The issue: LH 2.4 requires a Vehicle Speed Signal in order to detect engine load conditions, and to engage fuel supply cutoff on throttle over-run. Without it, the car will use fuel when coasting off-throttle and the idle will be all over the place.
The solution in the past for 240s changed to LH2.4 with these issues has been to change the rear axle to one with a tone wheel, and use an electronic speedo, so that LH2.4 gets that speed signal.
But what if you don't want to change the diff, and you wanted to retain your 240's original pre-1981 cable-driven speedo?
Following at suggestion from @timbo, I've just got a hold of a Brantz BR4 sensor, which connects straight up to a M40 / M41 / M45 / M46 / early AW7x speedo connection at the gearbox.
This device has a suitable female M18 x 1.5mm thread for attaching to the gearbox ferrule, and a male thread on the other side for attaching the speedo cable.
Here t'is, attached to an M46 ferrule.
It provides 4 pulses per revolution with a 200mA sink capacity and requires a 5v to 13v power supply. For the pulse conditioner (and in order to get the required 8,000 pulses per mile) I'll add a home-brew circuit incorporating a Schmitt trigger and pulse multiplier, fashioned out of couple of 7400-series ICs. Thanks to cleanflametrap's research on the signals generated by a genuine LH2.4 speedometer, it should be pretty easy to get the circuit to get the correct pulse shapes that will make the ECU happy.
These BR4 units will come in very handy for folks wanting to put LH 2.4 into their pre-81 240s and retaining the original speedo cluster, and who don't want to go through the tedious process of changing the rear axle.
Available here:
https://www.brantz.co.uk/product-page/european-gearbox-sensor-br4