Ignition - Ignition Relay, UK (78-on)

Hover over a wire to confirm the colour

Note: This is one of three multi-way plugs behind the dash.

Updated October 2023: This version is on 78 and later cars, several circuits having been removed from the ignition relay and put back on the ignition switch, possibly because of problems with relays sticking on. However the fuel pump is still on the relay - not ideal if it sticks on! It's not known exactly when the change to this version occurred but Geoff Turner's Oct/Nov 77 built 78 model has this version, so possibly part of the wiring changes for dual-circuit brakes on RHD cars in May 77 at chassis number 436465 (GT) and 437181 (roadster).

There are now three separately fused green circuits: The original circuit fed from the 2nd fuse up in the 4-way block, and two others each fed by their own in-line fuse under the fusebox - one feeding the cooling fan switch, and the other feeding indicators, heater switch, tach, and GT heated rear window switch. Both these in-line fuses have white/brown (unfused) one side and green (fused) the other.

There is also the in-line fuse for the hazard flashers under the fusebox with brown wires both sides.

There are three separate white circuits: One from the ignition switch feeding the ignition relay and ballast resistance for the coil (which then changes to white/brown for one of the inline ignition circuit fuses), another in the rear harness feeding the fuel pump (which changes from white/brown at a bullet connector in the main harness), and the third feeding the ignition warning light (changing from white/brown at a multi-way plug behind the dash).

There are two separate white/brown circuits: One from the relay contact feeding the fusebox, cooling fan in-line fuse, overdrive, fuel pump and ignition warning light, the other feeding the in-line fuse for indicators, heater fan switch, tach, and GT HRW.

Multiple uses of the same colour and several changes from white/brown to white and back again does not help in diagnosing ignition problems.

This clearly shows the two white/brown to green ignition circuit in-line fuses nearest the camera and the brown to brown hazard fuse further away. It also clearly shows that the way the wires have been routed and the orientation of the ignition circuit in-line fuses that it's possible to erroneously connect white/brown to white/brown and green to green, which was how a new harness came to me from the supplier and caused some head-scratching when I first powered it up!

The schematics show the original rectangular Lucas relays with W1, W2, C1 and C2 terminal numbering, but in practice cylindrical relays (and then cube-type SRB 520) with ISO numbering were fitted. The relationship is as follows:

Wire colourOriginal numberingISO numbering
White from ignition switchW185
Black earthW286
Brown 12v supplyC230
White/brown to ignition powered circuitsC187

Information on the ballast resistance can be found here.