Electrical Explanation Requested

The Hallsensor has a magnet... this has a magnetic field.. this field is disturb by tooth on the flywheel and this disturbtion is "seen" by the Hallsensor
The Hallsensor (HS) is a semiconductor and gives a small signal... not really a signal, it makes connection (the semiconductor, just like a transistor, connectts Emittor and Collector ... the Basis is the magnitic field)
To get a better signal there are two transistors in the compete sensor as bought. These two transistors make the signal stronger.
you are right that its strange that there is no ground.
I think there is not 12V going in, but 12V coming out of the speedometer and that 12V is switched to ground (brown wire??) .. the more its switched to ground in a period of time , the higher the speed i suppose?
(sorry for my bad english)

PS do you have a pic of the rear with the pcb layout? I see three incoming connections of the Hallsensor... 2 resistors 120 and 12k Ohm and then 2 condensators and 2 transitors.. finally a Zenerdiode... withe the PCB layout a can make a circuit diagram.

In your schematic of your car ... where is the Yellow and where is the brown wire going to?

 

I took a look at the wiring diagram. One of the wires is V (green) and is the battery plus via the oil meter and a fuse. So thats power supply.
The other one is RN and is the signal wire towards the speedo connector 1>> This wire will be 12V -0V (I think it will be higher then 0V... approx 4,7V or so... but the voltage drop 12-4,7 is enough to trigger the speedo) when driving and the frequency will depend on the speed.
I will think over your initial question: why is it blowing up so fast? (Is you Alternator voltage when running a nice 14V? Or much higher?) (First reaction on saving the sensor is put a condensator of 10uF 12V on the V=green wire to dampen the power supply>> is you oilmeter giving a constant pointer or is it continious moving a bit?)

(I'm typing on a Mac... how can I change to English grammar correction fast??)
Greetings from Holland

 

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Here is an idea of how it can work with two wires... right of D3 the two wires to the coil are now going to your Speedometer.
The higher one at the anode is the plus, the lower one at kathode is minus AND signal... your second transitor is for power instead of B1 in this schematic.
(But it can also be grounded via the gearbox..) I have to see the sensor for that

 

It's a magnetic reluctance device, but I believe that there is no permanent magnet involved (current flowing in a coil of wire generates the magnetic field). On the diff pinion shaft there is this 4-pole metal "trigger wheel" 116352:

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item 2 here:

https://www.ferrariparts.co.uk/diagram/ferrari/328-gtb-gts-(1985)/027-lay-shaft-gears

When one of the 4 metal arc-shaped "leaves" passes close by in front of the two metal pieces protruding from the end of the 116987 sensor it changes the reluctance of the magnetic path between the two metal pieces (and the sensor generates 4 pulses for each revolution of the diff pinion shaft from this change in magnetic reluctance).

Adrian (member alhbln) has done a bunch of work repairing, understanding, and reverse engineering the 116987 sensor. Try a search on "116987" posted by member "alhbln" for his prior threads.

 

I'd suggest re-flowing those solder connections even if they look okay. Failing solder joints would account for intermittent operation of the unit, and sometimes a solder joint can fail even with no visual evidence of a problem. Re-flowing is easy to do and worth a try.

 

Hall effect sensors need a constant current that is then deflected by the magnetic field to create a voltage perpendicular to the current flow. Reflowing the solder joints is a good idea, particularly where the wires attach to the board. Constant flexing can break solder joints. make sure your wires are supported.
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Don't follow you here -- are the rear wheels rotating (signal should be present) or not rotating (signal should not be present -- maybe your Fluke meter is autoscaling and just picking up a small noise EMI signal from the ignition system)?

 

The transistors - what are they ? The zener is at 33V ? I guess it is meant to protect against overvoltage spikes, but maybe it has blown up and taken the transistors with it.

Depending on fault mode in those transistors, they might still work, but not according to spec.

 

Welcome to the club -- my speedometer stopped working about 5 months ago so I bought a new 116987 sensor (which I had replaced once before about 10-15 years ago) thinking I'll just replace it again as the first "fix". Next day, it started working again, and has been fine since. Can't fix it if ain't broken -- but I'm prepared .

 

My speedo stopped working. It would start working again as I drive by a Lexus dealership on the freeway as if it were saying "please don't trade me in." It continued doing that for a few months until I changed the sensor and then all was well. And then my tach started displaying half speed. I removed it and "adjusted" it to be much more enthusiastic. And of course, the fuel pump died soon after.

 

What I told before...its a full time job to keep our cars on the road.