DIY - Testing the Stop Lamp Switch (S9): ML320 2000
DIY - Testing the Stop Light Switch (S9) (aka Brake Light Switch)
Car Model 163.154 /157 /172 /174 /175
The Stop Light Switch (S9) can be the cause of problems in a number of systems – ABS, ESP, ETS, to name a few.
Before installing a replacement switch, one can do a testing of the switch, to make sure it is faulty.
The Stop Lamp Switch (S9) is in fact an assembly of two switches: one SPDT – single pole double throw - and one SPST NC – single pole single throw normally closed. The SPDT is known as Switch A, or the Release Switch, while SPST NC is known as Switch B, or the Brake Light switch.
Testing:
a. The switch can be tested without a full removal, with a Scanner, if the scanner can collect BAS live data.
Collecting live data of the Switch in the BAS system will show the two switches state changes from Closed to Open, when activating/deactivating the brake pedal during the data collection. See bellow an illustration of the BAS live data collected with Auto Enginuity Scanner (laptop software and USB hardware connector):
b. The switch can be tested with a Digital Volt Ohm Meter (DVOM) after removing it, by testing continuity at the contact tabs, while moving the switch pin from Normal/Relaxed to Pushed In/Compressed. The Open/Closed States of the Switch should follow the illustrations bellow, for correct functioning.
A DVOM test need be complemented by a Live Data Test, because the timing of the switch state change is important. The state change is checked against the pedal travel – an excessive pedal travel (more than 3.5mm) before switch state change may be considered as a defective switch.
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