Whilst in between the two stages of upgrading my A/C panel to LEDs, as per the previous two blog posts,
I got into a conversation with club member Nick T regarding the method by which I could test the Cooling Fan Fail bulb. He mentioned that there was a mod I could do to enable the light to come on when the fans operate, rather than when they fail. As I've had many overheat issues due to a couple of Cooling Fan Relays failing I would welcome the peace of mind this solution would bring me, to know my fans were activating during a journey, and if not I could then switch on the fans via my new override switch. This solution is possible since the Fan Fail Module relay in the fuses and relays compartment (blue socket on the left) has been removed during restoration and a common modification installed, or at least this is when I believe this mod to have been installed. The mod can be seen in the image below and comprises the black wires plugged directly into specific relay socket connector slots - The logic for this will be explained further in this post.
Remembering that this blog is primarily to document my learning journey as I work on 16606, it is therefore also useful for others, similar to me who are not experienced and requiring to gain the benefit of my experiences, to help them complete similar tasks. Therefore I feel it necessary to explain what I first had to learn about what "relays" actually are. This will help understand the very simply modification I have installed.
Relays
I found the following site extremely good at explaining to me what a relay is and can do. I will extract the pertinent parts of their article below whilst linking to the original.
Relay Guide
Overview
What is a relay?
A relay is essentially a switch that is operated electrically rather than mechanically. Although there are various relay designs, the ones most commonly found in low voltage auto and marine applications are electro-mechanical relays that work by activating an electromagnet to pull a set of contacts to make or break a circuit. These are used extensively throughout vehicle electrical systems.
Why might I want to use a relay?
There are several reasons why you might want or need to use a relay:
Switching a high current circuit using a lower current circuit
This is the most common reason and useful where an in-line switch or the existing circuit does not have the capacity to handle the current required. For example, if you wanted to fit some high power work lights that come on with the headlights but there is a risk that they would exceed the capacity of the existing loom.
Cost saving
High current capacity wiring and switches cost more than lower current capacity versions, so by using relays the requirement for the more expensive components is minimised.
Activating more than one circuit from a single input
You can use a single input from one part of an electrical system (e.g. central locking output, manual switch etc.) to activate one or more relays that then complete one or more other circuits and so carry out multiple functions from one input signal.
Carrying out logic functions
Electromagnetic relays can be put to some quite clever (and complex) applications when linked up to perform logical operations based on certain inputs (for example, latching a +12V output on and off from a momentary input, flashing alternative left and right lights etc.). Although these logical functions have now been superseded by electronic modules for OEM designs, it can still be useful, fun and often more cost effective to use relays to perform them for some after-market projects (particularly where you have a bespoke application).
Note: In this article we are going to focus on ISO mini or 'standard' relays which have a 1" cube body and are the most commonly used in vehicle electrical systems.
Construction and operation
Inside a relay
This is what the inside of an ISO mini relay looks like:
A copper coil around an iron core (the electromagnet) is held in a frame or 'yoke' from which an armature is hinged. One end of the armature is connected to a tension spring which pulls the other end of the armature up. This is the relay in its de-energised state or 'at rest' with no voltage applied. The braided bonding strap provides a good electrical connection between the armature and yolk, rather than relying on contact between the armature pivot point alone. The coil and contact (or contacts) are then connected to various terminals on the outside of the relay body.
How they work
When the coil is supplied with voltage a magnetic field is generated around it which pulls the hinged armature down onto the contact. This completes the 'high' current circuit between the terminals and the relay is said to be energised. When voltage is removed from the coil terminal the spring pulls the armature back into it's 'at rest' position and breaks the circuit between the terminals. So by applying or removing power to the coil (the low current circuit) we switch the high current circuit on or off.
Note: It is important to understand that the coil circuit and the current-carrying (or switched) circuit are electrically isolated from one another within the relay. The coil circuit simply switches the high current circuit on.
The following simplified circuit diagram is often used to easily understand how a relay operates:
Relay terminology
The ISO mini relay we have looked at above has 4 pins (or terminals) on the body and is referred to as a make & break relay because there is one high current circuit and a contact that is either open or closed depending upon whether the relay is at rest or energised. If the contact is broken with the relay at rest then the relay is referred to as Normally Open (NO) and if the contact is closed with the relay at rest then the relay is referred to as Normally Closed (NC). Normally Open relays are the more common type.
ISO mini relays with two circuits, one of which is closed when the relay is at rest and the other which is closed when the relay is energised, have 5 pins on the body and are referred to as changeover relays. These have two contacts connected to a common terminal.
