Fuel Pump Relay
My 1969 BGT has a number of subtle modifications that aid safe driving on modern roads but which would be only apparent to the keenest eye and scrutiny when viewed externally. Early on in my ownership I replaced all lights, with the exception of the headlights, with LED units and for the indicators and hazard lights also changing the relays. This has increased my visibility to other motorists on the road enormously and gives me assurance when driving on fast roads at night or in poor visibility. The second upgrade relevant to this article was the replacement of a faulty fuel pump of unknown origin with a Facet ‘Silver Top’ electric fuel pump. A combination of the two created an issue where there would be a noticeable flicker in the LED lights, and my belief was that the pulsating draw on current on the fuel pump was causing a voltage drop to the lights, which was noticeable on sensitive LED units, but less so when the engine is running and the alternator is feeding the circuit.
In the first instance I cleaned, and/or, replaced all bullet connectors on both the relevant wiring circuits, in case one poor connection was causing a voltage drop. This didn’t solve the issue, but did remind me to check the connectors regularly, as those behind the grill in particular, are exposed to the elements.
On a ’69 MGB the fuel pump is fed by the white wire from fuse terminal 3 and the various lights by the green wires from terminal 4, so they are connected via the common fuse terminal. My thinking was that by separating the fuel power feed from terminal 3 would remove the voltage drop and rectify the flicker. The obvious way to do this is to provide a direct power feed to the fuel pump via a relay, so with that plan in mind I purchased a few relatively cheap components on-line, comprising: a 4-terminal relay, 1.5mm gauge cable, 5 amp fuses and an in-line fuse.
For my GT, with various modifications, the battery is in the boot and the right hand battery bay houses the fuel pump. I ran the new power feed through the grommet orifice on the right hand side of the boot space, and a short route length to the fuel pump and connected it to relay terminal 30. The ‘old’ power feed was disconnected from the pump and re-connected to terminal 86 so as to act as the trigger i.e. ignition on. The fuel pump feed wire was connected to terminal 87, and a ground wire to terminal 85. The simple wiring diagram is shown below.
All of the connections were made using bullet type connectors and once all tidied up, weather sealed using heat shrink sleeves. In the end I used a relay with a built-in fuse as opposed to the in-line fuse. If that proves susceptible to corrosion where I’ve placed, but I think it’s high enough up in the compartment to be largely free of road spray, I may swap it out for the weather sealed in-line type. The current draw on the Facet fuel pump is circa 2 amps, but internet research had shown that some found 3 amp fuses tended to blow and that a 5 amp fuse was the way to go. All of the wiring from the battery to the fuel pump was tidied up with cable ties and the finished wiring at the pump end is shown below.
Sure enough, the direct power feed to the fuel pump has fully rectified all flickering LED lights – indicators, hazards, driving (spot) lights and reverse lights. An added benefit is that the power to the pump no longer runs through the ignition switch, reducing loading on it. I already carry a spare relay in the ‘spares box’ of the car as I have them on various other components (headlights, spot lights and cooling fan) so all I have to add are a few spare 5 amp blade fuses, just in case. In conclusion, I recognise that it’s a problem brought about by using modern, as opposed to factory original, components, but it’s my wish to make my B GT drivable and as safe as practicable regardless of conditions, and this little upgrade is one further step in that direction, for me at least.
Neil Brant
MGB Register – May 2026
