Modern Vespa

In the meantime, assuming we don't change the calibration, I suggest to run the gauge at 6Vdc in production. To achieve this, you'll need to build a circuit illustrated. This can be built without a PCB, but you'll need some soldering tools and know-how. Components required are in this basket: https://eu.mouser.com/Tools/SavedCart/Share?AccessID=18ec23e100

Edit:
Forgot to mark the stabilizer pin numbers

Yikes! It gets complicated!

Thanks very much for the suggested circuit; I have an electronics store a short drive away I can swing by tomorrow. (Edit: I see 5 components on the circuit but 6 in the basket; is the extra an alternative if another is unavailable? The only part I recognize is the ubiquitous 1N007)

However, are we certain that hooking up the gauge according to the last successful test won't work without the mods? I have to say I'm tempted to try.

Regarding calibration, the needle followed the normal range on vespas: hovering a little past E when empty and under power, and a little past F when full. I'll check again later today, but having done quite a few tests on these the range seemed the same as usual vis a vis the faceplate screws. If it's off a few nanometers it won't be noticeable

The extra component not seen on the diagram is a snap-on heatsink for a TO-220 package (voltage stabilizer). Your store may have that too. I picked the one you can clip to the component, but even the one with a fastener will do. But then you need the fastener.

Re. the complexity, it really isn't. Just seems so. Before you build it, I need to give you some pointers regarding the placement of components.

Re. testing in in production without the stabilizer, well it's up to you really. I advise for a stabilizer, but you do you. I welcome both choices. So what you wanna do is connect the 12Vac to the diode in this case. You conveniently it kept in place, good call. You could add an electrolytic capacitor to it at least. You'd connect the capacitor positive to where diode and resistor are joined, and capacitor negative to GND.

Something like this

I just came back from the electronics store, and all they had was this:

Not sure it fits in the small space the gauge resides in; not even taking into account the other parts....

I see plenty of space 😂

But if you can't figure it out, you can put both diode and capacitor at the back of the cluster. Given that it's made of plastic-like material, it won't provide any protection from radiated EMI, so your rectifier can be anywhere close to connections.

So? Have you made up your mind? Stabilized voltage or not, which way are you gonna go? If you can't decide, I have a nice coin to toss. What's it gonna be, heads or tails?

Remember the 4 holes are what the gauge sits on. Unless the goliath of a capacitor can fit under the speedo dial, I don't see it....

That being said, I almost thought the ordeal was over. I connected the gauge to the relevant wires coming from the loom (1 to ground, 2 to float, and 4 (diode included) to 12v), kickstarted it, and the gauge needle budged. I moved the float upwards and the needle followed. I gave it gas, and the needle stayed at full without wandering off. The video doesn't show much, but when I conducted tests with the camera hand free, it all worked as normal.

https://streamable.com/nv5xv5

I screw the gauge into the speedo, the speedo onto its cover and the cover into the handlebar and kickstart: no more movement. Either the connectors didn't plug into each other correctly, or in putting the gauge into the speedo a short was created

https://streamable.com/uxnkv2

Given the finnicky quality of the loose wiring in this headset, gremlins aren't unheard of. I've often had to unscrew the cover and push the wires around to get the Neutral light to work, other times it was the back light. I'll unscrew it and check again...

I call that a success regardless of minor setback. You're probably missing the Vac to the gauge or the float due to wiring issue.

Unless of course, now that it was connected properly for probably the first time ever, 12V was too much for it. Fingers crossed not... 🤞🏻

I'll try unscrewing the headset cover first, then start the engine and observe, and keep disassembling until it works again, and try to figure out what changed. Fingers crossed....

Once it's time to put it back together, I advise testing in stages. Test with everything open, test with instrument cluster closed, test with cluster in place.

That's the plan!👍

Special update: I'm now the beaming owner of a gauge that not only follows the level of gas in the tank, but whose needle no longer dips while revving.👍

It would be a great conclusion to the many attempts and long time spent trying to get the original gauge to work properly, but unfortunately after the false hope when the gauge was out of the speedo, I was never able to get it working again (except to perform as usual, dipping with revs).

It turns out that the cosa gauge I had laying around, which had provided mixed results with a 12v battery, and after noticeably heating and smelling like hot plastic before I finally gave up trying to get it to work, did well on the 6V tests, the "plug into the loom and see what happens tests", and finally building it into the speedo test.

https://streamable.com/8u6dax

Obviously I shouldn't jump with joy until seeing the gauge go through a few gas tank cycles to confirm it's working as intended, but while using it today, the needle goes way to the left when off, moves to the right when under power (only slightly at first because the tank was in reserve), and fully to the right when full. And most of all, it stays unusually stable while revving. 👍👍👍

Fingers crossed!

I'll report back once I can confirm it performs satisfactorily and reliably on a daily basis.

Did you do any of the voltage stabilization discussed in the topic, in the end?

No, I just tried the other gauge to see if it might work, and it did.

I copied the parts list you made and will likely order them if I can't find them locally in a different electronics store.

There's obviously less urgency now👍, but I still look forward to getting this thing to work, if only to write the epilogue of this story, and have the last word with this gauge.

It's a pretty basic component list, doesn't get any more basic than that, most electronics shops should have it. If they don't have exact voltage ratings for the capacitors, bigger will do, as long as they physically fit.

What's wrong with ordering from Mouser, I'm curious?

There used to be quite a few shops in Paris where you could find these parts, but many closed down for good during covid. There's still another place I intend to try.

Nothing wrong with Mouser apart from the cost of parts / cost of shipping ratio.

I still think that fitting that massive capacitor into the cramped speedo bay is likely fantasy, but given the total cost of parts and the fun expected soldering parts together, the challenge is accepted

Since there's no urgency, I'll wait until I'm on my next electrical project (T5 wiring harness and (hopefully not, but) possible issues with the speedo) to add onto the list before ordering.

I thought shipping within EU would be reasonable, but hey, anything is possible. 🤷🏻‍♂️ I've experienced that Mouser recalculates shipping fees after one enters the address, which may or may not distort the cost.

Re. your next project, let me know should you need some assistance, I know a thing or two about building motorsport harnesses...

I stopped using mouser cause of the shipping costs. I now order from tme.eu. I usually choose DHL Air Express (they have many options) and I get the packages in 2 days...

Yeah, I'm in no hurry now but I'll check out tme if the other possible local source doesn't pan out. In any case, given the low cost of the components, I'll most likely wait until I have a definitive list (i.e. once I've advanced on the T5 project) to place an order.

Rikko, thanks again for your help and your proposal for more help. The loom itself should be straightforward (you're welcome to take a look at the diagram for inconsistencies); the only caveats being the integration of the bgm rectifier, and its effect on the blinker idiot light in the speedo. I'll have to double check the latter point before threading the loom into the frame, but I'm far from there yet.

One thing I kinda forgot about but occurs to me now, is that the principle of an iphone charger fed by the P2's AC would be a great thing to build. The phone usually handles 5h rides with continuous gps with some power left, but it would be nice to be able to take off without worrying about not having a full charge, or to be able to consider longer trips without thinking about charge breaks. I have a blueprint somewhere to build such a device, but I'll likely need advice when time comes to tackle that.