Hi there,
So looking at that track and diode it does look like you have a similar problem as I did with the heater fan motor. My diodes failed to a short, so caused a fuse to blow each time. I assume that a fuse blew when you lost some of the functions?
Hi Nick,
Thanks for your input, until recently all I'd actually lost was the seatbelt warning tell-tale, this never worked since I bought the car and was never an issue for me as I'm perfectly capable of knowing if I've put my seatbelt on thank you very much Morgan!
However, more recently my Speedo and ignition tell-tale dropped out, TBH I can live with a few missing tell-tales, but I'm not keen on driving without a speedo so I needed to investigate why it had sudenly dropped out?
Did you try a larger fuse? Or was it quite a high current fuse anyway?
To get the speedo working again my first thought was to check the fuses, and as expected I did indeed find one had blown in the main fuse board under the dash in front of the heater, it was fuse 4 which according to my laminated fuse diagram is a 5a fuse responsible for the dash module. An interesting point to consider here being that it clearly does not protect the entire dash module as a lot of other elements connected to the PCB continued to work just fine despite the blown fuse!
However, my first replacement 5a blew immediately, then another in quick succession, so clearly there was a deeper issue. I then did what one should never do, while lying on my back looking up at the fuse board I noticed there was a spare 7.5a fuse staring right at me, the temptation to try it was overwhelming. I fitted it justifying my actions by convincing myself an additional 2.5a was minimal, it is of course in percentage terms quite a jump in value.
Anyway, hey presto, as soon as I fitted that spare 7.5a fuse the speedo, the handbrake tell-tale, and the ignition warning tell tale all returned and have remained reliable ever since. Unfortunately, while I was delighted to see the return of the ignition warning light and most importantly the essential speedo, the two green indicator direction tell tales, the green headlight tell tale and the blue high beam tell tale did not return. I believe this is also when I lost the illumination to the fog light switch and heated screen switch, looking back at the chain of events it would seem my ill advised fitment of the 7.5a fuse must have blown the tracking on the PCB.
That aside, these Morgan PCBs are definitely unreliable with some owners reporting up to four consecutive failures, it seems to have been built with tracks that are only just heavy enough to carry the expected loads, there appears to be no head space to deal with any small increase in resistance or current spike. Now, as it stands I could simply have the burnt track repaired and re-fit the correct 5a fuse, but for sure it'll blow immediately again and I'd be back to the unacceptable situation of being without a speedo.
In my defense and even with the benefit of hindsight, I would probably do the exact same thing again as I can live with a few missing tell-tales and some switch illumination, what I can't live without is a speedo, I also fear the issue may have nothing to do with the speedo, it could just as easily be blower motor or heated screen related. I strongly suspect the track that burnt out is a shared ground for some, but not all of the tell-tales, which is why I've only lost a few of them and only some switch illumination too, the handbrake & ignition warning tell-tales still remain and we now know that 5a fuse 4 only protects certain circuits on the PCB, the way it's wired and fuse protected follows no logic ????
That diode failing and a decent current through that track could have blown the track. I suggest just checking the resistance across the diode (check with the probes the right way round so that you are not forward biasing the diode) - it is in parallel with the relay coil so you will get quite a low resistance if all is well, or a dead short if it is blown. If it is blown you could try replacing it with a common or garden 1N4001 and fix the track with a bit of wire.
I'm not sure if you still have heater fan issues? I would ensure D6 and D7 are also changed at the same time to prevent that issue raising its head. What I don't understand is why this only affects some cars and not others - unless it is to do with the a voltage regulator or somesuch. Of course Rog is the real expert here, but hope my 2p'th helps.
Cheers,
Nick
As I say, the board only seems to be just about man enough to take the expected current, with a bit of age, vibration and moisture it's inevitable there will be a certain amount of increased resistance and this will vary from car to car based on use, climate, storage conditions and the 'law of sod' of course. The Carebont made instrument MFU is also a long way off what we would consider modern car maker quality, as Roger points out above its function seems to introduce unnecessary complication for no real benefit, it also has to work perfectly with the PCB that as I understand it was not made by Caerbont.
To my mind as Caerbont supplied the speedo, rev counter and multifunction gauge but Caerbont did not make the PCB, Morgan should have wired the gauges completely separately to the switches and tell-tales. Either way you look at it the setup is a bit of a pig in a poke, expecting a small company like Caerbont to work with the manufacturer of the PCB to deliver one integrated reliable system was a bit of an ask. I very much doubt either company had the kind of development budget and working practices to make a collaboration like that work, for example the Caerbont MFU could easily be overloading the PCB and it certainly seems possible this could be my very issue.
Anyway, it's all a lot of speculation, some definitely seem to have solved their issues by replacing the board mounted relays with a bank of traditional 12v automotive (diode protected) relays that are mounted remotely. This will be my starting point as even if the board mounted relays aren't my issue, going with the remote mounted relay solution should help to make the system more robust.... hopefully!
