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L-Jetronic Workshop Manual
This manual describes the trouble=shooting and repair of vehicles with the Robert Bosch L-Jetronic (EFI) System using mostly common workshop tools. You should understand this fuel injection system before you begin working on the vehicle. The Bosch Technical Instruction Booklet VDT-UBP 751/1B explains the L-Jetronic System in detail and will increase your understanding of this system. The will reduce repair times and facilitate the use of this Workshop Manual.
This manual is designed to assist the technician unfamiliar with Robert Bosch fuel injection systems in locating faults in the L-Jetronic System, isolating the component involved and testing the component for correct function. You will need a volt-ohmmeter and a pressure gauge as described in the Tool List. Electrical Measurements are made at the terminals of the large plug at the end of the wiring harness after it is removed from the control unit, or at the terminalsof the individual components themselves. Most components of this system cannot be repaired and will have to be replaced once you have determined them defective.
If you are already familiar with Bosch EFI systems and can identify and locate all the components in the vehicle, go directly to the Trouble-Shooting Chart to track down the faulty component according to the engine symptom given. Then go to the Component Test Instructions for directions on testing the suspected component. Technicians who are experienced in the various tests on this vehicle model can go directly to the Test Chart where the individual test steps and values are given.
An Electrical Wiring Diagram is included at the end of the manual to assist you in tracing cables and locating the various terminals. Be sure to check and if necessary reset the CO adjustment before returning the car to the customer.
- Never start the engine without battery cables firmly connected.
- Never jump the battery to start the car.
- Never remove the cables from battery with engine running.
- Always remove cables from battery before charging.
- Never remove or attach wiring harness plug to control unit with the ignition on.
- When turning the engine over to check compression, unplug the red cable from the battery to the relays.
- Before testing the L-Jetronic system, be sure timing, dwell and spark plugs check out OK and are within specification.
Is ignition system in proper condition?
Check battery voltage, ignition distributor, spark plugs, ignition coil, ignition timing, etc.
Is engine in proper mechanical condition?
If the engine still does not start or starts only poorly after the electrical test, check for mechanical problems: compression, basic adjustment of valves, and engine oil pressure.
Are all hose lines attached?Are these lines in proper condition?
Check whether hoses in the air-intake system are attached properly, tightened firmly in place, not kinked, and not damaged.
Check the fuel pressure according to the Test Chart.Is the fuel pressure correct?
- Crank engine, check whether the fuel pump operates (aural check). If yes , check fuel line and filter to be sure they permit throughflow. Filter in tank clogged. Corrosion in tank.
Check pressure regulator according to Test Chart. - Crank the engine. Check whether voltage is present at the diconnected pump plug. If yes, replace the fuel pump.
- Check whether voltage is present at the pump fuse and at the double relay terminals 88y and 88d. If no, replace the pump fuse and/or the double relay.
Is the start valve in proper condition?
Check cold start valve according to Test Chart. Voltage at cold start valve ok?
If no, Check cable 45 for continuity to thermo-time switch terminal G. Check cable 46 for continuity to thermo-time switch terminal W.
If yes, Test the start valve mechanically. Remove the valve from the intake manifold and hold it in a container (Caution: Fire Risk!). During starting and with engine temperature below 50° F the valve must spray fuel (max 20 sec.). At a temperature above 104° F the valve must not spray fuel. With the ignition switched on and with pressure built up, the valve must also not spray fuel.
Carry out the spray test at temperature above 104° F as follows: Remove the plug from the thermo-time switch and connect terminal “W” to ground. If a spray test cannot be carried out, see cold-start relay.
Is the thermo-time switch in proper condition?
Check thermo-time switch according to Test Chart. Does thermo-time switch check out ok?
If no, replace the thermo-time switch.
If yes, Check cables 45 and 48 from the cold-start valve for continuity (see Electric Wiring Diagram).
