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MG MGB Technical - Starter Relay

I have a new Starter Relay with marking 30, 85, 86 and 87. I couldn't find the numbers on my old one. I need to know where the White/Red, Brown, Brown/white, Black, wires go to the numbers 30, 85, 86, 87. Thanks
CDB Chuck

I think these are labelled as per DIN72552 standard.

From Wikipaedia (with my notes in brackets)

30 from battery+ direct (live feed probably brown)
85 relay coil - (feed from negative probably black)
86 relay coil + (feed from switch probably white/red)
87 common contact (feed to starter probably brown/white)

You can check your vehicles specific colours from the diagrams at http://www.advanceautowire.com/mgb.pdf

Regards

Paul
Paul1970

Correct, but best not use the word 'probably' with electrics guidance - although, it is better than 'possibly'!

Hal Adams

CDB-
Most 12 Volt relays have four terminals, each with a number. The numbers are normally 30, 85, 86 and 87; these are an indication of what should be connected where.

The coil side (the circuit with the On / Off switch) is connected as follows: terminal 86 is one connector on the control or coil side. Terminal 85 is the other connector of the control or coil side. It is normally connected to earth / ground. It does not matter which way around the relay is connected. You could connect terminal 86 to earth and the relay will work quite happily. However, convention dictates that we use terminal 85 connected to ground (earth).

The load is connected as follows: terminal 30 is the power input, and terminal 87 is the power output to the load.

When you supply power to the coil (terminals 85 and 86) it then becomes an electromagnet. The coil (electromagnet) then pulls on, which closes the connection between the contacts 30 and 87 which are normally open. Power can then flow through the relay. The connections on terminals 85 and 86 are attached to the magnetic coil inside of the relay. Think of it in this way: the coil pulls the switch contacts 30 and 87 together. If you remove the power from the coil, the coil then releases, and thus the switch contacts and the load circuit is broken.

One contact is probably going to be connected to an On / Off switch. The other side of the switch will be connected to the battery. The other relay contact will be grounded (earthed). When the switch is operated, current flows from the battery, through the switch, and then through the coil that is inside of the relay, and then onwards to ground (earth), which is in effect the other side of the battery. The circuit is then complete and the relay will operate with a "click" sound. You can create this simple circuit with a few pieces of spare wire.

Terminals 30 and 87 are used to switch the load. One terminal is connected to the headlights, and the other terminal is connected to the power source (i.e., the battery). When the relay operates, the circuit is from the battery, through the relay contacts, 30 and 87, to the system, and onward to ground (earth). The circuit is thence complete and the system will function. This will continue until the switch controlling the relay is opened. Current will then stop flowing and the coil will release the contacts inside of the relay.
Stephen Strange

I only used the word 'probably' to direct him to the wiring diagram for his vehicle, should I have used to word 'usually' instead or said 'usually on lucas / british wiring systems'. No exact vehicle information was given. It would be easy to check if my advice was correct before connecting. Having said that I'm sure I was correct anyway :).
Regards
Paul.
Hopefully litigation avoided
Paul1970

Paul, Thank you very much. Very helpful. Car in the question is a 1977 MGB. I did check the wire diagram and it referred 30,85,86,87 as W1,W2,C1 and C2, which wasn't to helpful. I appreciate the information. Chuck
CDB Chuck

W = Winding. ie, coil
C = contact

30 = C1
87 = C2
85 = W neg
86 = W pos

85 is connected to negative because some relays have suppression diodes fitted, so the relay will either not operate or go up in smoke if you hook it up ar*e about face.
Old Lucas relays with the C/W designations don't have diodes, so 85 & 86 are reversible on those. It is possible that connection of the C1/C2 terminals reversed could have some effect on very long term contact wear, but that is very arcane and I haven't looked into it; early pos earth cars seem to be so reversed from modern convention.

FRM
FR Millmore

This thread was discussed between 10/11/2012 and 12/11/2012

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