Modern Vespa

The fan will usually come on quickly after starting and runs on and off but not continuously. To me, this seems like a faulty temp sensor and not a fan relay.

If fan relay had contacts welded together, it would appear that it would run continuously.

The temp gauge stays consistently at the 1/2 way mark so engine is not overheating.

The temp sensor looks to contain 2 sensors from wiring diagram. One sensor goes to temp gauge on dash and the other to the ECU for control to turn fan on/off.

Questions:
1. Is this a failure mode of the temp sensor where only 1 out of 2 sensors will fail but not both at same time?
2. Could the fan relay still be a culprit with the pickup coil and contacts failing?
3. Until it gets repaired, any harm to continue riding in Florida?

My thoughts are to replace both the sensor unit and the fan relay.

TIA

I don't know what the connector(s) on the sensor look like but is it possible to switch the wire to the ecu with the wire to the dash as an experiemnt?

As you might have to buy a new sensor anyway, swapping the wires over is a good diagnostic manouver. Just make sure it's all watertight (heatshrink with glue inside or 'liquid tape') and all should be good.

Steelbytes, jimc - thank you for the responses.

Unfortunately the scooter is not near me to do much more diagnostics, such as hands on. When it was in my hands, the fan was doing the same, but seemed to have stopped for awhile after a good coolant flush, change and bleed. The coolant was low but I could find no evidence of a leak.

I am recommending that a new sensor and fan relay be purchased.

Since then, the person took it into a Vespa dealer and the dealer said the culprit might be an air bubble. She disputed that since she knew that I had bleed the system. She asked him if it could be a failed temperature sensor and he conceded it might be but would have to remove the windshield and headset to get to the circuit board. Why that is, I don't know as the sensor going to the gauge cluster works fine and no need to go to the instrument cluster. Makes me wonder about the dealer???

Although it's possible the relay has shorted itself out due to contacts arcing once too often, it's extremely unlikely for the fan relay, though not unheard of for the starter relay which has much higher currents.

Just swap the relay out for one beside it to test - no need for a new one.

If the temp gauge doesn't give any indication of getting warmer than normal, then I'd definitely point a finger at the temp sensor circuit - double check the blue/yellow wire isn't intermittently shorting to frame.

I doubt it is the fan relay as it goes on and off, although it did run continuosly recently but on cool down and next start, fan did not come on and coolant temp ran up to 1/2 way and stayed there.

The wires are a good possibly but I still think the sensor is funky. I don't have a logic diagram or something that shows what happens in the ECU but it would have to drive the blue/yellow wire to ground to pick up the fan relay, so grounding that wire prior to ECU would cause that.

Too bad scooter is in Florida and I am in Vermont. Hmmm, gets much colder a house call may be in order!

UPDATE

Since the last post, the scooter has been in to a Vespa dealers shop for repairs.
and per the invoice:

Connected to PADS and temperature reading not accurate
Replaced Thermic Sensor p/n 640485
Replaced Thermostat p/n 82831R5
Replaced Assembly Thermostat Cover p/n B013477
O-rings and filled and bled system
Test rode and final PADS check, fan cycles as should

Well my friend rode it back home from shop and fan was almost constantly running. This was a 20 min, 9 mile ride in 65 deg weather. She called the shop immediately, was told they had taken it on a 7 mile ride, told to bring it back in and he will check the relay.

She was able to bring the scooter back a couple of days later, on ride there gauge got to 1/2 way mark, fan turned on and stayed running constantly. Again outside temp was 65 degrees. Got to the shop, told tech she was there and waited 1.5 hrs outside until he came out. Temperature sensor was indicating 125 deg C from PADS. Left scooter there as tech said it could be a shorted wire or the ECU.

There are several relatively simple troubleshooting steps that can be done and I suggested those to her to see what the tech does.

