Component Tests and General Diagnostics

The frequency valve is operated by pulsating voltage from the electronic control unit. By measuring this signal, certain functions of the system can be tested. A special tester (Bosch KDJE 7453) is recommended, but a high quality analog dwell meter or suitable "Duty cycle" meter can be used instead.
1. Connect dwell (or duty cycle) meter to two terminal test connector.
2. Start engine and run until warm. Disconnect oxygen sensor and observe needle reading (should not fluctuate). Place a piece of tape on the meter face to indicate the 50% position.
3. Remove fuel pump relay and connect jumper wire across sockets corresponding to terminals 30 and 87. Turn ignition "ON". Reconnect oxygen sensor.
4. The frequency valve should operate, making a buzzing noise. dwell meter should indicate 45-65°. Disconnect oxygen sensor and touch the wire to the ECU to ground. Readings on the meter should rise. Ground one end of a 1.5V flashlight battery, and touch positive end to the ECU wire. The readings should drop.
5. Connect oxygen sensor and start engine. With a cold engine, meter reading should be stable. When engine warms up, meter needle should fluctuate equally 5 - 10° to either side of the reference mark. It may be necessary to run the engine faster than idle to heat up oxygen sensor and cause needle fluctuation.
6. Connect a CO meter to exhaust test point (upstream of catalytic converter). With oxygen sensor disconnected, the reading should be stable on the meter. Note the CO% reading. With ECU lead grounded, meter reading should rise, and CO% increase. With lead connected to the flashlight battery, meter reading and CO% should decrease.
a. If meter reading does not rise with lead grounded, check wiring and replace ECU if wiring is ok.
b. If meter reading does not change with battery connected, check wiring and replace ECU if wiring is ok.
c. If meter reading rises but the CO% does not, check frequency valve wiring and replace valve if wiring is ok.
d. If meter reading decreases but CO% does not, check frequency valve wiring and replace valve if wiring is ok.
7. Adjust CO% to a rich level (about 3%) with oxygen sensor disconnected. Reconnect sensor. Reading should drop at least 1%. If it does not, replace the oxygen sensor.