Help me out with my new Megasquirt harness

[leesfer]

I have been drawing out a new harness for my car but having trouble with a few details...

 

I'm not exactly sure how to wire in the alternator and starter with a kill switch and Megasquirt, I also am not sure what to do around the MSD tach adapter.

 

Note: the car is not using any part of the old harness.

 

http://i.imgur.com/OOjU0kn.png

As for the starter and alternator, this is correct, right?

 

http://i.imgur.com/kD6yg9z.pnghttp://i.imgur.com/hYquxcF.png

Edited February 17, 2014 by leesfer

[ZacMan]

The alternator needs to go to the battery side of your kill switch, or when the engine is running, and you flip the kill switch, it will continue to run off the alternator alone.

 

The main power to the starter motor should travel through the kill switch, which is why they are rated for such high currents.

 

I like to run the main battery cable to the kill switch, and then use the other side of the kill switch as the new main distribution point for all power supplies within the vehicle. The only exception to this the alternator wire, which is hooked to the battery side of the kill switch, for the reason mentioned above.

 

There is an easier way to get your tach working on an aftermarket ECU. I'm making an assumption that you can configure one of the spare outputs of the megasquirt to provide to provide a 0-12v square wave, with one pulse per ignition event. You connect this directly to the original tach wire that went to the negative post of the factory coil, which leads directly to the input on the back of the tachometer in the gauge cluster. This 0-12V signal from the megasquirt doesnt have enough energy to correctly trigger the tachometer however, but you can make it work by removing the tachometer from the cluster and soldering a jumper over the first resistor in line on the tachometer PCB. I've personally done it a couple of times, and it works well. This way, you don't need an external MSD adaptor.

 

Have a read of this thread:

http://forums.austarion.com/viewtopic.php?t=14013&postdays=0&postorder=asc&highlight=tachometer+square&start=15

However, when I say 0-5V signal originally, I do mean 0-12V. Might still work with a 0-5V signal, but I haven't tested it.

[ZacMan]

Actually, hang on, are you still running a distributor? If you aren't using the megasquirt to control your ignition (you should, by the way), then the above method won't work, as the megasquirt won't really know when your ignition events are. I don't have any experience with the MSD boxes sorry, so I can't help you there.

 

Are the MSD boxes some form of CDI?

 

If they're not, you should still be able to run the factory tachometer off the negative port of the coil, as per the factory setup.

[leesfer]

Yeah I am using a coil and dizzy connected to the MS ECU so I need to tach adapter, plus I am using all new gauges too.

 

The kill switch I am using is a 4-prong so the alternator is connected to it's own prong which attaches to only the battery on the other side... when the switch is flipped it will disconnect all prongs from each other.

 

Does anyone have a link to a simplified diagram of the starter and alternator wiring?

 

For the alternator, all I need is for it to be attached to ground and the battery, correct?

Edited February 16, 2014 by leesfer

[ZacMan]

I still don't know why you need to run a tach adaptor? If you're running just one coil, and it's not a CDI setup, then the big inductive voltage spikes tach's usually require will be present on the negative side of the coil at each ignition event. Should work with either a factory, or most aftermarket tachometers.

 

Your alternator also requires switched power to the field winding. Most cars provide this through a charge warning light, but out cars don't have a charge warning light, so you can run it from you 'on' switched circuit (not the 'acc' switched circuit). You don't need to provide a ground for your alternator, it grounds via its bracket and the main engine ground.

[importwarrior]

talk to scottstar87 he can help

[leesfer]

I still don't know why you need to run a tach adaptor? If you're running just one coil, and it's not a CDI setup, then the big inductive voltage spikes tach's usually require will be present on the negative side of the coil at each ignition event. Should work with either a factory, or most aftermarket tachometers.

 

Your alternator also requires switched power to the field winding. Most cars provide this through a charge warning light, but out cars don't have a charge warning light, so you can run it from you 'on' switched circuit (not the 'acc' switched circuit). You don't need to provide a ground for your alternator, it grounds via its bracket and the main engine ground.

 

So no tach adapter needed? Okay that makes things easier

 

And for the alternator, would it be best to run switched power from another fuse?

[scott87star]

Tach adapter is there because the MSD is a CDI so the coil is running 540 volts, not compatible with the factory tach. BUT, the Zac Man has a neat tach mod that will let you use the MSD tach out to trigger the tach so you still don't need the adapter. If you already have it you can go ahead and use it instead of making the tach resistor jumper mod.

 

No tach adapter: run wire from MSD 6AL tach out to the factory tach in, bypass tach resistor per Zac's post on the aussie site.

 

Tach adapter: red wire to +12V switched, white wire to MSD 6AL tach out AND factory tach in.

[leesfer]

Tach adapter: red wire to +12V switched, white wire to MSD 6AL tach out AND factory tach in.

 

And if I am using an aftermarket tach, same thing?

[leesfer]

I've updated the wiring diagram image, thoughts on the alternator and starter wiring?