Will keep this thread updated with final resolution.

love replacing other parts as well 125C after sitting for 1.5 hours with the engine off - seriously ouchy feeling. Normal riding is 85-87C when there's decent airflow. Fan should turn on at 105 or 110 (can't remember which). Wonder what was the peak when running (no way of knowing). Not going to say the next thought in my head.

Makes me wonder if they actually put in coolant and bled properly after replacing the thermostat/thermistor (yes they claim they did). iirc have to bleed when cold and then bleed again after cycling the temp above 85 so as to make the thermostat open and allow flow.

I'm doubting it's the ECU's fault. Have seen many who don't understand things terribly well jump to such conclusions before. Although the wiring between the ecu and the thermistor could be worn.

If you look at the table of degrees C vs ohms for the temp sensor, the highest reading is 80 deg C which is at 300 ohm as the temp sensor is basically a variable resistor. I graphed the temp vs ohms and since the 125 deg C is off the charts, it indicates a short.

Now that can be tested as the light blue/green wire from the sensor goes to pin 9 on the ECU. The other wire, grey/green, goes to ground and pin 18 on the ECU. With a multimeter, the wire going to the ECU pin 9 can be checked to ground. Of course if it is an intermittent ground...

Another possiblity, which would confirm operation of the ECU, is to put a 2.5k ohm resistor on pin 9 and pin 18 on the ECU. This should read about 20 deg C, so that would confirm the ECU is reading the input properly.

Suggested the above to my friend who sent it to the tech with a food for thought note. We will see.

what table? I'm not sure you reading it correct (or the wrong table) as I'm 110% sure of the temps I said above (for a gts300 which I assume is the same as for a 250). Maybe there is a x 2 missing in your reading of the table, ie max of 160C at 300 ohm, or there is a+40C offset (suggesting this as the value on the canbus is +40C offset). Also the workshop manual does say the thermostat doesn't open until 85C

edit: add offset 40

Haynes Manual Chp 4 on the cooling system has a table of temp vs ohms, starting at 0 C for 5.9k ohms up to 80C for 300 ohms.

Also states normal operating temp for GTS250 is 69.5-72.5 C

ok, interesting to see the 250 thermostat is different. but doesn't mean the temp sensor is different. Also the table might be incomplete eg the temp sensor might for example give 150 ohm at 120c

regardless though, my earlier about 125c being ouchy ...

Yeah to me the 125 C reading indicates a short or close to 0 ohms. I am leaning towards a short either in the connectors themselves or somewhere in the wiring.

I doubt it is the ECU.

I don't know what troubleshooting this shop will do.

I'd just edited my reply ... more speculation

although the highest in the table is 80C/300Ω doesn't mean that it's the highest valid value from the sensor. eg could be 120C/50Ω or something like that.

True but when it is graphed, it is a very steep line from 30 to 80 C and extrapolating that it goes to 0 ohms around 90 C if that line slope is followed.

Had a call from the dealer working on my friends scooter. He wanted some more information concerning the troubleshooting items I sent him.

He said when PADS is connected and scooter running the temperature is bouncing all over the place.

One item I suggested was to connect a resistor of like 1.7k ohms across the sensor pins and see what PADS reads. He found at 1.8k ohm resistor connected it across the sensor pins and PADS read 28 C, so that is reading correctly.

I suggested that he start the scooter with the 1.7k ohm resistor and see what PADS reads. Waiting to hear back.

I also explained about testing the sensor wire to ground and shake the wire harness to see if intermittent ground. Waiting to hear back.

It may be an ECU gone bad. Not a common occurence from what I see.

Just ask the tech to measure the resistance of the sensor with the engine cold.

It is a new sensor so should not be an issue.

so cable or connector. maybe as simple as tarnished or loose connector (probably at the sensor end not the ecu end). unplug the ecu and stick the meter into the correct pins on that then wiggle the wires and connector

We believe it is fixed. The tech did all the testing I suggested and cleaned all connectors. Since that time, the cooling system fan has been coming on and going off at the correct temps and temp reading thru PADS is stable.

It really appears that this was a connection issue.