1994 F-150 5.8L 4WD. I just figured out that my Airbag Diagnostic/Control Module is dead (see my post earlier today about blown fuse). I have removed the module. My questions for you gurus:

1. Is this the kind of thing I can get at a junkyard? eBay?
2. How finnicky do I have to be about getting a 1994 module?
3. I assume that the airbag won't work without the module? Do I have to worry about the airbag spontaneously going off in my face for no reason?
4. Any notion about what might of made the module fry out? There has been no electrical/wiring work of any kind on this truck.

Thanks!
Bruce in Austin

 

Fixed the Airbag Module. The varistor (marked MOV on the PC board) had done it's job and shorted out. I removed it and the module works fine and is back in the truck. I need to replace the varistor but for now all is OK.

 

what is a varistor?

the pc board is where?

 

The Airbag Diagnostic Module in my 1994 F-150 is under the dash on the driver's right. The module is a black and blue plastic case that pops open to reveal a PC board. A varistor of the type used in this module is sort of like a reverse fuse. A regular fuse is in-line with the current flow and will open when the current exceeds a certain value. A varistor sits across the +12V and ground lines. It shorts out when the voltage exceeds a certain value. You use a varistor when you know that there is a fuse somewhere in the wiring which will open when the varistor shorts. Similar to a fuse the purpose of the varistor is to sacrifice itself to save a more valuable component/circuit. The varistor is marked MOV on the PC board. If you think it is bad then use an ohmeter across the pads where the varistor is soldered in with the PC board unplugged. If the ohmeter reads less than a few ohms then unsolder one of the legs and measure across the pads again. The resistance should now read very high, >10kohms. This indicates the varistor is bad. Unsolder the other leg and throw away the varistor. You can use the module without the varistor but run the risk of now damaging the module if an over-voltage situation occurs. You can get a varistor at RadioShack I think. Try to get one in the 18-20 volt range.

 

thanks for the details.