The SRS control unit sends a voltage signal to the seat belt buckles, since there are different resistance values buckled vs unbuckled a return voltage for one or the other value is *expected* in return. If the ECU sees a different voltage than expected the light stays on for 60-90 seconds. All it takes to damage a buckle is a drop of coffee or other substance. Sometimes the resistor just goes bad!


ON the 1984 thru 1987 the light(test time) is supposed to stay on for 10 seconds. 1988 & later the test period is only 4 seconds.

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mbdoc is right about the resistors going bad. Took the seat belt latches apart, exposed the two resistors and found on both latches opened 1/4 watt 100 ohm resistors (the larger of the two resistors). I suspect the 1/4 watt resistor goes bad because current is always passing through it when the car is running. The smaller wattage 100 ohm resister is placed in series with the 1/4 watt resistor when the seat belt is unlatched and shorted out when latched. With an ohm meter one can check for proper resistance without taking anything apart; with no key in the ignition, under the front of the seat is an orange connector with two wires, brown and brown-green. Place the leads of the ohm meter into the wire ports of this connector. If the SRS circuit is functioning correctly about 177 ohms will be read when no seat belt is in the latch. Insert the seat belt and the resistance should drop to about 98 ohms. It's worth a try to replace the resistor since a new latch runs about $150 and a replacement resistor costs a few pennies. Removal and installation of the 1/4 watt resistor requires simple soldering techniques. The most difficult step was removing the two small screws that hold the latch assembly together. The screw head has a tri-cut, almost looks like the MB emblem, so I used a drill to remove the screws. A small easy-out screw extractor would work instead if one is available. I replaced the drilled out screws with #4 3/8 screws when reassembling the latch. Of course, the seat must be removed but this is straightforward. There's no need to pull the latch wire leads completely out of the seat after disconnecting the wire connectors (there is also a black two wire connector for the seat belt buzzer) from the housing attached to the seat; there will be enough slack to work on the latch when it's disconnected from the seat and any cable ties are undone (don't cut the cable ties, using a pair of pliers wedge the locking stub off the serrated strap and push the strap free with a screwdriver). The driver side seat will have both connectors in use to remind the driver to buckle up.