Talk Morgan Forums Morgan Sports Cars Morgan Three Wheeler & Super 3 Main Beam fuse blowing

This morning I went to the garage to give the Mog a check over prior to its MOT on Monday.
It had been used for a couple of weeks.
I found flicking on the main beam lights is causing fuse 16 ( main beam ) to blow instantly , with an audible click.
Replaced the fuse , same thing.
I pulled the plug on the back of the headlights to eliminate any issues with the bulb - long shot I know but desperation setting in at this point - same thing.
I am beginning to think faulty relay ? Any suggestions welcome.

So anyone got any idea which is the main beam relay please ?
I am guessing its in the cluster behind the fuses ?

Thanks in advance.

Without delving into the not very clear circuit diagram the only advice I can give, based on my own experience, is to check the wire loom at the rear boot lid hinge point, under the leather strap. I know it's the other end of the car bit bear with me. The loom gets crimped here every time the boot lid opens and closes, the wires fatigue and short out causing all sorts of strange issues. Really not sure if the headlight circuits link with the tail lights from memory but it may be worth checking. I remember the two tail lights are independently fused as it was always one side that blew the fuse. Your car is a similar age to mine so it could have the same degree of wire fatigue?

There are two relays in the main group that control main beam, dip/main changeover and main flash but both are wired before the main beam fuse so couldn't cause it to blow.

The fault is a likely a cable short after fuse 16 which feeds a blue/white wire (UW74) to power headlamp main beam. There is a three way split in this wire at the inboard end to feed each headlamp (UW75/76) with third wire (UW77) feeding back to give high beam warning on the panel.

Hopefully this gives some guidance as to where to look for a short.

it sounds like a dead short so doubtful relay is causing the issue unless its internally melted itself.
Rear lights shouldn't affect you as its a separate circuit and any issues there should blow the side light fuse.
Look at the plugs/cabling located above the starter motor and under the steering column shaft (RHD) coupling area and of course where the cabling comes out of the large locating nuts on the bottom of the headlight units - all places where cables rub.

Here is a somewhat related question. Does the low beam stay on when the main beam is activated thus adding more current to the circuits? I notice the main and the low are on separate fuses.

Did you find the fault?
If not, it could be a faulty headlamp bulb.
I had a similar problem on my Commando caused by a defective bulb socket.

Not normally with filament bulbs. A headlamp flash whilst on dip will momentarily have both on though. As you suggest they are indivdually fused though.

An update from OP might be helpful

I have no idea if low beam and main are simultaneously active when main is selected, outside the scope of my problem.
My issue turned out to be a faulty relay, found by substitution with new relay units until normality was achieved.
The head light relays are the two rearmost in the array you can see when removing the fuse cover.

Thanks for heads up nippymog and glad issue appears to be sorted with replacement relay.

I still cannot see how the main beam fuse 16 could blow though as it's after the relays, and the wiring schematic for both early and late M3W's is very clear on this. There is a main beam relay fuse 7 (20amp) however, that feeds the flash relay (both coil and main contact) which could blow this fuse if it caused an internal short though.

The issue of main and dip beams on simultaneously is a red herring as each are fused individually.

Hi Richard,
I am logical when it comes to electrics , but in no way skilled at fault finding.
My approach was to simply swap relevant things until it worked, which fortunately it did !
In other words , I cant explain it either unfortunately.
If it hadnt worked I would have had to take it somewhere and lighten my wallet which doesn't sit well with a Yorkshire / Scot !.