420 "Field Isolation Unit"

Saloons
1

My 420 has a black plastic unit that looks like a regulator next to the 4TR alternator control. It has a single coil inside and terminals C2 C1 W1 W2. Nowhere on this device can I find any Lucas #s. No part # and no date. I have it out of the car, cleaned off, and in my hands now. It does say Lucas England on the top near the wire bale. No where in the parts or service manuals can I find any info or part numbers for this device. One of the terminals has come off from a failed rivet. I could possible just drill and rivet it back but now after a half century of use it might be time for a replacement. Can anyone tell me how to ID this and find another? Electrically it looks like I should be able to replace it with a typical Bosch type 40A relay.

2

It should be a Lucas 6RA and therefore a relais. Fits your description well and if the contacts are not burned out I’d try riveting. But if they are available new why not; or fit a standard relay in the can… depends on you. http://www.v8register.net/FilesV8WN/160229-6RA-relay-explained-NH3.pdf

3

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The 6 RA type is indeed a mechanical relay but very small and I suspect has limited capacity. This large lump illustrated has a plastic cover interchangeable with an old RB regulator. Also the coil is large. I have successfully replaced it now with a Bosch cube rated at 40 amps. For that I might have to avoid revving enough for a full output or maybe placing a fuse in circuit to readily replace if needed. With an ammeter in dash I can fairly well keep track of load and it only uses high charge for a short time after start. Even then, running on the starter carb does not exceed that. I still would like to ID the pictured lump. I think the 420 diagram does in fact show a 6RA but used for steering lock relay.

4

Maybe an older version of my 6RA. In other words just a relay. Do you know what it is switching exactly? Maybe the 40A is sufficient. Allso I think the 40A rating relates more to what it can reliably switch on and off than to its short term current capacity. If it is a good Bosch then there are data sheets and you’re guaranteed that it can do more than that. What would happen if the relay broke? If it works for now you could carry a spare for emergencies (as long as it isn’t a safety issue of course) and call it a day. Especially if the original is, as you described, small and limited in its capacity.

5

That unit looks like an aftermarket Lucas headlamp dimmer typically available in the mid 60’s. They were wired so that when the ignition was ON, and the side lights were ON, then the headlamps were dim. This prevented the vehicle being driven on side lights only.