Tacho output on Ign #4

[Leiden]

Hi guys,

I was reading through the Link manual and came across a couple pages that mentioned the spare ignition outputs should only be used for simple switched devices (see attached picture). On the Auxiliary Output Wiring page it shows that the first three ignition channels can be used for tacho up to 300Hz but the fourth output isn't supported? (see second picture)

I have a G4+ Atom on a 6 cylinder running waste spark off the first three ignition channels and have been using the spare 4th channel as the tacho output for the past couple of years, the Link software let me select Tacho as the configuration so I didn't think anything of it until reading the manual. So far everything has been fine except for having a strange issue with dwell times, I have it set to around 2.0ms but the dwell time jumps around a bit, anywhere from 1.85 - 2.10ms even though the voltage is steady, or at least the log isn't showing any changes. It doesn't seem to affect the vehicle in any noticeable way but I was curious as I also get lean spikes in the wideband reading when this happens (see third picture).

Just out of curiosity I was wondering if having the tacho output on the spare ignition channel is causing some kind of interference with the dwell timing which causes a minor misfire and that's why I'm seeing those lean spikes?

[Simon]

Tacho will be fine on the ignition drive.

The Dwell is calculated off the firing event and will vary a little as the RPM moves around.

Unstable dwell can however also point to a trigger issue. If the timing is also unstable then likely a trigger issue.

[Leiden]

Thanks for the reply Simon,

I ended up swapping the Tacho over to Aux 1 and using the spare ignition drive to trigger the fuel pump relay, the tacho sweep seems to be a little smoother but its probably just my imagination.

The engine is a RB30 with a RB25DET Neo head, I'm using a 36-1 on the crank and a single tooth on the exhaust cam for sync. Both sensors are ZF GS100701 Hall Effect and seem to be capable of up to 15KHz, I have tried adjusting the air gap in the past with no noticeable changes, its always had an odd lean spike here and there, I just assumed it was the Innovate LC1 wideband since they seem to be a bit unpopular and outdated, when I saw the manual state that the ignition outputs should only be used for simple switched devices I thought I was on to something. The dwell moving around seemed a bit off though, I've never noticed it before but then again I probably wasn't looking close enough at it.

I was actually thinking about the triggering being an issue however I'm not seeing anything on the Trig1 or Trig2 error counters. Would these be a good indication of trigger problems or only in a worst case scenario? Log wise the timing appears to be stable but I'm guessing you a referring to actual ignition timing rather than what the ecu thinks is going on?

[Adamw]

Some other ECU brands only log "commanded dwell" (what comes from the dwell table), rather than the actual dwell that was employed at that point in time.  Link records/displays the actual dwell.  Dwell will always have to vary a little from the commanded value because the ECU has to start dwelling the coil at the commanded amount of time before the next spark event.  But because that spark event is "in the future" the ECU can only predict when that next spark event is going to occur based on how fast the crank was turning at the initiation of the dwell period.  During the dwell time the ecu receives further updates from the crank position sensor (as more teeth go past the sensor) so if the ECU learns the crank has sped up or slowed down from its original prediction then it has to either lengthen or shorten the dwell to ensure the spark still occurs at the correct time/angle.

Your lean spike suggests a misfire, the dwell stability during a misfire will suffer as the crank quickly slows down due to the miss, then quickly speeds up again on the next cylinder.  In other words, your dwell scatter is a symptom of the misfire rather than the cause.

It is normal to see some "dwell scatter", generally +/- 0.15ms is about as good as it gets.  If you are seeing more than say +/- 0.4ms or large spikes of dwell that that is usually a strong indicator that there is a trigger issue - the ECU is not receiving tooth events with regular time intervals between them.