Some good news on the original issue I posted.

I was pulling the bulbs related to the 7/8 fuse, and when I got to the two license plate bulb, found that they were wired incorrectly, with the power line coming into the threads, and the ground out the bottom.  Swapped those around, no blown fuse, all running lights (and dash lights) now working!

Still have the turn signal/flasher issue, but at least I feel better knowing fuses aren't blowing.

Appreciate the responses here, as always.

Scott

Ideally you want to have great headlamps with an H4 bulb and you want them on all of the time you are driving. While the amperage draw is not particularly large, the switches available since the MK II cars aren’t particularly robust. Think of a relay as a remote control switch easily capable of carrying high current. Fitting relays for the high and low beam circuits allows the original wiring of the car to only carry the load required to trigger the relays, which is almost difficult to measure it is so low. The benefits of this are a greatly extended life of all related components: headlamps switch, dimmer switch, the entire wiring harness; and increased reliability.

There are many good resources for this on the web, often from MGB enthusiasts: note that their wiring has all of the same colours as ours.

Jeg, who does sometimes still look at this board posted some great diagrams of his relays installation which allowed him to keep his car within the fairly strict Danish regulations.

A set of relays for the headlamps is easily fitted without cutting any wires or doing anything that might be hard to switch back to the original configuration.

here is a good description

//www.mgtoronto.com/pdf/Tech/Protecting_MGB_Circuits.pdf

Mur, could you expand on your relay comment? Both theory and practice of why and how to do on headlamps?
Thanks kindly.

It would help if you took a relevant wiring diagram and enlarged it, so you could see how this should work.

The headlamps switch is fed via an unfused N or Brown wire, and it provides power to first the park lamps R red wire, and second the headlights U blue wire. Blue heads off to the dimmer switch and leaves there either blue with a red trace or blue with a white trace, for the high and low beams. The red heads to that fuse, the one that keeps blowing. This is how I know that the switch is fine.

I am not surprised that changing the hazards switch fails to correct the problem with the left rear signal. You need to fix whatever is wrong.

if you have not yet done so, now is the time, while you are gaining understanding of simple circuits; to fit relays to carry the current to the headlamps.

personally I almost never solder any connection. It is too easy to get too much heat into the wire and then it will fail. My go to is crimp connectors both butt and terminal that are covered with heat shrink, and I also use double barrel crimps that crimp to the wire and the insulation, as are common in modern automotive wire harnesses.

a 5 amp blade type fuse in a blade type fuse holder will be fine for your test rig. I think you may need a handful.

Thanks Mur - yes, not too experienced in electrics, but willing to learn, and armed with a multimeter and test light. 

Why would you say the headlight switch is not a potential issue - because it's feeding the power to the left side of the 7/8 fuse?

A few more details I left out - for the hazards, I've tried two switches with the same behavior.

Whenever I've needed to put a bullet connector on a bare wire, I've fluxed and soldered the bullet on and used shrink tubing.  All connections from the front to the back loom have similarly been protected with shrink tubing.

I've got the wire and will pick up a inline fuse holder and use a 17 amp BUSS fuse (per the British to U.S. conversion), as I've only got 4 of the "real deal" Lucas fuses, and don't want to blow them all on this investigation.

I'm going off of the '84 carb wiring diagram, which most closely represents what I've got in the car.

Thanks all, please let me know of any particular approaches I should try, will begin walking through that circuit in the coming week (only have a few hours each weekend unfortunately).

Scott

I had that same lights issue on my Clubman Estate and it was the wiring harness rubbing on the speedometer hole behind the air filter, repaired the wiring with the correct colours and it has been fine since.
Start at the power source with a test light and fused wire as suggested then trace the wire all the way to and from the switch it is shorting out somewhere along the line.

It certainly is not the headlamps switch. The fuse is blowing for a reason, and that is somewhere on the park light/dash lamps circuit. Isolate parts of the system till you are able to narrow down and ultimately find the issue.

The turn signal is probably easier to sort out. It will be the green with red tracer for the left side; and first unplug that rear signal and check the wire feeding it for power when the left signal is turned on. If no power there, then check where the rear harness connects to the front. Also check that you can illuminate that signal from another 12V source.

from your description it seems you don’t have much electrical troubleshooting experience. No one starts with these skills, and problems like these can be good teachers. I suggest you get a good 12V test lamp, a length of wire with a fuse and an alligator clip at the start that can be used to test run things, and a good wiring diagram that works for the harness you installed. 

Hey all,

Well, been trying to sort out the lighting gremlins for some time, and will give it one more go before taking it in to have someone else tackle it - I want to drive it in the summer, not troubleshoot. 

The lower 15 amp fuse keeps blowing (side & tail lights, panel lights)

Car - '68 (ext. hinges that I built from the ground up) with a few updated items like reverse lights, rear defroster, alternator, and later steering column so wipers on on the stalk.

New wiring harness for a late carb'd car, with center 3 gauge binnacle, alternator and 4 fuse main.  The car's only had ~300 miles on it since being "born again".

Issues:
No rear running lights (brake and backup are fine)
No left rear turn signal
Hazards on - all lights flash, save for the left rear as mentioned above
Left or right turn signal on (hazard not on) - same as with hazards on, all lights flash, save for the left rear.

Today, I removed the gas tank, and confirmed all connections are solid, and ground is good.
Took the steering column cowling off, and all connections look tight and corrosion-free, the three multiplugs look good (remember, this is a new harness, so I'd expect things to be "clean").
Everything else works - wipers, horn, healights, high beams, brake lights, gauges.  Engine runs fine.
All bulbs test ok

Fuse is fine, with everything off.  Blows when I put the headlight switch in first position (running lights).

Any thoughts on testing the headlight switch?  Something downstream?  Troubleshooting from here?

I'll try any troubleshooting that's straightforward, but if no success, I'll be taking it in to a British garage here in Portland for resolution.

Thanks all!

Scott