Technical Articles (during lockdown)

Installing relays into older classic cars


Why do enthusiasts fit relays in their cars? 

Well possibly for a couple of reasons, but the most benefit would be derived from modifying the headlight circuit. The benefits hopefully when relays are fitted to this circuit will be that the headlights will be substantially brighter. Could that be true? Well with a little testing the answer ought to be evident, after which we can look at how to fit the relays if required.

Besides benefits to the lighting circuit of brighter lights there is another benefit to fitting relays in other circuits. When switches are operated, especially in higher powered DC circuits, a flash can occur across the contacts of the switch. After a time this flashing (normally referred to as arcing) will cause severe burning to the switch contacts, causing poor operation and finally a complete failure of the switch. When relays are fitted into a circuit the power through the switch is drastically reduced and so arcing is suppressed and the switch will last far longer. This of course is beneficial to the headlamp switch, the dipswitch in the lighting circuit and also other switches such as the brake light switch all of which can be protected with the use of relays.

How does fitting relays protect switches? Well if the switch on the dashboard is only required to switch the load of the relay instead of the load created by the headlamps then the current flowing through the switch will be greatly reduced and the switch ought to be far more reliable. Let me point out at this point I am pretty sure that the vast majority of the dashboard switches on our cars are more than adequate for the job they do but I am simply describing the suggested benefits of relays. It can be pointed out that in modern cars most circuits are operated via relays so they must be better? Well of course that is true however in modern cars the relays perform other functions. They actually perform functions that we refer to as logic such as a number of different ways to operate a circuit over and above a simple switch. On top of this you will notice that modern dashboard switches are far smaller than the large toggle switches we find in older cars, the use of these smaller lighter switches is only made possible by the use of numerous relays.

 I hope that was not too complicated? Anyway back to the brighter headlights story. The brilliance of any bulb is determined by the voltage applied to it. Of course our headlights get around 12V and it would be reasonable to expect that this should always be the same. Well that is not the case because when the engine is running the voltage in the system rises and can be as much as 14,5 V, this is easily noticed when cars are used at night. When the engine starts and revs up the headlights can be seen to come quite a lot brighter. It is the way many of us check that our charging systems are working.

OK so that seems straightforward enough but if the headlights glow brighter with increased voltage, then they will also glow significantly dimmer were the voltage to reduce. What then could be the reasons why this voltage maybe reduced? If the light bulb was connected directly across the battery then it would be as bright as possible. However the light bulb is not connected directly across the battery but is actually installed at the front of the car. It is then connected to the battery by a circuit that contains a lighting switch, a dip switch and numerous connections (bullets). Each one of these switches and connections could reduce the voltage applied to the headlight bulbs due to poor connections leading to high resistance and reduced voltage.

Whilst my car is well modified in plenty of other areas I still make use of the original dip switch on the column and part of the original harness from just under the scuttle to the front of the car where there is all the original bullet connectors connecting wires to both headlamps. I decided to do some testing to assess any problems with my headlamp circuits and began by switching on the headlights, I then measured the voltage from the battery positive to the negative (body) (This demonstrates how much voltage is available). I then measured the voltage at a point in the front of the car where the original bullet connector splits the feed to both headlamps, if everything was working fine this ought to be the same value as the battery. However I was staggered the difference was more than 1 volt and was nearer 1.5 volts (that is more than a 10% reduction). I used a prong on my meter to poke into the bullet to make a connection to the meter as it is absolutely essential to have the lights switched on and operating to do this test. If the lights were to be disconnected to do the test and the measurement made then there would be absolutely no difference between the 2 readings and the test would be a waste of time. After doing the voltage reduction test I decided to do another test to be sure the readings were accurate. This time I connected a piece of wire to the battery positive, the other end of the wire I connected into the bullet where I had previously measured the reduced voltage. As I pushed it in and out of the bullet a very noticeable increase and decrease in brilliance of the headlight was easy to see, proving that voltage had reduced in the original circuit causing a dimmer performance of the headlights.

