A Swedish Gremlin?

Posted 3/15/2010
By Dave Cox

The following relates what may be the strangest issue I have ever faced.

Vehicle: 2002 Saab 9-5.

Complaint: Every time the horn is honked, the supplemental restraint system (SRS) light in the instrument cluster comes on.

The SRS light is commanded “on” by a message on the I-bus from the SRS control module. Since the only components the horn and the SRS system have in common are the horn pad/driver airbag and the clock spring, the technician decided to replace the clock spring and the airbag. The light continued to come on when the horn pad was pressed. 

Diagnosis: We started our diagnosis at the horn relay to determine which side of the system the problem was on, activation or output. I asked the technician to remove the relay and to check what happened when the horn pad was pressed. The SRS light did not come on. Then we moved to the horns. With the relay out, I asked him to put a jumper wire between pins 30 and 87 of the horn relay … the horns honked and the SRS light came on. Remember, this was with the relay out, so there was no connection to the activation side of the relay. This was getting interesting.

Next, I asked him to disconnect both horns and to put the relay back in. When he pressed the horn pad, there were no horns and no SRS light. I then asked him to provide external power and ground for the horns, thus isolating the horns from the vehicle’s wiring harness. The horns honked and the SRS light came on!

Somehow, the noise or vibration of the horn(s) was turning on the SRS light, which is supposed to be turned on only by a message on the I-bus. My conclusion was that the vibration of the horn was disturbing one of the front crash sensors. The technician said the sensors were mounted correctly and were not loose.

Dave Cox is an Identifix European specialist and is ASE master-certified.

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