Modern Vespa

Even after fixing the polarizer, the LCD panel in my '08 GTS250 is not really readable in bright sun. So I'm swapping in a speedometer/dash from a 2013 GTS300 Super, primarily to get analog fuel level and water temperature gauges.

I'll document the process here and if I make any changes so if any of you would like to do the same, you can. FWIW it should be less work than the polarizer replacement was to just make the swap and effectively does the same thing. You can still get new GTS300 analog gauges like this in km/h for about $120-150 from various places including Scooterwest. I found the used/1600-mile mph speedo on eBay for $77. Had I known how poorly the LCD performs in direct sunlight, I would have skipped straight to this step rather than doing the LCD.

In my year-old thread when I first started investigating this, jimc posted the wiring diagrams that I used to start this investigation:

https://manuals.wotmeworry.org.uk/Vespa/GTS250/GTS250%20Wiring%20Diagram.pdf
https://manuals.wotmeworry.org.uk/Vespa/GTS300%20Super/GTS300%20Super%20Wiring%20Diagram.pdf

This is not a direct swap. The two speedometers have different connectors, so you can't just unplug one and plug in the other. My plan is to cut off the GTS250 connectors and splice in pigtails for more standard automotive connectors. I got these: https://www.amazon.com/dp/B0BHN5MTF6?ref=ppx_yo2ov_dt_b_fed_asin_title&th=1

First job was to map the wiring to evaluate the best option to make the conversion. Attached is a pdf of my Excel doc that I made working on this. This is to map how I'll splice them into the new connectors, of which I will have four.

The GTS300 speedo/dash does not have a rev counter or an ambient air temp gauge, so those wires are not going to be connected. Among the rest of the wires, all but three are the same color on the GTS250 and the GTS300.

The GTS250 dash has separate left and right turn signal indicators, and the GTS300 dash has one indicator for both in typical motorcycle fashion. I noted that the GTS250 wiring diagram does not show a flasher relay, but routes wiring to left and right turn signals and the turn signal switch, which makes me think perhaps the flasher function is integrated to the dash. In that case I will likely have to add a turn signal relay.

The idea is to splice the female side of the four new connectors onto the GTS250 wiring harness and the male side to the speedo/dash. The pigtail connector on the speedo will just be cut off and I'll solder and heat shrink the new connector on there. For the two "connectors" that are molded extrusions in the speedo body, I searched the world over and apart from chopping them off of a GTS300 wiring harness, I couldn't find any way to fit connectors that will mate directly to the speedo. The current plan is to just pull up the exposed buss ends and solder wires to each one, heat shrink the joint, and then zip tie the wires to the back of the speedo for strain relief using adhesive tie-downs. I may revise this plan once I have the new speedo in hand (it's due tomorrow). I figure someone's going to suggest that this has to be waterproof, but the original connector is not waterproof so leaving some exposed conductors is not going to be any more of a problem. It's not like I have to operate this submerged.

I'll lose the rev counter which I couldn't see anyway in the sun and also the ambient temperature sensor that reads about 8 degrees high. But I'll be able to see the water temp and fuel level which is a good trade IMHO.

Watch this space for more progress, most likely to be updated significantly either by this coming weekend or in about three weeks.

The 2013 speedo came in, and I took another detailed look at the wiring diagrams.

In fact the 2008 scooter doesn't have a conventional turn signal flasher relay, so I'll have to add one for the 2013 speedo to work.

Small chance I'll get to work on this today, otherwise I'm leaving tomorrow for a trip and won't be back for another three weeks, then I'll get on it.

Maybe time to switch to led bulbs and a led compatible flasher

I'm definitely putting an LED flasher in it. I'll get some LED bulbs if I can find them with the right base. On a separate topic I'm trying to figure out how to switch the tail/stop light to LEDs, it's not super straight forward for an old GTS. Already switched the headlight. I have a brake light flasher to install, needs LEDs.

While you have all this apart maybe consider adding leads to the digital clock and wiring it to a step down transformer. Horrible choice on Piaggio's part having this powered by a watch battery.

Trying to make a DIY Euro Turn Signal Kit (Post 2569440)

I'll tuck the turn signal flasher under the horn cover, should fit there without issue.

The turn signal bulbs have a weird base, but I managed to get a replacement bulb to work when I had a bad one on one side of the rear. I have a bunch of replacement LED bulbs that can be made to fit but they are all white, and the lens is white so I need amber LEDs. I haven't gone down the rabbit hole of finding the right brake/tail LED lamp, I'll do all that when I get back home from my current trip at the end of the month, convert the speedo and work out the LED lighting and brake light flasher.

The clock battery is dead on this speedo I have, so that's a good tip. Probably needs the battery for backup, but should definitely get power from the scooter's batt.

OK can anyone confirm these will work?

turn signals:
https://www.amazon.com/gp/product/B07S6SGH4Z

tail/stop:
https://www.amazon.com/gp/product/B0CLXXZ8ZQ

what base type are they? eg are they the correct BAZ9S or?

They are BAU15S ... is that not the correct one? I'll dig through the parts list and verify.

Back in town, looking closely at this with an ohmmeter.

The flexible PCB on the back of the '13 speedo appears to be polyimide/mylar substrate which Gemini says I can solder to with low-temp. The fuel level, coolant temp, switched +12V and ground all have a PTH with a screw going into the body, so I can put a ring terminal on them and prevent having to solder to those pins on the connectors.

