Engine faults diagnosis methods in conventional ignition system



What are sympotoms of  engine failures and how to diagnose ? Why does engine fail to start? What causes engine idled unsteadily? Why the engine shut off immediately after starting?  
High-tension lead, spark plug,ignition switch should also be tested in conventional ignition system. In this article, we will learn about it. 


When engine have faults in conventional ignition system ,usually we should do as follows


(1)\tVisual Inspection

First, Check for loose high voltage wires and low-voltage circuit between the ignition coil and distributor. If so, restart the engine and see if it can be started after troubleshooting. If the\tvisual examination does not find any problem, proceed to the next troubleshooting step. 



(2)\tSpark jump testing of the central high-tension lead

Pull out the central high-tension lead on the distributor, and make the high-tension lead terminal 5-8mm away from the engine block, then turn on the ignition switch, when start the engine with the starter, see whether to spark jump between the high-tension lead terminal and the engine block. There are three cases; The spark is strong; the spark is weak or the spark jumps.

A. If the spark is very strong, it indicates that the low-voltage circuit and
ignition coil is basically normal. While the fault is in the high-voltage circuit or spark plug, proceed to carry out the next step fault diagnosis.

B. If the spark is very weak, it may be caused by faulty components, such as breaker, capacitor and ignition coils, or poor connection of high-tension lead. Further diagnosis can be made in step (4).

C. If there is no spark, the possible causes of the fault include poor contact of the breaker, the movable contact point ground or short circuit of capacitor, short circuit or open circuit of the ignition coil, or poor contact of the ignition switch and ignition coil low-voltage circuit. Step 5 can be used for further troubleshooting.

(3)\tHigh-tension lead test

Insert back into the central high-tension lead, pull out the high-tension lead on a cylinder spark plug for test, and make the high-tension lead terminal 5-8mm away from the engine block. If the spark does not jump or the spark is very weak, it indicates electric leakage or open circuit in distributor cap, rotor or high-tension lead, these parts need to be repaired. If the spark is still strong, the spark plug shall be removed for inspection. If the spark plugs of all cylinders are in good condition, then check the oil circuit, ignition timing and other possible faults that may cause the engine to fail to start.



(4)\tDiagnose the cause of weak spark with instantaneous short circuit method

When the breaker contact is open, turn on the ignition switch, and conduct jump spark test of the central high-tension lead by instantaneous ground between the "-" terminal of ignition coil (leaving quickly after wire the terminal is grounded), to see whether the spark at the terminal of the high-tension lead becomes stronger. If the spark becomes stronger, it is necessary to check the contact of the breaker, capacitor and other components; If the spark is still weak, check or replace the ignition coil.

(5)\tDiagnose the cause of no spark jump with instantaneous short circuit method

When the breaker is disconnected, the ignition coil "-" and conduct jump spark test of the central high-tension lead by instantaneous ground between the ignition coil. If there is jump spark, it is necessary to check breaker contact, capacitor and other components. If the spark is still not jumping, then see if there is a spark when grounding at the moment. If not, check whether the low-voltage loop circuit of the ignition system is open circuit or the primary winding of the ignition coil is open circuit; If there is a spark when contacting. check whether the secondary winding of the ignition coil is open or short circuit fault, and whether central high-tension lead is open.

The sympotoms of  engine failures  


1.\tEngine idled unsteadily

The engine can start, but the engine runs unsteadily or even stops at idle speed. When accelerating, the exhaust pipe sounds like rur ..rur.. rur…Engine power is obviously insufficient.
This kind of fault phenomenon is generally due to individual cylinder not working or working bad, ignition timing is abnormal and other causes.


(1)\tCheck the working condition of each cylinder of the engine

When the engine is running at idle speed, adopt misfire detection method (make the high-tension lead of the tested cylinder short circuit or open circuit, result in the cylinder misfire). Observe the changes in engine operation. If the engine speed does not change or the change is small, indicate the cylinder does not work or work bad. If the engine speed drops obviously, it indicates that the cylinder works normally. Then, check other cylinders in turn. If the engine speed drops when each cylinder misfire, the next fault diagnosis shall be made.

(2)\tCheck the strength of each cylinder spark


Pull-out high-tension lead and conduct the jump spark test to see if the spark is strong. If the spark is strong, check and adjust the ignition timing and spark plug, if the spark plug is in good condition, the ignition timing is correct or the engine idle speed is still unstable after the ignition timing is adjusted, then the oil circuit shall be checked or adjusted. If the spark is weak, check the breaker contact, ignition coil, distributor cap, rotor, etc.

(3)\tCheck whether the cylinder spark jump

Pull out the high-tension lead of the cylinder and conduct the spark jump Spark jump test to see whether spark jump. If not, check distributor cap, high tension lead; If there is spark jump, check the spark plugs and repair or replace them if necessary.

2.\tThe engine shut off immediately after starting

The engine starts smoothly, but as soon as you release the ignition switch (disconnect the ignition switch) the engine shuts down. The possible causes for this phenomenon include:

(1)\tThe connection of the additional resistance of the ignition coil is broken or the additional resistance itself is burnt out (ignition coil with additional resistance).

(2)\tThere is an open circuit between the ignition switch "I" terminal and the ignition coil "+" terminal.

(3)\tPoor ignition switch contact.

The fault diagnosis method is as follows:

Check the ignition switch

Turn on ignition switch. see if the charging indicator is on. If the charging indicator is not work, it indicates the contact point of the ignition switch is not in good condition, the ignition switch should be repaired or replaced. if the charging indicator is on, then proceed to the next step of fault diagnosis.

Check additional resistance

Check the resistance between ignition switch "+" "and two low voltage ignition coil terminals with multimeter. In breaker-point Ignition System, the additional resistance about 1.5 Ω. If the measured results are unavailable, it indicates that additional resistance have burn out and need to change the ignition coil. If additional resistance test is normal. You need to troubleshoot the circuit between the ignition switch and the ignition coil" + "terminal.


Updated Products

Delcoribo Motor Products Co., Ltd.

Economic Development Zone, Jinhua, Zhejiang, China

Tel: +86-579-8275-8180

[email protected]

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