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MG MGA - wiring harness tips?

I'm installing a new wire harness, does anyone have any tips to offer? The section from the engine bay to the rear, along the frame rail, is installed, and I have wired the tail lights and license plate light. The section for the front lights, under the bonnet latch bar, is also installed. No problem with these, but now I have to tackle the main harness. I plan to try to follow the schematic as found in the shop manual, is anyone aware of any errors in this? Any help from others' experience is appreciated!

Thanks, George
George G.

I just finished mine. Unfortunately, I’m away on vacation and not near my notebook of resources. The first place I would look is Barney Gaylord’s site

http://mgaguru.com/mgtech/electric/electr_1.htm


Frank Bruns
F. J. Bruns

Good time to put in fuses and relays. I found a fuse block that had mounting holes that matched the holes for the voltage reg so I did not have to drill any new holes in the fire wall it holds four fuses. So you can fuse the horn, head lights, other lights, coil and fuel pump. Install relays for the horn head lights, and brake lights.
I mounted a terminal block (same hole spacing) on the drivers side and moved the three wires that are jammed onto the one connector of the ignition switch to the terminal block. Then I ran one larger (#12) wire from the terminal block back to the ignition switch.
I wired the fog light switch to control the power to the coil and fuel pump. I can now turn the power to these off when I park (anti theft) or need to work on the wiring with out the fuel pump or coil running.
Use wire from your old harness so you can match the color coding.
When you are first checking your wiring use a power supply that only puts out a couple of amps (battery charger). That way you will not fry your new harness if some thing goes wrong. This might not be enough amps to trigger the flasher unit on the turn signal.
Todd Clark sells a large lamanted wiring diagram.
Good luck Steve
S E Bryan

While you are doing the rewire, why not fit a battery isolator switch in the kickboard behind the passenger seat if you don't have one. That makes the odd electrical job much easier, particularly as getting to the batteries on the A to isolate them is a real pain if the hood is folded.
Graeme Williams

I agree with all the suggestions. I would have one change, though. Using a low amperage 12V supply, you may get some confusing results as you approach the limit of the supply with the voltage drooping considerably from 12V. What I did was to buy an inline fuse and fuse holder at the local Auto Zone and isolate the battery from the rest of the car through the fuse. I disconnected the main battery line at the starter switch and put the fuse between the starter switch and the main line. Using a 3 amp fuse will insure that no more than 3 amps will be carried by any of the wiring in the car, but maintain full battery voltage.
F. J. Bruns

Frank, I want to be sure I understand your suggestion. Did you put the in line fuse on the large battery cable at the starter switch, or in the brown smaller wire that also attaches to the starter switch? Is 3 amps sufficient for the starter to turn?

Thanks, George
George G.

THere are a couple of threads on the Sprite section at the moment about fuses in the electrical circuit. There seemed to be a reluctance in the 50s and 60s to fuse lighting circuits and I see the A in no different to the sprites in that matter. Was there a deliberate decision to remove the risk of a fuse failure cutting out the lights at a critical moment, or merely a matter of keeping the cost down? At least if there was a direct short taking out the lights, the resulting flames would illuminate the road ahead until you came to a stop.
Graeme Williams

I used die-electric grease on all connections to prevent corrosion. I put in a fuse box inside the car and bolted it to the firewall using holes already there. Use relays to carry low current to the dash switches. Break down every circuit with a fuse to protect the wiring harness. As stated, use an inline fuse to power the car BEFORE the battery is hooked up to protect your new wires. I used a resettable fuse so I did not have to replace fuses during testing of the whole system. Hook up the negative wire to your car battery and hook up the inline fuse to your main wiring harness at the starter and then hook that wire to your battery to protect your new system so the smoke does not leak out. Good luck

Jeff Becker
JEFF BECKER

George -- As Jeff said, the fuse goes in between the large wire from the battery and the starter switch. The idea is to have the fuse blow (instead of smoking your wiring) if you make a mistake and short something out. No, the fuse is not big enough to support the starter. The starter is the last thing you test, and the fuse is removed and the wire re-connected.
Frank
F. J. Bruns

My secrets to a fast and easy wiring loom exchange

1. Sit down at a well lit table with a wiring diagram and the loom. Using a test meter, identify each cable and mark it with a flag of masking tape saying what it is. This makes life much easier when attaching the various cables in the car. I still do this although I can pretty much label a loom without the diagram these days. I have just labelled up the 1600 wiring loom like this in 30 minutes, which will be time easily saved when installing.

