Electrical Tests

1. Test overvoltage protection relay.
a. Check fuse on overvoltage protection relay, and if it's bad, replace it and locate the short circuit.
b. Unplug the relay.
b. Bridge sockets 1 and 2 in the relay plug.
c. Check voltage between sockets 9 (+) and 11 (-) on the idle speed control unit plug. Battery voltage should be present. If not, check power supply wiring.
d. Turn preglow/starter switch to position '2' and check voltage between sockets 5 (-) and 6 (+) of the relay plug. Battery voltage should be present. If not, check the power supply wiring.
e. If all tests check out good, replace the overvoltage protection relay.

2. Check TD-signal. Remove EGR control unit. Run engine at idle, and check voltage between sockets 2 and 11 of EGR control unit plug. The reading should be greater than 2.8 vdc.

3. If voltage is not within specification, turn engine off, and remove idle speed control unit.
a. Check for continuity between socket 2 of the EGR control unit plug, and socket 6 of the idle speed control unit plug.
b. Check resistance between sockets 10 and 12 of the idle speed control unit plug. Resistance should be 1.9 ± 0.2 Ohms.
c. If resistance is not within specification, pull off ring gear speed sensor connector and measure the resistance between the pins.
1. Resistance should be 1.9 ± 0.2 Ohms. If not, replace the sensor.
2. If wiring is in order, replace ring gear speed sensor.

4. With engine running at idle, check voltage between sockets 10 and 12 of the idle speed control unit plug. Voltage should be greater than 4 vdc (voltage will increase with increasing rpm). If voltage is not within specification, check ring gear speed sensor for dirt accumulation, and clean or replace as necessary.

5. With the engine off, turn preglow/starter switch to position '2', and check the voltage between sockets 9 (+) and 11 (-) on the idle speed control unit plug. Battery voltage should be present. If not, repair wiring.
g. If all other tests are good, replace the idle speed control unit.


6. Connect a vacuum gauge with a 'T' fitting, to the EGR valve.
a. Run the engine at 1200 ± 50 rpm.
b. Pull off the 2-pole connector on the temperature switch and bridge the sockets in the plug (simulating temperature above 97°C). The vacuum should drop to 0.
c. If not, check:
1. Switchover valve
2. EGR control unit
3. TD-signal
4. microswitches

7. Connect vacuum tester with Y distributor onto EGR valve. Start and operate engine at 1200 RPM for approximately 1 minute, then disconnect temperature switch electrical connector and hold against a good ground. EGR valve vacuum reading obtained should be zero. If not, check:
a. Switchover valve
b. EGR control unit
c. TD-signal
d. microswitches

8. With ignition switched OFF, disconnect micro switch electrical connector and check resistance readings between terminal 2 and 4 or 1 and 3. With engine idling, zero ohms should be obtained. At full throttle, infinity should be obtained. If not, replace micro switch.

9. With ignition switched OFF, disconnect electrical connector from micro switch. Connect a suitable ohmmeter onto terminals 1 and 5. With engine idling, ohmmeter reading obtained should be infinity. At full throttle, ohmmeter reading obtained should be zero ohms. If not, replace micro switch.