Smoking Headlight Switch

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autonomousbronco

autonomousbronco

If it ain't broke, fix it until it is.
7173 Mustang Supporter Member
Joined
Nov 4, 2018
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Location
LA
My Car
'72 Mach 1. 351C, 4 Speed Toploader w/ Hurst Shifter, 9'' Rear End (3.9). Power windows, rear defroster. I’m
I was driving earlier today and smelt the telltale aroma of burning electronics. Luckily, I was able to quickly pull over and then via a generic quick-disconnect (highly recommend) quickly disconnect the battery; I then readied the fire extinguisher I keep behind my seat. Luckily, nothing appeared to smolder nor burn. I searched under the dash and the headlight switch was very hot to the touch and still had some residual smoke. The wires appeared intact. I debated calling AAA but felt confident after testing the other systems (radio/heater blower), that as long as the headlight circuit was off and no more smoke was seen, the car was ok to putt home without the lights. At the house, I remembered that I had recently upgraded the turn signal and emergency flashers to solid state relays for the LED turn signals (I remember having to switch the polarity on one of the connectors for the emergency flasher to work but the circuit worked post installation and has for several hours of operation). 

Any suggestions on how best to troubleshoot this electrical problem...if it might be the headlight switch at fault or it was just the weakest link following too much of a current draw from another part of the circuit being shorted etc. 

Thanks in advance!

 
I was driving earlier today and smelt the telltale aroma of burning electronics. Luckily, I was able to quickly pull over and then via a generic quick-disconnect (highly recommend) quickly disconnect the battery; I then readied the fire extinguisher I keep behind my seat. Luckily, nothing appeared to smolder nor burn. I searched under the dash and the headlight switch was very hot to the touch and still had some residual smoke. The wires appeared intact. I debated calling AAA but felt confident after testing the other systems (radio/heater blower), that as long as the headlight circuit was off and no more smoke was seen, the car was ok to putt home without the lights. At the house, I remembered that I had recently upgraded the turn signal and emergency flashers to solid state relays for the LED turn signals (I remember having to switch the polarity on one of the connectors for the emergency flasher to work but the circuit worked post installation and has for several hours of operation). 

Any suggestions on how best to troubleshoot this electrical problem...if it might be the headlight switch at fault or it was just the weakest link following too much of a current draw from another part of the circuit being shorted etc. 

Thanks in advance!
Click to expand...
Hello,

When this event occurred, were you running with headlights on?  I assume so based on the story.

So, what headlamps do you have in the car?  If you have Halogens or other higher output lamps they tend to put a higher current draw on the system due to a higher wattage lamp.  If you have added any lighting upgrades to your system that run off the main lamp circuit you are likely over driving the switch capability.  This explains the temperature and the smell.  Vendors sell a plug in headlamp relay harness that is a plug and play, no drill, reversible upgrade to your system to allow you to run halogens or other higher wattage lamps and relieve the electrical load from the switch.  (Cant post a link to the kit right now)

Other things to check:

1) Corrosion in lamp sockets at the headlamps.  You can clean these out and coat with a grease to prevent corrosion and improve conductivity.

2) If you do not have standard headlamps(non-Halogen) you need to order from a Mustang or classic parts dealer if you are not going to update to the relay kit.  All auto parts stores carry Halogen as a minimum.

3) I would buy a new headlamp switch either way.  If that is the original switch it could have been overloaded for years and may have broken down.  It is cheap insurance to upgrade to a new Motorcraft Headlamp switch. https://www.rockauto.com/en/moreinfo.phppk=43501&cc=1132670&jsn=12227&_nck=Ekt6BqBF1pwlXdx0gYlm1esw98pF26vehMm%2FGG3C%2Fl2QOXSJubi7Wbtv3UuYQU5UmTy3q3lNg0n0sNYXcqGVd0I7F1ZVlApVc%2FeGbjLg9bwtVfG%2FcpPEWxGEKmL6SZuKylhhtnWftozmV%2BLmT3elIJkP%2BeiaAULbH%2FEmfgBCLinu%2FfCBG%2FqaZ%2BbMxdCosL6mjp1YsywH1DE3YYFemynRu0hEP5ypVvFzzRtnc22X141ppwyAc631jTkG8wh5AypSKo4%2FRtGdn7d4D23b9VNy7Y4VWBlyPZF8bAkev%2FDJJMeFNb9UX6UJi%2F3orWchaMTauSNkcgMD5CuaezTqnyDL8w%2BPKItLz6OHtFfwbR9En3LD%2BeW6hyeg4bkjKNWP066AI9N9c0kOEZQJdb1TVOK1e7wXmftOoGO9RPye96jxbT%2BMKorbgg21pnj1uV%2FlUiaO7qP4n0w5MoR%2BkPIjbEB98rRKuorWXrmCfmNGPQgMdlfJ7iVRMxC0iB%2BR9j3Q9DQgyqpPYfp%2FlV1vihYky%2BXAEsPooyTbBs4b

I am no electrical guru, but my car had a problem in the 90s with my switch failing.  I didn't know the problem then, but I had been running Halogens on the original system.  Mine would have the headlamps suddenly cut out and in as I was driving down the road.  then it got to where the lights would turn on and off when the car was sitting.  I found the same thing, a very warm switch and replaced it.  I thought it was just the switch, but recent literature shows that due to the older system design with all the electrical load going through the dash switch you are at high risk with any light upgrades.(Including the halogen lamps that were NOT OEM equipment in 71 when these cars were designed and built.

kcmash

 
+1 on everything kcmash said. The only thing I would add is that the headlight switch has a built-in automatic resetting circuit breaker, which was the causing problem that kcmash described, with his lights going on and off, due to the overloading.

