So my 1990 Regina-equipped 740 is now giving me the code 3-1-1, "signal missing from speedometer." However, speedometer works fine. Does the signal travel from the speed sensor in the differential to the speedometer gauge and THEN to the computer? I don't get it.

Could this be related to the idle problems I've been having?

Any thoughts? Can anyone explain how the speedometer signal influences the computer's engine management?

Yes. Code 3-1-1 indicates the Regina system ECU on your 700 series vehicle is not receiving an indication of vehicle velocity.

Your 700 series has a sensor (magnetic pick up coil), probably mounted on the rear differential or transmission (not sure on 700-900-90) that sends the impulse signal to the speedometer via the vehicle wire harness.

The speedometer interprets the impulse count per period and presents that as the vehicle speed. The speedometer also outputs that value to the odometer motor and, via pin mount connection on the dash head PC (printed circuit) board, to a connector on the board that interfaces with the wire harness and sends the velocity value to your Regina ECU. I'm not familiar with Regina systems, so I'll guess the Regina ECU that controls fuel / air ratio and some emissions controls.

I'm familiar with the Bosch systems. Never owned a 700.

If your have the 3-1-1 code and the speedometer works and the odometer works, then the fault lay anywhere between the PC board connection inside the odometer, the connection to the PC board inside the dash head, the wire harness connector and connection through the monolithic 700 series wire harness.

The vehicle speed sensor information is used by the Bosch or Regina controls systems to further refine fuel trim at speed. The faster you travel, if you floor the gas pedal, the trim puts less fuel into the engine. At low speeds, more fuel is dumped. Sort of the horsepower versus torque matter that varies between transmission gear, engine RPM, and vehicle speed.

My 1992 240 GL with Bosch LH-jet 3.1 now has this fault. When coming off at high speeds the idle remains high. The speedometer works yet the odometer has failed. I though I had screwed it up replacing the large clock with a tach as the odometer failed sometime afterward.

I traced a broken solder weld (actually several) across the main 240 dash head circuit board and on the speedometer board the carries the odometer motor. Restored continuity. The two-decade old odometer gear is perfect. Whether it works in a half hour, well, we'll see.

cheers,

Man Friday.