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post #1 of 30 (permalink) Old 01-23-2012, 05:04 PM Thread Starter
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Marelli Plex photo

Here is a photo of my refurbished Marelli Plex I'm about to install on my Super. Two things to note are the generic GM trigger module which can be bought at any parts house and the MSD adaptor which is plug-compatable with the GM module. This allows you to easily connect a MSD6A ignition to the Marelli Plex thereby producing a very powerful ignition system.

Overall the Marelli Plex is a very robust ignition. In contrast with other aftermarket systems, the Marelli Plex supports a high output low primary resistance non-ballasted, coil. Although it isn't mentioned in the literature that accompanied the ignition, the GM trigger module must be insulated from the finned heat sink or it will overheat and ultimately fail. A computer part thermal barrier like Artic Silver provides a good fix for this problem.

Marelli Plexes haven't been made for a number of years but they can sometimes be found used on Ebay. The Hall-effect sensor (white wire) in the distributor, the rotor, distributor cap, and heat sink are still avaliable, however.
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Jim . . . '72 Super 1300, '70, 1750GTV, 2nd series,
'62, Lancia Flaminia Zagato3c, 2nd series

Last edited by 180OUT; 01-23-2012 at 05:18 PM.
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post #2 of 30 (permalink) Old 01-23-2012, 06:57 PM
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I think that the pick-up is inductive, not Hall effect. I have owned a couple of them that were re-curved by John Shankle and someone else in California, maybe John Norman. The trigger must must have a good thermal connection to the heat sink as it generates quite a bit of heat. The special grease is for that purpose.

My Marelliplexes made good strong sparks for the reasons that you give but the weakness of them is that they are old and many of them have developed play in the pivots of the advance weights and/or the bushing on the drive shaft. This results in erratic timing and is what caused me to switch to a 123ignition distributor. I also was twice stranded when the trigger unit failed without warning.

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post #3 of 30 (permalink) Old 01-23-2012, 08:18 PM Thread Starter
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The GM based trigger is indeed inductive. However, the sensor mounted in the distributor is Hall effect and uses a magnetic field to generate timing events. The dielectric grease supplied with the new trigger is OK but, in retrospect, I think the Artic Silver paste is probably a better insulator. Distributors do wear out, of course, but good examples and even NOS examples do turn up occasionally. I have a second distributor to use for parts.

Jim . . . '72 Super 1300, '70, 1750GTV, 2nd series,
'62, Lancia Flaminia Zagato3c, 2nd series

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post #4 of 30 (permalink) Old 01-23-2012, 08:24 PM
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The GM trigger module, the thing that is fastened to the heat sink, is solid state. Based upon resistance measurements that I have made on pickups and observing the electrical pulse that they develop, I am pretty sure that they are wound coil inductive pickups. Do you have a reliable reference that says that they are hall effect?

The Magneti Marelli plex201 brochure has an electrical schematic that shows the pickup as a coil with a core. In my experience this has always been a symbol for an inductor. It also refers to the GM module as an "electronic module". So I suggest that the inductive pickup produces a current pulse that turns on a solid state switch - the GM module, that can switch the higher current demanded by the low resistance coil.

Ed Prytherch
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76 Suzuki GT500
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post #5 of 30 (permalink) Old 01-24-2012, 04:09 AM
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Quote:
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The dielectric grease supplied with the new trigger is OK but, in retrospect, I think the Artic Silver paste is probably a better insulator.
Memory tells me that the 'grease' is a conductor, not an insulator. It is supposed to draw heat away from the module and into the distributor (which acts as a heat sink to keep the module cool).

Jim

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post #6 of 30 (permalink) Old 01-24-2012, 05:54 AM
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Yes, thermally conductive. Important to provide an effective path for heat flow. Not (significantly?) electrically conductive although not specified in Arctic Silver 5 description.

'73 GTV - '78 Alfetta Sprint Veloce - '88 Milano Verde
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post #7 of 30 (permalink) Old 01-24-2012, 08:03 AM Thread Starter
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Quote:
"Arctic Silver 5 is optimized for a wide range of bond lines between modern high-power CPUs and high performance heatsinks or water-cooling solutions."
Although, some replacement GM modules come with "grease" others do not and that can be a fatal error for someone refurbing an old Marelli Plex---when the new module is installed without thermal protection and then overheats and fails.