So where was the point of high resistance that was potentially causing my dim headlamps? Well to determine where the voltage is being lost sounds an easy task but is quite difficult in practice. What we need to do is to do the test described above at all connection points within the circuit remembering that the lights have to be switched on whilst doing the test. That means we need to access various switches and connections, many hidden behind the dashboard or tucked into the steering column. I managed to do most of them and I found out where my headlamp circuit was losing voltage and it was in one place only. The voltage was disappearing across the “dipswitch” which of course is located as a switch on the steering column. Those of you who are familiar with these switches will of course not be surprised by this loss and would probably have guessed that it would indeed be the problem area. I suppose I could have purchased a new switch but considering popular opinion on the quality of these items it is far easier and far cheaper to regain maximum headlamp brilliance by simply fitting relays.

What is a Relay

 A relay is a pretty simple device used to reduce the amount of current flowing through switches such as those on our dashboards. In modern vehicles this allows the switch to be constructed far smaller and whilst there are other reasons, this is certainly the most important. The switches fitted to most MGs however were designed to be able to operate at high currents and would therefore not require the use of relays, but installing relays in lighting circuits could give us more light and less switch damage if the original circuit is less than perfect.  

The relay consists of 2 circuits and therefore generally has 4 connections. The first circuit is known as the coil and it is the circuit that will be operated by the original switch. Within the relay this coil when energised (powered) becomes magnetic and attracts a metal former towards the core. This is connected to a contact in circuit 2 and therefore when this occurs the 2 connections on circuit 2 join together and will operate/power/switch whatever is connected to them and thus replicate the action of the original switch.

With reference to the photo of a relay it can be noted that 4 connections can be seen on the base. On inspection it can also be noted that most relays will have each of these connection numbered, these numbers are almost always the same and so will dictate how the relay should be connected. However what is not the same on all relays is the physical arrangement of the connections and so reference to the connection numbers is always necessary to ensure correct connection and operation. The numbering system is easy with the coil being on connections 85 and 86, whilst the contact is on connections 30 and 87. A very cheap supply for these relays is for you to take a trip to a scrap yard and you will find under the bonnet of all modern cars a fuse box that will contain a number of relays. These can be pulled out of the base of the fuse box and can be used for our modifications.

Fitting relays

To change the lighting circuit to relay operation requires 2 relays, one for the main beam circuit and the other for the dip beam circuit. First let’s concentrate on the main beam circuit. In both modified circuits we use the original wiring to operate the coil of the relay this on pin 85 with pin 86 now connected to earth. The original main beam wiring is coloured Blue with white tracer from the dip switch to the main beam headlight. This has to be connected to pin 85 on the relay so it either needs cutting at some point to make the connection or perhaps disconnected from the headlight and routed back. On my car I mounted the relays on my inner wing about halfway between the switch and the actual headlight. I stripped back the loom insulation and found the blue wire with the white tracer. Cutting the wire here I then needed to extend each cut end to make it long enough to connect to the relay on the inner wing. Looking in my spares/rubbish draws I found some blue wire with white tracer that I could use to make the connections, so for me I simply connected the extra wire to the relay terminals with soldered female spades and these wires were then soldered onto the cut wire in the existing loom. To insulate these new soldered connections I used heat shrink that makes a reasonably neat job and then re wrap the loom with a few turns of tape. So the connections on the new relay need to be connected these need the blue wire with the white tracer connected to pin 85, pin 86 to a new wire connected to the body earth, a new power wire to be connected to pin 87. This I ran from an always live connection on my fuse box alternatively this could be connected to the battery and an inline fuse used for protection. Then finally a connection from pin 30 requires connection to the original Blue wire with white tracer that connected to the headlight unit.

So to recap the existing blue/white wire was cut in the loom and a new piece was soldered to it and connected to pin 85 on the relay base with a soldered female spade. From pin 30 another new piece of wiring is then soldered onto the other cut piece of wire in the loom that is connected to the headlight.  To finish the wiring installation a new live wire is connected to a new always live fuse and connected to pin 87 and finally the new earth wire is connected to pin 86 and the car body earth.

This effectively completes the wiring for the Main beam and now the dip beam can be modified. This is very similar to the modification of the main beam circuit except instead of working on the blue with white tracer wiring simply substitute blue with red tracer. The new power wire that was installed from a new always live fuse can be split near the 2 relays and used to power both circuits. The source of this new supply should be from an always live fuse in the fuse box or better still connect a new wire to the battery cable (probably best fitted at the starter solenoid) then through a new inline fuse before connecting to the relays.

The connections I make with the spade connections on the relay I always do with a soldered connection. I find the mechanically crimped connections very poor and to make these a quality soldered joint is very easy. Instead of crimping these simply solder the wire into the crimp and then I use a short piece of shrink wrap to insulate the spade terminal.

