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pocketscience · Posted: Thu Jun 14, 2007 8:56 pm Post subject: MSD 6A Question

I've got my MSD 6A installed and it works great - engine fired up nice first time! However when I turn OFF the ignition the engine continues to run!!

The doco talks about engine run-on and installing a pull down resistor to cure the problem - but it doesn't mention what size resistor to use... anyone know?

Cheers,
Gavin.
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RC · Joined: 25 Mar 2007 Posts: 2637 Location: Australia

Shouldn`t need any resistor if the B+ (12V in) to the unit is on the switched line (15).
Or/ and the coil is switched and this unit grounds it.
If you can post a circuit I`ll work it out for you Gav.

Roger

pocketscience · Posted: Thu Jun 14, 2007 10:37 pm Post subject:

Yup, it's sure on a switched line... and after some more reading its a fairly common problem with the MSD units (and others such as Crane). You can either install a diode, or a Chysler Dual Ballast Resistor. I've found some on Ebay locally, so I imagine they are also available from AutoOne etc...

G.
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Whoever said you can't buy happiness forgot about Porsche!
81 924 N/A, GTS lights, Saratoga, interior, headers, Integral cam, EFI (sold)
95 993
69 911

RC · Joined: 25 Mar 2007 Posts: 2637 Location: Australia

A silicon diode will give you a voltage drop of 0.6v (GP types). Any resistor will too depending on its value and the current drawn if it is constant, which it probably wont be. This can be calculated according to ohm`s law.

I`ll keep my opinion of high current systems such as msd to myself.
As msd says it draws little current from the battery and actually suggest using a diode or resistor in series there is no reason why you cant connect it direct to the switched line 15. You could if you wish, connect a relay, coil to 15 and earth and NO contacts to the msd and battery, however it is not necessary and only another potential problem.

You havn`t shown the circuit to the coil. I am assuming the + is switched and - goes to the unit. The unit grounds the coil through a transistor and the battery connection provides power for the electronics. As I`m not familiar with the operation of your unit there is a slight possibility that it may require constant unswitched 12v but this would be a design fault as this is not even needed with programable units. (unless theyre crap too)

If not, can you detail the terminals on the unit. Bet a carton msd dont supply a circuit of the internals.

Roger

RC · Joined: 25 Mar 2007 Posts: 2637 Location: Australia

So its wired like this Gav?

Whatever msd suggest I dont know, but looking at that you want to connect both red wires to #15 at a suitable point. If the unit is under the hood go to the black/blue wire on your removed unit. Just run the black to a GOOD ground,

Your next question will be why doesnt my tacho work.
If you play the pokies hook your green tacho wire to the black - coil connection.
It probably grounds - to the coil like the bosch system but if it switches + well

Suggest measuring V at coil + when running. If constant 12-14 VDC youre OK If not you will need to buy an interface/adapter or make one yourself.

Roger

pocketscience · Posted: Fri Jun 15, 2007 9:48 am Post subject:

Pretty much yes, but my wiring doesn't match the Haynes for an '81 model, so I have no green/white on the Bosch unit (no idle stabiliser?), hence why I went for black/blue that it hooked to 15.

The + and - leads go straight to the battery terminals - only the on/off goes to a switched supply.

And yeah, the tacho no go, so I'll look into that later - I think someone mentioned you can hook the MSD tach output to the green/black that runs to tacho.

G.
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Whoever said you can't buy happiness forgot about Porsche!
81 924 N/A, GTS lights, Saratoga, interior, headers, Integral cam, EFI (sold)
95 993
69 911

Min · Posted: Fri Jun 15, 2007 10:02 am Post subject:

pocketscience · Posted: Fri Jun 15, 2007 10:17 am Post subject:

Cool, thanks Min. Once it's all running then I'll start tinkering for optimal setup.

Cheers,
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Whoever said you can't buy happiness forgot about Porsche!
81 924 N/A, GTS lights, Saratoga, interior, headers, Integral cam, EFI (sold)
95 993
69 911

RC · Joined: 25 Mar 2007 Posts: 2637 Location: Australia

Pocketscience wrote:

pocketscience · Posted: Fri Jun 15, 2007 11:25 am Post subject:

Just standing on the shoulders of others Roger, and following the doco that came with it. I can see your reasoning though...

I'm using the MSD Blaster 2 coil, and Magnecore leads.

My new meter turned up today actually - sitting on my desk as I type this...! So will do some probing over the weekend.

Oh and to answer your earlier question - no they don't provide a circuit of the internals...!

G.
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Whoever said you can't buy happiness forgot about Porsche!
81 924 N/A, GTS lights, Saratoga, interior, headers, Integral cam, EFI (sold)
95 993
69 911

RC · Joined: 25 Mar 2007 Posts: 2637 Location: Australia

Well mate that coil would explain why msd reccomend direct connection to the battery. Uses 50 year old technology with a low impedence primary and high turns ratio.

Cant just use amps=volts/ohms (since it has an ac component that is affected by frequency (rpm) to calculate current.

Measure total current in series to battery. If <10A connect to #15. If it is more than that suggest using a relay to isolate unit from battery rather than rely on its internal switch(mechanical or silicon)

Measure the V on #15 in the off position too Gav, so we can get a better understanding of any leakage.

Roger
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pocketscience · Posted: Sun Jun 17, 2007 9:14 am Post subject:

924RACR · Posted: Fri Aug 31, 2007 1:48 am Post subject:

Hey all, a question on MSD install, and this seems like a good thread to do it on. We're trying to put an MSD6AL into Chris's racecar, to improve reliability as much as anything... question comes up, is the 924 "even-fire" or "odd-fire"? What's the difference, anyway?? This affects how you wire the rev limiter...

EDIT: never mind, I guess that's a distinction that only applies to 6-cyl cars...
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Min · Posted: Fri Aug 31, 2007 2:15 am Post subject:

Smoothie · Posted: Fri Aug 31, 2007 4:37 am Post subject:

Here's a long explanation of it -
http://home.off-road.com/~merls_garage/oddfire.html
The V6 odd-fire engine used for the example there fires at 90-150-90-150-90-150 degrees. An even-fire 6cyl would go at 120-120-120-120-120-120 degrees of crankshaft rotation.
(I didn't know about this before today - found it with a Yahoo search.)

Obviously our 4-cyls go evenly at 180-180-180-180.
An even-fire 8cyl would go at 90-90-90-90-90-90-90-90 degrees.
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