So, is the grease dielectric or something else? Can it be purchased over the counter? And what about thermal paste like the Artic Silver mentioned above? That would seem to provide a better, longer lasting, bond than grease.

My point in starting this thread was to provide a description of the Marelli Plex ignitiion. There are more than a few of these floating around, in various stages of repair, and most likely (unless you know how to search the bb) with scant information concerning their assembly or installation.

The first thing most people will do with one of these is install a new GM module but, as mentioned, not all replacements come with protective grease. (An Echlin module I purchased from NAPA several years ago did not contain the thermal protection, for instance.)

So, to simplify things: what kind of thermal protection should be used between the GM module and the heat sink? This is a pretty important question since it could easily mean the difference between a reliable Marelli Plex and one that isn't.

Jim . . . '72 Super 1300, '70, 1750GTV, 2nd series,
'62, Lancia Flaminia Zagato3c, 2nd series

Last edited by 180OUT; 01-24-2012 at 08:06 AM.
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post #8 of 30 (permalink) Old 01-24-2012, 09:44 AM Thread Starter
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From an earlier post: Here are some relatively obscure specs on the Alfa Marelli Plex. I found them in a '80's book on electronic igns. (that I have subsequently lost . . .). Of particular importance is the air-gap which must be properly set when you install a new sensor. Also, note that the 'Plex doesn't use a ballast resistor while many other after-market systems do. If you replace the coil, be sure to purchase one without a internal ballast resistor. They're commonly avaliable but you have to ask. Most coils are ballasted.


Quote:
Coil Resistance: primary (Ohms): 0.75-0.81 secondary (Ohms): 10,000-11,000 Ballist Resistance: none. Pick-up coil resistance (Ohms) 700-800. Pick-up coil air-gap (inches): .012-.015

Jim . . . '72 Super 1300, '70, 1750GTV, 2nd series,
'62, Lancia Flaminia Zagato3c, 2nd series
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post #9 of 30 (permalink) Old 01-24-2012, 10:11 AM
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It's all about thermal conductivity and specifically, the conductivity to heat flow across the module's flat metal surface to the heat sink, be it extruded aluminum, the distributor base, etc.. Thermally conductive paste, heat sink compound (HSC), etc fills microscopic air voids between these two surfaces and effectively increases heat flow capability as the paste has a much higher thermal conductivity than air. The conductivity of HSC is much lower than that of metal though so a sufficiently thin layer to fill the voids in the metal surface, but no thicker, is best. HSC can be found many places: Radio Shack here, here, here, Home Depot (check application), Napa, etc. The higher the thermal conductivity of the paste, the better. Arctic Silver 5 thermal conductance is specified at >350,000W/m2 C (0.001 inch layer).

The term 'grease' is a bit of misnomer since it provides no lubrication. Its electrical conductivity is of no consequence either so it does not provide 'dielectric protection'. From memory, the electrical connection of the module to ground and back to the distributor is very important and should be formed with the module mounting screws and dedicated wiring, not relying on the heat sink-to-module interface.

'73 GTV - '78 Alfetta Sprint Veloce - '88 Milano Verde
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post #10 of 30 (permalink) Old 01-29-2012, 05:12 PM Thread Starter
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The best part about the Marelli Plex is its GM ign. module. These modules are from common, garden variety, HEI ignitions which were used on many GM cars in the 80's and 90's. They're avaliable at any parts store for about $15 up. Summit and the other race performance shops sell hotter versions of these modules, also. Even better, a quiet afternoon at PickNPull will get you a handfull of these for almost nothing. I found three, two Marelli Plexes and one oem GM in one of my parts boxes. A call to AutoZone told me that they'll check these modules for free. Their module tester showed that one of the three I brought in was bad.

One thing I discovered at AutoZone was that, even with brief tests, these modules definitely generate heat. Although, the replacement modules come with dielectric compound, this doesn't do much for heat conductivity. On Draglines suggestion, I decided to apply Antec Formula 5 silvered heat sink compound. If you haven't done this before apply a thin covering to the back side of the module and an even smaller amount to the surface of the aluminum heat sink. There are some excellent instructions on how to do this on YouTube and elsewhere so I won't go into more detail here.

