Generating LH2.4 VSS from Cable Speedometer Cruise Sensor
The early 240s use a cable driven speedometer. When cruise control is installed, a VR sensor in the back of the cable driven speedometer is used. (click on pics for full size)
The later 240s use an electronic speedometer with a VR sensor in the rear differential. The electronic speedos also supply the VSS vehicle speed signal to the LH2.4 ECU. It's possible to use the circuit board from an electronic speedo with the cruise control VR sensor on an early cable speedo to generate a VSS signal for the LH2.4 ECU.
Here's a picture of the circuit board on an electronic speedometer (a 91 in this case), showing the main 3 pin connector in the center and the 3 single pin connector sleeves on the right. +12v power comes in the main connector and goes directly to the leftmost single pin connector. This needs to be jumpered to the center single pin connector (or you can just wire +12v directly to the center single pin connector). One side of the VR sensor goes to ground, the other side to the leftmost pin on the center 3-pin connector. Polarity of the VR sensor doesn't matter. The VSS signal to the LH2.4 ECU is the rightmost single pin connector. In the center of the circuit board is the stepper motor that drives the odometer gears. The blue and brown wires at the top of the board go to the speedometer needle galvanometer.
Here are pictures showing a hand drill driving a cable speedo, with the cruise VR sensor connected to an electronic speedo. Notice that the electronic speedo reads ~2.5x low. I don't know if this will work OK with the LH2.4 ECU for idle control, or if it needs to have a higher rate. (The electronic speedos use either 12 tooth differential sensors, or 48 tooth in the later ABS 240s.) If this doesn't work, it's possible to use an ABS front wheel hub sensor (48 tooth) with the speedo circuit board from a ABS speedo.
There are 2 screws holding the circuit board to the speedomter, and a couple pairs of wires. I unsoldered them but you could just clip them off. The rotor for the odometer stepper motor just falls out.
By itself, the circuit board draws ~1.4 watts of power. Most of this goes into the odometer stator coils. I think that you'd be fine wiring up the circuit board as shown above and wrapping it in electrical tape. The stator housing does get hot from the power (~115 ?F after an hour on the benchtop at 60MPH).
I went further on mine and removed the stator - clip the 4 plastic heat stake pins flush and unsolder the 3 winding pins. Without the stator, power was only ~0.4 watts. I also had a scrap of big heat shrink tubing but it wasn't quite big enough so I trimmed off the edge connector and wired directly to the circuit board.
If anyone tries this, please post an update on how the LH2.4 behaves -- does it have good idle even though the VSS rate is ~2.5x slower than standard? Does the Shift light (if equipped) do anything weird, or does it just not work?