[emcon5]

This happened to me once in my old SVO Mustang. Luckily, Ford was smart enough to not have a fuse in the circuit. Instead they have a self resetting circuit breaker (I think they called it a fusable link). I was a little shocked when my headlights went out on the freeway, halfway between St Louis and San Diego, in the middle of the night.

In my case, it was corrosion in the headlight dimmer switch. With headlights and fog lights on, the load was too much, and they would cut out.

I know one fuse of the proper amperage will handle the load, but wouldn't 2 be better for redundancy? If one blows, you still have a working headlight.

Tom

[Early_S_Man]

Tom,

That is precisely why I have been saying to install four relays and four fuses/circuit breakers ... for more than 2.5 years on this board!

[targa80]

I don't know if this applies to standard automobile stranded wire. In electrical circuits (as in house wiring) a 15 amp fuse would require a 14 awg wire, 20 amp fuse would require a 12 awg wire and a 30 amp fuse would require a 10 awg wire. My best quess would be you would need a 10 awg wire to run a 25 amp fuse to be on the safe side. So , if you run two seperate fuses of 15 amps with each having 14 awg wire you would be doing a correct and safe modification.

[Wil Ferch]

Guys:
Please look at my post prior to this one...there is still some confusion here.
- If you followed my wiring logic, you would have noted there is a 20-25A fuse in the load wiring leading to the relay kit...and the kit then feeds and uses the 4 individual headlight fuses in the original fuse box. This may ( or may not) be what Chris has..he'll have to check , as he said he would do. The 25 A main fuse would only serve to protect the wiring from the battery to the relay kit....or maybe the relay itself if it goes nuts....it is NOT intended to be the first fuse that blows if the headlight draw is too high....yes...there would still be 4 fuses for that. If you replaced your original 60/55 watt headlights with 80/100 W versions, then the hi beams ( low beams, too, for that matter) would EACH need to replace the original, white, 8A fuses, with 16A fuses ( blue?), since they would draw over 8A under steady-state load. Metric "suppository" fuses go from 5 to 8 to 16A ratings, so once we go beyond 8A, 16A is next up in size. Some people don't even fuse the main power supply from the battery to the relay kit, so some of the later comments made by some people miss the points made entirely.
Having said all this, you would need to check the wire gauge that exists from the original fuse box that leads to the headlights. IMHO, it's a bit light for the added draw. And to answer another posters' question, no, the convention for automobiles is different than the National Electricm Code ( NEC) basis for house wiring. Auto industry standard allows more aggressive ( smaller) wire diameter for a given load..although I don't fully agre with the rationale. Cars have a weight concern and end up with smaller gauge wire. My point? don't expect NEC standards, but on the other hand....don't go too light on wire gauge. There are handbooks for auto wire gauge that relate total wire length to appropriate gauge, given the insulation quality used.

---Wil Ferch

[Doug Zielke]

Wil Ferch sez:
"Metric "suppository" fuses ......"

That's a good one, Wil. Please allow me to use it from now on!

[Type911]

Wil,

What is the Marcus Sucro relay kit? Based on the limited amount of information I have provided, do you know why I cannot flash my headlamps unless the headlamp switch is on.
Before p-tech installed the sub-par relay system, I could flash without switch being turned on.

thanks

[targa80]

Allow me to chime in one more time on this thread. My suggestions are based on the National Electric code as Wil had stated. Wil also clarified that the auto industry uses a more aggressive setup (is that wire wire harnesses in cars fry). My statements still hold true if you use those wire sizes with those size fuses protecting them no matter where or what is connected to them they will protect that circuit accordingly. I don't know what was done to the wiring in this perticular car and I don't have the schematic of what it should be from the factory sitting in front of me. Whether it is with or without a 25 amp fuse followed by 1,2,3 or 4 15 amp fuses. Fuse the circuit for the appropriate capacity of the circuit or modify the wiring to handle the load.

[RoninLB]

this is a winded thread..why no discussion of circut breakers instead of fuses??
.....and flashing without the sw on/if the highs work from the column sw, the column sw should allow the flash unless the column sw is_ _ _ _ _ _..............Ron

[Craig Stevens]

Chris, Do a search for headlight relay. you will find a thread that includes a e-mail address for Marcus Sucro. His kit is a good one. Craig