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Vespa electrical help...

Posted: Thu Jul 06, 2017 3:12 pm
by Nelsonmd
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!)
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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.

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