Ignition timing is the measurement, in degrees of crankshaft rotation, of the point at which the spark plugs fire in each of the cylinders. It is measured in degrees before or after Top Dead Center (TDC) of the compression stroke.
Because it takes a fraction of a second for the spark plug to ignite the mixture in the cylinder, the spark plug must fire a little before the piston reaches TDC. Otherwise, the mixture will not be completely ignited as the piston passes TDC and the full power of the explosion will not be used by the engine.
The timing measurement is given in degrees of crankshaft rotation before the piston reaches TDC (BTDC). If the setting for the ignition timing is 5° BTDC, the spark plug must fire 5° before each piston reaches TDC. This only holds true, however, when the engine is at idle speed.
As the engine speed increases, the pistons go faster. The spark plugs have to ignite the fuel even sooner if it is to be completely ignited when the piston reaches TDC. On all engines covered by this information, spark timing changes are accomplished electronically by the engine and ignition control computers.
If the ignition is set too far advanced (BTDC), the ignition and expansion of the fuel in the cylinder will occur too soon and tend to force the piston down while it is still traveling up. This causes engine ping. If the ignition spark is set too far retarded, after TDC (ATDC), the piston will have already passed TDC and started on its way down when the fuel is ignited. This will cause the piston to be forced down for only a portion of its travel. This will result in poor engine performance and lack of power.
Timing marks consisting of O marks or scales can be found on the rim of the crankshaft pulley and the timing cover. The mark(s) on the pulley correspond(s) to the position of the piston in the number 1 cylinder. A stroboscopic (dynamic) timing light is used, which is hooked into the circuit of the No. 1 cylinder spark plug. Every time the spark plug fires, the timing light flashes. By aiming the timing light at the timing marks while the engine is running, the exact position of the piston within the cylinder can be easily read since the stroboscopic flash makes the pulley appear to be standing still. Proper timing is indicated when the mark and scale are in proper alignment.
Because these vehicles utilize high voltage, electronic ignition systems, only a timing light with an inductive pickup should be used. This pickup simply clamps onto the No. 1 spark plug wire, eliminating the adapter. It is not susceptible to cross-firing or false triggering, which may occur with a conventional light, due to the greater voltages produced by electronic ignition.
INSPECTION & ADJUSTMENT
There were three different types of ignition systems used on the Ranger and Bronco II vehicles; the Dura-Spark II (DS II), the TFI-IV and the distributorless systems. The DS-II system uses a distributor with a vacuum and mechanical advance module and the timing can be adjusted. The TFI-IV ignition system distributor has no vacuum advance module, and was used on most fuel injected and some feedback carburated engines. Only the initial, or base, timing can be set. The distributorless ignition system has no distributor (thus its name) and therefore, no provisions for adjusting the ignition timing.
Dura-Spark II Ignition System
This procedure should not be used as a periodic maintenance adjustment. Timing should only be set after the distributor has been disturbed (removed and re-installed) in some way.
Specific instructions and specifications for setting initial timing can be found in the Vehicle Emission Control Information (VECI) label in the engine compartment. Because this label contains information regarding any specific calibration requirements for YOUR vehicle, those instructions and specifications should be followed if they differ from the following.
Locate the timing marks on the crankshaft pulley and the front of the engine.
Clean off the timing marks so that you can see them.
Mark the timing marks with a piece of chalk or with paint. Color the mark on the scale that will indicate the correct timing when it is aligned with the mark on the pulley or the pointer. It is also helpful to mark the notch in the pulley or the tip of the pointer with a small dab of color.
Start the engine and allow it to run until it reaches normal operating temperature.
Once normal operating temperature has been reached, shut the engine OFF.
Firmly apply the parking brake and block the drive wheels. Place the transmission in P (A/T) or NEUTRAL (M/T, as applicable).
Make sure heater and A/C, along with all other accessories are in the OFF position.
Disconnect the vacuum hose from the distributor and plug it.
Attach a tachometer to the engine.
Attach a timing light according to the manufacturer's instructions.
Check to make sure that all of the wires clear the fan and then start the engine.
Adjust the idle to the correct setting.
Aim the timing light at the timing marks. If the marks that you put on the pulley and the engine are aligned when the light flashes, the timing is correct. Turn off the engine and remove the tachometer and the timing light. Connect the vacuum line to the distributor. If the marks are not in alignment, proceed with the following stops.
