'87 and newer 928 no-start diagnosis
05-22-2004, 11:24 PM
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'87 and newer 928 no-start diagnosis
I am seeing many more failing LH Jetronic fuel injection modules this season. I've given a lot of diagnostic advice one-off and thought I'd share it with the group.
Many people jump to the most expensive part when a no-start or bad running problems occur. That isn't the way to go. If you or your mechanic have eliminated the "easy to find things" ... this list of diagnosis goes a lot deeper.
All functionality with the EZK spark system must be confirmed prior to testing the LH Jetronic system.
Sometimes the diagnosis can be more expensive then the repair! So feel free to share this information with your shop, it could save you money.
===========
928 Model year 1987 – 1988 System No Start Diagnostics (later model diagnosis is similar, consult the wiring diagrams for specifics):
Diagnostics should be only be performed by a trained technician. Always check for proper relay functionality and power/ground connections before undertaking further diagnostic testing.
a. Testing the engine RPM sensor. The EZK spark control is the system master, if engine rotation signals are not provided to the EZK, spark will not be generated and EZK will not provide a turn-on signal to the LH Jetronic fuel injection system. The RPM sensor input to the EZK is from an inductive RPM sensor. Inductive RPM sensors generate voltage so a VOM can be used for testing the sensor. EZK pins 23 and 6 are connected to the RPM sensor; pin 6 is ground and is connected to master-ground pin 18. EZK pin 24 is the shielding of the sensor cable and is connected internally to pin 18.
b. The Hall Effect engine position sensor can be tested with an LED tester. The EZK provides the power supply for the Hall Sensor at pin number 5 (12V). The EZK provides a 5V level at pin number 22, which the Hall sensor can pull down to ground. Pin EZK pin 22 is the "hot" signal cable and therefore is shielded. The cable shielding is connected to EZK pin 4, and pin 4 is connected internally to master-ground EZK pin 18.
c. The EZK spark module provides the signal to turn on the LH fuel injection module; it is the circuit from EZK pin 13 to LH control Pin 1. Check for continuity, if the wire is broken or the connection isn't clean, the car will not start.
d. The circuit from LH Pin 20 goes to the Fuel Pump Relay XX pin 85, check for continuity between these two points.
e. From the Fuel Pump Relay XX pin 87 the circuit passes through fuse 42 then to the fuel pump motor. Pull the fuse, check the fuse, and clean the connectors. Check the continuity of this path. Bridge Fuel Pump Relay XX socket points 30 and 87, the fuel pump should run.
f. There should be 12V (nominal battery voltage) at the fuel pump and LH relay pins 30 at all times. This same circuit connects to LH connector Pin 4, check for 12V there too.
g. The LH relay XXV pin 85 connects to LH connector pin 21, check for continuity between these two points.
h. The LH connector pin 18 connects to all the fuel injectors. The LH functions as the switch from the fuel injector to ground. With the LH relay jumpered between pins 30 and 87, there should be 12 V at pin 18. [Model Year 1989 and newer 928s have a spark monitoring system, consult the wiring schematics and workshop manuals.]
i. Power and Ground connections are key points to inspect and clean. The LH is grounded via Pin 17 to chassis ground point VIII. Use an Ohm meter to measure the resistance between LH connector Pin 17 and a chassis ground point. It should read as close to zero as possible. If it reads a high resistance, a corroded ground connection at MP VII or cut in the wire could be the problem.
j. If all of these items check out OK, and the fuel pump is functioning, then potential problems could be:
- The system fuel pressure is too low; fuel filter or pipes are restricted, fuel injectors are not functioning correctly
- The LH Jetronic fuel injection module is not functioning correctly and requires a rebuild.
Common symptoms of a failing LH injection module
- Fuel injection valves click when the ignition turned is on and engine is not running
- Air-fuel mixture is too rich (this can damage catalytic converters)
- Engine will not run at idle
- No fuel pump operation, but the fuel pump operates when its relay is jumpered
- The engine will not rev higher than 3000 RPM
- A/T cars: no idle speed regulation when switching between drive gears and neutral or park
- Engine will not start, spark plugs are dry and the ignition system produces spark.
>> warning, if you replace a failed LH module with a used one, you may have a failure in the near future. The failure symptoms seem to be "module age" related, not number of operating hour related. I've spokent to people who've purchased used units and they've failed within six months. Insist on a replacement that has had the hybrid chip removed and exchanged.
