Arrangement for monitoring the performance of a vehicle stop-light switch
BACKGROUND OF THE INVENTION 
     Stop light switches are actuated when operating the brake whereupon a signal is released which activates the stop light. In the presence of an anti-lock brake control and/or drive-slip control, however, this signal is also supplied to the control unit of the controller where it is used during control. The presence of this signal can thus serve to activate an ABS-control or interrupt an ASR-control. 
     Including this signal therefore in the control is of particular interest in order to determine whether the stop light switch is functioning or disabled due to short circuit or line damage. 
     SUMMARY OF THE INVENTION 
     Monitoring the stop light switch according to the invention ensures that short circuits as well as line damages are detected and indicated. This can be done by turning on a warning light and/or setting a fault memory such that even when the fault disappears, its occurrence is recorded. 
     The stop light switch signal is monitored by two circuits. In both circuits the presence of a stop light switch signal is checked when the ignition is actuated. 
     When the stop light switch signal is present, a first fault memory is set, which memory is cleared when the signal disappears. When a prescribed vehicle speed is exceeded, a drive signal memory is set. A fault indicator is activated if the first fault memory and the drive signal memory are both set when the ignition is subsequently actuated. 
     When the stop light switch signal is not present, a second fault memory 15 is set, which memory is cleared when the stop light switch signal occurs. When a prescribed vehicle speed is exceeded, a drive signal memory is set. A fault indicator is activated if the second fault memory and the drive signal memory are both set when the ignition is subsequently actuated. Preferably, both monitoring performances are applied parallel. The signal line leading to the control unit is hereby included in the monitoring. The circuits are preferably integrated in the control unit and memories of the control unit are used. When a monitoring of this kind is employed it is also possible to include the stop light switch signal in the control to a larger extent. 
     In vehicles having an automatic gear shift where the operator&#39;s foot remains in most cases on the brake pedal when the vehicle is started, it is advantageous to provide an occurrence switch to set the fault signal only after a prescribed number (e.g. 3) of fault detections. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The sole figure is a schematic of the system for monitoring the stop light switch. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As shown in the figure, a stop light switch 1 is connected to a control unit 2 via a signal line. The control unit 2 may be part of an anti-lock brake control, a drive slip control, and/or an engine drag torque control which evaluates the stop light switch signal as an auxiliary value. 
     The actuation of the ignition switch 3 when starting generates a signal that is differentiated in a differentiator 4. The generated pulse temporarily closes an AND-gate 5 when a signal from the stop light switch 1 is available. This is only the case when the operator actuates the brake pedal or when there is a short circuit against the battery potential or when the stop light switch sticks. 
     The output signal of the AND-gate 5 sets a first non-volatile fault memory 6. If the stop light switch 1 does not supply a signal in case the pulse occurs at the output of differentiator 4, which is the case when the brake is not operated and also when the line is damaged, a second non-volatile fault memory 15 is set in the same way via an AND-gate 14 with inverted input. 
     If the signal of the stop light switch 1 drops out during subsequent driving, an invertor 7 and a differentiator 8 generate a pulse which resets memory 6. Reversely, memory 15 is also reset when a stop light switch signal occurs because of braking after the setting of the fault memory 15. 
     A comparator 9 is additionally provided which compares the vehicle speed V ref  or the reference speed necessary for measuring the slippage in the control to a preset speed value. If this speed value is surpassed, a non-volatile memory 10 is set which indicates a driving of the vehicle. 
     If one of the fault memories 6 or 15 was set and has not been reset during driving, but the driving memory 10 was set, a fault indicator 11 will be set via one of the AND-gates 12 and 16 and the OR-gate 13 when the ignition switch is operated again. This fault indicator is here configured as a memory so that a signal which was supplied remains stored at one of the AND-gates 12 and 16. The output signal of this memory can actuate a warning light 18. With a slight delay, memory 10 is deleted via a timer 19 by a signal from the ignition switch 3. 
     If the vehicle in consideration has automatic gearshift, it is logic to release the fault message only after the fault memory 6 has been set several times. For this purpose, a counter 17 is connected to the output of AND-gate 12 which activates the fault indicator 11 only after the memory 6 has been set three times.