Topic: '67 door light switch connection hot..'67 with interior decor&courtesy lights

[socalgt]

One of the male wire connections(black w/blue) to the driver side door light switch is very warm to the touch.  The other terminal is properly connected to the green & yellow wire ( from the overhead console)  I assume rather than try to find out the problem, I should just replace the switch.  I am curious what would cause it and assume there is a fuse that would protect against any serious problem.

[67gtasanjose]

Wow, yet another "wiring issue"! Truthfully, with all of the electrical issues you have been discussing, it might be a good idea to do a bit of "exploratory surgery" to open things up enough to look around and see what all has been tampered with on every harness of the car. It might sound drastic, but so is a melted harness, should something inadvertently be connected to a non-fused circuit or a high amperage fused circuit that prevents a shorted wire from blowing the fuse.
When these cars are together, (especially a SelectAire equipped car with multiple add-on options) you cannot see too much of the wiring harnesses. This can be a real mess under there WHEN IT IS ALL CORRECT!  Then enter +40 years of tampering and repair work of previous owners and you end up with these nightmares you are discovering. Everything is repairable but the BEST repair is when you know EVERYTHING THAT IS WRONG all at once, so you can make the best choice on repairing or replacing the harnesses that need attention.
As you peel back the layers of tape the PO has put on, you may find more and more things wrong to the point of  POSSIBLY considering if it is wise to replace the whole underdash harness. These old connections get corroded, even in arid climates so everything can be suspect on condition, but how could you know if you do not inspect them all? I was really surprised at the condition of my wiring when I tore down my car for restoration. I went over every single connection and every single wire and CLOSELY inspected every taping of the wiring harnesses I planned re-using. Layer, by layer, I found more and more wrong, just as you are finding on your example.
It IS POSSIBLE yours is NOT as bad as mine was, but how could you know? I didn't know till mine was taken apart far enough to see all the trouble areas, but once seen and known, I HAD TO REPAIR THEM OR REPLACE THEM! Nearly  ALL OF THEM!
I ended up deciding on a good used (needing very minor repairs) underdash harness, re-using my underdash SelectAire harness, Repaired Convenience Control harnesses, Reused tilt column main harness, replaced starter circuit tilt harness, repaired Fog Light underdash harness, NEW Taillight harness and all new under hood harnesses (everything).  That is only two sub-harnesses that were usable as they were! None of the rest passed muster! Many others were simple, minor repairs but because they were pulled from the car, much, much easier to do. For example, I didn't know there was a problem in the feed wire to my overhead console but I found out the wire had been pinched under the sill plate at one time or another,  the wiring insulation was cut and only two strands of the copper wire were still connected! It still worked, but obviously, this wasn't good.  I could solder, heat shrink and conceal most of the repairs like this one mentioned, under the harness re-taping. (read as all harnesses NOW look like new or near new)
Just a suggestion, but for me, I know it was well worth having a close-up look at ALL of my wiring and I know now that I can trust everything I will be using.

Richard

[67gta289]

That's good advise.  In other words, you found one hot connection by chance.  Is that one of one, or one of "x"?  There have been enough classic car fires due to electrical problems, including Mustangs, that we should all perform due diligence on all things electrical.   And I don't think it safe to assume that there is proper fuse protection for every scenario, especially as Richard pointed out as modifications have taken place.  Consider the recent Ford F150 example from a few years ago, where there was a cruise control cut off switch issue that caused some fires.  Back in the 60's, there was a lot more liberty taken from a safety perspective.

[midlife]

I heartily endorse the notion that every wiring harness needs to be thoroughly examined and fixed if required.

Y'all won't believe some of the stuff that I see.  The item that bothers me the most are the use of wire nuts. 

[socalgt]

I did  some troubleshooting and found.....The black w/blue wire(rear courtesy light) that goes to one of the connections on the door light switch has a pigtail to accept another courtesy light male fitting and then connect to the door light switch.  This is the one that gets hot.  If I temporarily connect the two black w/blue courtesy light wires  to the door light switch without using the pigtail, the wire does not heat up.  It sure appears that the pigtail is the problem....am I missing something? 

[midlife]

I did  some troubleshooting and found.....The black w/blue wire(rear courtesy light) that goes to one of the connections on the door light switch has a pigtail to accept another courtesy light male fitting and then connect to the door light switch.  This is the one that gets hot.  If I temporarily connect the two black w/blue courtesy light wires  to the door light switch without using the pigtail, the wire does not heat up.  It sure appears that the pigtail is the problem....am I missing something?

