Basic Operation and Troubleshooting of the EZ 117K [Don Willson]

This discussion is based on my 3 cars all of which have the EZ-117K Jetronic Ignition which is for the 4 cylinder engine, with distributor on the rear of the cam shaft, the timing signal is from the Hall effect sensor within the distributor or an RPM sensor on the flywheel, air is measured by an air mass meter, it has a knock sensor, and throttle position switch.
I have had three 740s and though I am not a professional mechanic I’m a pretty good DIYer. I seldom use a dealer or independent shop unless time or tools require it. I am an engineer and like to know what’s wrong and how that affects the engine My basic reference are the Volvo shop manuals, especially the electrical, engine and ignition manuals.
[Editor's Note: see the section on Maintenance Manuals for more details on how to obtain the correct versions for your car.]
I will try to show a systematic approach to based on a chronological order where applicable.

Starting: Turn key to “run”
Power is supplied to the IGNITION CONTROL UNIT , hot side of the COIL, POWER STAGE, and most accessories.

Turn key to “start”
Power is removed from most accessories and applied to the STARTER SOLENOID. (if the car is equipped with automatic transmission it must be in Neutral or Park)
The starter pinion gear meshes with the ring gear, the high ampere connection made and the starter motor operates.

Engine turns over. The sequence of events is as follows.
IGNITION CONTROL UNIT (ignition computer) sends power to the DISTRIBUTOR.
The HALL EFFECT sensor in the DISTRIBUTOR (LH2.2 systems) sends a square wave electrical signal that varies from 5 to 0 volts back to the IGNITION CONTROL UNIT. As the signal rises from 0 to 5V the control unit starts to compute the timing of the next ignition pulse. As the signal falls to 0V the control unit commences ignition countdown and delivers ignition pulse as computed. [Editor's Note: this is true for pre-88 cars; newer Volvos with LH 2.4 use an RPM SENSOR at the flywheel for the same effect.]
When the IGNITION CONTROL UNIT gets this signal it says “the engine is turning over, let’s give it some fuel and spark” (I presume this is a safety function so that in case of an accident the fuel will not be delivered to a dead or damaged engine.). It sends the appropriate signal to the:
POWER STAGE (ignition amplifier). This feeds the coil which then send high voltage to the center tap of the distributor where the distributor sends the high voltage on to the correct spark plug.
grounds the FUEL RELAY which (hopefully) turns on both the IN-TANK FUEL PUMP and the HIGH PRESSURE FUEL PUMP.
Fuel then flows under pressure, in the 30 to 40 PSI range (though the pump is capable of pressures up to 80 PSI), into the fuel rail. At the front end of the fuel rail is a PRESSURE REGULATOR that maintains a pressure in the 30 PSI range though it varies according to engine vacuum. A higher vacuum, as when idling or running lightly, allows more fuel to flow back to the tank and the pressure is reduced in the fuel rail. When under higher pressure or even turbo boost the pressure in the fuel rail is higher and more fuel is delivered for the same amount of injector open time.
Opens the INJECTORS for the appropriate time. Note, since this is not a sequential fuel injection system, all INJECTORS fire at once and timing is not an issue other than that they fire each half resolution, so that fuel is in the intake manifold ready for any intake valve to open.

Now the engine starts, however, there is more to it.
If the engine is cold, like the first start in the morning, the AIR CONTROL VALVE opens and acts like a fast idle cam on old non fuel injected cars. The controlling signal on this is the COOLANT TEMPERATURE SENSOR. Generally only a few seconds is needed for this to be open, however, under some conditions it closes, the idle speed drops significantly and it opens again.
There is no choke but since the choke only forces a rich mixture the FUEL CONTROL UNIT will open the injectors more times and/or for longer periods.
As the engine warms up various signals are supplied to the FUEL CONTROL COMPUTER to modify the INJECTOR open time. For example:
The COOLANT TEMPERATURE SENSOR sends a signal to the FUEL CONTROL COMPUTER that less enrichment is needed (like a choke backing off)
The AIR MASS METER supplies the engine load as it measures the amount of air being delivered to the engine, as a function of the THROTTLE position, and sends this signal to the FUEL CONTROL COMPUTER for use in determining the optimum mixture
The OXYGEN SENSOR warms up and begins to send a signal to the FUEL CONTROL COMPUTER that in real time varies the mixture to maintain a mixture that delivers power with minimum emissions.
The KNOCK SENSOR ‘listens’ for the characteristic sound of a knock and sends a notice to the IGNITION CONTROL UNIT which retards the spark in steps of 2.8° up to 10° to 16°, until knocking ceases. Then it advances the spark in steps of 0.37° until, if possible, it return to the normal advance.
After fully warmed up and running the COOLANT TEMPERATURE SENSOR is continuously monitored and if it indicates an overheat condition it advances the timing by 13° if the throttle is closed on the B230FT engines.

The THROTTLE SWITCH senses when the THROTTLE is closed, foot off of the gas, on of two things happen:
If the engine is turning over rapidly, above idle speed, the spark and fuel are adjusted to give the maximum engine braking.
When the engine drops to near idle speed the spark is retarded so as to provided a smoother idle.

After Shutting Down
One item occurs after the key is turned off, that is that a voltage is sent to the AIR MASS METER to burn off contaminants that have accumulated.