Engine Controls - Revised Chart A-3, Cranks No Start
88chevy30Number: 89-36-6E
Section: 6E
Date: Nov., 1988
Subject: ENGINE "CRANKS BUT WON'T RUN"
Model and Year: 1987-89 CELEBRITY WITH 2.5L (VIN CODE R) ENGINE
TO: ALL CHEVROLET DEALERS
This bulletin has been reissued to revise Chart A-3 (Page 2 of 3), "Engine Cranks But Won't Run", used in the diagnosis of 1987 through 1989 Celebrity models with 2.5L (VIN Code R) engines. All copies of 88-329-6E dated August, 1988, should be discarded.
When measuring crankshaft sensor resistance, as described in "Test Description" step number 5, make certain the crankshaft sensor is at room temperature. High resistance readings could result if the crankshaft sensor is warmer than room temperature.
Also, a "Diagnostic Aids" section has been added to the facing page which addresses the correction procedure for an intermittent crankshaft sensor condition. The diagnostic chart has been updated to reflect this change as well.
CHART A-3
(Page 2 of 3)
ENGINE CRANKS BUT WON'T RUN
Circuit Description:
A magnetic crank sensor is used to determine engine crankshaft position, much the same way as the pick-up coil did in HEI type systems. The sensor is mounted in the block, near a slotted wheel on the crankshaft. The rotation of the wheel creates a flux change, in the sensor, which produces a voltage signal. The DIS ignition module processes this signal and creates the reference pulses needed by the ECM to trigger the correct coil at the correct time. If the "Scan" tool did not indicate cranking rpm, and there is no spark present at the plugs, the problem lies in the direct ignition system or the power and ground supplies to the module.
CHART A-3ENGINE CRANKS BUT WON'T RUN:
Test Description: Numbers below refer to circled numbers on the diagnostic chart.
1. The direct ignition system uses two plugs and wires to complete the circuit of each coil. The other spark plug wire in the circuit must be left connected to create a spark.
2. This test will determine if the 12 volt supply and a good ground is available at the DIS ignition module.
3. This test will determine if the ignition module is not generating the reference pulse, or if the wiring or ECM are at fault. By touching and removing a test light to 12 volts on CKT 430, a reference pulse should be generated. If rpm is indicated, the ECM and wiring are OK.
4. This test will determine if the ignition module is not triggering the problem coil, or if the tested coil is at fault. This test could also be performed by
substituting a known good coil. The secondary coil winding can be checked with a DVM. There should be 5,000 to 10,000 ohms across the coil towers. There should not be any continuity from either coil tower to ground.
5. Checks for continuity of the crank sensor and connections as well as sensor magnetism. These cheeks must be performed with the crank sensor at room temperature in order to obtain accurate readings.
Diagnostic Aids:
An intermittent problem with crankshaft sensor could cause a "Cranks But Won't Run" condition. Therefore, the crankshaft sensor must be replaced first. If the "Cranks But Won't Run" condition persists, then replace the DIS module.