Once you have the module in place, attach the wiring harness. The red wire attaches to the "B" side of the module and then to the + side of the coil. The black wire is not used. I taped mine off and pushed it behind the coil.
Install the heatsink onto your firewall, hook up the green wire from the ign. switch to the + side of the coil and you're ready to plug in the distrubutor.

The next photo showed the top of the 'Plex distributor with a genuine piece of Alfa archeology installed. Shown is the lightweight secondary spring from a 427 Hemi 4bbl carb. This will speed up the advcance curve, reportedly going to full advance at around 3-3500 rpm. Note that it is installed on the side with the square hole.

When installing the distributor you may find that the drive key is 180 degrees out. This is easily discovered: with the distributor all the way in you'll be able to rotate the distributor drive causing the key to jump out of the drive slot. The solution is easy. Remove the spring on the distributor drive, tap out the pin and rotate the drive key 180 degrees. Reassemble and you'll now see that the drive key is a much more solid fit.
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Jim . . . '72 Super 1300, '70, 1750GTV, 2nd series,
'62, Lancia Flaminia Zagato3c, 2nd series
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post #11 of 30 (permalink) Old 03-28-2012, 06:38 PM Thread Starter
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Marelli Plex Install

Full of confidence I installed the refurbed Marelli Plex . . . only to find that I didn't have a spark. To shorten a long story I checked everything using the published specs (they are correct, btw) and found that everything was within spec. I changed to another of the 3 good amplifier modules I had. Still no spark.

The problem turned out to be driver error. The Marelli Plex instructions rather ambiguously state that you should connect the ". . . lead wire to the +B side of the coil and . . . the electronic rev counter wire, if any, to the other terminal. If there is no rev counter, leave the terminal bare." So I did, taping off the black wire from the ign amplifier until I had everything else connected. Still no spark. Time for consultation with a higher authority. After a great Mexican food/killer Margarita lunch my friends Bob and Howard and I returned to Bob's shop. About the same time Bob found the taped off connector, Howard looked at another Marelli Plex in Bob's parts stash. Sure enough the black wire was connected to the "-" side of the coil. We connected it to the "-" side of the coil, applied some power, spun the distributor and were rewarded with a nice, fat spark acorss about a .5 inch air gap. It wasn't quite an MSD arc but it was pretty impressive for an inductive ign.

So, despite what the printed Marelli Plex instructions seem to say: hook up the black wire from the ign amplifier to the coil.

The remainder of the install is easy although there are some things that are useful to know about.

1. Air gap. The air gap between the reluctor and magnetic trigger in the distributor isn't particularly critical. If you are setting up a refurbished distributor set the air gap using a business car as a feeler guage. A loose fit is just about right. You can bench test the air gap by applying power to the coil and spinning the distributor. If you get a spark you're OK.

2. GM amplifier module. These are probably the most commonly avaliable ign. parts anywhere (at least in the States). You can buy them anywhre or simply visit a Pick N Pull and harvest a few for almost no money. When installing the amplifier make sure you use good quality heat-sink compound which you can buy at electronics or computer parts stores (Artic Silver is very good but there are lots of other choices). The grease supplied with aftermarket amplifiers isn't good enough so use the good stuff.

3. The Marelli Plex requires 2 separate grounds. The through bolts holding the amplifier heat-sink to the body is one but you should also run another ground from the heat-sink to the engine. As you can see in the photo, I ran one to a carb manifold stud.

4. Since the ign. supplies a substantially more powerful spark it's safe to open up the plug gaps a bit. I set my trusty N4C Champions to .035.

5. While I was at it, I replaced my plug wires. The Cavis wires were OK so I saved them, but I had a nice set of wires from Kingsborne Ignition so I decided to use those instead. I don't know if you know about Kingsborn but they're a small company in San Diego with excellent products. Their wires are ready-made for Alfas and have tightly fitting, numbered boots. Something I especially liked: the coil wire is a bit longer than normal allowing me to run it underneath the air cleaner.

Kingsborne Wire Werks Inc - Spark plug wires, Automotive, RV, Motorcycle, Marine, Racing, Industry

6. The 'Plex is apparently set up for 2.0 engines and so only advances to about 32-34 degrees. Mine has a lightweight advance spring so it goe to full advance at about 3k rpm and, frankly, after driving it awhile I can't tell the difference in the way the motor performs. Time will tell, however, and, besides, I can always have the distributor recurved or move on to a programmable ign.

