Why the timing light can fool you
A dual pickup distributor gives the ECU two signals: a trigger for each firing event and a home (sync) pulse once per cycle. When nobody has documented where that home pulse lands relative to cylinder 1, you have to find the TDC offset yourself.
Here's the trap. The distributor rotor mechanically routes each spark to the next cylinder in line. So if your TDC offset is out by exactly one firing interval, or two, or three, the spark still arrives at the right plug at the right time. There are as many "working" offsets as there are cylinders. The engine starts, the timing light matches the laptop, and you'd think the job was done.
What isn't right is everything the ECU does per cylinder. It thinks cylinder 1 is a different cylinder, so sequential injection is aimed at the wrong intake strokes, and any per-cylinder fuel or ignition trims land on the wrong cylinder. The engine may run, but it won't run as well as it should, and per-cylinder tuning becomes meaningless.
The idea behind the test
If you change the timing on cylinder 1 alone, the ECU applies that change to whichever cylinder it believes is number 1. Put a timing light on the real cylinder 1 lead. If the light shows the change, the ECU and the engine agree. If it doesn't, the offset is pointing at another cylinder, and you step it along one firing interval at a time until the change shows up.
What you need
- An inductive timing light clamped on the cylinder 1 plug lead
- A laptop online with the ECU so you can watch the
Ignition Anglechannel - The engine able to idle steadily
Step by step
- Clamp the timing light on cylinder 1's lead and go online.
- In Main Setup, under Engine then Ignition, turn on cylinder correction and press OK. The software may ask you to power cycle the ECU.
- Start the engine and confirm the light agrees with the Ignition Angle shown in the software. If it doesn't, sort out base timing first.
- Open the ignition cylinder 1 correction table and fill it with
-15. Now only the cylinder the ECU thinks is number 1 is retarded by 15 degrees. - Look at the light. If the mark retarded by about 15 degrees, the offset is correct and you're done.
- If the timing didn't move, add one firing interval to the TDC offset (see the table below) and check again.
- Keep stepping until the light shows the 15 degree retard on cylinder 1.
- Set the cylinder 1 correction table back to zero. If you don't need individual cylinder trims, you can switch cylinder correction off again.
| Cylinders | Firing interval (degrees) |
|---|---|
| 4 | 180 |
| 6 | 120 |
| 8 | 90 |
Worked example
Take a V8 that starts with the TDC offset at 50. With the cylinder 1 trim at -15, the light on cylinder 1 doesn't budge. Step the offset to 140 (50 + 90) and check again. Still no change? Try 230, then 320, and keep adding 90. A V8 has eight candidate offsets, one per cylinder, and at one of them the cylinder 1 mark will jump back by 15 degrees. That's your offset.
Notice that overall timing on the light doesn't change as you step through. Shifting by exactly one firing interval keeps the spark timing the same; the only thing that moves is which cylinder the ECU believes it's firing.
Run this at idle with a steady engine. Keep hands, the light's leads and loose clothing clear of belts and fans, and don't forget to zero the trim table before driving.
Troubleshooting
| Symptom | Usually |
|---|---|
| Timing never shifts at any offset | Cylinder correction not enabled or not power cycled, or the trim went into the wrong cylinder's table |
| Timing shifts, but the whole engine retards | Global ignition table edited instead of the cylinder 1 correction |
| Light and software disagree from the start | Base timing not set; fix that before this test |
| Timing moves on the wrong lead | Timing light isn't clamped on the real cylinder 1 lead, or plug leads are out of order |
FAQ
Does a wrong TDC offset matter with batch injection?
Much less, because batch fires injectors without regard to each cylinder's stroke. It still matters for per-cylinder trims, and it matters a lot once you go sequential.
Why -15 degrees?
It's a big enough step to see clearly on the light at idle, and it's retard, not advance, so the cylinder is never pushed toward knock during the test.
More trigger reading: engine trigger systems explained, distributor rotor phasing and EFI Hardware Hall effect distributors.
