Replacing a car’s brake lights and CHMSL (centre high-mount stop lamp) modules with 12V LED replacement lamps is a popular upgrade which improves brake light visibility and response time. However, it can cause serious problems with cruise control circuitry or lamp failure circuits which sense the low resistance of the cold lamp filaments.
We have even heard of a case where LED replacements caused problems with fuel switch-over in a car with an LPG tank.
Even if your car does not have lamp failure monitors, LED replacements are likely to cause problems with cruise control operation. Cruise control is normally deactivated when the brakes are applied and this can be sensed by sourcing a small current into the stoplight filaments.
With incandescent lamps fitted, the cold resistance of the filaments holds the respective cruise control input low and it goes high (to +12V) when the brakes are applied. However, when LED replacements are fitted, their forward voltage will typically be 3V or more, even at low sense currents of a few milliamps or less. Hence, the cruise control input is not pulled sufficiently low and so operation is inhibited.
This solution to the problem involves using a relay to provide a low resistance (4.7Ω) path for filament sensing when the brakes are off. When the brakes are on, the relay switches the resistor out of circuit. The resistor is rated at 5W so that it can handle the initial 3A current surge that momentarily occurs each time the brakes are applied, before the relay contacts open. Overall resistor dissipation is very low.
Two versions of the circuit are provided. One suits CHMSL replacements where the lamp is separately sensed from the main stop lamps. The second version shows how to connect a DPDT relay when the lamp filaments are sensed separately. In this case, both lamps circuits are isolated with separate relay contacts.
Note that a 12V automotive relay should be used to ensure long term reliability. The 5W resistor should be housed in a small sealed diecast box. This is specified in case the relay coil wiring goes open circuit or if the relay fails; the resistor will burn out within the confined space of the box without the risk of starting a fire. Grommets should be fitted to protect the wire entry to the diecast box.
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Editor’s note: while using a relay may seem like a crude approach it has the advantage of simplicity and also prevents the LED replacement lamps from glowing faintly when they should be off.