Is the auxilliary-air device in proper mechanical condition
Visual check of the auxilary-air device: In the cold condition the auxiliary-air device must be open but when the engine is warm it must be closed. Replace the auxiliary-air device is these conditions are not met.
Is the air-intake system leak tight?
Check the intake manifold, components attached to the intake manifold, and all hose connections for leaks. Using compressed air and soapy water localize any leaks present.
Is the air-flow meter in proper condition?
Check pump contacts in air flow meter according to Test Chart. Remove the upper section of the air filter. Open the air flow meter flap by hand. It must be possible to open the air-flow meter flap with uniform ease from its fully closed position to its fully open position, and then this flap must close completely by itself. When the air-flow meter flap is opened it must not catch at any point. Watch for any indications of abrasion or rubbing.
When air-flow meter flap is opened: If the inside of the air-flow meter is very dirty, clean it.
Is ignition system in proper condition?
Check battery voltage, ignition distributor, spark plugs, ignition coil, ignition timing, etc.
Is engine in proper mechanical condition?
If the engine still does not start or starts only poorly after the electrical test, check for mechanical problems: compression, basic adjustment of valves, and engine oil pressure.
Is the fuel pressure correct?
Test according to Test Chart
Fuel pressure drops again after the end of starting process.
Test the fuel pump contact in the air-flow meter: Remove the upper section of the air filter. Detach the plug from the air-flow meter and connect the ohmmeter to terminals 36 and 39 at the air-flow meter. Press the air-flow meter open slightly by hand. When this is done the resistance must change from ∞Ω to 0Ω.
If not, replace the air-flow meter.
Is the start valve in proper condition (no leaks)?
Test the start valve for leaks. In order to do this, clamp the fuel intake line closed with a clip. Does the engine then start when cranked? If yes, replace the start valve.
Is ignition system in proper condition?
Check battery voltage, ignition distributor, spark plugs, ignition coil, ignition timing, etc.
Is engine in proper mechanical condition?
If the engine still does not start or starts only poorly after the electrical test, check for mechanical problems: compression, basic adjustment of valves, and engine oil pressure.
Is the throttle valve closed?
Determine whether the throttle valve can be closed farther and whether the engine speed decreases when this is done.
If yes, remove the throttle valve clamps: if the throttle linkage is bent, straighten it; reset the adjusting screw at the throttle valve.
Is the throttle valve switch set correctly?
If no, Adjust the throttle valve switch.
If yes, set the idle speed with the idle screw, according to the Test Chart. If the test value was not reached or the engine idle uneven, perform exhaust gas test using CO tester with the engine at normal operating temperature according to the Test Chart. Can the value be reached?
No leaks in the air-intake system. System in proper condition?
For testing, see under “Starting motor operates but engine either does not start or starts poorly”.
Is the auxiliary-air device closed when the engine is at normal operating temperature?
Using a clip, clamp one hose leading to the auxiliary-air device closed. The engine speed may drop only slightly. Or: visual check with the hose detached: when the engine is at normal operating temperature the auxiliary-air device must be closed. Speed falls sharply or auxiliary-air device open: replace the auxiliary-air device.
Is the start valve in proper condition (no leaks)?
Test the start valve for leaks. For this purpose clamp the fuel intake line closed with a clip. Does engine then run evenly? If not, replace the start valve.
Are the fuel injection valves in proper mechanical condition?
With the engine operating, detach the valve plugs individually one after the other from the fuel injection valves. With a good injection valve the engine speed must drop as the plug is detached. If no drop, replace a defective injection valve.
Is the fuel pressure correct?
Test fuel pressure according to Test Chart.
Is the air-flow meter in proper condition?
For testing, see under “Starting motor operates but engine either does not start or starts only “poorly”.
If yes, Exhaust gas test (repeat) using CO Tester with the engine at normal operating temperature: see Test Chart for value.
If the concentration of CO is too high, turn the bypass screw in the air-flow sensor a half turn counterclockwise. Measure the speed and the concentration of CO again. Make corrections in several steps.