This work I did on my Midget, however fitting relays is very similar on all of our models. The colours on the Midget are typical Lucas colours and are the same as many vehicles of the era. However what ever vehicle you intend to modify it is important to make sure of the relevant colours of the wiring before commencing the task.

Finally this technique can be used to modify any such circuit in the very same manner as the headlights. As an example if you wanted to protect the brake light switch with a relay then simply find the green wire with a purple tracer and modify it exactly as you have done to the headlight circuit.


Replacing the metal coolant pipes on an MGTF – Robert Taylor

The MGF and MGTF are renowned for overheating issues and one of the reasons is that the volume of coolant is small especially as it has to be piped from the rear engine to the front mounted radiator and back again. A key part of this process is the approx. 2m long metal coolant pipes that run under the car. These pipes are prone to rusting  and, as stainless steel replacements are readily available, I decided to tackle the replacement. It is easy to get an air lock in the coolant system so before tackling the work ensure that you are aware of the correct bleeding procedure as explained later.

I first of all raised the car front and back using ramps and axle stands. Set the heater controls to maximum so as to drain all or most of the coolant which will later be replaced with fresh coolant. Remove the expansion tank filler cap. There is no drain plug so release one of the hose clips on the metal coolant pipe and allow the coolant to drain into a container.

There is a thick underfloor strengthening plate secured with approx. 24 bolts (see photo1).

Photo 1

Photo 2

Photo 3

Photo 4

Photo 5

Photo 6

Photo 7

This needs to be removed to gain access to the metal coolant pipes. Next, cut the cable ties that secure the main feed cable from the battery that runs the length of the underside. Having allowed the coolant to drain the other hose clips can be released and moved back on the rubber hoses. I purchased replacement jubilee type hose clips in case they were needed but in fact the original spring type hose clips were serviceable and retained for originality. It is now possible to undo the 3 retaining bolts and remove the metal coolant pipes (see photo2). The cooling system can now be flushed with clear water and the radiator reverse flushed.

New cable ties were supplied with the stainless steel coolant pipes but check before fitting that when the main feed cable from the battery is cable tied in place there are no sharp edges likely to cut into the cable. I filed down some sharp edges before fitting. The new coolant pipe can be bolted into place, the rubber hoses fitted and secured with the hose clips and the underfloor strengthening plate refitted. Ensure all the bolts are replaced and correctly tightened as this is a structural item evidenced by the number of bolts used.        See Photos 3 and 4 showing the front and rear connections to the rubber hoses. It goes without saying that the rubber hoses should be checked for condition and replaced if necessary at the same time. However, the hoses on my 2005 TF were in good pliable condition with no sign of cracking.

The recommended antifreeze is ethylene glycol based containing OAT corrosion inhibitors which is pink in colour. The amount required is 5.25 litres and a concentration of 50% is required to ensure the anti-corrosion properties are maintained and not above 60% as cooling efficiencies will be impaired.

It is important to follow the correct re-filling and bleeding procedure otherwise an air lock could occur resulting to overheating.

1.      Turn the heater control to max heat.

2.      Remove the bleed screw from the top of the radiator (see photo 5)

3.      Open the heater bleed screw (see photo 6) found behind the black plastic cover at the rear of the front storage compartment.

4.      Fill the system from the expansion tank taking care to keep the expansion tank full to prevent air entering the system.

5.      When a constant flow of coolant is being emitted from both bleed points close the radiator bleed screw and then the heater bleed screw.

6.      Ensure the expansion tank is topped up, fit the cap and run the engine until the engine reaches full operating temperature. (Note: Do not operate the air conditioning if fitted.)

7.      Check the cooling system for leaks and that the heater is emitting heat. If the heater is not emitting heat there may be an air lock and the “Additional bleed” procedure should be undertaken.

8.      This involves removing the engine compartment access panel, releasing the clip and removing the inlet air hose from the throttle housing. This gives access to the additional bleed screw on the radiator return valve.

I suggest taking the car for a short test run to ensure the cooling system is working correctly before embarking on a journey.

Whilst I had the black plastic cover removed in the front storage compartment I took the opportunity to remove the screen washer bottle and give it a thorough clean out to prevent future blockages of the screen washer jets. A minor maintenance task that saves much frustration if overlooked.