Diagnosing car problems can be expensive, especially when relying solely on guesswork or replacing parts without proper testing. However, with the right tools and knowledge, you can achieve Cheap Car Diagnosis and pinpoint issues accurately. This guide focuses on using an oscilloscope to probe the ignition system of Volvo 240, 740, and 940 models equipped with Bosch LH-Jetronic fuel injection systems. By analyzing waveforms at different points in the ignition system, you can efficiently identify faults and save money on unnecessary repairs.
These notes detail how to probe various components, specifying the connector, pin, and wire color for each test point. Remember that while pin positions are generally consistent, wire colors might vary slightly between model years. For grounding your oscilloscope, any clean chassis or engine block point will work. If you encounter noisy waveforms, ensure your ground connection is clean and secure. Persistent noise might indicate a poor ground in the sensor or component itself.
When probing connectors with T-pins or sewing pins, insert the pin from the back of the connector, alongside the metal connector pin. Gently wedge it to establish good electrical contact. For Volvo LH connectors with rubber boots, resting the pinhead on the boot often provides sufficient side pressure for a reliable connection and prevents the pin from slipping out. Crucially, avoid forcing the pin into the wire or the connector pin itself to prevent damage.
For signals below 40 volts, you can use either a 1X alligator probe or a 1X/10X scope probe. Ensure the 1X/10X switch on your probe matches the setting on your oscilloscope. However, for higher voltage signals, particularly the ignition coil primary (-) and injectors, you MUST use a 1X/10X scope probe set to the 10X position. This precaution is essential to prevent damage to your oscilloscope input.
For each waveform described below, the oscilloscope setup is provided directly above the corresponding image. While these settings are also visible in the images, they may be less clear.
Ignition Waveforms: Understanding Volvo LH-Jetronic Systems
Most Volvo 240, 740, and 940 models in the USA utilize Bosch LH-Jetronic electronic fuel injection. LH versions 2.0, 2.1, and 2.2 were common from around 1983 to 1988, while LH2.4 was implemented from 1989 onwards. All LH versions employ separate control units for fueling and ignition.
Early LH 2.0, 2.1, and 2.2 ignition systems (early 1980s) used either a Chrysler ignition box (240s) or a Bosch EZ117K ignition box (740s), paired with a distributor featuring a one-pulse-per-cylinder Hall-type position sensor. The ignition box adjusts spark timing based on crankshaft position, engine RPM, and either intake manifold vacuum (Chrysler box) or a load signal from the fueling ECU (EZ117K box).
Later LH 2.4 ignition systems (1989 onwards) utilize a Bosch EZ116K ignition box, incorporating a VR-type sensor and a 60-2 “sixty minus two” toothed flywheel/flexplate for crankshaft position detection. In LH 2.4 systems, the distributor solely directs the spark to the correct cylinder without a position sensor. The EZK box manages spark timing using crankshaft position, engine RPM, and a load signal from the fueling ECU.
Ignition – Distributor Position Sensor (LH2.2): Square Wave Analysis for Timing
LH2.0, 2.1, and 2.2 distributors incorporate a Hall-type position sensor with a rotating 4-flag shutter wheel. This sensor generates a square wave pulse for each cylinder firing, crucial for timing. Adjusting the distributor body in the block fine-tunes the idle timing.
The distributor connector has three pins: “-” ground, “O” output signal, and “+” 12 volts. To test the signal, probe pin “O” (center pin), which is connected to the yellow wire.
[Waveform image for LH2.2 distributor position sensor is needed. A typical waveform would be a 0-volt to 10-volt square wave with two pulses per revolution and approximately a 40% duty cycle. Expect a frequency of around 30Hz at idle and about 70Hz at 2000rpm.]
Ignition – Crankshaft Position Sensor (CPS) LH2.4: VR Sensor Waveforms and No-Start Diagnosis
The LH2.4 EZ116K ignition system employs a Variable Reluctance (VR) type sensor for crankshaft position sensing. This sensor consists of a coil of wire, a magnet, and an iron core. As each tooth of the flywheel/flexplate passes the VR sensor, the changing magnetic field induces an alternating +/- voltage in the coil. The magnitude of this voltage is directly proportional to engine RPM – lower RPMs like cranking produce a smaller voltage, while higher RPMs generate a larger voltage.
A damaged VR sensor, perhaps due to internal shorting from corrosion, or an incorrect air gap (too far from the flywheel/flexplate), can lead to insufficient voltage generation, preventing the EZK box from detecting the crankshaft position. This often results in a no-start condition, making CPS testing vital for cheap car diagnosis of starting issues.
