Does any one know what this (automotive thing) is ?

No idea. Playing around, could it be some regulation for an alternator?

Thanks - That was my first guess but I'm looking for more confidence. If it was mine I'd open it up.

I found this by searching for "transorb symbols"

So my guess is that its a 14.5V transorb.

Won't be able to tell for sure without opening it unless someone recognises it.

MK

I'm going with a Transient Voltage Surge Suppressor.  Probably a Zener back-to-back with a standard diode since it's on a DC circuit.  Basically a crowbar circuit.
We use anti-spike diodes all the time with relay coils, right?

Varistors and MOV's are used regularly on field windings of alternators.  I think javagoza is on the right path.

https://en.wikipedia.org/wiki/Transient-voltage-suppression_diode#:~:text=A%20transient%2Dvoltage%2Dsuppression%20(,spikes%20induced%20on%20connected%20wires.

My first guess is its a load dump protection module. (which could be performed by a beefy transorb)  But again, this is just a guess.

back to back schottkey diode used for voltage suppression 

https://www.electronicshub.org/types-of-diodes/

12. Transient Voltage Suppression Diode
In semiconductor devices, transients will occur due to the sudden change in the state voltage. They will damage the device’s output response. To overcome this problem, Voltage Suppression Diodes are used. The operation of voltage suppression diode is similar to Zener diode operation.

The operation of these diodes is normal as p-n junction diodes but at the time of transient voltage its operation changes. In normal condition, the impedance of the diode is high. When any transient voltage occurs in the circuit, the diode enters in to the avalanche breakdown region in which a low impedance is provided.

It is very spontaneously because the avalanche breakdown duration ranges in Pico seconds. Transient voltage suppression diode will clamp the voltage to the fixed levels, mostly its clamping voltage is in minimum range.

These are having applications in the telecommunication fields, medical, microprocessors and signal processing. It responds to over voltages faster than Varistors or gas discharge tubes.

The symbol for Transient voltage suppression diode is as shown below.

Wow great reference link.

I showed the photo to a vehicle mechanic just now (approx 30 years experience) in case he recognized it, and he's never seen such thing : (

Hello,

It's wired across the battery supply between the alternator and the battery.

The symbol is for a 14.5 V bidirectional transient suppressor (what I call a "transorb").

It makes sense.

If the battery is accidentally disconnected from an alternator driven system then , depending on the alternator regulator you can get a big surge in the alternator voltage. (Google "load dump")

When I designed automotive stuff we routinely fitted load dump protection, usually a unidirectional transient suppressor across the nominal 12V supply  (like a zener diode but designed for lower slope resistance and the ability to deal with large single pulses).

I've never seen one installed in the vehicle on the wiring harness before - but this is a tractor !

And the symbol was new to me:

One of the symbols shown on Wiki is similar to the one on the mystery part.

https://en.wikipedia.org/wiki/Transient-voltage-suppression_diode

Thanks to all who offered suggestions.

MK