Hi. Can anyone tell me how to test for heat-related faults in Visa flywheel sensors?

Apart from the two sensors fitted on the engine, (one of which fails in hot weather, when fully laded, after a few miles at motorway speeds), I have six second-hand spare (2-wire) sensors, so I would like to test those to see whether any are better than what's currently fitted. I have tested them all in accordance with the Citroen workshop manual with a volt-meter, (0.5v to 2v without the magnet activated and then 5v to 7v with the magnet activated). At ambient temperature all but one passed that test, as did the (faulty) one on the car.

Someone told me that CX sensors might be the same. I found something on on a CX forum about putting the sensor in water and checking the resistance across the terminals while bringing the water up to boiling point. Apparently the resistance should remain at around 70 ohms regardless of temperature. If not, the sensor is faulty. However, on the Visa sensors I get no reading (open circuit) between the terminals or a much higher reading (about 500 ohms) between the brown wire and the sensor body (earth). So it seems that CX sensors are different, so that's not much help! (I've tried heating the faulty one up to boiling point: the resistance stays the same).

Can anyone help to clarify how to heat-test Visa sensors? Thanks.

I can only say this;

Are you sure it is the sensor and not the black-box or coil????
Are you sure it isn't a (nearly broken wire) in a sensor or a bad rusty connector???

Some people are using a modern replacement sensor... or a new ignition

Thanks lpgo. The black box is mounted inside the car, where it's not affected by heat. I tried another coil when the fault happened, so I'm fairly sure about that. The connections are all good (I made sure of that when I fitted everything). A nearly broken connection in a sensor seems the most likely cause. When I fitted the engine to my van last year, your 1-2-spark (for Visa) was still in its early stages so I used the original system plus made an adapter for 2cv points, as a backup, (which I needed, and gives better performance than expected). I don't trust 123: I've seen too many faulty ones on 2cv engines. When your system is ready for Visas, I will buy one, but for now I need the Citroën system to work.

Update:
I decided to do the voltage test in the w/shop manual again, (0.5-2v & 5-7v) while in a pan of water, moving the sensor nearer to and further from the metal of the pan to activate the switch inside, back and forth, watching the volt-meter flicking between the two readings, while the water got up to boiling point.
I started with the one that had been fitted in the lower position on the car: The one that I suspected had been failing when hot. (This one is located right behind where the upper and lower barrel/head cowlings meet, so gets hottest). The voltage flicked back and forth as it should at first but before the water got to boiling point, (maybe around 85 degrees?), it stuck at the higher voltage (just under 7v) and only returned to the lower reading (about 0.7v) several seconds after taking it out oif the water. All my spare ones working properly right up to boiling point, so I concluded that I was right about it being that sensor failing when hot. Then I removed the upper sensor from the car and tested that: it lasted a bit longer than the the first one but it too failed in the same way just before boiling point (maybe 90-95 degrees).

It's difficult to guess how hot the sensors will get in use (and whether they would get hotter under a 2CV bonnet than in a Visa), but I have decided to fit two of the ones that worked up to boiling point and I have made up a cooling duct around the sensors, by adapting a disc cowling, (& fed in the same way).

Now, having found that both the sensors on the engine are faulty, this leads me to another question!:
When I put this engine in last year, I just picked two sensors from several second-hand ones that I had taken a chance on at flea-markets etc. It seems a bit stange that the two I picked at random are now faulty whereas all but one of the others appear to be ok. Could a fault elsewhere in the system have caused this fault in the sensors? I know all the earths are good. The control unit is mounted inside the car in a place where it can't get wet. I have checked the charge rate: it doesn't go above 14.4v. Any ideas? (or am I just being paranoid?!)

Has any welding been done to the car, without disconnecting the 'black box' as well as the battery?

No but thanks for the warning for future reference.

Also a possibility... Did you ever had a failuire in your connection to the coil when the engine ran??, The ignition will backfire into the sensors then... (did cost me a black box and sensors once)...

When it fails at motor way speeds does it still tick over .. Is the vacuum advance working driving fast 10degrees retarded may cause misfires after a while

When vacuum advance comes in it will ADVANCE the ignition by 10 degrees (not retard)

Vacuum advance will NEVER come in at full throttle only if the throttle is released a bit (lets say cruising at around 100 km/h or less it will kick in..

You can easily check it with a strobe...
disconnect the vacuumhose rev the engine to about a steady 2500 rpm, connect the vacuum hose and then the flywheel needs to move for about 3 tooth against the clock (standing in front of the engine looking at the flywheel)...

I don't think so.

The vacuum unit is working.
It wouldn't tick over or start until it had cooled down.
The first time it happened (about 1000 miles after fitting the engine) I was stuck on the motorway with the traffic moving at walking pace. After about 15 minutes, just as the traffic started to move, it died and wouldn't restart. I pushed it to the side and, although it had died more suddenly than a 2cv nomally does with a hot coil, I tried a spare brown coil, as that was the quickest option. This made no difference, so (as we were already late for a ferry) I just disconnected the electronic ignition (& brown coil) and connected the points (& black coil). (I had prewired it to make it easy to swap over). It started instantly so I left it on points until we got off the ferry in France. I put it back on electronic ignition and it was fine until for a couple of hundred miles of mixed driving but then died after about half an hour of flat out motorway driving in Germany. I put it back onto points and that night tried a spare sensor. That lasted about 100 meters before dying, so I put the original sensor back on. We did the rest of the trip on points until queing for the ferry to return to England, when the condenser failed, so I put it back onto electronic and drove home ok (120 miles, taking it easy).
It great isn't it: I fitted two ignition systems and they both failed on the same trip! When I buy a 1-2-spark, I'll have 3 systems to choose from!

I drove it today (with my new cooling ducting fitted over two of the sensors that passed the boiling water test yesterday) about 85 miles of foot to the floor motorway driving. It was ok but then the air temperature low and the car wasn't fully laden. La Rochelle will have to be the real test.