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trigger errors above 5300rpm

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while tuning the long tube header car (Porsche 944 Turbo) today, I couldn't really rev past 5300rpm.

Looking at the Trigger/Limits runtime values in the G4+ software, I quickly found out that the RPM signal becomes erratic at that point and that I get a lot of Trigger 1 Errors.

The brand new crank trigger sensor is the factory Bosch 3-pin VR sensor (VE+, VE- and shield), so is the crank trigger pattern - factory 132 teeth Porsche.

I didn't really have an oscilloscope handy, so the best I could do is rev the engine up and get a trigger scope capture within PCLink. That was my first scope with PCLink, so I'd guess and looks OK. The only thing that I noticed is, that the max. values of the crank trigger sensor are -6.5V and 6.5V.

Which I personally think is a bit on the high side - compared to the cranking RPM scope of not even 0.8V in both directions.

Whilst setting the base ignition timing, the timing mark didn't drift at all and I also made sure that the polarity is correct, so that _shouldn't_ be the culprit.

Setting the arming treshold table to a ridiculous value of 0.8V above 4000rpm actually improved the situation, as it was set to 2.5V before that and I had a hard time revving past 4500rpm.

The trigger filter setting didn't really change much.

Could my problem be related with an incorrect sensor gap?

I know that 132 teeth are a lot, but even the 25 year old factory ECU was able to read the RPM's from that ;)

Any thoughts?

thanks a lot,


Look at your ignition dwell times, twice this year already I've had issues that the dwell table was set at over 5's when is should be 2.5 or less. Both times I was limited about 5500

I would actually consider that a 6.5V to -6.5V peak to peak value to be on the low side for a VR sensor, you would normally see a higher value. This does not really make a difference to the ECU however, as it is the 0V crossing and the direction that it is heading that an ECU uses with a VR Sensor. If the Arming Threshold value is the same as the Trigger Voltage value in a MoTeC Hundred Series ECU, then for a VR sensor then this should be 0, unless there is an offset of the crossing point.

Can you post the trigger scope capture?

If my memory is correct, the engine speed pickup on the 944 is off the flywheel? I've had a few Porsches over the years running on Link ECUs and found triggering issues based on the high tooth count. I haven't tried anything of the G4/G4+ era though and I would have expected these ECUs not to struggle. Having said that though, the Porsche trigger pattern in my experience does seem quite fussy to set up.

What rpm are you doing your trigger capture? If that's at idle it's probably pretty good. If you're only seeing 6.5 volts at 5300 rpm then it's not enough. The air gap from the sensor to the trigger teeth is another thing to look at. If the gap is excessive it will drop the magnitude of the voltage. Of course too close and any run out in the flywheel or thrust movement could take out the sensor.

BlackRex, on the G4/G4+ the Arming Threshold table is the same as the MAG Sensor Levels table in an Mx00. The Link ECU doesn't cater for offset signals and triggers by default at the zero crossing.

Dwell table is set for LS1 coils, as we're using them ... and the table is straight from Mark McCoy from MoTeC ;)

Yes, it's off the flywheel and the signal is also in form of a wave ... so basically there is a runout on the flywheel, or starter ring gear for that matter.

I talked to Simon from Link earlier today and apart from the sensor gap and upgrading the firmware, he also mentioned to wire in a 10k ohms resistor across the wires.

They had the problem on a customers 944 Turbo, and the solution was to change the upgraded Aluminium flywheel against the original steel one.

Bad luck for me, the car in question is also equipped with an aftermarket aluminium flywheel. Fingers crossed i can fix it today without the need of a new flywheel.

So I'll try all of the suggestions and see what's happening.

I've also attached the Trigger Scope from around 5000rpm plus

Attached Files

I wouldn't expect to see problems from an alloy flywheel provided the actual teeth are still made from a ferrous material.

The trigger scope file you posted is not long enough to be useful unfortunately. You really need at least one full engine cycle showing the relationship between the sync and reference signals. The amplitude of that signal however is quite low for 5000 rpm.

Would changing from a VR magnetic signal to a hall signal be viable? Motec do a Dual Magnetic Converter, you could try.

We were able to fix it, and we only had to sacrafice one of the spare VR sensors :)

The trick was, to realign the sensor bracket, so that the sensor is able read on it's complete surface, not just the "first part". Basically the bracket can be rotated up / down, but changes the angle of the sensor and how it is facing the starter ring gear.

I guess Porsche made shims in various sizes to get the correct gap back in the day.

We sacraficed one sensor to get a 0-gap idea as you can't get a feeler gauge anywere near that part :)

Then we made up a shim to get as close as possible while cranking over the engine.

The signal is now absolutely perfect until about 3000rpm, there's basically no difference between the highest and lowest voltage of the amplitudes (about 8V to 9V). Above that rpm, the ring gear obviously starts swinging away and back to the sensor, so at around 6000rpm the highest amplitude is around 12.5V while the lowest beeing only around 2V.

Setting the Trigger 1 Arming Treshold table acordingly then made as able to rev the engine past 6800rpm, which is good for now.

To be on the safe side though, a new front pulley with a 60-2 trigger wheel is already in the works.

I'm glad you sorted the issue. It sounds like you must be getting a reasonable amount of runout on the flywheel though. Have you measured it?

Me too, thanks for all the hints :)

Not yet, we only have a week and half left to get the car ready for a Rally Legend Event over here and there's still loads of prep work to do.

We'll have two more test days at the track this Thursday and Friday, if everything works out good we'll go with the stock trigger system and will move on to the 60-2 afterwards.

If not, then we'll go straight to the new trigger pattern this weekend, as the pulley should be done on Wednesday or Thursday.

After the Rally, there should be some time to check the runout and deciede which flywheel route to go.

Good luck with everything.

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