Talk Morgan Forums Morgan Sports Cars Morgan Three Wheeler & Super 3 ELECTRICAL GREMLIN

Looking for guidance on how best to trace an electrical short which keeps blowing my Main Beam Fuse.

I have checked out the relay and all is fine.

Cheers
Gordon

Assuming you have an M3W there are two relays that control the main beam Gordon, one that flips between high and low beam and a second that gives main beam flash, both controlled by stalk. There are also two fuses concerned with same, fuse 7 20 amp feeds the light switch and main beam flash relay. The output of this relay together with output of main/dip relay (when main selected) both feed through fuse 16 15 amps and on to main beam bulbs.

If you can tell us which fuse is blowing and under what circumstances further help can be given. Year of car would help as there were substantial changes in 2014.

My first port of call would be to check inside the headlamp bowls. It's relatively common for wires to chafe and break in there thus potentially causing a short to ground. Also, check the rear harness under the boot liner for wear at the pinch points around the fuel pump and fuel sender. Lastly, check the wiring at the flex point of the boot hinge where they have a tendency to break.

Problem Solved.

Thanks Richard and Bunny points noted.

Richard great information on the control relays etc. I thought only one relay was associated with the headlights. On further investigation it transpired that the Main Beam was not operating under any circumstances and when I tried to put the main beam on it was taking out fuse 17 (15Amp)? Dipped Beam.

I replaced the relay LH side in fuse board (same side a steering wheel) and my problem was solved. (I take it the relay I replaced was the one flipping between High Low Beam}Now I know what the other does.
Thanks again Richard

Cheers
Gordon

Yes regarding the relay Gordon. It's wired so that when that relay is not energised dipped beam is selected which is useful when headlights first selected in not blinding oncoming. Unusual fault with relay I would have thought, but glad you got it sorted

I have read in the past of headlamp bulb upgrades to provide better illumination of the road ahead, which seemed then to be followed on by dip switch failures, the cause perhaps being the much higher current required by by the brighter lamps...?

I wonder if the relay contacts spec might be a tad tight relative to headlamp demands over time...

Just thinking in type..

The standard 12 volt 30/40 amp changeover relay should comfortably be able to cope, the dual rating being related to break and make of contacts. Important it's internally diode protected though to avoid back emf spikes upsetting the ECU. I recommend Durite as replacements.

The way in which the OP's failed though according to his report is surprising.

Thanks for the heads -up on the relay spec Richard.
Interesting to read that the relay has diode protection acting not only to protect the relay contacts from possible arc damage when breaking the circuit but also protecting the ECU from spikes further down the circuit interconnections..?

The relay on my starter motor circuit began to fail a while back, suspected due to arcing at the contacts, perhaps not an unusual fault on pre digital trads given the lack of diode protection and the possible level of back emf generated by the inductive load of the solenoid etc.

While every day has potential to be a school day, the problem these days is trying to remember that which I have learned...!.