Hazard warnings not warning of anything

Here with my cautionary tale of how I forgot basics and spent a long time chasing a very simple fault!

Instrument light sin the MGB are poor to very poor- and as I have a fair number of LEDs left over from my Lotus conversion I decided to use these in an attempt to improve matters. I managed to swap most of them (speedo and rev counter new bulbs awaited), but during the process I became aware that the hazard warning flasher was inoperative. The steering column indicator switch was working normally or I would have noticed sooner!  However, straight away this tells me that the wiring from the bulbs to earth is obviously fine and the fault is unlikely to be serious- although something clearly is wrong.

Working on the basis that I have probably upset something with a previous job I need to investigate  past activities. I have swapped the starter (and so all its positive post connections), Ive moved the relays (I fitted a new ballast bypass system) and of course I could have dislodged any connectors behind the dash whilst I struggled with the instrument lights.

Wiring diagram for flasher circuit: Power comes in via the brown wire which originates at the starter relay where its joined to the positive post connection, power doesn't pass through the relay. The brown wire has an internal fuse (not shown) and terminates at the flasher unit. Power is then fed via a light green wire to the hazard switch. When this is operated all 4 of the top contacts are connected supplying power to both R and L flashers as well as the tell tale light in the switch... note my car doesn't have this switch, the tell tales on the dash are used to show the flashers are working and the internal bulb is connected to the panel lights via external connectors (not shown in this or and diagram I have found!).

I first checked I have power to the flasher unit. Both direction and hazard flasher relays are below the dash on the right,  in my case both were rectangular 2 pin units each served by two wires.  The hazard warning flasher has to dissipate 84 watts in use and is larger than the side flasher unit which deals with only half this power. The hazard unit  has a brown permanent live feed and a green/brown connection to pin 3 on the hazard switch.  The flasher relay receives a green brown ignition controlled live and a green feed to the column switch.

Flasher units below dash on right. The smaller (lower) box is the direction indicators and the larger box for the hazards.

I checked for 12V at the brown input lead and found it using a voltmeter.

In order to test the relay I swapped the connections from the flasher relay to the hazard warning relay making sure the power in connection was made to the correct pin. A better test would be to connect the hazards to the flasher relay but the power would probably overload it so this is second best.   The switch indicators then still worked suggesting the hazard relay is able to cope with that load at least, so I turned my attention to the switch.

Switch pulls out of dash- note 4 pins at top, 2 at the bottom. There are side terminals which connect the internal bulb to the panel lights.

The switch fits into this back plug. Note that socket for pin 3 is missing as this isnt used in my car. Also the two wires with spade connectors which connect the panel lights to the switch's internal bulbb

The switch should have 6 pin terminals, I'm numbering them 1-4 at the top and 5-6 at the bottom.  The bottom two power the direction indicators for normal driving and the top 4 are concerned with the hazard warning function. These pins connect to the loom via a rear plug which has corresponding sockets. Note that the socket for pin 3 is missing; this would normally flash the switch's internal light as a tell tale when the hazards are working. However in my car the dash tell tales are used and this pin has been disconnected, the switch's internal light now being connected to the panel light circuit.

I next tested for continuity between hazard relay green wire and socket  4 which was OK, and 12V from the relay was detected here.

The lower pair of pins (5 and 6) are simply a pass through for the ignition controlled feed from the flasher relay. These are connected for normal driving  to feed power from the flasher unit to the column direction indicator switch. When the hazard warning button is activated this power feed has to be interrupted so that the flasher units don't interfere. I tried removing these wires in case the internal switch disconnection mechanism wasn't working.  This made no difference and the continuity meter confirmed that the switch was operating correctly on these contacts.

For the hazards, when the switch is operated power in pin 4 is connected to pins 1,2 sending power to the left and right indicators to flash them simultaneously. I tested these actions with the continuity meter and as far as I can tell,  the switch functions exactly as intended. I checked that pins 1 and 2 are connected to earth (through the flasher bulbs ) and tried bridging terminal 3 to each in turn. This should provide power to the bulbs and light one pair of indicators- but the bulbs didn't light. However the 12V was no longer detectable suggesting that it was being earthed???

At this point I began to suspect that 12V input isn't always a 12V input and rigged a bypass lead directly from the battery. Connecting this to terminals 1 or 2 on the switch plug gave steady illumination in the relevant indicators which eliminated any weird shorting between the switch and the lights. Further- bridging this feed directly to  terminal B on the hazard flasher box restored full function!

12V feed direct from battery to hazard flasher unit restored function.

Obviously this throws suspicion on the power feed to the hazards- and as I have been moving the relays about I could have disturbed something. I checked all were still secure and connected properly.

Starter relay removed- all wires had good connections. The attachment strip is corroded and I cleaned it but it isn't used as an earth. The feed for the hazard flasher unit is via one of the two brown wires entering a single spade just above my thumb.

I disconnected the fly lead from the battery and reconnected it to the brown wire spade disconnected from the starter relay. The flashers still worked so power was clearly arriving at the starter relay as expected from the  fact that I can start the car. Any problem must lie between the relay and the flasher unit. This is essentially a single stretch of wire, although it does pass through a 17A  inline fuse. I removed the fuse and checked it- continuity was fine although the contacts were perhaps a little dirty. I cleaned it, refitted it and Hey Presto-  my problems were cured! I conclude that when I fiddled with the relays I must have disturbed an already poor connection worsening it to the point where although V were passed the resistance was so high that no current could actually flow when demanded.

In line fuse holder- brown wire feeding the hazard warning flasher unit

I was surprised that such a small amount of dirt could cause the problem so took  my cue from this to clean the other in line fuses. All were dirty to some extent and one (35A feed to cooling fan) had a cracked cap, although still functional. I changed all fuses and refitted them with a smear of dielectric grease which I'm hoping will prevent this problem in the future.