Harry

Consequent to using Alfa dipstick replacement I had a problem with “leak” from the speed sensor unit located on the top of the gearbox. At first I thought my trans shaft seal was leaking but after much cleaning I tracked down the source to the speed sensor unit. Expensive piece of kit that. I tried new O rings in the unit to no avail. Every test run I has a leak. Then after one run I shone my super bright LED torch down at the very clean speed sensor and a bubble appeared! It was coming from a tiny crack in the top of the unit and nothing to do with the seals.
Then it struck me. THIS BOX IS NOT SUPPOSED TO BE PRESSURISED.

I had kicked an own goal. The Alfa stick has a sealing O ring whereas the original dipstick has a convoluted path to atmospheric pressure. I solved the problem with a Dremel cut behind the O ring and all is well. Without the pressure the speed sender doesn’t leak

Still a good cheap fix but make the above modification