How to test GM ignition module?
In brief, testing a GM ignition module involves confirming power and ground to the module, verifying the trigger input, and ensuring the module correctly drives the ignition coil. Using a multimeter and an oscilloscope, you can read signals and timing to determine whether the module or related components are at fault.
GM's ignition systems have evolved from early HEI designs to distributorless arrangements, but the core diagnostic approach remains consistent: verify supply, grounding, triggering, and output to the coil, then interpret whether the module itself is healthy or if replacement of the pickup, coil, or wiring is required.
What you’re testing
These checks cover the core electrical paths and signals involved in GM ignition control modules.
- Power supply and ignition feed: confirm 12V is present at the module’s power input when the key is ON, and that any separate constant power feeds are intact.
- Ground path: verify a solid ground from the module to the engine block or chassis with low resistance.
- Trigger input from the pickup: ensure the module receives a clean, pulsed signal from the pickup sensor when the engine is cranking or running.
- Output to the coil: check that the module is switching the coil primary circuit on and off as the engine runs or cranks.
- Control signals and timing: compare the timing and waveform (if using an oscilloscope) to specification or a known-good reference.
If these checks pass, but you still have no spark or misfires, the fault may lie with the pickup, coil, wiring, or a faulty module necessitating replacement.
Tools and safety
Gather these tools to perform bench and on-vehicle testing safely and accurately.
- Digital multimeter (and test leads or back-probe adapters)
- Oscilloscope (highly recommended for waveform analysis)
- Wiring diagrams or service manual for your GM model
- Spark plug tester or spark gap tester (optional but helpful)
- Spare ignition coil and, if possible, a known-good ignition module for swap testing
- Basic hand tools (screwdrivers, wrenches) and protective gloves
With these tools, you can inspect pins, measure voltages, view signal waveforms, and perform swap tests to isolate the fault without replacing multiple parts unnecessarily.
Step-by-step testing procedure
Follow these steps in order to diagnose a GM ignition module with engine off and under running conditions as appropriate. If you are not comfortable performing some checks, consult a professional.
- Locate the ignition module and identify its wiring harness pins using the service manual; note power, ground, and signal pins.
- Test the module’s power supply: with the ignition ON, measure voltage at the power input pin; it should read close to 12V. If there are multiple feeds, confirm each relevant line is present.
- Test ground continuity: check that the module ground is solid by measuring resistance to chassis/engine ground; it should be very low (near 0 ohms).
- Check the trigger input from the pickup: using an oscilloscope or a high-sensitivity meter, observe the sensor’s pulses when cranking and running; there should be clean, regular transitions.
- Evaluate coil primary output: back-probe the coil primary circuit or use a dedicated coil tester; there should be rapid switching of current and a sharp voltage change when the module fires.
- Perform a swap test if symptoms persist: replace the current module with a known-good unit (same part number) to see if the problem clears; if it does, the original module is at fault.
If no spark persists after these steps even with a known-good module, assess the pickup, wiring harness, distributor components (for HEI), and the coil or plugs themselves; misfires or no-spark conditions point to a broader ignition system issue rather than the module alone.
Interpreting results and next steps
Clear signals point to specific causes. For example, proper power and ground but no trigger pulses usually indicate a faulty pickup or a faulty module’s input stage; presence of trigger pulses but no coil output points to a module failure; consistent coil output with misfires can reflect bad spark plugs, wires, or ignition timing issues unrelated to the module.
In many cases, replacing the module is the final step after confirming the fault isn’t the pickup or coil. Always consult the vehicle’s service manual for pinout diagrams and testing specifications, and consider professional diagnostics if the vehicle is today's complex model with integrated engine control systems.
Summary
Testing a GM ignition module involves verifying power, ground, trigger input, and coil output signals, ideally with a multimeter and oscilloscope. Follow a structured approach: check wiring and power, confirm a solid ground, verify sensor signals, observe coil drive, and, if needed, swap in a known-good module to confirm fault isolation. Always reference your vehicle’s manual and prioritize safety when working around live circuits.