The gauge illumination power and ground are all supplied by the pigtail connector, so that's easy to splice in my connector.

All of the indicator lights except the check engine light are only accessible via the connector, so I'll try to solder to these terminals. The oil light looks like it's connected to common +12V and the terminal switches ground. The rest, except the turn signal, are common ground with swiched +12V. Looks like I'll only have to solder about five of the mylar. I'm tempted to drill a little hole in the connector mylar where it's exposed and use a #3 or #4 screw and a ring terminal like the +12V/GND on the pigtail connector C. I guess I'm a bit worried that I'll melt the mylar substrate and ruin it, but the drill/screw method can be a backup plan in case I do ruin the substrate. When I solder this on, I'm gong to do it on the exposed end.

This project stalled because my temp-controllable soldering iron was broken and I had to send it back for warranty replacement.

During the time I have had to think about this, I am rethinking my original plan. I'm considering trying to fab a connector somehow. I have some ideas. But mostly I'm beginning to be convinced that I can't likely solder to this mylar flex PCB without melting it.

Alright, I got the '13 speedo installed and everything works except the fuel gauge.

According to the wiring diagram, the '08 and '13 have the same wiring colors coming to the speedo. There are two wires: white/green which looks to be fuel level from top of the resistor in the sending unit, and gray/black which looks like a switch to ground for the low fuel light. They appear to be independent of one another.

My GTS has a pretty much full tank of gas. I measured about 4 ohms between these two wires coming from the sending unit, which doesn't make much sense. Coming FROM the speedo, I measured 12V on the white/green wire and about 6V on the gray/black wire.

Anybody have any info on how this is supposed to work? Or is it possible my new-to-me speedo just has a faulty fuel gauge? This would be extraordinarily annoying since that's the entire reason I replaced it, so I can read the fuel gauge.

With any luck the low fuel light might work. IDK. I'm going to unhook the sending unit end and try doing some algebra on the connector to see if I can get it to do stuff.

FWIW this speedo is massively easier to read, not just the gauges but the speedo itself. Huge improvement. I just need to get the fuel gauge to work and I'll be pretty stoked about this mod.

Alright, I know nobody cares, and believe me if I could snap my fingers and have my '08 GTS250 be transformed into a brand new GTS300 I would. But I'm determined to make this resurrected and abandoned scooter into a reliable, usable and fun bike.

After thinking about this fuel gauge problem I think it is most likely a ground problem, that is >50 ohms of resistance to ground between the gauge and the sending unit.

The gauge I'm guessing is driven by a voltage divider. When the fuel level is high, resistance from the sensor to ground is low (under 5 ohms) so the voltage at the sensor input on the speedo is low. As the fuel level goes lower the resistance to ground increases up to about 50 ohms or so when it's empty, and the voltage at the pin is high. I measured the voltage open circuit to be like 12.1V.

In any case I'm going to take it back apart and armed with the knowledge of the range of resistance that the sending unit should provide, I can stick resistors on the pins of the sending unit connector to ground and with any luck get it to light up.

I did verify, Piaggio used the same fuel sending unit on the old GTS250 I have and the analog-gauge '13 GTS300.

FWIW, I'm following along. I'm interested to learn what you figure out to get it sorted!

I know it's been a long while but I have a slight update.

I thought I must have the two fuel gauge wires mixed up. So flipped the wires, tested it, did a bunch of voltage measurements, studied the schematic, etc.

My conclusion: the fuel gauge in my 300 gauge cluster is borked. Or the wiring diagram is wrong, like the pin on the connector that is documented to be fuel level is not actually fuel level. But either way, I am getting between 0 and 12 volts on both fuel level wires. With about half a tank of gas, one wire was at about 7V and the other about 9V.

One appears to be wired as ground for the fuel light, so the fuel light will glow brighter as that voltage approaches zero. The other is for the fuel gauge, which must be internally grounded, and should drive the meter towards F as the voltage is higher. The one with the lower voltage must be the warning light's ground otherwise the light will never light until the fuel gauge is below E. The higher voltage is for the needle gauge.

But in any case, connecting either wire to the fuel gauge should result in SOME movement of the gauge. It stays at baseline and doesn't move at all no matter what. So the gauge must be hosed.

I read elsewhere on this forum that there is some known fault with GTS300 fuel gauges that is fixed via some warranty repair. Maybe that's the magic. I'll do more research to see if I can sort this out.

This is pretty annoying because the entire reason I swapped this was to get a readable fuel level gauge, since the LCD "gauges" stock on a 250 are not readable in bright sun. At least I also have a readable odometer with the 300 gauge so I can use that as a fuel gauge, I just fill up when it gets to round 100 mile increments. I know the range is at least 100 miles on a tank, so I should be in good shape.

Meanwhile I don't really like the goofy way they do the low fuel light, and that's something I might be able to fix. I'm going to crack this open again and see what I can do to fix the fuel light weirdness and chase down what's wrong with the fuel gauge.

recalls on fuel pump? yes.
warrantee replace on some fuel senders? yes.
bad fuel gauge on dashboard? no.

Now I'm thinking I must have been measuring it with it connected and the key on. I'm going to take it apart again and check some more, maybe even rig some resistors to see if I can get the gauge to move at all or get the fuel light to come on.

Interesting post for sure, any update on the fuel gauge issue?
I'm about to do the conversion myself I got a used GTS 300 speedometer to use