Is ignition system in proper condition?
Check battery voltage, ignition distributor, spark plugs, ignition coil, ignition timing, etc.
Is engine in proper mechanical condition?
If the fuel acceptance of the engine is still in poor condition after the electrical test, check for mechanical problems: compression, basic valve adjustement and engine oil pressure.
Is the throttle valve set correctly?
Adjust throttle valve switch.
Is the air-intake system leak-tight?
For testing, see under “Starting motor operates but either does not start or only starts “poorly”.
Is the auxiliary-air device in proper mechanical condition?
Visual check of the auxiliary-air device: in the cold condition the auxiliary-air device must be partly open but when the engine is warm it must be closed. If not, replace the auxiliary-air device.
Is the air-flow meter in proper mechanical condition?
For testing, see under “Starting motor operates but either does not start or only starts “poorly”.
Is composition of exhaust gas (concentration of CO) correct?
Check according to Test Chart.
Is ignition system in proper condition?
Check battery voltage, ignition distributor, spark plugs, ignition coil, ignition timing, etc.
Is engine in proper mechanical condition?
If the engine misses occur after the electrical test, check for mechanical problems: compression, basic valve adjustement and engine oil pressure.
Are the plug connections in the L-Jetronic wiring harness in proper condition?
Check for engine misses caused by loose contacts as follows: Start engine with tachometer connected. Hold the engine speed constant while observing the reading and/or listening for the engine misses. Move the wiring harness and the plug connections, the reading must not vary and engine misses must not occur. Check the plug connections for the correct seating and corrosion. Test ground cables 5, 16, 17 and 49 for continuity and loose contacts.
Is the voltage supply for the fuel injection system in proper condition?
Detach both wiring harness plugs from the double relay. Check that all plug sleeves and electric cables are in proper condition. Check all connector cables for continuity. Move the wiring harness when doing this (suspicion of broken cables). See also preceding testing steps with wiring harness.
Is the start valve in proper condition (no leaks)?
Using a clip, clamp the fuel pressure hose closed in front of the connection at the start valve. If this eliminates the engine misses, replace the start valve.
Are the alternator and regulator in proper condition (engine misses caused by voltage peaks)?
With the engine stopped, remove the plug from the alternator. Start the engine. If this eliminates the engine misses, check the alternator and regulator.
Is the quantity of the fuel delivered by the fuel pump correct?
In order to test the quantity of fuel delivered detach the fuel hose at the start valve. Extend the hose and lead it to a measuring vessel of 5 liters capacity with a graduated scale. Remove the top section of the air filter, open the air flow sensor flap by hand until the pump operates. Measured value for the correct delivery (after 1 minute of operation) about 1.5-2.0 liters/minute.
Is the quantity of fuel delivered by the injection valves correct?
Remove one valve at a time and inject fuel into a graduate for 30 seconds while engine idles. Compare quantity with that delivered by a known good injector. If delivery quanity deviates from good injector, orignal valve was defective.
Is the air-flow meter in proper mechanical condition?
For testing, see under “Starting motor operates but engine either does not start or only starts “poorly”.
Is the control unit in proper condition?
Connect tachometer. Let the engine run. Tap lightly on the control unit and move the multiple plug. Watch the reading and listen for engine misses. Replace the control unit with a known good one. If the problem is solved, original control unit was defective.
Is ignition system in proper condition?
Check battery voltage, ignition distributor, spark plugs, ignition coil, ignition timing, etc.
Is engine in proper mechanical condition?
Check for mechanical problems: compression, basic valve adjustement and engine oil pressure.
Is the fuel pressure correct?
Test fuel pressure according to Test Chart.
Is the quantity of fuel delivered by the injection valves correct?
Remove one valve at a time and inject fuel into a graduate for 30 seconds while engine idles. Compare quantity with that delivered by a known good injector. If delivery quantity deviates from good injector, original valve was defective.