Make & break relays are also known as Single Pole Single Throw (SPST) and changeover relays as Single Pole Double Throw (SPDT). This is based on standard switch terminology. There are other contact configurations discussed below but make & break and changeover relays are the most commonly used.
Terminal numbering convention
The terminal numberings found on a relay body are taken from DIN 72552 which is a German automotive industry standard that has been widely adopted and allocates a numeric code to various types of electrical terminals found in vehicles. The terminals on the outside of a 4 or 5 pin mini relay are marked with numbers as shown below:
Terminal/Pin number | Connection |
85 | Coil |
86 | Coil |
87 | Normally Open (NO) |
87a | Normally Closed (NC) - not present on 4 pin relays |
30 | Common connection to NO & NC terminals |
According to DIN 72552 the coil should be fed with +12V to terminal 86 and grounded via terminal 85, however in practice it makes no difference which way around they are wired, unless you are using a relay with an integrated diode (see more info on diodes below).
Tip: you can use a changeover relay in place of a make & break relay by just leaving either the NO or NC terminal disconnected (depending on whether you want the circuit to be made or broken when you energise the relay).
Terminal layouts
The automotive ISO mini relays we have been looking at above are typically available in two types of pin layout designated Type A and Type B layouts. These layouts are shown on the two 5-pin relays below (pin 87a not present on 4 pin relays):
You will notice that on the Type B layout pins 86 and 30 are swapped over compared with the Type A layout. The Type B layout is arguably easier to work with as the connected terminals are in-line, making the wiring easier to visualise. If you need to replace a relay make sure you use one with the same terminal layout as it is easy to
overlook if you're not aware of the difference.
Terminal sizes
The terminal widths used on 4 and 5 pin relays are almost always 6.3mm wide, however some more specialist relays can have terminal widths of 2.8mm, 4.8mm and 9.5mm. The 9.5mm wide terminals tend to be used for higher power applications (such as for starter motor solenoid activation) and the smaller terminals tend to be used for electronics signalling where only very low currents are required. All widths will be compatible with the standard female blade crimp terminals of the corresponding sizes.
Relay body markings
Relays can look very similar from the outside so they normally have the circuit schematic, voltage rating, current rating and terminal numbers marked on the body to identify them.
Circuit schematic
This shows the basic internal circuits (including any diodes, resistors etc.) and terminal layout to assist wiring.
Voltage rating
The operating voltage of the coil and high current circuits. Typically 12V for passenger vehicles and small craft but also available in 6V for older vehicles and 24V for commercial applications (both auto and marine).
Current rating
This is the current carrying capacity of the high current circuit(s) and is normally between 25A and 40A, however it is sometimes shown as a dual rating on changeover relays e.g. 30/40A. In the case of dual ratings the normally closed circuit is the lower of the two, so 30A/40A, NC/NO for the example given. The current draw of the coil is not normally shown but is typically 150-200 mA with a corresponding coil resistance of around 80-60 W.
Tip: Knowing the coil resistance is useful when testing the relay for a fault with a multi-meter. A very high resistance or open circuit reading can indicate a damaged coil.
Terminal numbering
The numbers 85, 86, 30, 87 & 87a (or other numbers for different relay configurations) are normally moulded into the plastic next to each pin and also shown on the circuit schematic.
