AIRBAG LIGHT FIX
for 1999 Chevy Silverado/GMC Sierra

- These instructions were originally written by a Chevy trucks forums member, Shawn Lin in April 23, 2004. Some years back I wanted to do this same mod and was able to get in touch with him. He fortunatley still had the instructions and graciously sent them to me. Since this seems to be nowhere to be found on the web anymore I'm rehosting them in case anyone wants to do this. I put an Escalade cluster in my 99 GMC several years ago and used this mod and it has worked perfectly ever since. - Rich (MixManSC)

Problem: You bought an expensive new 2000-02 Escalade or Denali instrument cluster, put it in your 1999 truck, and the airbag light stays on! DOH! The reason is because the 1999 trucks use an earlier SDM (airbag computer) that grounds pin A11 on the cluster to turn on the airbag light, and open circuit to turn off the airbag light. In the 2000-02 trucks, pin A11 is not used because the SDM sends a command on the Class 2 Serial Data Bus telling the cluster to turn off the light.

Solution: To make my 2001 GMC Yukon Denali cluster's airbag light work properly in my 1999 Chevrolet Silverado, I decided to rewire the cluster so that the SDM (airbag computer) can control the airbag light directly as it does in a 1999 cluster. This required only an NPN transistor, 2 10k ohm resistors, and a little soldering. I was able to find all 3 parts in my junk bin, so total cost of the project was $0.

Technical Details: Most people will probably want to skip over the technical stuff, but here it is anyway.
The airbag light is a high-brightness red LED which is connected to a positive voltage feed at all times. A logic circuit grounds the LED's negative feed when it wants to turn on the LED. In a 1999 truck, this logic will keep the LED's negative feed grounded, so the LED negative feed needs to be disconnected by disconnecting jumper wire W51. The SDM grounds pin A11 in the 1999 clusters to turn on the LED. However, the SDM also keeps the A11 pin grounded when the truck is off, so if pin A11 were directly connected to the negative pin of the LED, the LED would be lit constantly when the truck is off. A solution to this is to use an NPN transistor. The NPN transistor in this application works as a switch. The Emitter is connected to pin A11, the Collector to the negative of the LED, and Base is connected to +12V switched (wire jumper W23). When the ignition is turned on, the Base gets a positive charge which allows current to flow from the Emitter to Collector. If the Base sees 0V, the transistor is OFF so the LED will remain OFF. This takes care of the problem of the LED staying on even when ignition is off. When the ignition is on, the transistor is energized so the SDM can control the LED as it normally does. Two resistors are required in this circuit, a 10k ohm resistor is placed inline between the transistor Base and the switched +12V to reduce base current (this protects the transistor). A 10k ohm resistor is placed across the switched +12V and the Emitter. It acts as a pull-up resistor that pulls pin A11 high when the SDM turns off the LED. The pull-up resistor is required because without it, some leakage current from the SDM keeps the airbag light on very dimly when it is "off".

Simplified diagram

Required Parts and Materials

• NPN Transistor. I used a KN3904 (equivelent to 2N3904) from my junk bin. Equivelent transistor from Radio Shack is Cat. #276-1617, $2.49 for 15pk (4/22/04).
• 2 10k ohm Resistors. Radio Shack Cat #271-1335, $0.99 for 5pk (4/22/04).
• Pencil Tip Soldering Iron, 15-40W (too big and you might burn the circuit board)
• Solder, rosin core 60/40
• Small piece of small gauge wire
• Something to use for insulation like electrical tape or heat shrink.

Instructions

WARNING: Attempting this instrument cluster mod could potentially damage it! If anything goes wrong, it's not my fault! Proceed at your own risk.

1. Remove back cover of the cluster with a flathead screwdriver. Be gentle so you don't break any snaps. There are 4 on top, 2 on the sides, and 2 on the bottom.
2. At this point, you can remove the circuit board from the cluster. It pulls straight off, but be careful not to bend/flex the circuit board too much. Practice on your old cluster if you're concerned about damaging it. If you don't mind working in small spaces, you could probably do everything you need to from the little space next to the connector. I took the circuit board off to make it easier to experiment with different circuit designs.
   
   

   Small space next to connector

   
   
3. Cut or desolder one end of jumper W51. I chose to desolder and lift one side of it in case I ever want to return the cluster to its original state.
4. Insulate the jumper W51 with electrical tape if you desoldered just one side like I did.
   
   

   I insulated it with electrical tape just in case.	You can see both jumpers from this top-view pic

   
   
5. Looking at the front of the transistor (flat part facing you, pins facing down), the pinout is E-B-C. Solder a 10k ohm resistor to pin B (center one) and solder a short piece of wire to pin E.
   
   

   Resistor and Wire soldered to transistor.

   
   
6. Insulate transistor pin B and solder a resistor onto pin E. The resistors are both 10k ohms, this one is just a darker color because it's a different brand (both resistors were salvaged from junk electronics).
   
   

   Both resistors soldered on, pin B insulated.

   
   
7. Insulate the circuit board with something like electrical tape so the transistor and resistor don't accidentally short circuit anything. I cut 2 small holes in the electrical tape so I can access jumper W51 and W23's solder pads.
   
   

   I insulated the circuit board just in case.

   
   
8. Solder the transistor's pin C to the W51's lower solder pad.
9. Solder the free ends of both resistors to either one of jumper W23's solder pads (I used the upper solder pad).
10. Scrape the clear varnish off of the solder pad for pin A11 of the cluster connector. This would be 2nd solder pad from the left on the bottom row. Now solder the free end of the short piece of wire from transistor pin E to A11's solder pad. Make sure you don't accidentally bridge any adjacent solder pads.
    
    

    Soldered to the circuit board.

    
    
11. Use electrical tape to insulate anything else you think needs insulating.
12. Double check your work, then reassemble cluster.
13. Install and test the airbag light. It should work just like it did with your original cluster, which is flash 7 times then shut off. If it works, woohoo!!! If it doesn't, you must have screwed up somewhere!

I'm not an electronics engineer, just a nerdy computer programmer that likes playing with electronics, so there may be better ways to design this circuit. I'm also not a particularly good writer.