Relays, Spot-lamps, Headlamps & Voltage Drop!

Page last updated:- 10 July, 2005

This article attempts to do three things:-

1) Explain how a relay works, 

2) Show how to use relays to get brighter headlamps on the MK1 Cortina, and

3) Show how to find voltage drop in your wiring loom

Relays -- Itís "Relay" not too difficult !

If youíre one of those people mystified by electrical relays, who canít tell their Ohms from their Amps, the following may prove useful.

A relay is an electrical switch which allows a low-current circuit to control a high-current circuit. For example you want a pair of spot lamps on the car (high current) to be turned on and off by a dashboard switch (the low current circuit). The relay allows a thin cable to be used on the control side, and a (hopefully!) shorter but thicker cable on the load side. This helps to preserve your on/off switch from arcing, and reduces voltage drop in thin, tired old cables.

A basic relay for this purpose will cost between £2 and £4 each, be rated at 12 volts and either 30 amps or 40 amps. You probably want the kind with a mounting bracket to allow a self tapping screw to mount the relay on the inner wing or wherever.

Most 12 volt relays have four terminals, each with a number. (Weíll look at five terminal relays later in the article). The numbers are normally 30, 85, 86 and 87; not winning lottery numbers, but an indication of what should be connected where. (The numbers originated from a German specification).

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The problem was, I could never find the information I needed to tell me how to connect a relay. I hope this will help:-

The coil side (the circuit with the on/off switch) is connected as follows:-

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The load which could be spot-lamps, etc are connected as follows:-

When you supply power to the coil (terminals 85 and 86) it becomes an electromagnet. The coil (electromagnet) then pulls on and that closes the connection between the contacts 30 and 87 which are NORMALLY OPEN). Now power can flow through the relay to your lamps etc.

85 and 86 in more detail

Connections 85 and 86 are to the magnetic COIL inside the relay. Think of it this way, the coil pulls the switch contacts 30 and 87 together. Remove the power from the coil and the coil releases 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 power (that is the battery). The other relay contact will be earthed. When the switch is operated, current flows from the battery, through the switch, through the coil in the relay and then to earth, which is in effect the other side of the battery. The circuit is complete and the relay will "click" and operate. You can create this simple circuit with a few bits of spare wire and try it out for yourself! It is a very satisfying click!

30 and 87 in more detail

Contacts 30 and 87 are used to switch the load, such as spot lamps. One contact goes to the lamps, the other to the power source (e.g. the battery).

When the relay operates, the circuit is from the battery, through the relay contacts, 30 and 87, to the lamps and hence to earth. The circuit is complete and the lamps will light. This will continue until the switch controlling the relay is opened. Current will stop flowing and the coil will release the contacts in the relay.

Additional data

Use a maximum of 2 lamps per relay (so as not to exceed the capacity of the relay Ė a normal relay would be rated at 30 amps.)

You should always mount relays with the contacts pointing down, to prevent water collecting, entering and damaging the relay contacts.

Some relays have fuses, which are used to protect the circuit being switched (in our case the spot lamps). If a short-circuit develops in the circuit, for example the "live" main cable may short onto the chassis, the fuse in the relay will blow. The fuse size should be close to, but more than, the current drawn by the load. (Watts divided by Volts = Amps) So, for example 130Watt Lamps, 130/12 = 10.8, so use a 12.5A or 15A fuse. (Note:- Always use "12" volts to calculate your amperage. The voltage of your battery should be nearer to 13.2 volts, but it can be lower so this gives you a safety margin when calculating wire sizes and fuse capacities).

A fused relay is going to cost around £8 but gives you peace of mind and saves you having to install extra fusing in the lamp circuit.

Wiring sizes

For loads up to about 17.5 amps you will need 28/030mm, 2 sq mm cable which will cost around 35-40 pence per metre. This translates as 28 strands of copper, each strand being 0.30mm diameter. The cross-sectional area of the cable is 2 sq mm, which guides you towards fitting the correct terminal. The load quoted for this cable, 17.5 amps, is a continuous load of 17.5 amps. If you think you need a higher capacity cable, youíll have to move up to a thicker cable, the next one being 44/0.30mm 3sq mm and 27.5 amps.

If in doubt, ask!

Terminal colours

Youíve all seen the nice coloured packets of pre-insulated terminals for sale in shops and at autojumbles. But what do the colours mean? Aha, you thought they were just decorative! The colour indicates the cable size which will fit the connector. So Red is up to 1.5 sq mm, Blue is 1.5 to 2.5 sq mm, and Yellow is 3 to 6 sq mm. From this, we will probably be using Blue insulated connectors. However, I would always solder my terminals now, so we need to look at non-insulated terminals and buy the insulations separately.


I wired one car with "crimp" terminals but I would never do this again. Iíve had to go round and solder most of the terminals as they went "intermittent disconnected" giving me all sorts of wiring faults. Connections were made with the correct crimp tool, which simply squeezes the connector round the copper wire. It may work for others, but I will always solder my connections from now on. It only takes half an hour to learn how!