[ZacMan]

Ahhhh, yeah, I was wondering if the MSD was a CDI unit, or just a fancier ignition module, if it's a CDI, then yup yup, tach adaptor needed... Unless you wanted to configure one of the MS outputs to do the same job. What sort of coil are you using with the MSD box? Do they supply one?

 

Yes, run the switched power to the alternator field winding through a fuse. a 5A one should be enough, it doesn't have a massive current draw, and infact only draws current until the alternator starts generating current, once that happens, the alternator powers it's field winding itself.

 

Wiring diagram looks good, I like the 4 pole kill switch, will have to look into that, would make things easier in future! It looks like your running the main power wire to your starter motor through an 80A fusible link? Don't do that, as the starter will draw 200+A when cranking under load. You should have a nice big beefy power cable running directly from your kill switch to the starter motor terminal, with no breaks of deviations along it's length. You can use either the kill switch, or the starter motor terminal as a distribution point to get power to your 80A main fusible link, and then ignition switch and 'main' relay.

 

How is your MS ECU getting an RPM signal? Does this come from the Tach adaptor, or do you have it hooked directly to the reluctor inside the distributor?

 

My recommendation would be to lock up the factory distributor (remove/lock advance weights, and disable the vac advance/retard dashpot), and connect its reluctor output to the MS ECU. By doing this, you;re using it as a rudimentary type of crank angle sensor. Then use the MS ECU to trigger the MSD CDI box. That way, you're taking advantage of the mappable ignition the MS ECU can provide. Massive power and driveability gains are there for the taking by moving to a mappable ignition system, and not relying on the old distributors advance weights and advance/retard dashpot to be in perfect functioning condition to provide a semi decent timing curve.

 

My other recommendation is to listen to whatever scott87star tells you, I've read his threads and seen photos of his work, he knows his s*** :-).

[leesfer]

How is your MS ECU getting an RPM signal? Does this come from the Tach adaptor, or do you have it hooked directly to the reluctor inside the distributor?

 

 

Luckily for me, Scott made my MS for me... but I'm not sure if the RPM signal is from the dizzy (which is locked) or from the coil because the harness is connected to both the dizzy and also has a wire for the negative on the coil as well.

 

But it came from Scotts hands so it's definitely good

Edited February 17, 2014 by leesfer

[leesfer]

I've dropped the 80a fuse to be after the starter

[leesfer]

Thanks for MASSIVE help guys

[ZacMan]

Just noticed your starter picture. the connection you have labelled as ground is actually the main battery input. the starter grounds via the engine block / trans, so it has no external ground lead.

 

The stud on the starter you currently have labelled as battery shouldn't have anything connected to it, except for the small lead it comes with that goes down to the actual motor itself. The smaller cylinder ontop of the starter motor is the solenoid, it takes the main battery input, and connects it to the actual motor via this lead.

 

Your alternator wiring looks correct, is that a picture of the actual alternator you're using? If so, it's a bit later model than I was anticipating, and you'll need to run a 'Battery Sense' wire, in addition to the 'Field Energise' wire. The Battery Sense lead is a separate wire that should connect to the battery sense terminal on the alternator directly to the positive terminal of the battery. I don't know which is which, but there are usually some letters on the alternator, something like 'B' for 'Battery Sense', and 'L' for 'Light' (which is the field energise line).

 

I know it seems a bit odd, and you'd think you could just connect it to the output stud of the alternator, as that's connected to the battery anyway but...... You shouldn't.

 

WARNING - EXCESSIVELYjz DESCRIPTION BELOW, READ AT YOUR OWN RISK.

 

The 'Battery Sense' wire is the feedback signal the alternator regulator uses to determine how much current it should generate.

 

The alternator generates current, which it passes to the battery via its output stud, along a reasonably large wire. However, this can be a reasonable amount of current, and there will be a measurable voltage drop along this wire. Consider this situation:

 

You've connected the 'Battery Sense' alternator terminal directly to its main output stud. The alternator regulator is configured to generate enough current to provide a voltage at the battery of 13.4 volts. If there is a 1V drop along the main charging wire between the alternator stud and the battery, then there is only 12.4V at the battery. The alternator knows nothing of this, as it's seeing the 13.4V present at it's output stud. The alternator thinks everything is fine, and it's providing the right amount of current, when it really, really isnt.

 

As the alternator battery sense input circuit has a very high impedance, it draws very little current. This means, that you can run a relatively small diameter wire from the battery to the battery sense terminal, and as very little current flows through this wire, there is a very small voltage drop, so the alternator sees the true battery voltage.

 

It would probably be acceptable to run the battery sense wire to the kill switch, as the cable leading from here to the battery is so large that the charging current will cause next to no voltage drop, so the true battery voltage will be present at the kill switch.

 

I really hope that made some sort of sense, and I don't sound like too much of a pretentious wanker, I don't mean to be, honest! (It just comes naturally ;-) )

[leesfer]

Thanks for the info!

 

That's not the exact alternator I am using, it's just the first picture I found on Google images

 

But I do believe the alternator in my car has a similar plug, I will have to check when I get home.

 

As for the starter wiring, that is what I was most worried about... only changed a starter a few times before on a 350 but when you have to create the harness for it I was a bit over my head!