Why a classic car won’t start
Common causes engine fails to start: battery power is insufficient; Incorrect ignition timing; Start system failure; Electric spark quality is not good enough; Fuel system failure and so on. In this article, we will learn how to eliminate the ignition system failure.

Ignition coil
There are open circuit and short circuit in the primary winding and secondary winding

Distributor(1) Rotor burning loss and leakage of electricity
(2) Distributor cap is dirty, damaged and leakage of electricity

High voltage wire
(1) Damage of high voltage wire result in leakage, loose, fracture
(2) The high-voltage wire is inserted incorrectly

The spark plugs
(1) Spark plug carbon deposition
(2) Spark plug electrode burning loss or the gap is too large

Ignition signal generator
(1) Ignition signal generator is faulty
(2) There is a fault in the connection circuit or components of the signal generator

Electronic igniter
(1) Internal circuit fault of components in electronic igniter
(2) The electronic igniter is not well grounded

Switches and circuits
(1) Faulty ignition switch
(2) Open circuit and short circuit in the ignition coil primary circuit

Ignition timing
The initial ignition advance angle s too large

How to diagnose
1. Visual examination
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. Spark jump test 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. High-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.  Diagnose 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. Diagnose 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.

James Smith        July 15, 2020
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