Automobile Maintenance: Identification of Electrical Relays

Here I am discussing a topic which is very important for automotive safety and yet is rarely discussed. This write-up concerns the general-purpose automotive electrical relays. The specialized ones like timer relays etc are not discussed here.

Electrical relays allow the use of a low-amperes circuit to control a high-amperes circuit. They can have either five or four terminals but mostly four of them are used. Ignoring the fifth terminal, a relay has two ground and two power connections. The low-Amps circuit is the control side and high-Amps circuit is the feed side of the relay. Looking at the specific positioning of the terminals, one would think that plugging-in of the relays is fool-proof and that they can only be inserted one way but in reality it is not the case.

The reason is that relays have two types of terminal layouts, namely layout A and layout B. These two layouts are not the same. Both the terminal numbering and positioning vary. Here we would take a look at the relays according to their layouts and terminal numbering. Terminal position and schematic diagrams of these relays are shown in the figure. You would note that the schematic diagrams of these different relays are similar (with different terminal numbering) but the actual terminal positions are different. Electrical diagrams in repair manuals use these schematic representation of relays and one does not see the actual layout of terminals.

French and Italian Relays (Type B Layout)

These relays have terminals 1-2 as the control side and terminals 3-5 as the feed side. The layout of terminals and schematic diagram is shown in the figure. We can see that control-side terminals are located opposite to each other, just as the feed-side terminals.

American Relays (Type B Layout)


These relay have terminals 85-86 as the control side and terminals 30-87 as the feed side. Although the pin/terminal numbers are different, they are interchangable with French/Italian relays because the layout or positioning of the terminals is the same.


German Relays (Type A Layout)

These relays have terminals 85-86 as the control side and terminals 30-87 as the feed side. In this case, the pin/terminal numbering is the same as American ones but layout B of terminals is used. The control-side terminals are positioned diagonally instead of opposite to each other. It is the same for the feed-side of the relay. Therefore, these type B relays are not interchangable with type A relays. Changing a type A relay with a type B relay can result in serious damage or even vehicle damage from fire. You either have to replace it with same type relay or interchange wires corresponding to terminals 30 and 86 on the relay mount.