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

Phew, thank goodness for that! As for an explanation...it’s anybody’ guess.Possibly, with your repeated switching, the switch contacts may be separating a little quicker and cleaner but that’s a shot in the dark really.

So with 30 disconnected you still get the spike albeit random. That kind of rules out the clever circuitry in the switchback bulbs generating the spike so it still looks like it’s the relay.

What now….. Well I guess you need somebody that knows what they are talking about here and that’s not me

But thinking out loud and for the sake of a couple of quid you could try a relay that might uses less power. If you have used a 40 or 30 amp relay and for the sake of a couple of quid you could try a smaller 20 amp relay? (and upgrade to speedy delivery for 60p )

Thanks for all your help this far Rog, I will invest in a smaller relay and see what happens.

I will post the results on here.

As luck would have it I have an MOT booked in with BHM on the 28th of this month so if the problem persists I will run it past Jim who is their resident auto electrician and see what he thinks.


Thanks for all your help once again mate.


Rob...

What a journey Rob! Glad you have proven absolutely it's an issue with the relay coil though. My only thought when reading your quoted post, was try another diode relay but kept quiet as it seemed such a shot in the dark. Would now think this a good idea especially a lower power or better brand one as have a feeling this issue will eventually rear it's ugly head again as is.

Not sure if I've mentioned it before but have in the past seen a very clear demonstration of the back emf effect from a relay coil when it's de-energised. A non diode relay on my narrowboat to power the tunnel lamp, had a small led warning light wired across its coil. It initially didn't indicate although the lamp came on fine. Something of a mystery until I noticed a brief flash of the indicator when I turned the lamp off. Reversed the ambiguous indicator connections and all came good.

I would be surprised if a lower rating relay would have a different coil. The relay rating is usually based on the rating of the output contacts. However, I support Richard’s suggestion above but make sure the relay is from a different manufacturer (if thats possible as everything nowadays seem to emanate from China)

Well chaps I have ordered another relay (number 4) using a non diode one and blowing another accounts for 2 of them, this time the seller states that they are used especially in Morgan Roadsters, so lets see where this road takes us.

Watch this space, as I mentioned in my earlier post I have an MOT booked at BHM on the 29th so if all else fails I will chat with Jim the auto electrition and post the results.

I know its desperate Doug but I'm working on the flimsy premise that size matters
If the contact size/mass comes down the power to generate a lower mag flux density to move the lighter mechanical assemble (and lower spring force) would reduce too, as with a smaller coil in a micro relay.

Agree with your thoughts Roger. Another possible solution would be wiring an extra external diode, reverse biased across the relay coil connections.

Sound engineering principles Roger but with such miniature relays not sure the difference would be significant. But you never know of course.
However, I would not be surprised if these things come off the mass production line identical in all respects, the only difference being whether they stamp 20 A or 40A on them!

This looks like a common problem when relays and electronics are both included in the same system. The cause will be one of two things. With a decent oscilloscope, you can get to a firm diagnosis. Either one or both of these could be true.
1) There is a signal spike being induced on the signal wires to the speedo.
Or
2) The relay coil operation is changing the supply voltage to the speedo.

Without knowing more about the signal shape to the speedo, I would be reluctant to mess about with it although I have seen reference to people fitting a small ferrite bead here (sorry, cannot remember where I saw this).

A safe experiment, is to wire a small capacitor across the power supply to the speedometer - unless you short things out, you cannot make things worse. As to value, I would start with 0.1 µF wired between speedometer power in and speedometer earth as close as practicable to the device. If this makes any improvement at all, gradually increase the value and see what happens. If you get to 0.5 µF and there has been no improvement, then this isn’t the problem. Make sure you buy capacitors with a voltage rating of at least 35V as the spike might be a fair size. Buying 5 0.1 µF will work as an experiment - when you wire them in parallel, the values add so 2 x 0.1 µF wired in parallel gives 0.2 µF.