Talk Morgan Forums Morgan Sports Cars Maintenance & Servicing Dash PCB-ectomy

I think the only MOT requirement is that rear fog light(s) have a warning light when on. Sadly this is insufficient for many

I like the one touch latching system on later cars which means the rear fog selection is lost when ignition is turned off.

Indeed it is !

For anybody with the dreaded ‘stuck on’ rear fog issue, a bit of an update re the faulty PCB.

My board is looking like it has suffered a couple of failures. The first snag was the 2n3904 transistor that drives the fog relay coil had failed on. The fog relay and it’s snubber diode are fine so whipped the tranny out and stuck in a new. Unfortunately that only made a partial improvement. The ‘stuck on’ fog supply was now at least off, however actuating the fog light switch does nothing. That ‘clever’ latching logic appears to be shot as I cannot detect any control voltage on the base of the transistor despite the board still having its 5v supply working . The control tracking winds its way back to the microcontroller so maybe there's a partial micro failure too, unless I’ve missed something obvious? If it’s the micro then this add a new twist and an obstacle for home repair

If... I can maintain the enthusiasm I’ll put a Pickit 3 on my Christmas list and attempt to salvage the hex file and flash a new chip, that might be fun. Assuming of course that it hasn’t been locked….

The offenders


Testing the fog light

I have been trying to get my head around this but all I can find on the Rutter parts book is a chassis harness. No dash details etc. Nowhere could I find the resistors and the led indications either.
So, am I missing something here; are there actual wiring diagrams that I have missed or did you really do it from the harness diagram and guesswork from fighting your way through the actual wiring behind the dash?
If so, it appears to be much more difficult than it looks!
EDIT..
Another issue just spotted: in the photo of the dashboard sub harness connector, there are 3 red wires attached to the resistance board, and yet at the other end only 2 red wires going on to the warning light pcb socket! And no mention either on the diagram, of 2 wires going into position number 1, (one of which is red and could be the mystery one?)

Doug, I don’t think that you are missing anything. The level of published electrical info on our cars is really quite poor, considering. I’m not aware of any relevant dash wiring details, only as you say the chassis and engine harness’s from the parts book. They aren’t particularly reliable either when you start delving into them. I’ve seen no information either on the PCB other than the various photo’s floating around, that’s all there was to go on.

I originally dropped the dash to do an initial recky. I took loads of photos and measurements. I then went away and built the replacement bits in slow time over the winter. I tested it at leisure on the bench. When it was ready it took less than a day to fit.

Regarding the instrument warning lights, it’s fairly obvious just by looking that they are in fact LED’s and the current limiting resistor array can me identified on the PCB photos.

The mystery third red wire in the pic is a spare 12v supply that I just cut off and left inside the loom. I had one too many wires connected, sorry for the confusion. The second pin.1 is now pin.9….

Now sorted. Thanks.

Rog, thanks for your write ups & pictures. When a component “let’s go” on circuit boards it would seem that the spike or over voltage causes surprising failures elsewhere on the board.
During my education at A level. Me & a mate did a project testing transistors to destruction, testing their heat tolerance. Using different manufacturers & working out what heat syncs would be required.
Simple circuit used on a universal circuit board. Only the transistor was being heat treated.
70% of the time another component failed when the transistor gave up. Often included the circuit board, sometimes difficult to see.
IIRC some transistors failed at temps around 70c. Was in 1970 so the components were quite large compared to todays.

[quote=Rog]
Dashboard PCB


[quote=Rog

When my heater fan stopped working recently this was the state of the 9 way Molex which Roger refers to as a washing machine connector.

I've seen the Rad fan connectors on the front engine harness 'cook' like that too, mucky poor contact increases resistance on what is a high amperage circuit

That green wire with burnt insulation on its dash connector is main heater fan supply Ken. A dirty contact on the connector has caused resistance such that over time with fan on enough heat has built up to melt contact insulation, all without blowing the feeds 15 amp fuse.