Installation Tip Information on Lucas (SFB105) Flasher
The information in this technical sheet involves the identification and installation of the Lucas SFB105 flasher relay in positive- and negative-ground vehicle electrical systems. Always consult the vehicle workshop manual to verify the correct wiring, terminal connections, and indicator circuit configuration. If you are unsure of any procedure, lack the proper tools, or are uncomfortable performing electrical work, do not proceed and seek out a qualified Classic British Repair Facility.
Incorrect wiring or improper terminal connections may cause the flasher, indicator lamps, pilot lamp, or vehicle wiring to malfunction or become damaged. Always verify the B (or X), L, and P terminal functions before connecting the flasher and confirm the vehicle's ground polarity before beginning work. Disconnect the battery before making or changing electrical connections unless specifically instructed otherwise.
The person performing the work assumes all responsibility for the use of the information contained in this document.
The Lucas SFB105 flasher relay will operate the same for positive vs. negative ground vehicles because it’s polarity-insensitive by design.
The SFB105 is a 3-terminal thermal flasher (marked B, L, P) and is specifically described as working on both positive and negative earth vehicles.
Internally it doesn’t depend on chassis polarity the way some electronic components do.
The terminals don’t change meaning:
The wiring stays the same regardless of ground polarity:
Refer to your vehicle’s workshop manual for the correct wire-to-switch relationship.
- B (or X) = feed (power in) from fuse box
- L = load (out to indicator/hazard lamps)
- P = pilot (dash warning lamp)
Those functions are identical whether the chassis is positive or negative.
What does change (but not the flasher itself)
In a positive-ground car:
- The chassis is +12V
- The switching happens on the negative side
In a negative-ground car:
- The chassis is ground
- The switching happens on the positive side
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