Hello All

I have been asked about Lucas SB20 and 6RA relays.

I'm on holiday at the moment and can only read emails, not send them so have to reply this way. That said the info may be useful to others so perhaps it should be on here anyway.

I cannot find any information on the SB20 relay. The description sent is;

“The middle terminal is marked B and fixes to the bottom of the coil.”
“The right hand one is marked P or F? and the wire from the coil winding goes to this.”
“The left terminal is marked H? and goes to the contact that the coil contact pulls to make the circuit.”

From the description I would imagine the following is possible

B = Battery, P = Pushbutton, H=Horn

If this is the case then the 6RA relay would connect as follows;

B =C2 (and W2 on a 4 terminal relay)
P=W1
H=C1

Regardless of the SB20 unit markings, when fitting the 6RA, C2 is the main feed, C1 is to the horn, W1 is to the pushbutton.

It should be noted that the 6RA is a group of relays that have different configurations and voltages so do not assume you have the correct type. In particular the difference in 3 and 4 wire systems is important.

Also for information

When changing from 4 terminal 6RA to modern cube relay;

C2=30
C1=87
W1=85
W2=86

Where changing from 6RA 3 terminal to modern cube relay;

C2=30 and looped to 86
C1=87
W1=85

Hope this of use

David

My bus number (if any): RML2276 M1001 T806

why bother with the horn relay? Just wire the horn direct from the push button. It's unlikely to burn out in the next 20 years.

But useful information for the purist.

My bus number (if any): RML2532

and that is the very reason that bus drivers/train drvers/ mechanics etc do what they do and electricians are the elite trade.

My bus number (if any): RML2276 M1001 T806

David

Although I personally wouldn't bypass any relays I think your comments are a bit harsh tarring us all with the same brush. There are good professional drivers as well as bad ones and there are good mechanics as well as bad ones as there are good electricians and bad ones.

Martin

T'was I that emauled David about this given his offer of help/assistance when subject mentioned in the past on both 'channels'

Thanks for the taking the time and effort to make a reply whilst on holiday David, most appreciated.

In this instance its not on a RM and its not the horn but the relay is the same as the original style RM horn one.(start inhibit circuit that uses 2 relays off the oil pressure light but needs to retain latching when cranking)

Very useful to have the full info

MArk

My bus number (if any): RM1414 (sort of)

You are welcome Mark P. Hope it helps although my thoughts on the terminal designations are purely supposition. I would like to see the circuit it is used for.

Martin, I am not actually tarring all with the same brush in fact the brush was definitely used only the once. If you as a mechanic offered someone mechanical advice I would not presume to simply dismiss that advice publicly. I am after all just an electrician. I, however, offered advice and someone who has claimed to be either formerly or currently a train driver, bus driver and mechanic but not an electrician has publicly dismissed my advice. Not argued against it or debated it providing current ratings or the relevance of continuous or intermittent use as evidence, which I would actively encourage, but simply dismissed it as unnecessary. Unless your a purist of course.

My bus number (if any): RML2276 M1001 T806

Not particularly bothered about the brush tarring etc., but taking advice from a very well known electrician and an even better known omnibus consultant.

The horn relay is reckoned to be typical LT overkill on the RT/RF/GS etc classes. My GS has run for the last 30-odd years without it with no problems, considering the number of times the horn is actually used! Otherwise I would not have proffered the advice.

Which is why I made the comments. I often have to consult electricians/consultants/others for advice. No-one can know everything, and there's a lot of faults and failures out there that are sometimes unique to preserved buses that simply wouldn't happen in service.

My bus number (if any): RML2532

Leyland used relays as standard for the horn on half cabs and on rear engined vehicles for the horn, low air and high temp as well as start inhibit some have them on the headlights to avoid the dim dip switches on the column getting hot - moving onto the plug in clear ones and later the little square plug in ones.
They must be there for a reason and continue to be used in modern heavy vehicles.

Mark

My bus number (if any): RM1414 (sort of)

Diesel bug in particular.

Problem sorted but not relay. Found this info in manual;

My bus number (if any): RM1414 (sort of)