Is the pigtail connected to anything?  If not, I have seen two-wire molded bullet connectors having high resistance (means heat when current flows through it).  It seems that the crimps inside the molded connectors have loosened up or have become corroded.  At times, I can restore low resistance by tugging tightly on the wires going into the back of the molded connector.

[socalgt]

With the pigtail left unconnected...no heating problem...when connected to the additional courtesy light it gets hot.  I'll try your suggestion....Thanks

[midlife]

Ahhh...it could be that corrosion on the door lamp is causing the higher current/resistance.  I'd check out thoroughly whatever is being attached to that plug as well.

[67gta289]

Mid, you must have been in a bit of a hurry.  Corrosion would indeed cause higher resistance, and higher resistance would cause a lower current, not higher (V=IR).

The corrosion, creating increased resistance, will result in heat generated based on the "I" squared "R" calculation.

The light bulb itself has resistance, and will generate heat.  However, the housing and surroundings should be designed in such a way as to properly manage (dissipate) the heat.

If the light bulb is disconnected, the current (I) goes to zero, and so does the heat.

The wire insulation provides a degree of protection against heat (in addition to its primary role as a high resistance insulator), but not enough in all situations unfortunately.

If we think about how much heat was generated to get the original poster's attention with the load from a little interior light bulb, imagine how bad it can get on a high load circuit, such as a horn, blower motor, starter motor, etc.

A good representative picture is attached.

Back to the problem at hand.  There is a problem in the wire or connector where the heat is being generated, not downstream.  If there is increased resistance downstream, there would be a heat problem at that point of resistance. 

However, if there is a problem in one location, there very well may be multiple issues.

[midlife]

Right you are: high resistance means lower current.

I was thinking more of a engine starter, which demands high current.  With a faulty or corroded starter wire with high resistance, that sucker gets real hot in a hurry.  When resistance is high, the voltage drop becomes high, and the voltage drop turns into heat.  As long as there is enough current going to the device that is being powered, the corrosion or high resistance at that point generates heat. 

You are correct, and my thinking was a bit off.  My wife says that a lot as well...

[socalgt]

At the risk of talking this problem to death.....I checked the resistance on the wire going to the rear courtesy light without the front courtesy light attached to the pigtail  and the resistance was "0".  Likewise, I checked the resistance on the wire going to the front courtesy light by itself and it was "0" as well.  Running out of ideas...Guess I can try running new wire to both the front, passenger courtesy light, the passenger rear courtesy light, join them and attach to the door light switch and see what happens. 

[67gta289]

If the problem has not been solved, it is not dead yet.  When you checked resistance, can you be more specific regarding the lead connections?  You measured the resistance of the wire referenced to what?  To ground?   

If possible, disconnect the wire near the rear courtesy light, and measure to ground.  You should read the resistance of the bulb if all is well.  If that is not accessible, pick an under dash light and measure the short lead, through the bulb to ground.  If you don't read anything, and hook it up and the light works, we have to suspect the meter itself.

With a good bulb, good ground, and good connections you should not be reading zero when referenced to the body (ground)

[socalgt]

With the ground wire from the meter attached to the same place the ground wire goes from the door light, and the red lead to either the wire that goes to the rear courtesy light, or the passenger underdash, the reading is zero.  I'm taking that as meaning both the wires are OK.

[67gta289]

If the bulb is installed and good, you should read the resistance of the bulb (plus what should be negligible resistance from the various connection points).  If there were no bulb, or a burned out bulb, you would read infinite ohms.  However, zero (ohms) indicates a short circuit, which should blow a fuse.  I would expect to see 20-50 ohms depending on the bulb wattage.

Not to be offensive, but let's make sure that you are reading ohms and not volts or amps.  Take the lamp out, or get a different bulb, and measure the resistance.  If you are not registering anything, and the bulb is good, either your meter or your technique is incorrect.  Not all meters are of the same quality, and as the old saying goes you get what you pay for.

If you are not sure about any of these checks, no worries, as we are here to help.  If possible take some pics of what you are doing including the meter display and controls.

John

[socalgt]

John....I really appreciate yours and others' inputs on this.  I've been working on this for over a week now, and since the only problem with it hooked up as it was, although incorrect,  was that the map lights could not be used independently of the courtesy lights.  There was no heat problem with any of the wires. (I guess you could say there was a good side to the incorrect wiring.....you could not leave the map lights on accidentally after the car was shut off and run the battery down).  I don't normally give up on correcting things in my cars, but  I have other things more important to continue working on and I know the wiring is safe, though be it incorrect.

                                           Thanks for all the suggestions and I'll probably revisit this and try to correct it at some future date.