The circuit breaker works by heating up a bi-metal strip which then causes the contacts to separate. That bi-metal strip may have had oil on it, causing the smoke. The contacts in the circuit breaker may be stuck together, causing overheating. The contacts in the switch may be dirty and corroded, which also causes heat. I agree, the first thing you need to do is replace the switch. Even if you don't have aftermarket headlights I would recommend installing a headlight relay circuit, even the light output from the stock headlights will improve.

 
Thank you all for the input!

@Kcmash, yes they are Halogens and were on when the "magic smoke" appeared. Regarding the headlight sockets or a poor ground being part of the chain of events that contributed to the problem...That morning, I had just had the engine coolant flushed and the (I'll use a euphemism and just say the "inexperienced") "tech" at the shop used a device that pressurized the radiator hose whilst suctioning the radiator cap...well, he messed up and managed to "bathe" my engine bay with Toyota red antifreeze (I've used it due to the heads being aluminum) anyways, it soaked a good amount. I figured all would be fine as the wiring in the engine bay is in tip top shape (I've restored much of the harness) but didn't consider the sockets and I now suspect the grounding points, as I did the engine bay restoration over a decade ago and didn't apply dielectric grease at the grounding tab's chassis junction at the time (live and learn). I also will replace the headlight switch (thank you again for the in depth post and common culprits)!

@Not A T5 and @Tony-Muscle, regarding the aftermarket harness to reduce load, I'll be in contact with @Rocketman to take the load off the switch. Thank you @Don C for the description of the headlight switch...I have no doubt that bi-metalic strip failed on and is most likely welded itself together due to the fault and age. I'll keep you all posted on the progress and resolution of the issue!

 
I've heard stories like this and noticed that my headlight switch gets a little warm when used, so I ordered a headlight circuit relay kit from NPD: 11653-1A  https://www.npdlink.com/product/headlight-harness-extension-custom-relay-style-use-with/186973/202877

It direct-wires the headlights to a circuit from the battery via relays, with the headlight switch only handling enough juice to trip the relays - no more 'hotter than a two-dollar pistol' head light switches.

I think it's something the factory should've done all along.  It's also pretty much how modern lighting systems have been wired up for at least the past 20-30 years.

 
I've heard stories like this and noticed that my headlight switch gets a little warm when used, so I ordered a headlight circuit relay kit from NPD: 11653-1A  https://www.npdlink.com/product/headlight-harness-extension-custom-relay-style-use-with/186973/202877

It direct-wires the headlights to a circuit from the battery via relays, with the headlight switch only handling enough juice to trip the relays - no more 'hotter than a two-dollar pistol' head light switches.

I think it's something the factory should've done all along.  It's also pretty much how modern lighting systems have been wired up for at least the past 20-30 years.

 


- Do you know if I need a special tool to remove the trim bezel and headlight switch assembly from the dash? 
Click to expand...
 
If you push the little button on the underside of the switch, you can pull the switch knob out along with the shaft.  I didn't use any special tools to remove/replace mine... I was just careful with it, but I think there's a special tool recommended to use.

 
autonomousbronco,

As Eric posted there is a release button you depress and then firmly pull the headlight switch knob and shaft straight out. There is a tool made to remove the switch retaining nut but if your toolbox is like mine you'll only find it when looking for something else a week later. I have used a large blade tipped screwdriver but bring your patience as these are easy to damage. To my knowledge, there are no repo retainer nuts for the 71-73 Mustang/Cougar, only the 65-70's (Imagine that)!



 
autonomousbronco,

As Eric posted there is a release button you depress and then firmly pull the headlight switch knob and shaft straight out. There is a tool made to remove the switch retaining nut but if your toolbox is like mine you'll only find it when looking for something else a week later. I have used a large blade tipped screwdriver but bring your patience as these are easy to damage. To my knowledge, there are no repo retainer nuts for the 71-73 Mustang/Cougar, only the 65-70's (Imagine that)!



-Thanks for the photo @secluff (I used to own a '65 and the difference in parts availability is something else by vendors...that said, it is improving in very recent years)
Click to expand...
 
One thing we should all consider, is getting a direct powered headlight harness. It makes our 50 year old headlight switch work with hardly any load going thru the switch. The power from the switch only operates a low power relay, and the power comes straight from the battery. You could also just replace the switch, but ford headlight switches have a history of melt downs. Its happened in most every old for I have owned, including twice in my 94 Bronco. All mine use the headlight harness/relay setup now.

 
Last edited by a moderator:
It seems that I've found the culprit. I thought I'd play it safe and decided to directly visualize/inspect the headlight wiring harness as well as the connectors to the headlight switch. So, I pulled the dash and luckily everything appears to be intact! The headlight switch on the other hand was burn't as it literally fell apart as I removed it. I've uploaded the picture of the switch with its replacement.

I suspect that the copper tag in the image, after many years of fatigue, broke off and shorted the switch assembly thus causing the plastic center of the switch to burn (I've circled it in red). I'd say the heat from the shorted tang caused the ceramic backing for the resistor to then fracture (or perhaps it cracked first thus dislodging the tang). Either way, I'm going to take the rest of the dash apart to check the entire length of the wiring loom as a scant amount of smoke also arose through the passenger side dash between the glove box and the door jam.



 

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