Relay related lunacy

Puuh – planning the future wiring took much longer than expected. And at first glance the new plan might look wild. And it is not the final. But once you study it – you might see the method behind my madness. If not – don’t worry and read on – this is going to be a long explanation. In case you are also considering some (not necessarily all) changes to a MGB wiring – some of the ideas might be helpful.

Last year, while working on the wiring plan for the Morgan, I used a freeware called TinyCad and tried to create my own plan. Even though I tried hard to document all the changes I had to make – I know that the final wiring has some quirks not documented in my TinyCad file. Especially the fact that it is not possible to draw multicolored lines is a limitation of the software. When following BSAU7 – multicolored cables are the standard

Holger Beck, who has an excellent blog on his MGB restoration, posted his wiring diagram almost exactly 12 years ago. He modified a Bentley or Haynes version to his needs. I don’t know what software he used to do this. I played around with Microsoft Visio – but either my Visio skills or the usability of the software were not good enough. I also started to create all symbols from the Haynes plan as TinyCad symbols – but setting the connector points is a tedious task. After a few hours I gave up on this.

So, the weapon of choice was paint.net. The starting point of this plan is a copy of the 73/74 UK market MGB plan from Haynes which is available as PDF on the web. I printed the required page as PDF and opened this file from the explorer with Microsoft Word. Sidenote: In the past you could open pretty much any filetype from the file open dialog in Word – but for some frickin’ reason this is not possible anymore. Explorer right click “open with” works.

Once opened in Word I could save the wiring diagram as PNG, which gave me the best quality to start with. In Paint.Net I created a background layer (just white), an original plan layer and one for my modifications. What followed was an extreme cut, copy & paste orgy that lasted for a few nights. This included a major detour and a return to the beginning in terms of relays and fuses. Just before Christmas I already bought two inexpensive, semi-modular relay & fuse boxes. I started off with one box with 4 relays and 12 fuses – then had the idea to use 5 relays and 9 fuses and bought a second one.

It was again a comment on Holger Beck’s blog that helped me to decide how many relays I need and why. He also bought a new fuse box. While this box it not available at AutoSparks anymore – I was able to identify it as an MTA modular main distribution unit. For two nights I planned to get a 4 unit MTA box. Two 6-way micro relay panels, one 2-way maxi relay and one with 20 micro fuses – called Minival in MTA product terms.

The dimensions of this box are 160mm by 131mm – so roughly twice as large as the fuse box I bought. If I read his blog right Holger Beck wants to put the box inside the passenger footwell and bought a custom build loom at AutoSparks.

My plan is to keep and modify the existing loom. As I wrote before it is in relatively good condition and after working on the Morgan and Mini wiring, I feel confident enough to do this myself. But I want to keep the fuse box in its location in the engine bay. Mainly because this will require less changes to the existing loom. As I need to modify the small metal panel that the fuse box is mounted to anyway – I could create a larger version for the MTA box.

But in the end, I decided against this for multiple reasons. Firstly, even if it fits – the large box MTA box would look awkward in the engine bay. OK – you could argue that the one I will use is also not original – but it is only twice as big as the Lucas one. Secondly, if you look at the left fuse box in my plan, you will see that the old wiring in this area is not changed. Instead of 4(+2 inline) I now have 12 fuses. Four of them being used for the relays that are all necessary – well at least beneficial – in the engine bay.
The left side of my wiring diagram is a rearrangement of the existing plan for the most part.
Finally – getting the MTA box might take a while. I found multiple places in the UK but not vendor in Germany where I could order online easily.

Back to the loom: Besides the relays the headlight, the added cooling fan and the fog lights – the biggest change is the hazard and indicator cabling. For this I used the same design that can be found in the late MPI Mini Coopers from 1997 to 2000. I prefer this design over the old one. I just noticed that I forgot to add two P600A diodes to the cable from the hazard switch. The late Mini hazard switch has these built-in afaik.

The fuel pump will be connect to an added Inertia switch and will get a fuse – somebody added an inline fuse just in front of the fuel pump on Mildred – they must have had a reason. The heater motor will get a PVM motor speed controller – an idea/recommendation I got from a MGDC forum post.

The right fuse box will be added behind the dashboard. This is basically for all “gimmicks” that I might add or not. I decided to use slate as main color for everything that is not “standard”. Slate as cable color was used in BSAU7 as “windows lift main lead”. Slate plus an additional color not used at all.

To get enough power to this secondary fuse box while avoiding feeding everything through the ignition switch – I had to add another relay. This in turn, however, will mean I need more than 8 relays – which are the available combined slots in the fuse boxes.
To solve this – I will remove the relay I added for the horns. This is the least important one. And if I ever get a heated windscreen for this car – the necessary relay will get mounted outside the fuse box behind the dash.

Next up is to create a new mounting bracket for the engine bay fuse box. I need to determine the correct position to get the correct place for the connector changes required at this location.