Mechanics

AIRBAG MODULE RESET

1994 Lincoln Town Car • V8 2WD Automatic • 130 miles

I have a 1994 Lincoln Towncar that hit a deer and deployed airbags. I've replaced airbags and module but "Airbag" light when turning on ignition stays on for 7 seconds pauses then flashes 3 pause 5 repeatedly. The car runs just fine but how do I get the airbag system to reset?
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Mikeychar
January 25, 2011.




Find a mechanic with a scanner that can access the air bag computer to read the codes directly. That will direct you to the circuit or system with the problem, not necessarily the defective part. You might have to go to the dealer as their test equipment is much too expensive for most independent shops to invest in.

Given the history, the likely suspects are the clock spring under the steering wheel and the wires going to the front crash sensors. The clock spring has the connector for the air bag assembly and usually melts when the bag fires. Wires to the front sensors can be cut. The diagnostic fault codes are sophisticated enough to differentiate between cut wires, one wire that's grounded, and the two wires shorted together.

Don't overlook a blown fuse. There are always two for an air bag and an anti-lock brake computer. That's so when one blows, there is a second circuit to run the warning light.

Caradiodoc

Caradiodoc
Jan 25, 2011.
I have located a Ford Airbag Fault code list and 34 means: "Driver Side Air Bag Circuit Low Resistance or Shorted." Once the code problem is eliminated the light operates properly!

Mikeychar
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Tiny
Mikeychar
Jan 25, 2011.
What do you mean by "Once the code problem is eliminated the light operates properly"? Do you mean the light goes off as it should but turns back on later while driving? Or is that part of the code description saying everything will go back to normal once you fix the problem?

You're going to have to make measurements to locate the problem, but it might help if I explain how that circuit is constructed.

There are two wires going up the steering column to the air bag. Once they get to the steering wheel, they go through the clock spring which is a wound up ribbon cable in a plastic housing. Years ago they used a spring-loaded "nail" head that pressed against a brass ring for the horn circuit. That method of making an electrical connection is not reliable enough for the air bag because a very small dirt particle could cause an open circuit for just a fraction of a second when the air bag must pop. There has to be a permanent electrical connection at all times. A wire would break off after turning the steering wheel just a few times, so they use the ribbon cable instead.

A pigtail wire comes out of the clock spring and plugs into the air bag. The wire going up the steering column either plugs into the bottom of the clock spring housing, or that wire is part of the clock spring and you have to fish it down the column where it plugs into the harness. That wire also plugs into the air bag computer. All of those connectors below the clock spring will be bright yellow and the terminals will be gold-plated. Also, there will be a gold-plated shorting bar in every one of those connectors that shorts the two terminals together when the connector is disconnected. Those shorting bars are on the air bag side of the connector. They aren't needed on the controller side but sometimes you'll find them there too.

When the connectors are plugged together, the shorting bar is pushed out of the way. Those shorting bars are used because without them, static electricity can pop the air bag. With a shorting bar, the same voltage will appear on both wires. Since the difference in voltage between them is 0 volts, nothing happens.

There is also a resistor across the terminals inside the air bag canister. A common value would a 1000 ohm resistor. When you turn on the ignition switch, the computer performs a self test of the system, then it continuously monitors the circuitry after that. As long as it sees 1000 ohms across that pair of wires, no related fault code will be set. That's how it can tell there is a good connection on both wires. There are two other possible conditions. A break in the connection between two mating pins would cause an open circuit. The computer will detect that as too high resistance and shut the system down. It will turn on the warning light, and the air bag will be susceptible to static electricity.

The other condition is where the two wires are shorted together. The computer will see 0 ohms in the circuit, shut the system down, turn on the light, but static electricity won't pop the air bag. That's what is happening to your car.

The problem comes when troubleshooting the wire for that fault code. If the problem is intermittent, some systems will rearm and the light will go off when the problem goes away, but the code will remain in memory. Some systems rearm only after the ignition switch is turned off. If the system is working properly during your testing, and you unplug a connector to measure the resistance between the wires, it is very possible to pop the air bag from the battery in the ohm meter. For demonstration purposes, a nine volt transistor battery works just fine to pop an air bag, and that's what's in a lot of digital volt / ohm meters.

What you can do is unplug the air bag, then unplug the wire somewhere under the clock spring to measure to see if the wires are shorted together, but you will have to lift up the shorting bar in the lower connector. If you find 0 ohms, the logical place to look would be where the wire connects to the bottom of the clock spring. If no problem is found there, suspect the ribbon cable itself. Most shops replace the clock spring anytime they replace the air bag but not because of the ribbon cable. It's because the air bag connector is usually melted.

Caradiodoc

Caradiodoc
Jan 25, 2011.

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