2. Cut off and keep all the dash bulb holders from the old loom, complete with their cable stub. Then using a soldering iron, remove the small brass ferrule from the end of each cable stub. Then remove all the new bulb holders from the new loom (again using the soldering iron and then you have a few ferrules over to cover the lost ones)- on the 1600 loom I will have to cut these off, as they are crimped not soldered..... The old holders are good and hold the bulbs nice and tight, the new ones are in my experience cr*p. This is confirmed by the new 1600 loom.

3. Cut the old loom apart to make removal easier. I try and rescue the longer lengths of original wire with the colours for front sidelights and indicators, and for bridging between the horns. This is because these wires are never in any kit, even if it claims to be a full harness. This again is the case with the newest loom on the bench, which has cables as far as the inner wings and no further.

4. Buy the correct tool for the connectors. It makes life MUCH easier, and causes less potential for injuries than when the alternative small screwdriver slips and spears a hole in your palm. See the image attached

5. Buy extra connectors - every MGA loom I have ever installed has always been short on connectors, some have none at all!

6. The baffling left over wires at the dash which have no connectors go to the horn button, which has to be unmounted to get at the screw terminals!

7. For the lighting switch, there are too many red wires to be able to get them all easily into the same terminal at the light switch. But one of them goes to the F switch, so you can make life easier by moving one of the the cables to the same terminal at the F switch, and that saves a lot of fiddling

8. The red wire to the map light switch is often missing, it is just a bridge wire from red terminal of F or L switch to the map switch.

9. Be prepared to find other electrical components are beyond saving as you disconnect them. They are usually indicated by bodges around them.

10. I always modify to -ve earth and add an under dash 12V socket. With modern phones, GPS etc, it makes life much easier for the owner.

11. I always add a fuse for the lighting circuits behind the dash.

12. Avoid Lucar push on terminals wherever possible, use a good crimp tool and add a ring connector for a goo connection (e.g. at the coil). The flat metal things are NOT a good crimp tool. I prefer to crimp and then solder. Don't use the crimp connections with a coloured plastic collar - if that is all I can get, I cut the collar away before crimping for a much better job.

13. Use heat shrink tubing to add resilience to any modifications or additions to the loom. This also looks much better than a raw crimp.
dominic clancy

The forgotten picture


dominic clancy

Thanks Dominic. I'm doing #1 now, and have the tools needed, #4,5. I'm labeling the wires by matching the colors to what's shown on the schematic, so far so good I think. This is a new harness being installed as part of a complete restoration. I had left several inches of wire from the original harness on instruments etc for later (now!) reference. I do intend to fuse the lighting circuits, I had planned to install in line fuses, do you have an opinion on those? Did you add fuses to any other circuits?
Your comments are greatly appreciated!

George
George G.

I do what the owner wants. I either add on-line fuses behind the relevant switch (which although not always easy to get at is at least logical). I have also added fuses below the starter switch - this is a good location as it removes the need for any mods to the loom itself, just add bullet connectors and wire to splice in - old wiring loom remnants provide the correct wire colours. You can never have too many fused circuits, although it is only now on the winter list for my own car! I need to tidy up the various changes behind the dash and make it a bit less of a rats nest.

For the 12V auxiliary connections I have now given up on the cigarette lighter sockets and started to use a sealed module which has two USB connectors on pigtails : see eBay item 141614621473 These have the advantage of delivering more current than the cigarette lighter adapters, and there's a much more secure connection. Cheap too!




dominic clancy

For me there is a step 1A in Dominic's list:
"Get wife to check colour"
I am starting to rewire my Frog and being colour blind adds a new dimension to the problem. Some wires are black, some are white.... but the rest?
Graeme Williams

Graeme, best of luck, I feel sure you'll get it right!
George G.

This thread was discussed between 18/08/2015 and 26/08/2015

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