How these GM systems work
On many older GM engines, the trigger signal from the distributor goes straight to the ignition module (the ignitor), and the module fires the coil. That lets the engine run even with no ECU input, at a single fixed timing. For the ECU to control timing, it needs to break into that path.
That's what "bypass" means here. When bypass is active, the module stops using its own timing. The trigger signal goes to the ECU, the ECU calculates timing, and it sends a firing signal back to the module, which still does the work of switching the coil.
| Bypass off | Bypass on | |
|---|---|---|
| Who sets timing | The ignition module, at a fixed value | The Elite ECU, from your ignition tables |
| Who fires the coil | The ignition module | The ignition module, when the ECU tells it to |
| Can you tune timing? | No | Yes |
Setting it up in ESP
The function has one connection to set: the Ignition Bypass Output.
- Assign it to DPO-1. That output is used because its pullup voltage is programmable.
- Set the pullup to 5.00 (volts).
- Set the Active State to High.
With those three settings the bypass line sits at the level the module expects once the ECU takes over. Any other output or setting and the module may stay in its own fixed timing mode.
Once it's set up, check timing with a light at idle and confirm it matches what ESP says the ECU is commanding. Then change the timing a few degrees in the software and make sure the light follows. If it doesn't move, the module isn't in bypass and is still running its own timing.
Troubleshooting
| Symptom | Usually |
|---|---|
| Timing doesn't change when you edit the map | Bypass output not on DPO-1, pullup not 5.00, or active state not High |
| Engine won't run with bypass on | Trigger or ignition output wiring to the module, or trigger settings in ESP |
More on ignition: base ignition timing and trigger setup. The Nexus version of this function is covered in Ignition Bypass (Nexus). Working on an older GM? Ask us for help wiring the module.
