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Fault with the starter 6RA relay

The familiar bright alloy case of a 6RA relay on the offside inner wing of an MGBGTV8 greets you every time you go into the engine bay. The 6RA forward of the fuse box controls the twin cooling fans and the other on the bulkhead side controls the engagement of the starter motor. In both cases the 6RA relay is managing the switching on and off of a high current device with a signal from a switch on a low current circuit - so in effect the 6RA is relaying the signal from the low current circuit into an action on a high current circuit. Recently Steve Newton replaced the 6RA relay for the starter motor as part of the refurbishment of an MGBGTV8 owned by a longstanding V8 Register member, John Gay. Within a day or so a fault developed where having turned on the ignition key to engage the starter motor to fire up the engine, the starter would not stop turning once the engine was running. What was the problem? (Feb 15)

Usually once the engine fires up the starter motor automatically stops and disengages, but with John's V8 even turning the ignition off left the starter motor turning! The only way to stop this was to tap the alloy case of the 6RA to free the contacts or to break the power supply to the starter motor from the batteries. With John's car he has a battery isolator switch so that was a readily available option. Steve replaced the 6RA but the problem was repeated a few days later. So he opened up both 6RA relays to investigate what was going on and causing this problem. What he found was sadly not unusual with many replacement parts in the classic car spares market today - poor quality parts often imported from Far Eastern suppliers and manufacturers.

On examining the faulty 6RA relay Steve found the contacts in the switch came together under the magnetic force of the coil in the relay but did so at an angle and not parallel to each other as they should, so there was a reduced area of surface contact between the contacts. With the large current drawn by the starter motor, the result was a fusing in one spot with the limited area of surface contact between the fixed and flexing contacts. This could only be broken by tapping the 6RA case to free the contacts. Steve also detected a lack of sufficient spring tension on the sprung blade (the lower of the two contacts in the

photo to the right).

Where a 6RA relay is used in less demanding roles, for example with the twin cooling fans, the surface fusing problem above is less likely to arise. So a serious operational fault may not develop although we have seen a case of a "burn out" with a 6RA relay used for the twin cooling fans - an example below.


How does a 6RA relay work?

A relay is just a remote control switch, but it can have many variants. The first is a single pole switch that,
depending on which terminals are connected to which contacts, is Normally open or Normally closed when the electromagnet is not energised when no current is passing through the coil (W1 - W2, see the diagram to the right).

In the case of a 6RA relay C2 and C1 are Normally Open. When the coil in the relay (we will refer to the electromagnet as simply the coil for brevity) is energised by passing a current between terminals W1 and W2, it produces a magnetic field which draws a sprung, iron armature towards the iron core (see above right). In doing so a contact at the end of the armature (C2 terminal) makes a connection with a fixed contact, the C1 terminal. So then C2 and C1 which were open are now closed and, for example,


the cooling fans will run. See our detailed note


Why were 6RA relays used?
As with many other cars of the period, MGBGTV8s used Lucas 6RA relays to control the power supply to various electrical ancillaries, specifically those that draw larger currents like the starter motor and twin cooling fans fitted to V8s. The benefit of using a relay, an electrically operated switch, is it allows a high current circuit (for example the circuit supplying power to the twin fans) to be controlled by an isolated, low current circuit (the circuit with the switch). This enables all the wiring handling the highest currents to be located within the engine bay but controlled by low current wiring routed from a switch. With the twin fans the switch is the Otter thermostat switch in the engine bay but in cases where the switch is near the dashboard, removing the high current wiring from the dashboard area reduces the potential fire hazard and also allows a lighter grade of wiring to be used.
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