Originally Posted by Rog
Originally Posted by Rog
Originally Posted by IcePack
Good work Roger. As a matter of interest, did you manage to find what had caused the original fault on the PCB ?
Thinking that it would be either a component failure or dry joint/track continuity fault. Both should be an easy fix. However if the fault is impossible to find their in shows the problem of these type of faults.
I haven’t actually confirmed the original fault yet Bob. However I have quickly powered the pcb up on the bench and can see that the fault it still there, the fog light is permanently on. It appears to be using a regular 2n3904 transistor as a switch/driver for the fog light relay coil and that looks like it may have failed in the on state. I’ve ordered up a few transistors (15 pence each, ouch!) and have a few diodes here just in case too. It’s low priority at the moment but would like it back working as it gives me the option to swap back. I will report back!

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 frown

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…. ooo

The offenders
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Testing the fog light
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PCB update….. oops

I must admit, this is a bit embarrassing. My PCB is functioning flawlessly now. In fact, it has been working ever since I replaced the faulty fog relay driver transistor. The issue was that I hadn't properly connected it up when testing out of the car on the bench. rofl

Things took a turn when curiosity got the better of me and I decided to sketch out some more of the circuit. It turns out that the micro doesn't receive its cue from the sidelights supply on the PCB to enable the momentary fog switch’s relay latching routine, as I had initially thought. Instead, its uses a single 'common headlight supply' directly from the column switch with a resistor in series directly into the micro. A feature I confirmed by probing my spare Disco 2 column switch, as it wasn't something I could deduce from the circuit diagrams. Thankfully a simple fix on my test setup. As soon as moved the supply to pin 5 instead of pin 4 on the 15 pin header it burst into life with the relay clicking and latching. So now my PCB is fixed, should I ever decide to sell my car, I can easily revert it back to its original state within a couple of hours if necessary.

I hope this will be helpful to anyone in the future with rear fog issues. It seems that this fault is not uncommon, as I have seen similar reports several times on the forum. The 2N3904 transistor appears to be yet another weak point, but looking on the bright side, there is a positive. Yes the introduction of these PCB’s into our cars is a bit of a nuisance. As discovered, a shot 15 pence tranny can be a real headache come MOT time. However, these older 70’s tech double sided PCB’s with discrete components are still relatively simple and as proven, can even be fixed by somebody with a schoolboy knowledge of electronics. It’s the next generation of multilayer boards with surface mounts that I fear will be the real spoilers, they will be an order of magnitude more challenging…. ooo

Working again. The off state and the on state with relay latched on.
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Roger
2011 Plus 4