Art, that sensor in the bell housing isn't ringing any bells other than the Monotester name I've probably seen in an old manual. Reading elsewhere on the UK Club site it seems 1984 may have been the last model year that the sensor was fitted on the 240s and earliest 740 years with the connector living on a bracket up above on the firewall. 1984 was the end of the K-Jet years, so that kind of suprises me we're talking about an '89 780 LH 2.2 here that has one. The first 240 I had intimate relations with was our '85 240 LH 2.2 B230F, so I don't have your 240 K-Jet experience. I don't recall anything magical about that area of the bell housing or a connector that I didn't know what it was for. Previously I was K-Jet on my '74 140 B20F so it appears I may have skipped over the Monotester years? Mind you, as long as I could figure out how to hook up a dwell-tach and adjust the valves I was good to go for most of the advanced maintenance I needed to do way back then. I'm wondering if the OP's 780 possibly has an older engine transplant. Also, the low production 780s did not come off the normal production lines, so there's another possibility.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now