Wiring and Electrics

The wiring is seen by many to be the most difficult part of a K series build.  This is true.  I got mine right the first time (apart from the fan relay) but that was after a lot of research.  At some point I may endeavour to create a wiring diagram when I have some time.  I made my own satellite fusebox for the engine parts while retaining the Midget’s original fuses for all the non-engine related electrical components.

The engine loom is stripped from the Rover back to the Grey multiplug (loom connector 1).  This wiring forms the basis of the loom, and actually only needs to be connected at a couple of places.

The left over bits of loom looks a little like this:

Here’s a copy of my scribblings in Excel format with regards to wiring: Connectors_ECU_MFRU_loom_connectors

and a list of how my fusebox is wired in:fusebox

and now a picture of how it’s wired in:  (which I think is right)

Power Distribution

The 12V live to power the engine loom needs to be taken from the un-fused side of the original Midget fusebox.  This is then connected to a new satellite fusebox.  The lives from here then set off to the various parts of the Rover loom.  It’s wired like this:

Live side of Midget Fusebox Fuse Rating Colour What’s it do? Where’s it go?
20 NS O2 Sensor O2 Sensor
30 NS Starter Starter
15 NW Fuel Pump Fuel Pump Relay (MFRU)
N 10 P 5AS 5AS box
10 NK ECU Common Live
20 PU Fan Live Fan Relay Live Pin

 This fusebox lives in the engine bay.


All my earths go to one place on the bulkhead.  By the time the thing is wired in, there should be about 5 or 6 different earths from various places, so it needs to be a good one.  The engine earth must not be forgotten either, mine goes from the original place under the car and bolts to one of the lower bellhousing bolt holes.

Ignition Switch

The switch was a decision between whether to use the Midget’s original switch or the Rover one.  I chose to use the Rover one for the more secure key and the fact that the immobiliser ring already fitted the body of the switch.  Both switches fit the steering column so that the steering lock works.  The Rover switch is a confusing thing, having two lives in that supply different parts of the switch, and also by having all the colours on the wires mixed up, presumably to make hotwiring it hard.  This caused a bit of confusion, so here’s a table of what’s what:

Ignition Switch wiring

Midget Rover Midget
WB RW Pos 1 Aux GW Join TO Midget Loom
Feed TO ignition switch from Midget Loom N W BY Pos 2 Ignition W Join TO Midget Loom
W BW Pos 3 Crank RW Join TO Midget Loom

Join the W wire that powers the Midget ignition switch to the three in the IN column.

The Rover outputs different colours than normal – Join the OUT wires to the CHANGE TO wires present in the midget loom.  These wires used to plug into the old ignition switch.

The switch body needs a little modification to fit the midget column – the steering lock needs making smaller to fit in the hole in the column.  This is sort of easily achieved with a chisel and some patience.

Don’t forget to fit the immobiliser ring on the outside of the switch.

5AS Immobiliser Box

This little box lives (hides) under the dash in the Rover, and controls the immobilisation of the engine.  It needs to talk to the ring on the ignition switch and the ECU.  It has an unfused live, which I took from the live side of the original Midget fusebox.

A lot of the wires on the 5AS are not needed; they control things like electric windows, central locking, and alarm stuff that just aren’t used.  The connections needed are (pin numbers from left to right):

Which Pin? What colour is it? What does it do? Where does it come from?
Pin 1 W Ignition sense Ignition switch, Pos 1
Pin 2 B Earth Body
Pin 4 B Earth Body
Pin 9 US Immobiliser signal ECU
Pin 19 B Earth Body
Pin 26 N 12V live feed Fusebox, unfused side

It probably doesn’t need all those earths, but without being sure which one does what bit, it’s safer to get the lot of them.

Rev counter

The Midget one is currently in use, it stays wired in exactly as it was in the old system.  My rev counter is the RVC variety that measures current in the white ignition line.  Currently under investigation in the use of the Rover rev counter as it is the voltage variety.  My Midget counter seems to be inaccurate above 6Krpm – hence using the Rover one.  This is currently being constructed.


I’ve used the electric fan from the Rover.  It’s controlled by the ECU with a wire COLOUR that goes to the relay along with the live from the satellite fusebox.  It’s mounted inside the front valance – the horns have been moved slightly.

Fuel pump

The fuel pump wiring uses the wire that was the live from the old SU pump.  The front end of this wire is joined to the fuel pump relay out wire of the MFRU (COLOUR).  The ECU controls the fuel pump relay switching – this is already in the engine loom so doesn’t need to be cut/rewired.  The same relay controlled feed powers both Low Pressure and High Pressure fuel pumps.

Starter Motor

This is wired the same as was in the rover.  The main battery feed comes to here from the battery in the boot, and then from here meets the old midget feed to the live side of the fusebox.  The solenoid is sat on top of the starter motor and is fed with the start signal wire from the MFRU.  The MFRU gets the relay input from the ignition switch


The alternator is joined to the old midget loom exactly the way the A series alternator did.

There needs to be a diode in the brown and yellow wire that feeds the ignition/charge lamp.  If this diode isn’t there, then the car will not stop running, even after disconnecting the battery.  The diode I used was one I found – you can buy them for 100 for 50p from various places.


The battery (from the Rover) has gone in the boot, as the space where it used to be is full of heater.  It lives over on the passenger side allowing both a spare wheel (145/80/13) and a spare fuel can.  The electricity reaches the front via a fat cable from a mini, and is earthed in the boot.  The cable is carefully attached so it can’t rub on anything.

Emerald ECU Upgrade

I managed to obtain an almost brand new Emerald ECU (the K6) for not as much as brand new.  This coincided with a full re-wire – a few years of use with the “wire it up and be surprised that it works so chuck it all in and carry on” wiring had resulted in a trip to a car show that was only successful with a plastic takeaway lid jammed under the loom to stop fuses blowing.

I rewired the whole lot, buying a loom connector for the Emerald, a load of connectors and wire. I also built a fuse box that brought all of the fuses into one place (instead of having cylindrical glass fuses for the car systems and blades for the engine).  There is also a relay pack in there too, providing individual 30A relays to replace the ones in the Rover relay box, one of which burnt out when overusing the starter motor when a fuel pump earth went bad.  The Emerald is a great piece of kit, I still need to get the cold start to play properly.

If you like tinkering, then an ECU you can tinker with is great.  The MEMS is a good ECU, and learns and adapts nicely, but the Emerald is quicker to react and makes a bit more power.  It’s also fun to mess with.  The rev limit is set at 7200prm, which is where the power drops off. It regularly sees this, and the little 1400 is happy to get there on just about every trip out.