# Megasquirt - severe misfire with alternator connected



## TheTimob (Sep 13, 2003)

Okay - I've been running Megasquirt on my Scirocco since 2005. Car is a 1.8L 8v JH with a G-grind cam, digifant fox manifold and rail with 19lb injectors, rabbit dual downpipe, eurosport exhaust, passat 16v TPS throttle body, digifant distributor. Innovate MTX-L Wideband.
*
In 2011, I started getting a random misfire that would come and go.*

Parts I replaced:
I started replacing parts and changing things, and each time it would seem to go away. Then it would come back.
I upgraded to MS2/extra. 
I built the new version of of the logic spark circuit and switched to a mk3 coil (built in module) because I thought I was having VB921 problems.
I changed the distributor, the starter, the battery, redid all the grounds. I even rewired the entire engine, and switched to a relay board wiring setup. 
My hall sender now has a shielded signal wire grounded at only the megasquirt end. 
My ground cable goes from the battery to the transmission top lug, and then to the body. 
Megasquirt is grounded at the transmission lug on the same stud. The ignition coil is also grounded here.
The fuel pump is new - it's an E2471 Airtex in-tank pump meant for an explorer. I even ran all new fuel lines and wiring for it. There's plenty of pressure - got 50PSI at the rail. New fuel filter recently.
I even did a compression test - all good. Checked the timing. Good. 
I TOOK THE CYLINDER HEAD OFF - nothing wrong internally. New head gasket, new downpipe, put it back together - all good.

When the alternator field wire is disconnected and the alternator is not charging, it runs smooth. When it's connected and charging, the car misfires like CRAZY. It sounds like its running on two cylinders when it's in this mode.

I re-did all the power supply tests on the megasquirt - checking for 5v on the processor pins - everything tested good there. I did not try to do this with the motor in the misfiring state - it's too hard to keep it running when it's doing it.

I took the alternator off, and tested it at the parts store - failed for "Stator Volts" - so I got a new one. It tested good. When I installed it? Nope - same issue when it was actually on the car. The alternator case is grounded to the block with a short ground strap - it's grounded in the bolt hole where the knock sensor goes on later cars.

So I thought maybe there's a short in the field wire. I disconnected it, engine idling - not charging - running fine. Put 12v on the stator terminal right at the alternator to start it charging? *MISFIRE*!

So I'm not sure why having an alternator causes a misfire.

Remember when vortex was Pics 4 clicks?


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## weejunGL (Feb 2, 2005)

is the misfire seen in the logs?


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## B4S (Apr 16, 2003)

Is the exciter wire the stock blue one back to the cluster, or is it a 12v to the alt from another source?


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## TheTimob (Sep 13, 2003)

B4S said:


> Is the exciter wire the stock blue one back to the cluster, or is it a 12v to the alt from another source?


The blue wire is the stock one to the cluster. I tried 12v directly, but same result.



weejunGL said:


> is the misfire seen in the logs?


I'm not sure about the logs, I'll have to take a second look and post it here.


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## need_a_VR6 (May 19, 1999)

Watch battery voltage and dwell values in the log. Also if you are driving a mk3 coil off the led instead of an internal driver the output needs to be inverted.


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## TheTimob (Sep 13, 2003)

Here's what I'm working with:

Here's a log file

At startup, the car runs fine. When I rev it up - the alternator self-excites, and the voltage comes up. Everything goes to hell and the car misfires like crazy when it's high on voltage.

This is my current MSQ










I'm not triggering directly from the LED - my Mk3 coil is controlled with the FET driver method from the msextra manual - *5.3.1.3 Logic spark outputs*
with this setup, it needs to be non-inverted.

Looking at that log - I can see that the dwell on the coil is dropping with the alternator charging... what's happening?


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## B4S (Apr 16, 2003)

Looks like the dwell battery correction is dropping the dwell when the voltage comes up.
Your batt voltage is really inconsistent...which might be playing havoc with everything.


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## rodperformance (Oct 9, 2010)

*Dwell*

dwell as the fellow guys have mentioned,also there is a relation between bat voltage and injector pulse that i have set @, .20-.23 and it changes every thing concerned to fuel,injector pulsewith? ,it could be the same mentioned before,good luck mate!


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## need_a_VR6 (May 19, 1999)

Dwell should be around 3ms at 13v, whats your nominal set to?


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## TheTimob (Sep 13, 2003)

need_a_VR6 said:


> Dwell should be around 3ms at 13v, whats your nominal set to?


In that log, I had it set to 2.5ms. I raised it to 3. It acted the same. I changed all the values in the Dwell Battery Correction dialog to 100%.

I also tried raising it to 4 and seeing how it ran. Still backfires and is inconsistent when the alternator is charging, but not as bad.

Also tried putting it on fixed dwell - it ran about the same as the 4ms - but the dwell gauge was showing crazy random swings and it was scary so I put it back on normal dwell.
'
In one run - the engine was running crappy - and it shot smoke out of the intake and ran backwards for about 7 seconds and died.

My mk3 coil is powered by 12v from the relay board's injector 12v outputs. The ground goes directly to the transmission lug on the opposite side of the same stud the Megasquirt is grounded to. It's firing non-inverted from the FET driver circuit I built in the proto area of my V3 board.

I need to make a trigger log to see if I'm loosing rationality of my hall sensor signal in the distributor.


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## TheTimob (Sep 13, 2003)

Here's a new log:










The log file: http://www.timbox.net/Link/2015/Megasquirt/trig.msl


I tried to capture a trigger log, but I didn't save it for some reason. The bars on the trigger logger were shorter when the alternator was charging though. Maybe the voltage is pulling the hall signal too much?


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## need_a_VR6 (May 19, 1999)

You driving the aba coil from the led or a bip?


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## TheTimob (Sep 13, 2003)

need_a_VR6 said:


> You driving the aba coil from the led or a bip?


Top of R26 to a TC4427 FET driver - for a logic level output. The Bip is still on the heatsink.

Now that I'm thinking about it - what the heck am i getting out of Pin 36? Did I jumper Spark A to IGN? I have to check my board now. I need to remove my bip and double check everything.


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## TheTimob (Sep 13, 2003)

I'm not crazy. The output of the FET is connected to IGN. That's fine. 


















The big resistor on the bottom is the pullup for the hall. There's a burned trace on the ignition select jumper- fixed by just running from the db37 to optoin


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## need_a_VR6 (May 19, 1999)

Hm double check the inverted/non setting for use with the 4427, I have only used those to drive 1.8t/2.0t coils.


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## Prof315 (Jan 5, 2009)

need_a_VR6 said:


> Hm double check the inverted/non setting for use with the 4427, I have only used those to drive 1.8t/2.0t coils.


A 4427 needs going high with any logic triggered coil


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