While checking the wiper switch for an intermittent fault (one connector was partly pulled out) and I saw a botched repair job on one wire in the head light switch.
I pulled it apart to solder on a new wire end and saw that the plastic plug was partially melted and bubbled up and black around a couple of connectors.

Is there a TSB on this problem?

Almost every truck I have ever seen with this type of plug/switch seems to have this same problem with overheating.

The replacement is only $5, but I would like to fix the problem that caused it, not just repair the plug.

 

Pretty common problem. Ford didn't use large enough wire for the headlights and it melts the switch. My 94 Mustang did it as well as my 92 Explorer is starting to. A heavy duty headlight harness will solve the problem.

 

Hey one question... is that Heavy Duty Headlight Harness a spare part that can be obtained at parts shop? or should be built?

 

It can be built. I made a thread about it. I'll find the link for you.

 

Most people do a relay conversion that fixes the problem. Look up in the articles on this site for relay/headlight conversions. It's makes the headlights brighter too.

 

Well, thank's for the lead, but I searched (relay/headlight conversions) here and all I got were over 475 postings of everything but the headlight relay conversion!

Perhaps if I google it I might find something.

 

https://www.ford-trucks.com/forums/5...t-harness.html

I'll get you more info tomorrow after I head out to the track

 

I was looking at LMC truck on line and they have the relay kit w/wire harness for sale,I forget the $ but it wasn't bad.

 

The "articles" I spoke about are a link at the top of the page. Sorry I didn't mention that. Here's one of them.

https://www.ford-trucks.com/article/...r_Brights.html

 

The reason the plugs melt is because of the inductive properties of the bulbs. When you have current flowing through an element with high inductance and you break the circuit, current will arc across the switch to attempt to keep flowing. This arcing is what melts the plugs. It is not related to wire gauge size. Relays fix this problem because the inductive current is routed to the contacts of the relay which are farther apart and less likely to arc. The only inductance seen on the headlight switch with this setup is that of the relay coil, which is small.

If you look at a lot of old fan speed switches underneath the dash, you will see the same issue. The arcing in this case is caused by the inductance of the blower motor.

 

I don't know much about the inductance of a bulb, though I read there is a surge on an incandescant bulb when the filament is cold and it's first turned on.

But the melting would have to be from long term heating. It's hard to believe so much heat is occuring from a short arc when the switch is turned off. Possibly the arcing you are talking about is damaging the contacts in the switch, which then would cause heating during the time the lights are being used?

There is another theory that the slide on connectors in the plastic housing of the plug do not make very good contact with the terminals on the switch, which can cause the heating.

 

I think both are possible. If I look under the dash at night with the dome light off and turn the blower motor off and on with my old Fords, I can see a pretty nasty arc that lights up the back of the A/C panel. It is my opinion that 30 years of that will cause some melting of the plug housing. It also makes sense about how you mentioned the contacts can become damaged which will contribute to heat. I took apart the fan speed switch on my '73 Galaxie and there was barely any copper left, and it was very, very nasty looking.

 

The inductive properties of a incandescent bulb are almost nothing. This is a 99% resistive circuit. The spark produced when switch is open is just because a continuous Direct Current (DC) is been interrupted.

After 15 years working on Ford's electrical systems, I can say this plug is usually overheated because a poor connection between wire terminal and the switch terminal. Sometimes also the terminals doesn't connect all the way, making things worse.

The engineering calculations for this connector are right, but sometimes a trick is required to keep the connector in good shape.

1º Remove the connector and inspect for heated terminals. If heat/melt signs are found, replace burned terminals AND the whole light switch.

2º If no damage is found, the disconnect and reconnect the plug at least 3 times. Be sure ALL terminal pins are totally plugged to light switch, sometimes pushing wire by wire in with pliers makes wonders.

3º By design, all the lights are calculated to work from 12.5 to 13Vdc. then installing auxiliary relays and making external connections to lights will put ALL the battery voltage (13.5 to 14.5 Vdc), reducing light bulb average life a lot.

4º Inside the head lights switch, there is a protective circuit breaker with auto reset function. Then, using external relays; will disable this protective function for the lights wires

 

Not trying to start an argument here - I'd just like some clarification on this one. If a circuit is almost purely resistive (99% as you mentioned), then if you take a voltage away, current has to stop. Immediately. Ohm's law. There is nothing in the circuit to "keep current moving." Current is current. It can't move forward in a resistive circuit without a voltage to make it happen.

However, if you have inductance in the circuit, that is what makes the current keep moving momentarily after the voltage disappears. Current lags voltage in a circuit with more inductance than capacitance. It doesn't have anything to do with it being DC. AC can do it too - sometimes you can see a light switch light up blue when you flip off the light in a room without windows.

In the case of these headlights, the inductance "charges" as voltage is applied, similarly to how the inductor in a DC\DC converter charges. Voltage across an inductor is proportional to the rate of change of current - if you try to force current to zero (by opening a switch) the inductance will see a voltage spike which appears across the switch. This is why a snubber circuit is used in almost every application of any transistor circuit in electronics where an inductance is possible in the loop path. Voltage spikes lead to arcs which lead to switch failure, whether we're talking about transistor switches as in my example, or all the way up to headlight switches.

Whether or not arcing causes the plug melting is debatable, but there is no question that arcing is caused by the inductance. You have heavy wire going all the way from the battery to the switch, all the way along the front clip of the truck, to the lights, then to ground. That wide-area loop combined with the bulb filaments acts as a GIANT inductor. Keep in mind - when I say "giant," we're talking micro-henries, but micro-henries are significant.

 

DC does like to keep going once the circuit is broken. That's why fuses are rated for AC or DC. The DC fuses are special made to create a large gap when they blow, to break the DC current and stop the circuit from flowing. Also when stick welding, the arc is much easier to keep going using DC instead of AC settings on the welder.


True, the bulb life will be shortened. But most people feel this is a trade-off they can live with given that the light output is so much better.

This is also true. But if good wiring practices are followed, the relay circuits should be fused when they are added. I always thought the factory used the circuit breaker because the headlight circuit was so important, that they wanted it to work as much as possible, without nuisance fuse blowing. But it seems that was the wrong theory, or they threw it out the window, since all the new trucks have fuses going to the headlights.