Talk Morgan Forums Morgan Sports Cars Classic Morgans (pre-CX) Speedo jumping.

This one shows the polarity with respect to the diode

Yes you have the relay coil wired the wrong way around. Non diode relays are polarity insensitive which is why you will see some diagrams (incorrectly) indicating coil should be wired as yours even though this doesn't conform to pin codings. As Roger correctly suggests though the diode, there just to quash the the back (reverse) emf as the coil is de-energised, is now providing a direct short when energised. Hopefully you haven't damaged the diode so just reverse coil connections ensuring ground is connected to 85 and all should be good.

Morning chaps,

The latest update in my quest, I have swapped the connections and made 85 ground, after visiting Halfords for fuses and waiting to get in, unfortunatly even with this switch of treminals the fuse blew as soon as the side lights were switched on.

I am now out of ideas as to what to try next, I think that I should start to clean and service the original indicators to put them back.

I wonder if the diode may have been damaged because of the wrong wiring in the first place as suggested by Richard and should I get a replacement and try before giving up?

Any ideas would be welcome.

Seems highly likely that the diode has melted internally as it’s peak inverse voltage has been exceeded. As a result, if the diode has shorted then in turn, it is causing a short across the relay coil, so excessive current will flow and blow the fuse. Simply buy another relay and install the correct way round.

Sounds like you may have blown the diode to a short circuit in the reverse bias direction, as I intimidated above, although hoped you would get away with it. They are only spec'd to dissipate the tiny amount of energy (back emf) from the collapsing magnetic field of the relay coil when it de-energised, the cause of your speedo jump.

Fortunately they are not too expensive and you have already proven the solution is at hand, so don't be disheartened and just remember 85 is ground. Worth keeping a spare BTW for the others that are OE on your car as a quick plug-in solution.

ETA: posted before I saw similar answer above.

Thanks chaps, relay number 3 has been ordered and should be here mid week. must remember "85 IS GROUND" will keep you posted.

I also ordered 20 fuses "just incase". The hardest part of all this is getting to the fuses, oh!! to be double jointed.

Just a thought,
can you explain why a relay with no diode doesn't blow the fuse but the one with a diode ( knackered ) does?

It makes no logical sense to me as to why the fuse doesn't blow. I'm sure there is an explanation but my logical mind tells me that without the diode the fuse would blow probarbly quicker?

This is just out of interest.

This is because we suspect that the diode has failed as a short circuit. If this is what has happened, your relay is acting in the same way as a piece of wire (as a short circuit) and is therefore causing the fuse to blow.

you may find it useful to visit www.autoshop101.com where there is a VERY easy to follow explanation of relays, diodes etc.

Just to add, a diode conducts current on one direction only. In the case of a diode relay its wired such that it won't conduct current in the normal direction of flow when the relay coil is energised. It's purpose is only to quench (short out) the blip of reverse current (back emf) when the relay is de-energised, which can otherwise cause issues with sensitive electronics in modern cars - as you are now aware

As you can see therefore a diode relay must have its coil wired correctly to work without issue whereas with a plain relay it will work with coil current in either direction. All the factory fit relays in your car are diode ones as well.