I dissected the failed taillight on the 2002 over the weekend and it is small wonder it failed!

I live in the Pocono Mountains, and the rear fog lights are pretty much a way of life.



If all of these lights are on, the circuit can draw up to about 10 amps.

The wiring to the socket is 0.8mm, roughly 20 gauge. it is 3.3 meters (10 feet) from the fuse junction. Considering the maximum current draw, we will only get about 11.2 volts at the socket. This was measured at the fitting and found to be correct; with the rear fogs off, there were 12.0 volts available. The ground lead was noticeably warm.

The socket itself is designed for a maximum of 5 amperes.

I think we can see the problem here...we have a cable that can deliver, at best, 8 amps at the outside of its capabilities to a fitting that can deliver, at the outside, about 7.5 amps to the inner fitting. We are asking the car to deliver around 10 amps to the circuit.

Now the rub: as the wire heats up, the voltage drop becomes greater; the filaments in the lamps, once they are warmed up, will have a more or less fixed resistance. Ohm's law tells us that voltage is equal to resistance times current (V=IR), so as the voltage drops, the current rises, which creates more heat, and so it goes.

At some point, the weakest link in the chain...the terminals in the socket...heat up enough to melt the socket, the pins carbonize, drawing MORE current...eventually melting into the fitting and breaking contact with the grounding post on the lamp housing.

At that point, the lamps will find ground wherever they can, usually through the filaments of another lamp circuit. THAT is why your lights go crazy.

I will never understand why the Ford bean-counters require their engineers to use components that meet the absolute minimum specifications. Tail light grounds that are inadequate, headlight sockets that are inadequate, suspension components that are non-adjustable, downgraded front calipers and wheels (our X-types were designed to have 17" wheels and ST-225 calipers!)

Anyway, there is a fix; if you look at the light plane up above, you'll see a large flat spot in the ground plane just below the reverse lamp. Drill a hole carefully through this point, and pass a stainless screw with a lock-washer UP through it. Add a second lock-washer and bolt and tighten the fitting down. This will give you a terminal that you can attach a 14 or 16 gauge wire lead to; make it about 18 inches long and attach eye terminals to both sides. On the lamp side, drop a flat washer, then the eyelet, then another flat washer and finally a bolt and tighten it down.

On the body side, you'll find several mounting nuts mounted in the trunk near the housing. Loosen one, and strip back the paint under it so we get a good electrical connection. place the eyelet from the other end of the wire onto the bolt, then tighten the bolt back down.

This will provide a nice, solid ground capable of handling the current demands!

It literally takes longer to describe than it does to do!

 

Great post Wa3ra

Now I understand why you were outside at the rear of your car chanting "Ohmmmmm.... Ohmmmmm...."

 

Those numbers are run using the European lamps...the actual lamps we use here are brighter and use more current...27 watts for the main and 8.3 for the secondary filaments...so one side can draw over 150 watts...almost 12 amperes assuming the rated 12.8 volts at the lamp.


Pretty outlandish.


The XKRs work out even worse...they must have a ball with their rear lights!

 

Is fitting LED lamps a possibility?
Would have lower current requirement.

 

Yes, fitted 1156 LED projectors for the reverse, 1157R LEDS for the tail and stop/tail, and an 1156 LED for the rear fog.

The directional needs either a canbus 1156 or a resistor added to keep the circuit from flashing too quickly; a little self-defeating since you are still heating a resistor, but it looks like the combo will draw about 6 watts for that lamp. I haven't done that yet, because I am going to replace the front turn signals at the same time.

That brings the total power consumption down to around 12 watts a side