Here is a schematic and test procedure for
ESC (electronic spark control).
I am not familiar with the ECM's in these trucks; specifically how they interface with the ignition system. I always thought they were only associated with the fuel system. See a description of the ECM at the very bottom of the post.
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1984-86 MODELS
Engine Cranks But Will Not Start
1.Check for spark at at the ends of at least 2 spark plug wires using tester ST-125 or equivalent. If spark is observed, malfunction is not in ignition system.
2.If no spark is observed, disconnect 4 pin connector to distributor, connect jumper wire between pins A and C on distributor side of connector, Figs. 5, 6 and 7, and attempt to start engine. If engine starts, proceed to step 3. If engine does not start, proceed as follows:
a.Remove jumper wire and reconnect 4 pin connector to distributor. Turn on ignition and measure voltage at pins F and K of controller 10 pin connector. If voltage is less than 11.6 volts, check feed circuit and repair as needed.
b.Set voltmeter on 2 volt scale and measure voltage between pins J and K of 10 pin connector while cranking engine. If reading is .75 volts or greater, ESC system is satisfactory.
c.Set voltmeter on 2 volt scale and measure voltage between pins G and H of 10 pin connector while cranking engine. If reading is .20 volt or greater, controller is defective. If reading is less than .20 volt, pole piece is defective.
3.Stop engine and remove jumper wire from distributor.
4.Connect suitable high resistance test lamp between distributor TACH terminal and ground, then turn on ignition. If test lamp lights, proceed to step 6.
5.If lamp does not light at TACH terminal, connect lamp between BAT terminal and ground and turn on ignition. If lamp does not light, repair feed circuit to ignition.
6.Crank engine with test lamp connected between distributor TACH terminal and ground. If test lamp flickers, system is satisfactory.
7.If test lamp does not flicker as engine is cranked, connect lamp between BAT terminal and pin A on distributor side of 4 pin ESC connector, then tap on coil. If coil makes a clicking noise when tapped, pole piece is defective.
8.If coil does not click, remove distributor cap an disconnect and reconnect pole piece electrical connector. If coil still does not click when tapped, ignition module is defective.
ECM Description of Operation:
The ECM determines proper air/fuel mixtures for engine operating conditions by monitoring oxygen sensor and other vehicle sensor signals. Air/fuel mixture is controlled through the MC solenoid by varying the amount of time the solenoid is energized (dwell period) during each cycle. This is called ``Closed Loop'' operation. During some modes of operation, such as when engine is cold or operating at wide open throttle, the dwell period will remain as a fixed value. This is called ``Open Loop'' operation.
MC solenoid operation can be monitored using a dwell meter connected to the MC solenoid test lead and set on the 6 cylinder scale for all engines. Normal dwell reading during ``Closed Loop'' fluctuates between 10°and 50°. If leaner mixtures are required, reading will fluctuate toward the higher end of the scale. If richer mixtures are required, reading will fluctuate toward the lower end of the scale. For proper ``Closed Loop'' operation, the amount of dwell fluctuation is not important, but the reading must fluctuate constantly.
During normal idle and part throttle operation, this system operates in ``Closed Loop.'' However, under certain operating conditions the ECM sends a fixed dwell signal to the MC solenoid, and operates in ``Open Loop'' mode.
During cranking below a certain RPM, the system is in ``inhibit'' mode and the dwell signal will remain fixed for up to 25 seconds, then drop to zero until a specified RPM is reached. During engine warm-up or when oxygen sensor is cold, the dwell signal will remain fixed until the sensors reach specified operating temperature. When engine is operated at wide open throttle, the Throttle Position Sensor (TPS) sends a higher voltage signal to the ECM, which provides a fixed enrichment signal to the MC solenoid.