Loosen the distributor lockbolt just a little. Some models may use a security-type holddown bolt. Use Distributor Holddown Wrench, Tool T82L-12270-A, or equivalent, to loosen the holddown bolt.
With the timing light aimed at the pulley and the marks on the engine, turn the distributor in the direction of rotor rotation to retard the spark, and in the opposite direction of rotor rotation to advance the spark. Align the marks on the pulley and the engine with the flashes of the timing light.
When the marks are aligned, tighten the distributor lockbolt and recheck the timing with the timing light to make sure that the distributor did not move when you tightened the lockbolt.
Turn the engine OFF and remove the timing light and tachometer. Connect the vacuum line to the distributor.
TFI-IV Ignition System
SETTING INITIAL (BASE) TIMING
See Figure 1
Fig. 1: The underhood VECI label is specific to YOUR truck and should be used if it differs from another source
This procedure should not be used as a periodic maintenance adjustment. Timing should only be set after the distributor has been disturbed (removed and re-installed) in some way. If problems are encountered setting the initial timing with this procedure and no mechanical causes are found, follow the spark timing advance check procedure found later in this section.
Do not change the ignition timing by the use of a different octane rod without having the proper authority to do so. Federal emission requirements will be affected.
Connect an inductive timing light, such as the Rotunda 059-00006 or equivalent, to the No. 1 spark plug wire, according the tool manufacturer's instructions.
Connect a tachometer to the ignition coil connection using an alligator clip. This can be done by inserting the alligator clip into the back of the connector, onto the dark green/yellow dotted wire.
DO NOT allow the alligator clip to accidentally ground to a metal surface while attached to the coil connector as that could permanently damage the ignition coil.
Disconnect the single wire in-line SPOUT connector which connects the control computer (usually terminal 36) to the ignition control module. This will prevent the electronic ignition from advancing the timing during the set procedure.
Using a suitable socket or wrench, loosen the distributor hold-down bolt slightly at this time, BUT DO NOT ALLOW THE DISTRIBUTOR TO MOVE or timing will have to be set regardless of the current conditions.
A remote starter must NOT be used to start the vehicle when setting the initial ignition timing. Disconnecting the start wire at the starter relay will cause the ignition control module to revert to Start Mode timing after the vehicle is started. Reconnecting the start wire after the vehicle is running WILL NOT correct the timing.
Start the engine (using the ignition key and NOT a remote starter to assure timing will be set correctly) and allow the engine to return to normal operating temperature.
With the engine running at the specified rpm, check the initial timing. If adjustments must be made, rotate the distributor while watching the timing marks. Once proper adjustment has been reached, make sure the distributor is not disturbed until the hold-down bolt can be secured.
Reconnect the single wire in-line SPOUT connector and check the timing to verify that the distributor is now advancing beyond the initial setting.
Shut the engine OFF and tighten the distributor bolt while CAREFULLY holding the distributor from turning. If the distributor moves, you will have to start the engine and reset the timing.
Restart the engine and repeat the procedure to check the timing and verify that it did not change
Shut the engine OFF, then disconnect the tachometer and timing light.
CHECKING SPARK TIMING ADVANCE
Spark timing advance is controlled by the EEC system. This procedure checks the capability of the ignition module to receive the spark timing command from the EEC module. The use of a volt/ohmmeter is required.
Turn the ignition switch OFF.
Disconnect the pin-in-line connector (SPOUT connector) near the TFI module.
Start the engine and measure the voltage, at idle, from the SPOUT connector to the distributor base. The reading should equal battery voltage.
If the result is okay, the problem lies within the EEC-IV system.
If the result was not satisfactory, separate the wiring harness connector from the ignition module. Check for damage, corrosion or dirt. Service as necessary.
Measure the resistance between terminal No. 5 and the pin-in-line connector. This test is done at the ignition module connector only. The reading should be less than 5 ohms.
If the reading is okay, replace the TFI module.
If the result was not satisfactory, service the wiring between the pin in-line connector and the TFI connector.
Distributorless Ignition System
The 4.0L and 1989-90 twin plug 2.3L engines utilize a Distributorless Ignition System (DIS). On this system, ignition coil packs fire the spark plugs directly through the spark plug wires. All spark timing and advance is determined by the ignition control module and engine control computer. No ignition timing adjustments are necessary or possible.