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Old 06-08-2012, 02:12 PM
StyxB2500 StyxB2500 is offline
Always fixing something
 
Join Date: Feb 2012
Posts: 9
Default Airbag Code 25 - PAD Switch repaired

If you get an airbag light flash code 25, it means "DTC B1871 - Pass Air Bag Deactivation (PAD) Switch Fault." For those who don't understand "PAD Switch" there is a small magnetic reed switch in the deactivation module.

Here is how the switch module works:
When the airbag activation key is in the "On" position, the switch will bring a small magnet away from the reed switch (which is soldered to the module circuit board). The reed switch is then in the "normally closed" (N.C.) position. In this position a 1.0k-ohm resistor is placed across two pins of the electrical connector of the module. When the activation key is in the "Off" position the switch will bring a magnet into proximity of the reed switch causing it to activate and switch to the "normally open" (N.O.) terminals. In this position a 470-ohm resistor is placed across the same two pins of the electrical connector of the module. I've included a very basic schematic of the switch portion of the module.
Airbag Deactivation Switch.JPG

Cause of the error
As the magnetic reed switch ages, the connection on the normally-closed side of the circuit tends to fail. This is caused by vibrations slowly degrading the very thin rhodium coating. Eventually, the switch will not connect between the common terminal and the normally-closed terminal, regardless of the position of the magnet (the normally-open terminals typically last longer as they see less frequent use). Sometimes exercising the switch can temporarily restore the connection but it is not typically a long-term solution. Other causes of this error could be that the resistors are out of tolerance. Using an ohm-meter measure the resistance of each resistor (be sure the reed switch is not common across all three terminals). If either resistor is out of tolerance by 5% then replace it. Acceptable ranges are from 0.95k-ohms to 1.05k-ohms for the 1k resistor and 446-ohms to 494-ohms for the 470-ohm resistor.

The fix
To fix my module (instead of paying $82 for a used one on ebay) I determined that I needed only one component: a single-pole double-throw (SPDT) magnetic reed switch. I found one at a local electronic parts store for $4.95 but you can also order them online: amazon currently sells them for little more than $10. The power capacity is negligible, only 500mW at most. The important factor is the AT rating, or magnetic pull-in sensitivity. I used a Philmore part No. 30-17154 but this switch is too sensitive at 15-30 ampere-turns. At this sensitivity the module will always read the keyed switch as if it were in the "off" position. To account for the difference, I mounted my switch higher above the PCB than usual (see picture). This meant that it would encounter less of the magnetic field from the keyed switch. Ideally you want to find a reed switch with an AT rating between 30-40, but the "stand-off" method also works. Instructions on how to remove the module from the dash can be found elsewhere on this forum.
IMG_4660.JPG

The vehicle
This repair was done on a 1998 Mazda B2500, 5-spd manual single cab but applies to several other ford & mazda models that use the passenger aigbag deactivation switch.
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'98 B2500, M5OD-R1, Standard cab, 160k and counting!

Last edited by StyxB2500; 06-12-2012 at 12:56 PM. Reason: update image attachment
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