Vespa electrical help...

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Nelsonmd
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Thu Jul 06, 2017 3:12 pm

So I haven't been here in a while, due to only having time to work on my daily rider V-strom, and my wife's daily rider, her adorable Vespa ET4. Anyway, I'm having electrical trouble with the vespa, and the vespa forum I'm on is not proving helpful, so I'm hoping some of y'all might have some insights.

Here's my problem, pasted from the other forum:

A little background:
I replaced the stator and flywheel after the whole thing grenaded itself stranding my wife earlier this year. Now last night as we drove to celebrate our anniversary, the thing just died, stranding us both in downtown Boston, and we had to uber home to get my car to carry it home (I'm getting good at loading the vespa into the back of my Outback...). It turns over on the starter, but never gets going on its own.

Currently...
I have run through the electrical tests, and the pickup resistance is correct, but the pickup voltage during startup is only 0.9V. I took the fan cover off, loosened the 2 pickup bolts, pushed the pickup towards the flywheel, and tightened the bolts down again. Put everything together and repeated the pickup test, got 1.2V and the engine started, but not well, and it required throttle. I tried pushing the pickup sensor again and I got the voltge up to 1.1-1.2V on startup, with it reaching 1.5V and starting if I give it some throttle. I also put the old pickup in and spliced it into the harness, and got similar results.

So to summarize:

New Pickup
Resistance: 120 Ohm
Voltage during startup: 0.9V - 1.2V
Voltage with throttle: ~1.4V (starts, but needs some throttle for a few seconds to get into stable idle)
Gap: 0.010 in

Old pickup
Resistance: 108 Ohm
Voltage during startup: 1.1V - 1.2V
Voltage with throttle: ~1.5V (starts, but needs some throttle for a few seconds to get into stable idle)
Gap: 0.008 in

Voltage spec: >2V
Gap Spec: 0.01-0.02 in
Resistance Spec: 105-124 Ohm

The manual says that the voltage on the pickup will increase with higher rev's, but it seems that I should be reading higher than I am getting, and I am having trouble getting it started and idling.

I have also measured the resistance of the wires going to the ECU, and they are good, maybe around 1.5 ohm additional, but still overall within spec for testing at the ECU connector.


The pickup is a single wire sensor that's grounded through it's mounts. Anyone got any thoughts?

And because we all love pictures, here's her on it this winter (she rode to school every day through the entire winter, longer than I did!)
Image

And here's the poor vespa crammed into the back of my outback to get it home the last time it stranded her. I got better the second time around. Not a skill I particularly wish I had.

Image
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radare
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Thu Jul 06, 2017 8:30 pm

I'm not familiar with the ET4. On old girlfriend had the Piaggio Fly which was a neat little scoot.

Before you start it, what voltage is measured across the battery terminals? Then after it starts (even with throttle), if you can keep it at 3k or so (without the automatic clutch kicking in), what voltage is present across the terminal?

Pickup coils are usually frequency devices, not voltage. As the engine rolls around, it creates a signal indicating the engine RPM. Based on that signal, the ECU knows where the engine is in rotation and knows what to do with timing and injection. What is the Vespa procedure for testing the pickup coil?

Oh, and is the ET4 a fuel injected bike or carbureted?

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Nelsonmd
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Fri Jul 07, 2017 6:29 am

It's carbed. The procedure from the vespa manual is to measure the voltage in AC mode and the voltage should be >2V. It does say that the faster the engine turns, the higher the voltage should be, and I think that is shown in the fact that opening the throttle a bit causes an increase in voltage on the pickup.

I'll check the voltages tonight. Thanks for the help!
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Sat Jul 08, 2017 11:37 am

Well I measured the voltage, and I get 12.5V with no load, and it goes up to 13.85V while idling, and it can just barely get to 14V when revving (rear wheel was spinning, so i was well beyond the auto clutch kicking in).

I did a diode test on the RR and it looks fine (no specs from vespa). I don't have any specs for a no load test on the stator (though the stator is brand new, so that shouldn't be the problem); stator resistances are in spec.

I can't get the HV coil test to work, so I'm still unsure about the HV coil. I should be getting over 100V, but I'm not getting any reading at all. The thing does start and run, so I know the coil can't be totally dead.
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Sat Jul 08, 2017 1:54 pm

I'm sure you have this, right? -> http://www.wotmeworry.org.uk/manuals/ET ... Better.pdf

What engine is in her ET4 (50/150, etc)?

Have you confirmed you have good blue spark when you are trying to start the scooter (and does it spark rapidly as it should)? Are you certain this is an ignition issue?

Anyhow, onto tests:

Plug Resistance cap - 5k ohm

Pickup coil - You've tested this and it's within spec. 105 to 124 ohm.
Voltage when starting - between terminal 2 and terminal 8 in the connector - >2V AC (make sure you test AC voltage).

HV coil Primary Circuit - Resistance between Violet wire (terminal 3) and black wire (terminal 8) - 0.4 to 0.5 ohm.
HV coil Secondary Circuit - Resistance between the plug terminal and the negative lead of the coil - 2700 ohm to 3300 ohm.
Voltage when starting - between the positive terminal and teh negative terminal on the coil, while not disconnected from the CDI ->100V

I am suspicious of your ignition coil. They can fail hot and exhibit odd behavior or have the secondary winding fail with usage. On my Triumph, the coil failure presented itself as a stumble at 2k rpm. It started fine, idled fine and accelerated fine. That made it a real PITA to diagnose.

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Nelsonmd
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Sun Jul 09, 2017 2:59 pm

Yeah, I've got that manual. Been through all the tests I can.

It's a 150.

I've been through all the tests and they have mostly checked out except for the pickup voltage, per my first post, and the HV coil voltage test, which I have been unable to get a reading for, perhaps because I don't have a peak reading adaptor. I've been using my DMM in min max mode for the max readings, but I'm guessing the adaptor does something different.

I remeasured this morning, and the spark plug resistance is now consistently out (high R), and the HV coil primary circuit is now measuring 0.9-1V. So I have ordered a new HV coil and spark plug cap. I've suspected that the spark plug cap was a bit off as it doesn't always hold onto the spark plug real tight (seems to be a common thing in some of these vespas). Hopefully that will solve my problem.
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Nelsonmd
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Mon Jul 17, 2017 9:51 am

Well, I got the new HV coil and spark plug cap installed and it is working much better. I suspect that the starter motor might be a bit worn out, and is not getting the full speed during startup, which is why my pickup isn't getting to the full voltage.

So basically I think it's "fixed for now". We're moving to Cambodia for a year starting this january, so I'll probably wait to replace the starter motor until we get back.

Thanks for the help Radare! Your questions made me go back and remeasure everything, and I got different results than the first time.
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