Dwell Time definition

Hi Guys,

I am having some trouble with my OEM Nissan Micra ignition and dissy module, trying to setup my Elite 1500.

This part is prone to just fail, and the part on the car is old with unknown background, but it was working fine until now.

The trigger side of things seems to be fine, since i get no errors and a reasonable rpm value during cranking. Spark was also OK for a while, even though the engine never started on the Elite. (a 4+1 slots optical sensor is integrated in the dissy, similar to the SR20s)

During the base timing setup process, the spark was just not there after a couple of trials. It is not a matter of distributor tolerance, since the problem occurred at a setting that was firing until then, just not on the pulley marks. I switched back to the OEM ECU and no spark again. In my understanding, this means that the coil or ignitor is damaged, a common problem on this car. Replacement parts are on their way, but i need to make sure that the settings are OK.

Q1: According to Haltech, the ignition edge should be set to falling. It it possible that setting it to rising can damage the system in a couple of runs?

Q2: How sensitive is a coil in non optimal dwell time settings? The values I am using are the base map data from Haltech and seem reasonable

Q3: Since the car is still running with a working dissy and the OEM ECU, it is possible to measure and reverse engineer the Dwell times?

Any hints?

Thanks!

It sounds to me like a failure from the ignition module. The dwell 'should' be set correctly but the reality is that in most instances we won't have proper dwell vs voltage values and in general a dwell time of around 2.5-3 ms at 14 volts is going to be sufficient for most coils without risking damage to them. If anything your dwell values may be a little low but not enough to give you no spark.

The ignition edge is very important and you can quickly destroy an ignitor module or coil if you have this set wrong. In almost all cases this should be falling edge (in my experience Honda and some Fords are the only ones that need a rising edge), however I'm not familiar with your particular ignition system.

If you have a scope then you can reverse engineer the factory dwell values, however if you have a scope you can test the required dwell using your Haltech ECU anyway.

Check your battery voltage. Many OEM ECUs will flat out refuse to start an engine if the battery voltage is too low.

Q1 -- yes, you can certainly destroy a coil by feeding it current for long periods of time (like when you set the trigger to rising, it is then supplying current to the coil when it it should be off. For get those milliseconds, you've been leaving it on for seconds or minutes. Check coil resistance to see if it's shorted.

Q2 -- not that sensitive to small errors (say 2x or 3x the optimal value). I would not be surprised if the dwell time at 6.5V would be 8-10 ms.

Q3 -- its possible to determine the dwell times using your ECU, a current probe and an oscilloscope. Engine doesn't have to be running, if the ECU has a mode to "test ignition". Easier to do on the bench if you have a variable power supply.

Hi guys,

Thanks for your input.

The wrong edge setting is a high possibility and a spare assembly is already on it's way. I will also try to measure the "damaged" one against the OEM service manual to verify what happened .

Ignition failures are very common with this car and one of the reasons i am switching to an aftermarket ECU, to get some freedom later on. But i need to get it up and running with the OEM ignition at first.

Reverse engineering should help to be more certain, so could you please give my some more guidance regarding the scope? Should I wire it in series with the ECU ignition output leading to the ignitor (power transistor in this case), or with the coil itself?

Thanks!

I think you will find what you need on this page:

http://dtec.net.au/Ignition%20Coil%20Dwell%20Calibration.htm

Bingo !

Thanks!

Update:

- Coil blown --> replaced

- Dwell times increased by 50%

- Base ignition set +/- ~2 deg

Outcome:

Car start perfectly, while warm (i didn't try a cold start yet, but i see no issues) and runs as expected for a non tuned fuel map.

How deep should I look into dwell times? is there an issue if it's a bit off?

Thanks a lot for your support!

Thanks for the update. Sounds like you did a good job!

I would only revisit the dwell times if I had issues such as a misfire at high load, or cold starting with low battery voltage.

Thanks!

My only concern is that the only think that breaks on this car is the ignition. So I want to make sure that I am not putting any non necessary load on it.

I will leave it as it is for now, during tuning, and keep an eye open for any reliability issues.

In the long run I plan to change the system to either a wasted spark, so I can still run the OEM ECU (1 ignition output + dissy) or to a coil on plug.