Diodes - and re-assurance please.

[MetBlue]

I've been investigating a long standing issue with my lights.
As bought, the head lamps wouldn't flash - NOTHING at all happens.
On last MOT, tester commented that side lights can't be on without the head lamps also rising, but he made it an observation only
Some time ago, I looked at the relay holder and could see some bodged links in various places, but until now had taken the view : Ain't broke, don't fix it.

But with the dash out and access as good as it will ever be, I decided now is the time to investigate.

So what do we have: Both the head lamp relay and the Rear fog lamp Relay are missing.
Across the Rear Fog lamp relay base, there is a link wire between pins 30 and 87, which I fully understand - Simply the relay function is removed and power will feed to the fog switch, irrespective of the relay being activated or not.

There are 2 links in the Head lamp relay base ( no relay as I say). There is a blue wire plugged into pin 87, which I now know is tapped into the Red/Pink wire of the Side light relay. So thats all very logical. When the side lights come on, the headlights get a power feed, and they come on at the same time - Just as the MOT man spotted.
So far so good - You can see the links in the Head lamp relay base in the photo below ( Side light relay removed for clarity)


But can anyone suggest what on earth is going on with the Red wire? It links pin 85 & 86, which by my logic, would make a dead short when ever the head lamp switch or flasher is switch is activated !! I just can't come to terms with the fact that someone with the confidence to link the output from the side light to feed the head light ( and also jump the fog relay) would also think it necessary to add the red wire - OR AM I MISSING SOMETHING OBVIOUS?

To help me understand the full circuit, I redrew the various sheets onto one, using red lines to show the relay switched power lines

.

The blue wire is across where I've written "link". The Red wire in the photo is across the 2 highlighted "A" 's on the Headlamp base.
There's no photo of it, but the rear fog link is across the 2 highlighted "b" 's on the Rear fog base.

So, first question : Is there any rational explanation for the red wire link on the Head lamp relay base?

Tony

[MetBlue]

I didn't want to confuse the above question, with this next bit about the actual diagnosis of my problem, so doing a seperate post.

Having removed the links, I checked and have 12 Volts at pin 85 on the head lamp relay feed ( as I should).
I was not getting a link to earth from pin 86 when the column switch was activated ( neither on flash or on rotary switch). I tried both polarities, as I knew there were diodes in the lines.

After much searching and scratching around, I eventually found all three diodes, and as best I can tell, the wires between each Diode has continuity.
I note one side had a large black plastic spade connector, which relatively easily unplugged in all three cases. The other side looks like a simple spade connector with a light plastic shroud. I assume it should just unplug, but on all 3 I couldn't budge it. On one of the diodes, I put a meter across it and saw 0.03V if I recall, irrespective of polarity.

I'm pretty sure all 3 diodes are toasted and that is the route cause of my problem. Where can I get new? Is there anything special about them?

Tony

[Excel SA]

Hi Tony,

Can't help with the relay issue, on my phone and can't see the detail on your drawing, and no real reason I might understand what's going on....

I did "rebuild" a diode after some research, one of mine was completely burnt out. There should be a post on my build thread somewhere, but it's nothing tricky, complicated or expensive....requires diodes, spade terminals - male and female and some soldering. I have not tested mine though.....

Neil.

[MetBlue]

Thanks Neil. Found it on page 3.

The OEM part number is Pektron A75-294. Googled it and found several links to other forums, Ebay and Pektron's own spares online shop.
It seems it is a 1 Amp, 100 Volts diode. There's various discussions about why the Pektron unit (also used on Deloreans BTW as well as MGB) is so relatively expensive (around £10 each most places, if in stock, or £60 plus Vat for 25 from Pektron), when std electronic components are pennies. Someone speaks of 2 types, silicon or Germanium, saying silicon is used on automotive applications.

Anyway, just ordered 25 off, 1 amp 100v silicon diodes from ebay for £3.84. There were no hits on germanium diodes, so maybe that's a red herring. I'll make some up and if that fails, probably go down a bulk buy with Pektron.

Any clever electronically type out there able to comment on my chances of success? Or why automotive diodes are so expensive.

Tony

[MetBlue]

Managed to get the D1 and D2 diodes out today.
Both showed open circuit either way with the multimeter, so decided to open them up



So I know my problem and also that a switch with the diodes on order shouldn't be a big issue. I'll repackage them in the same tube as these originals. Obviously I'll check out the 3rd diode before I re-assemble, as it is by far the hardest to access ( it's strapped along side the relay bank on the wrong side! I can only access it because I'm unbolted the relay rail).

What I can't understand though is how both are broken in exactly the same way. It's like two links in a chain both failing at the same time if I try analysing this with my mechanical brain - Puzzled.

Tony

[Excel SA]

I'm consciously incompetent at a lot of this stuff....but your diode is more complete than the one I had to replace. I suspect too much power went through it the wrong way - there are limits to how much current they can stop from flowing, too much and I suspect they go pop...May account for the dodgy relay wiring or vice versa....

Neil.

[Hawaiis0]

I would say the PO couldn't locate the diode so bypassed it.

Now you have them to hand you can try back to factory.

A short circuit could have taken them out in sympathy of each other.

[Excel SA]

Don't forget to solder the new diodes in in the same orientation as the originals....

Neil.

[MetBlue]

Pleased to report that the cheapo diode repair seems to have sorted the problems out. Only a quick test earlier (before I start building it all back in making access difficult,), but flash works for first time since owning car and headlights not coming on Simultaneous with the side lights.
Pods rise though with the side lights, which I guess is to do with the dim dip feature (currently linked out).
There's always one more job.
Tony

[Excel SA]

Good to know that the diodes work - no reason not to I suppose looking at what you originally had.

Can't say I understand the whole dim-dip thing - it's not something I've come across before getting my Excel, but seem to recall reading that there are good and bad ways of disabling or bypassing it....

Neil.

[MetBlue]

Disabling Dim Dip looks pretty straight forward.
Been digging into past posts on here.
A simple link on the plug connector is technically all that's needed (in fact my car already has it). The service manual has a wiring diagram page for Austrian cars, which is identical to the UK spec dim dip except for the DD module isn't fitted.
Seems I can stop the pods rising just by removing one fuse, but that's for tomorrow.
For my MY, technically the car will fail it's MOT if the tester realises, but ironically earlier MY cars would pass without DD. - What a crazy UK world we live in More reason to stop the pods coming up. Don't want to sign post the non compliance.
Tony

[TrevorK]

My Excel headlapm pods rose to allow you to wash the lights with them off. Then the sidelights come on and then the headlights, if I remember correctly.