Relay configurations and types
In addition to the basic make & break and changeover configurations above, ISO relays are available in a number of other common configurations which are described in the table below:
Configuration | Circuit schematic * | Description |
Make & break relay |  | The most simple form of relay. The circuit between terminals 30 and 87 is made on energisation of the relay and broken on de-energisation, known as NO (or vice-versa for a NC relay). |
Changeover relay |  | Two circuits (terminals 87 and 87a ) have a common terminal (30). When the relay is at rest 87a is connected to 30, and when the relay is energised 87 becomes connected to 30 (but never both at the same time). |
Relay with double output |  | Terminal 87 is linked to pin number 87b, giving double outputs from the single NO contact. |
Relay with dual contacts |  | The armature contacts both terminal 87 and (in this case) 87b at the same time when the coil is energised, creating a dual NO output |
Relay with integrated fuse |  | A blade or ceramic fuse is connected between terminal 30 and the NO contact, providing built-in protection for the high current circuit. The fuse is normally mounted in a holder moulded as part of the relay body so it can be replaced if it blows. |
Relay with diode across the coil |  | When voltage is removed from terminals 85/86 and the coil is de-energised, the magnetic field that has been created around the coil collapses rapidly. This collapse causes a voltage across the coil in the opposite direction to the voltage that created it (+12V), and since the collapse is so rapid the voltages generated can be in the order of several hundred volts (although very low current). These high voltages can damage sensitive electronic devices upstream of the +12V coil supply side, such as control modules in alarm systems, and since it's common to take low current alarm output signals to energise relay coils, equipment damage is a real risk. Using a relay with a diode across the coil can prevent this damage by absorbing the high voltage spikes and dissipating them within the coil/diode circuit (this is known as a blocking or quenching diode). The diode will always be installed in the relay with the stripe on the diode body facing towards terminal 86 (reverse biased) and it is important that +12V is connectedthis terminal (with 85 connected to ground) or the diode could be damaged. |
Relay with resistor across the coil |  | A high value resistor performs a similar function to that of the diode in the previous configuration by absorbing the high voltage spikes created by the collapsed magnetic field on de-energisation of the coil. The disadvantage of a resistor is that it allows a small current to flow in normal operation of the relay (unlike a diode) and is not quite as effective as a diode in suppressing voltage spikes, but it is less susceptible to accidental damage because resistors are not sensitive to polarity (i.e. it doesn't matter whether +12V is connected to terminal 85 or 86). |
* All schematics shown with the relay at rest (de-energised)
From the above article I identified that the type of relay that would be in the Fan Fail Relay socket would be a Change-over Relay.
Fan Fail Module Relay Sock Mod
So now, with an appreciation of what a relay is and how they work, let's focus on the Fan Fail module relay socket that has had a mod installed. Firstly, where in the circuit is this relay and what is its purpose?
The extract of the DeLorean wiring schematic, above, shows the cooling fan circuit controlled by the Otterstat. In my terminology, the circuit works as follows,
As the engine is running, should the coolant temperature reach a certain level the otterstat (which is located in the coolant flow and takes its temperature) activates the cooling fans via a switch within that creates a circuit of the Green wire to the black/orange wire.
With the power flowing through the activated Otterstat this feeds into the Cooling Fan Relay (I'm glossing over how this relay is wired)
From the Cooling Fan Relay power will flow via the circuit breaker ("Cooling Fan Thermal Trip) and on to the Fan Fail Module (outlined with the red box)
The power arrives at the Fan Fail Module at pin 87.
The module is energised to the NO - Normally Open state that then feeds the power to both Cooling Fan Motors.
Should the fans fail whilst power is being sent to the Fan Fail Module via the Otterstat then the circuit in the module will no longer be energised and it will move to a Normally Closed state with the switch moving from pin 87 to 87a.
Pin 87a has a wire feeding the Cooling Fan Fail light (outlined with the amber box) on the A/C panel. This will illuminate when the fans fail since the power source is diverted to this light via pin 87a
But I no longer have the Fan Fail Relay Module installed. Instead I have a modification, so what does this modification do?
There are essentially two black wires, one plugged in to the connector sockets where the relays' pin 85 and pin 86 would fit. From the underside of these sockets run the wires to each of the two cooling fans.
The opposite end of these two black wires are joined together into a connector that plugs into the connector socket where the relays' pin 87 would fit. Into the underside of this connector at position 87 runs the power feed from the circuit.
So the result of this mod is that the power is received at connector 87 and distributed directly to the fans via connectors 85 and 86.
The Cooling Fan Fail light is no longer operational since without the relay it can no longer receive power if the fans fail.
What is the mod I can do to illuminate the Cooling Fan Fail light?
I had previously been advised that if I ran a wire from the circuit breaker and plugged the opposite end into the connector socket at 87a and then turn my cooling fan override switch on, the cooling fan fail light should come on, but now indicating that the fans had been activated. Unfortunately this did not work for me, probably due to the fact that it was difficult to firmly attach the wire to the circuit breaker.
I had left it a couple of days and returned to it today. I stared at the changeover relay and realised that if I ran a bridging wire from connector socket 87 to 87a then when the fans receive power to activate them, power will also flow to the "Cooling Fan Fail" light. This solution allows me to make a simple connection by taking both black wires from position 87 and add the blue wire into a single connector and re-inster it into position 87. The other end of the blue bridge wire will also have a connector attached and plug into 87a. The resultansolution can be seen in the following image.
To test the solution I turned the ignition key to position two and then as I switched my cooling fan override switch on the "Cooling Fan Fail" light on the A/C panel illuminated. See image below.
With the up-coming journey to Belfast, this additional mod is very welcome for peace of mind during the journey.
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