My Repair of a Ford Ranger With a Code 27 Air Bag Fault

The infamous Ford air bag fault 27 code is rather well documented publicly by everybody but Ford.  Ford wants you to take your vehicle to a dealer for repair.  The cause of the fault is the indicator lamp for the passenger side airbag is burnt out.  Doing a quick search online will yield lots of repair tutorials are shown for replacing the light bulb with another like one or with a 12v LED.  Simple stuff, but all that required me to purchase something that I didn't have on hand.  My solution was to use a standard LED and a separate dropping resistor.  I had not seen anyone post this publicly before, so I wanted to document it.  To set this up, I soldered in an LED where the light bulb was and then used a razor blade to cut a trace on the circuit board that went to where the LED got soldered.  I then soldered in a resistor.  I believe I used a 470k resistor, but the exact one will depend on your LED that you use.  It does not mater which side of the LED you add the resistor to, but it does matter which way around the LED gets soldered. If it doesn't work, try de-soldering it and putting it in the other way around.  I did notice one issue with my repair and that is that the LED glows dimply when the light should be off.  This is not unexpected behavior, and could possibly be eliminated if I had used a higher value resistor.  LEDs take very little current to turn on, and there is always some current flowing thru the lamp with this circuit in order for the system to monitor if the lamp is burnt out.

Follow-up from a couple years in the future:

I was involved in a crash in this vehicle in January 2022 because of icy roads. While my tuck was a total loss, the air bags deployed properly. The sketchy repair I made likely saved my life.

Repairing IF Transformers (Silver Mica Disease/Silver Migration Disease)

Recently I took the time to work through a Hallicrafters S-53A receiver that I have had kicking around for several years.  While working through it, I encountered a common problem in witch the IF transformers had failed.  The failure is a common problem known as Silver Mica Disease or Silver Migration disease.  It occurs when the mica sheets used in the construction of the internal capacitors in the IF transformers corrodes and causes a path for shorts and arcing.  The symptoms of an affected receiver are either a static crashing like lighting (after all, that is what happens when these arc out), or no/very little signal passing through the IF can.  This is generally limited to the small (for the time), square IF cans commonly used from the early 1950's thru the early 1960's that have variable inductors inside them.  Ten years ago when I got started in the hobby of restoring vintage equipment, it was a sometimes the IF cans are good, and sometimes they are bad.  Nowadays, you can pretty much bet on any of this type of transformer being bad.

Identifying bad IF can capacitors is easy if you pull out the transformer and look inside, unfortunately, its not always easy to remove the transformers, but is a required procedure.  Look for black marks on the mica wafers in the bottom of the assembly, if the color of the mica sheet is not uniform, its bad.
To repair these, you remove the integrated capacitor from the transformer and replace it with an external silver mica or other highly stable type of capacitor.

There are several different methods of identifying the value of this capacitor described online, but I have found one that takes most of the guess work/assumption out of it.  One of the most commonly cited methods is to "just use 100pF capacitors".  This is by far the WORST method, as many IF cans use values other than 100pF.  Another is to use a capacitor substitution box.  That being said, I have found the best way is to use maths.

Breaking the principals of RF down a bit, an IF transformer is, at its core, an LC resonant circuit.  This means it has an inductor (L) and a capacitor (C) that work together to be resonant at a specific frequency.  This frequency is whatever the IF frequency at that transformer is.  In the case of the Hallicrafters S-53A, and most other standard broadcast band receivers it is 455kHz, but I have seen others such as 262kHz used in a Hallicrafters CB-3A CB radio.  Using math, it is possible to calculate the missing value from two known values.  For an IF transformer, the three variables are Frequency, Inductance, and Capacitance.  I'm not gonna pretend to even know how to manually calculate this, that's were web-based resonant frequency calculators come in handy. Here is one such calculator.

For these diseased IF transformers, we already know the frequency, and can measure the inductance.  To measure the inductance, use a good LCR meter that can read in very small inductance values (less than 1 mH).  After getting your inductance measurement, put in the values in the calculator (make sure you properly accommodate for the engineering notation so you get the right value output).  In the example of one of the transformers from my Hallicrafters, a frequency of 455kHz and an inductance of 0.396mH calculates out to a capacitance of 308.97pF.

Here are some additional tips for working with these types of IF transformers:

• Be careful not to break the very fine wires used for the coils
• After drilling out the rivet and removing the old capacitor, you will need to shorten and secure the tabs on the bottom.  I cut them back with side cutters so they don't overlap, then superglue them to the plastic frame.
• You can use the graphite from a pencil to lubricate the threads on the slug
• Consider taking multiple inductance measurements to identify the range of tuning that the IF can has to better narrow down what capacitor to use.  For example, measure the inductance as it was originally set, then once with the slug all the way in, once with the slug all the way out, and one in the middle of the travel
• measuring the inductance with the assembly in the can may improve accuracy.
• If you're really good at soldering, consider using surface mount type capacitors and putting them inside the IF can.
• xraytonyb on YouTube made an in-depth video on this subject, and is a good watch if you have the time.