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Will an alpine series V tach work with a mitsubishi coil pack?

J

jammont

Donation Time
Hey Guys
I would like to draw from your expertise. Would a smiths tach out of a series V work with the coil packs from a newer fuel injected engine? (mitsubishi 4G63 coil packs). I have a tach output from both my power transisitor which fires the coil packs and also a tach output from my megasquirt ecu. Is there anyone that could help me out with this?
Thanks
Jamie
 
T

Tom H

Platinum Level Sponsor
Jamie , I'm not familar with the specifics of any of the Mistsubishi. But I looked up the pack:
http://www.1aauto.com/1A/IgnitionCoil/Mitsubishi/Eclipse/1AECI00131/372703
It looks like there are two coils. So I assume this uses the wasted spark concept for a 4 cyl engine, and there is one pulse of current applied to the primary of each coil on each revolution of the crank. This is slightly different than on an Alpine where there are 2 pulses applied to a single coil on each revolution. The Alpine tach is designed to sense the current going to the coil, the actual wire delivering current to the coil loops through a "magnetic pick up" on the back of the tach. If you could route the wire carrying the current to one of the coils through that loop it would read 50 % low.(only getting half the pulses) You could probably adjust the tach to make it read correctly, or you might need to change the timing resistor inside the tach.

You say there is a tach output "from the power transistor that fires the pack ". It seems to me that there should be two such outputs, one for each coil. No? Maybe yu could route wires to BOTH coils thru the pick up on the tach. That would provide the correct number of pulses. You also mention a "Tach output" signal. Depending on what that signal is - voltage and current magnitude, you might be able to use that. But that requires and understanding of the ecu that I do not have. I do know that someone has written an article on how to convert an Alpine tach (actually Tiger tach, I think) to be triggered in a different way, not using the magnetic loop method.

Tom
 
R

RootesRacer

Donation Time
Tom and Jamie,

Waste spark still has the same number of sparks per revolution, in the case of a 4 banger, there will be 2 dwells per engine rev, yielding 4 sparks per cycle (720 degrees). The coils alternate dwell cycles. 1 and 4 get one coil, 2 and 3 get the other.

Now for the fun part...

Provided that the power feed to the coil unit is in the current transformer path,
at low RPM, the tach will in fact work to the same extent that these tachs work with any other electronic ignition.

The trouble however is how the tach responds if and when the two coil drivers are on at the same time.
On a wasted spark engine, this occurs at higher RPMs as the dwell% increases and the two dwell phases begin to overlap.
When the two phases overlap, the currect will shoot from say 4 amps to 8 amps. This may or may not trigger the tachs current sense when this occurs.
Very likely you will experience an anomaly when the RPMs rise into this regime.

In my experience, I ran a basic 4 post coil, which required a 3ms dwell time.
Each coil can get up to 10ms (100% duty at 6000rpm), The 2 coil dwells will begin to overlap at 50% duty cycle, which would have been around 10,000rpm in my case. This depends on the RPM the engine will run to and the specific dwell time that the coil needs for adequate spark.

In my case, the tach worked and did not hit the point where it was an issue, though the RPM where this happens reduces linearly with dwell and number of cylinders (IE a waste spark V8 will begin dwell overlap at 5000 rpm with a 3ms dwell time).

Hope this helps.
 
T

Tom H

Platinum Level Sponsor
Jarrid,

Of course a wasted spark engine still has the same number of "useful" sparks per rev. But with 2 coils doing the sparking each coil only get half the pulses. And if we trigger the tach with just the current to one coil we will miss half of all the pulses. I assumed that there are 2 paths- one for each coil. Or is there typically somewhere (externally accessible) where the current for BOTH coils is in the same path?

I understand your concern about dwell time and overlap. In fact, with an Alpine tach it occurs even before 50% duty cycle overlap. When I cal the tachs I do, I found I need to use less than 50% duty cycle from my signal generator for revs over about 3000 RPM. If this is dwell issue causes a problem with the spark circuit in question, it might prove useful to actually trigger the tach from just one coil, and adjusting the tach accordingly.

Tom
 
R

RootesRacer

Donation Time
Tom H said:
Jarrid,

Of course a wasted spark engine still has the same number of "useful" sparks per rev. But with 2 coils doing the sparking each coil only get half the pulses. And if we trigger the tach with just the current to one coil we will miss half of all the pulses. I assumed that there are 2 paths- one for each coil. Or is there typically somewhere (externally accessible) where the current for BOTH coils is in the same path? Tom
Click to expand...


Even if you have 2 absolutely separate coils, they would still common at vbat, so if the feed to the 2 coils is via the ignition feed that goes through the current transformer, each coil will in fact trigger the tach.

In actuality, that coil and every 4 cyl waste spark coil I am aware of has only 3 leads, CoilA-, CoilB- and VBat. This would ACT just like a single coil does as the ECU is doing the "points" multiplex.

Additionally, Coil on Plug (4 coils running sequential on a 4 cyl engine) would ALSO function in the same manner as it would have 4 spark impulses per cycle too. The same dwell overlap caveat applies though. The important point is that you power ALL coils from the tachs current loop feed.

As for triggering on one coil, using the above coil you cannot since both coils use the same feed. Even if you were able to trigger from one coil, you shouldnt as the low pass in the tach will have double the ripple and make idle twice as bouncy.
 
J

jammont

Donation Time
I appreciate the help guys..... i will try it with the tach output on the power transformer and see what happens.........Will let you know if i have any other questions.
Cheers
Jamie
 
R

RootesRacer

Donation Time
jammont said:
A megasquirt MS2 processor configured for 4G63
Click to expand...

Ah, good luck with that.

You should alter the wiring that powers the stock coil use the ignition "run" contact to power a relay, then use that relay to switch direct battery+ power to a wire that enters the tachs current loop, then makes its way to the coil.

That feed should be used by NOTHING else or the current impulses of that load can false trigger the tach.

You should connect a second relay wired as the first to power the injectors, ECU, wideband ETC (everything but the ignition).

Fuse each power feed of course.


This is a specialized recommendation due to the fact that the tach is current impulse triggered instead of ignition flyback.
 
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