The system serves to turn off and turn on the carburetor pneumatic valve (blocking the vacuum supply to the forced idle economizer) at forced idle. The main unit of the system is the electronic control unit (pic. 251) type 25.3761. It turns off the pneumatic valve 4 when the engine speed increases above 1600 min-1 and turns on the pneumatic valve when the speed drops below 1200 min-1. Control pulses (proportional to crankshaft speed) are fed to the unit from the end of the primary winding of the ignition coil.
The control unit can turn off the pneumatic valve only if the contacts of the carburetor microswitch 3 are open, that is, if the carburetor throttle control pedal is not pressed. If you press the pedal, the pneumatic valve will not turn off or turn on if it was turned off.
Before checking the control unit, you must make sure that the wires are connected to it correctly. Check the operation of the unit with a voltmeter (with measurement limits 0-15 V) in the following order.
Disconnect the wires from the microswitch mounted on the carburetor. Connect a voltmeter to the control unit 2 (pic. 252) using a special connector. Start the engine and, gradually increasing the speed, monitor the voltmeter readings: after starting the engine, it should show a voltage of at least 10 V, and at the moment the pneumatic valve is turned off, an abrupt decrease in voltage to a value of no more than 1.5 V. After turning off the pneumatic valve, the frequency is gradually reduced rotation until the pneumatic valve is turned on. In this case, the voltmeter should show an abrupt increase in voltage to at least 10 V.
It is allowed to check the operability of the control unit without a voltmeter and adapter connector by the characteristic sound of the valve when turning it off and on.