So, how does my new/old ignition work? Just the way I wanted it to. Start ups are quicker, the idle is overall smoother, and throttle response seems better. Except for the easier starting these changes aren't all that dramatic but they are descernable. A big advantage, however, is the fact that the 'Plex is breakerless. Since there are no moving parts connecting with one another as there is with a points distributor, the 'Plex timing should remain constant until you decide to change it. The last time I used this ign. my Alfetta went something like 20k miles without a miss. However we should be mindful of more recent users who experienced failures. I intend to carry my original coil and Bosch distributor around just in case.

But perhaps the biggest attraction of the Marelli Plex is its ease of connection to an MSD multifiring, capacitive discharge ignition---one of the more powerful in the marketplace. Using a simple adaptor cable to bypass the GM amplifier, the 'Plex is plug compatable with an MSD6A ignition which is an order of magnititude more powerful than the GM amplifier driven ignition. Even better, you can also easily connect one of the programmable MSD's which allows you to design your own advance curve to fit the particular needs of your car.

Watch this space.
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Jim . . . '72 Super 1300, '70, 1750GTV, 2nd series,
'62, Lancia Flaminia Zagato3c, 2nd series
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post #12 of 30 (permalink) Old 03-29-2012, 08:34 AM
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you can also easily connect one of the programmable MSD's which allows you to design your own advance curve to fit the particular needs of your car.
Yes, though you would need to lock up the advance mechanism in the MarelliPlex distributor. Otherwise, you'd get some weird advance curve!

Does the coil that comes with the MarelliPlex work OK with the MSD?

Jay Mackro
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post #13 of 30 (permalink) Old 03-29-2012, 08:58 AM Thread Starter
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The Marelli Plex is a low resistance coil (0.6/8 ohms) and is powerful, probably as good as you can get in a standard design coil. The 'Plex hardware is robust enough that you don't need a ballast resistor. it works well with an MSD. I'll probably move up to the new MSD programmable ign. but for now I plan to install my standard MSD6A. Mine's been sitting around for awhile so I'm going to sent it back to MSD to get rebuilt (for $28.00 last time I checked).

Jim . . . '72 Super 1300, '70, 1750GTV, 2nd series,
'62, Lancia Flaminia Zagato3c, 2nd series
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post #14 of 30 (permalink) Old 03-30-2012, 06:11 PM
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The Plex is a very good system.
But I found all of the after market Plex units to have much less advance than desired. If timed at idle (as usual done) substantial power can be lost from 5000 up. So you may increase the static advance (at idle) if your engine feels happy with it and get it right at the top again.
Another, more complicated way, if too much low rpm advance pings or has other negative effects, is to "dremel off" a bit of the part that limits the rotation of the advance mechanism in the two slots under the inner rotor (The part where both springs are fixed). A millimeter or two give your engine full advance at higher rpm back.
But please! Always check the maximum advance with a timing light!
The Plex is a very powerful ignition compared to other stock units, so use good HT wires. It does not overheat if the ignition is left on without the engine running like some other systems. I have tested the Plex systems on the bench and found no rpm limit. They can deliver as many sparks per second as the coil allows. Surprising for a stock part, making some more expensive units questionable. I have found them to be very reliable. I only replaced two modules in decades, where under the chip no or very few thermal compound was found. This dries out with time, so replacing it (silicon grease or any other stuff made for) is a good idea and takes only three minutes.
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post #15 of 30 (permalink) Old 03-30-2012, 08:31 PM
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MarelliPlex

They are a versitile system and easily made into a high energy ignition.

By modifying with the correct coils, these units can trigger coils with 100% more capacity of the Oil filled HP coils.

I modified the MP heat sink to mount Crane PS91 coils without needing a "CD" unit to trigger. I changed the "Gm type module"
to a better quality Accel "performance" version which has been trouble free.
The Crane PS91 coil is different in resistance to the PS92 which is for use only with "CD" ignitions (Crane Hi6/MSD7/MSD8 type units)

Great improvement in firing plugs in 14 to 1 motor.

See pics of unit in the Duetto, and spare unit (just in case)
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Last edited by Alfar7; 09-12-2012 at 05:37 AM.
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