Many people jump to the most expensive part when a no-start or bad running problems occur. That isn't the way to go. If you or your mechanic have eliminated the "easy to find things" ... this list of diagnosis goes a lot deeper.
All functionality with the EZK spark system must be confirmed prior to testing the LH Jetronic system.
Sometimes the diagnosis can be more expensive then the repair! So feel free to share this information with your shop, it could save you money.
===========
928 Model year 1987 – 1988 System No Start Diagnostics (later model diagnosis is similar, consult the wiring diagrams for specifics):
Diagnostics should be only be performed by a trained technician. Always check for proper relay functionality and power/ground connections before undertaking further diagnostic testing.
a. Testing the engine RPM sensor. The EZK spark control is the system master, if engine rotation signals are not provided to the EZK, spark will not be generated and EZK will not provide a turn-on signal to the LH Jetronic fuel injection system. The RPM sensor input to the EZK is from an inductive RPM sensor. Inductive RPM sensors generate voltage so a VOM can be used for testing the sensor. EZK pins 23 and 6 are connected to the RPM sensor; pin 6 is ground and is connected to master-ground pin 18. EZK pin 24 is the shielding of the sensor cable and is connected internally to pin 18.
b. The Hall Effect engine position sensor can be tested with an LED tester. The EZK provides the power supply for the Hall Sensor at pin number 5 (12V). The EZK provides a 5V level at pin number 22, which the Hall sensor can pull down to ground. Pin EZK pin 22 is the "hot" signal cable and therefore is shielded. The cable shielding is connected to EZK pin 4, and pin 4 is connected internally to master-ground EZK pin 18.
c. The EZK spark module provides the signal to turn on the LH fuel injection module; it is the circuit from EZK pin 13 to LH control Pin 1. Check for continuity, if the wire is broken or the connection isn't clean, the car will not start.
d. The circuit from LH Pin 20 goes to the Fuel Pump Relay XX pin 85, check for continuity between these two points.
e. From the Fuel Pump Relay XX pin 87 the circuit passes through fuse 42 then to the fuel pump motor. Pull the fuse, check the fuse, and clean the connectors. Check the continuity of this path. Bridge Fuel Pump Relay XX socket points 30 and 87, the fuel pump should run.
f. There should be 12V (nominal battery voltage) at the fuel pump and LH relay pins 30 at all times. This same circuit connects to LH connector Pin 4, check for 12V there too.
g. The LH relay XXV pin 85 connects to LH connector pin 21, check for continuity between these two points.
h. The LH connector pin 18 connects to all the fuel injectors. The LH functions as the switch from the fuel injector to ground. With the LH relay jumpered between pins 30 and 87, there should be 12 V at pin 18. [Model Year 1989 and newer 928s have a spark monitoring system, consult the wiring schematics and workshop manuals.]
i. Power and Ground connections are key points to inspect and clean. The LH is grounded via Pin 17 to chassis ground point VIII. Use an Ohm meter to measure the resistance between LH connector Pin 17 and a chassis ground point. It should read as close to zero as possible. If it reads a high resistance, a corroded ground connection at MP VII or cut in the wire could be the problem.
j. If all of these items check out OK, and the fuel pump is functioning, then potential problems could be:
- The system fuel pressure is too low; fuel filter or pipes are restricted, fuel injectors are not functioning correctly
- The LH Jetronic fuel injection module is not functioning correctly and requires a rebuild.
Common symptoms of a failing LH injection module
- Fuel injection valves click when the ignition turned is on and engine is not running
- Air-fuel mixture is too rich (this can damage catalytic converters)
- Engine will not run at idle
- No fuel pump operation, but the fuel pump operates when its relay is jumpered
- The engine will not rev higher than 3000 RPM
- A/T cars: no idle speed regulation when switching between drive gears and neutral or park
- Engine will not start, spark plugs are dry and the ignition system produces spark.
>> warning, if you replace a failed LH module with a used one, you may have a failure in the near future. The failure symptoms seem to be "module age" related, not number of operating hour related. I've spokent to people who've purchased used units and they've failed within six months. Insist on a replacement that has had the hybrid chip removed and exchanged.
05-24-2004, 08:56 PM
#5
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If you don't know which EZK and LH pins are which, the instructions can be confusing. If anyone would like a PDF with these hints and the pin-outs, drop me a line with your choice of a 57 kbyte PDF file that looks a little fuzzy or a 625 kbyte PDF version that will print out well. Send your return email address to
info@electronikrepair.com
and I'll send it out.
info@electronikrepair.com
and I'll send it out.
