Dec 092013

As with many other cars of the period, E-Types use Lucas 6RA relays to control the power supply to the various electrical ancillaries, specifically those that draw larger currents. The obvious benefit of using a relay, an electrically operated switch, is it allows a high current circuit to be controlled by an isolated, low current circuit.

This enables all the wiring handling the highest currents to be located within the engine bay and controlled by low current wiring routed from the dash area. Removing the high current wiring from the dash reduces the potential fire hazard.

So it’s odd why Jaguar didn’t use relays to control the main and dipped headlights. The addition of headlight relays is another popular modification which I’ll be making in due course. The mounting of the horns was relocated from within the bonnet to the picture frame during the production run of the S1 4.2. This has freed up a connection in the 8-pin bonnet plug which can now be used for the high current feed from the battery and the relays can be discreetly located behind the headlamp sugar scoop.

Four types of 6RA relays are used in the later S2 cars (those with the ballast resistor):
Terminals layout for the Lucas 6RA relays - applying 12volts to the W1/2 terminals switches the relay, connecting the C terminals

  • Alternator Relay – 33209F (SRB121) : 20A 4 pin
  • Starter Solenoid Relay – 33231E (SRB400) : double contact 5 pin
  • Cooling Fan Relay – 33232E (SRB501) : 3 pin
  • Horn Relay – 33252E (SRB111) : 20A 4 pin

Note: the SRB numbers are the modern replacement product codes

Internal wiring layouts for the Lucas 6RA relaysTo the right are diagrams detailing the external terminals and internal wiring for the four 6RA relays used in the S2.

The difference compared to the earlier cars is the starter solenoid relay has double contacts to enable it to provide power to the starter solenoid as well as bypassing the ballast resistor while the starter is in operation. All are simple electromagnetic type relays, which are ‘normally open’.

The various relays were all working fine but were showing the effects of decades of exposure to the elements and looked very scruffy against all the restored components.

The relay covers are only crimped in four places so it was possible to carefully undo them to enable the covers to be removed. These were dipped in a mild citric acid solution overnight to remove the remains of the zinc plating before being re-plated.

With the exception of the 3-pin cooling fan relay, the W1 and W2 terminals are used to energise a coil winding which has an iron core at its centre.

In the energised state the coil produces a magnetic field which draws a sprung, iron armature towards the iron core. In doing so a contact at the end of the armature (C2 terminal) makes a connection with a fixed contact, the C1 terminal or the C1 & C4 terminals for the double contact relay.

The 3-pin relay lacks a W2 terminal because it is designed for applications where terminal C2 is always connected to 12 volts. Internally the C2 terminal is also connected to the coil winding and so acts as the W2 terminal as well, delivering 12 volts. The coil winding is therefore energised by grounding terminal W1, resulting in the switching of the relay.

All that remained was to tidy up the electrical connections and re-crimp the covers back in place.

Ignition Switch
As I was sorting out electrical bits and pieces, attention turned to the Lucas ignition switch, which is marked 157SA 39415A. I’m not sure if this has been replaced at some stage but the terminals bore no correlation to the wiring diagrams and had intermittent connections when tested with a multi-meter.

The terminal connections on the wiring diagrams indicate:

  • 1 – Brown : supply from Battery, under permanent current
  • 2 – White : under tension only after ignition switch is on
  • 3 – White/Yellow : via starter solenoid relay, delivers power to the starter motor

Therefore, when the key is in position II, terminals 1 and 2 should be connected and when the key is in position III, terminals 1, 2 and 3 should all be connected.

To achieve this with my 157SA switch, the White/Yellow wire can only be connected to terminal 1. The other wires using terminals 2 and 3, in any order. I decided to take the switch apart to see if it was possible to ‘correct’ the terminal connectivity and address the intermittent connection problems.

The switch can be split in half by gently prising the retaining tabs outwards. The tabs are made of pot metal so I wasn’t sure they would survive the operation.

Inside the copper contacts were heavily ‘gunked’, which was the most likely cause of the intermittent connections, so they were cleaned up with good old Brasso.

The key lock engages with a nylon disc within the switch, which therefore rotates as the key is turned. On the underside of the disc is a sprung ball bearing which locates in dimples to differentiate the key positions and a spring which returns the key position from III (starter motor engaged) to position II when the key is released.

There’s no ability to change the terminal connectivity so I’ll just have to adjust the terminal wiring accordingly.

  2 Responses to “Relays and Switches”

  1. Hi Chris. I am preparing to change the ignition switch on my 1970 E-type. Is it removed from the barrel by releasing the three “tangs”? Are they likely to fail? Any advice would be much appreciated.

    • Hi Peter,

      The whole lock and switch is attached to the steering column via a security bolt. The head of the bolt shears once it has been tightened to a certain torque. I’m assuming you’re not talking about is bolt.

      The actual switch is secured to the lock mechanism by a tiny machine screw. Once removed, the switch should simply slide out. The orientation of the switch is set by the protrusion on the switch aligning with a channel in the lock.

      I think the three tangs that you are referring to are holding the metal and white plastic parts of the switch together – is this correct? If so, they are very fragile and prising them open is likely to damage them. I’m not sure what metal is used but it is quite brittle. Unfortunately one of mine snapped off when I pushed them back into position to retain the white part.

      I don’t think the switches are designed to be split. I had to put a dab of epoxy glue as a repair.


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