Five connector or change-over relays

Some relays have an extra contact, numbered 87a. This is used where a change-over is required. For example, two-speed fan motors, electric aerials up/down, or what used to be known as town and country horns. In this type of relay connections 30 and 87a are NORMALLY CONNECTED. When the relay is operated (energised) the changeover switch disconnects terminal 87a from terminal 30 and connects terminal 87. The load on terminals 87a should not exceed 20 amps. Refer to the specification when buying.

Up-Rated Headlamps

If you up-rate your headlamps from the normal 55 watts to say, 75 watt lamps, you increase the current drawn from 8.3 amps to 12.5 amps, which is quite an increase. Time to consider fitting a relay. This will save your headlamp switch contacts which may already be suffering (probably from 30 years + of old age!). If you use a relay , the existing switch contacts will only have to handle about  0.15 amps (12 volts divided by the resistance of the relay coil say 85 ohms = 0.14 amps). Also, Ford probably chose the thinnest wire they could reliably use for the wiring loom to save costs. That means that your wiring loom may be quite happy to work with 8 amps, but will complain (by heating up) when you push 12.5 amps through it. Push any more through it and you could end up with a fire, bearing in mind that there are no fuses to protect the circuits on the original car.

So, we have "thin" wires, old switches, and probably a lot of bad contacts where bullet connectors have 30 years of dirt and corrosion on them. We also take power from the battery, all the way to the dashboard switch, then all the way back again to the headlamps - double the necessary journey.

Time to fit relays!

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Where can I buy relays, terminals, cables, etc.

I recommend Vehicle Wiring Products at 9 Buxton Court, Manners Industrial Estate, Ilkeston, Derbyshire, DE7 8EF. Phone 0115 9305454, email They are always quick to respond to orders and are very helpful on the phone. They have a very nice catalogue available on request.


Hopefully I have given you enough information to get those extra lamps fitted and working safely. Remember, use the correct cable and connections. You donít want a fire. Run cables neatly, make up a bundle and wrap with insulating tape. You can buy grey tape and make up your new wiring loom and after "weathering" for a few months will look as though it was always part of your wiring.

Good luck, and remember its "relay" not too difficult.

  See useful formula, here.

  See fitting a fuse box, here.

Voltage Drop 

Note: See Ohms Law Calculator, here.

Ohm's law (E=I x R) states that even a very low resistance in a circuit will cause problems. For example, in a starter-motor system drawing 200 amps, 0.01 ohms resistance in the starter cable will cause a 2 volt drop at the starter. The sums are as follows,

Volts = 200 X 0.01 = 2 volts

So two volts is lost during starting. The Cortina starter may actually draw from 245 amps to as high as 340 amps at zero RPM. (Source; Ford Manual).

If we apply the same equation using 340 amps, we get 3.45 volts. Since you only started (excuse the pun) with 12 volts, that is a considerable loss.

Volts = 345 x 0.01 = 3.45 volts.

How to test for Voltage Drop

It is relatively easy to test for voltage drop. You need a multi-meter. The kind you get now are known as digital multi-meters, with the read-out being in numbers. Older meters were called moving coil meters, the most famous being the AVO.

Set your meter to 20 volts (See Note 1, below) and ensure that the circuit being tested is switched on. Put one meter wire on one side of the item being tested (for example the switch) and the other meter wire on the other side. You should get a very small reading, perhaps 0.3 volts. Anything more may indicate an excessive voltage drop.

In theory at least, voltage drops should not exceed:- 
200 mV (0.02 volts) in a wire or cable,
300 mV (0.03 volts) across a switch,
100 mV (0.01volts) across an earth/ground connection,
Note 1: Always select a range higher than the expected voltage. So, working on a car you might expect up to 13 - 14 volts. Normally meters will offer 2 volts or 20 volts -- so you must select the 20 volt range.

Some time ago, I converted my MK1 Cortina to Halogen headlamps. However, the lamps always looked "brown" when switched on and I always suspected there was something wrong. When researching this article, I used the multi-meter on the lamp circuits and found there was only 9 volts on the dipped headlamp circuit with the engine off, rising to 10.7 volts with the engine running. Clearly there was something wrong. (I measured 14.28 volts at the alternator output so it was doing its job all right).

I have a fuse box fitted, so I next measured the drop across the fuse for the dipped lamps. It was slightly above what I would expect, so I cleaned the fuses and contacts.

The problem turned out to be dirty connections at the "chocolate block".


The following table clearly shows the relationship between reduced voltage and reduced headlamp efficiency.

Nominal "12 volt system" is actually about 13.2 volts Efficiency of Halogen lamps
13.5 volts 100% efficiency
12.85 volts (95%) 83 % efficiency
12.15 volts (90%) 67% efficiency
11.5 volts (85%) 53% efficiency !!!


Using relays to Brighten-Up your existing headlamps

The following diagram shows how to wire relays into the main and dip circuits to provide an easier life for your headlamp switch and hopefully brighter headlamps.

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"Power" means a suitable power point such as the battery, alternator etc. The circuit uses the existing Cortina wiring from the main/dip switch.

If you want to fit spot lamps, which only come on when main beam is selected, pick up power from the wire shown in the diagram.

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