The CPS connector has 3 pins: ground, signal, and shield (grounded at the EZK connector). Probe pin 2 (center pin), connected to the red-yellow wire, to measure the CPS signal.
CPS Waveform Examples:
Idle – Zoomed Out View (1.5 Engine Revolutions)
Setup: AUTO trigger mode, 1V/div, X1, 10mS/div, DC coupling
Idle – Zoomed In on 60-2 Missing Tooth Section
Setup: Single rising-edge trigger mode, 1V/div, X1, 2mS/div, DC coupling, trigger level mid-waveform
2000 RPM – VR Sensor Details
Setup: Single rising-edge trigger mode, 2V/div, X1, 1mS/div, DC coupling, trigger level mid-waveform
Notice the peak-to-peak VR voltage: ~3.6v at idle, increasing to ~8.5v at 2000rpm. During cranking (~200rpm), expect around 0.9v. Once the ignition control box detects proper engine rotation from the CPS (LH2.4) or distributor sensor (LH2.2), it initiates spark signals to the power stage and generates a tach signal for the fuel control box. For safety, the fuel control box only activates the fuel pump continuously upon receiving a valid tach signal. LH2.4 fueling systems also briefly run the fuel pump at key-on to pressurize the fuel rail.
Ignition – Powerstage (Ignition Amplifier/Igniter): Logic Level Spark Signal
For LH 2.0, 2.1, and 2.2 ignition systems with a Chrysler ignition box, the ignition coil is directly driven by the ignition box. However, for LH2.2 systems with an EZ117K box, and all LH2.4 systems, a separate powerstage module (ignition amplifier or igniter) is used. The powerstage amplifies a low-current “logic level” spark signal from the EZK ignition control box into the high current needed to drive the ignition coil. One side of the coil (+) is connected to +12v, and the other side (-) connects to the powerstage.
When the EZK box sends a high signal (~5volts) to the powerstage spark input, the coil begins charging. When the EZK box signal drops low (~0volts), the coil discharges, generating the spark. The duration of the EZK spark pulse determines the dwell time.
The easiest way to probe the powerstage input is to unplug the connector, backprobe the EZK input pin (pin 5) with a T-pin, and then reconnect the connector. Alternatively, you can use a spare powerstage, leaving it disconnected from its mounting, to access the pins.
Probe: pin 5, gray wire
Powerstage Waveform Examples:
Idle
Setup: AUTO trigger mode, 2V/div, X1, 20mS/div, DC coupling
2000 RPM
Setup: AUTO trigger mode, 2V/div, X1, 20mS/div, DC coupling
Ignition – Coil Primary: High Voltage Pulse Verification
The coil primary voltage (negative terminal) is readily accessible for probing. To protect your DSO152 oscilloscope, always use a 1X/10X probe in the 10X position for this test. Also, ensure your oscilloscope is set to X10 to display accurate voltage levels.
Interpreting coil primary voltage waveforms is a complex skill – further resources on this are linked below. For basic cheap car diagnosis, the primary goal is to confirm that the coil is firing for each cylinder. The DSO152 is suitable for this basic check, though not for in-depth waveform analysis due to its limited storage and resolution. Displaying all four cylinders consistently on a single screen can be challenging, and waveform variations might appear due to scope limitations rather than actual cylinder differences.
Probe: coil “-” tab, red-white wire
Coil Primary Waveform Examples:
Idle
Setup: Single rising-edge trigger mode, 10V/div, X10 probe, 10mS/div, DC coupling, trigger level at about 25v
2000 RPM
Setup: AUTO (or Single) rising-edge trigger mode, 10V/div, X10 probe, 5mS/div, DC coupling, trigger level at about 25v
Single Pulse Detail
Setup: Normal (or Single) falling-edge trigger mode, 10V/div, X10 probe, 1mS/div, DC coupling, trigger level at about 5v
Ignition – Coil Secondary: Isolating Spark Plug Issues (Advanced)
Coil secondary voltage waveforms are valuable for pinpointing ignition problems to a specific spark plug. Using a specialized high-voltage ignition probe (like the Hantek HT-25, ~$20), you can safely probe the secondary or spark voltage (up to ~40KV). While coil primary voltage waveforms offer similar information, secondary waveforms directly isolate issues to individual cylinders.
[The original author encountered difficulties using a Hantek HT-25 probe with the DSO152, struggling to achieve stable triggering and meaningful secondary waveforms. Refer to specialized resources for detailed secondary ignition waveform analysis techniques.]
By using an oscilloscope and following these probing techniques, you can perform cheap car diagnosis on Volvo LH-Jetronic ignition systems, saving time and money while accurately identifying ignition faults. This method allows for targeted repairs, avoiding the costly trial-and-error approach often associated with diagnosing car problems.
