Marelli Microplex Help/Parts Needed

Hi John,

How did you test for vacuum? Did you apply a vacuum with a Mity-Vac or something like that and see that the vacuum would not hold?

I think it's the perfect excuse to go to an aftermarket ignition system that eliminates the caps etc. You can also get one and incorporate a map sensor to vary timing based on the vacuum signal but people have done without these also.

Are you certain you or your mechanic installed the correct fuel filter and in the correct direction? I don't think your old filter was so clogged that someone was able to successfully compensate for it. You probably would have had fuel starvation problems at high rpm under full load.

You probably know already but the CIS is all about correct fuel pressure so you really need to start by making sure that your idle pressure is correct for a cold and a warm engine and then go from there.

 

John ---

In the DigiPlex (308) and MicroPlex (328) ECU's, there is a device known as an "aneroid box". This is essentially a pressure transducer which measures intake manifold vacuum and converts this into current fluctuations which are used by the ECU to make adjustments to the ignition timing under changing engine load conditions.

This unit has a hose nipple sticking out of the ECU casing, and a vacuum line connecting it to a tap on the intake plenum.

In the MicroPlex setup, this device does not solely control ignition advance / retard, as the ECU processor also takes cues based on engine speed sensor and O2 sensor inputs as well. However, you absolutely need the aneroid for things to work properly --- without it, the ECU will think the car is at fully open throttle all the time.

Sounds like yours has been removed for some reason --- either because it went bad, or because some 'wing-nut' thought the car would run better without it --- I assure you, it won't.

There's virtually no chance that you will be able to locate a correct replacement aneroid box for the MicroPlex as a separate component --- I'm afraid you will have to replace the entire MicroPlex unit. And yeah, they are not cheap...

 

I don't have a MicroPlex unit handy, but here's a picture of the inside of a DigiPlex unit ---- the screwdriver tip points to the aneroid box...The MicroPlex unit is similar...

 

Oops --- pic didn't upload... here you go...
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In the 328, the (Bosch) lambda computer (ECU) and (Marelli) ignition computer (ICU) are separate devices. The Marelli ignition computer uses the vacuum line and the rpm from the TDC and RPM flywheel sensors. I think there's also an input from a simple on-off temperature sensor (if memory serves), and a really strange circuit to the catalyst control unit (CCU) (yet a separate box) -- but I think the latter isn't hooked to the Marelli.

Near as I can tell from the car's wiring diagram, the CCU circuit simply enables the injection to go "closed loop" with the O2 sensor after the catalyst warms up.

The ignition map is based on RPM and vacuum level, and is printed in the 328 tech specs. (It also appears to be way more complex than can be implemented with the 2.5 degrees of resolution from the RPM sensor. I'd like to see a scope trace on a working one, someday.)

On the 308s and 328s, I suspect that many of the vacuum connections aren't working. I've heard different opinions on whether it makes enough difference to detect.

There are a number of vacuum and pressure transducers on the market, but knowing what the computer wants for inputs is a challenge. There doesn't seem to be enough information floating around to reverse engineer a box that Marelli no longer supports.

 

You are correct --- I was making a system level "black-box simplification" --- hope that didn't mislead anyone...thanks for clarifying to those who may want the additional level of detail.

You can certainly detect it, at least with a scope --- note that each of the maps in the specs is based on a unique vacuum load input. Whether or not this has a negative effect on the engine performance is debateable. However, it will adversely affect the emissions output. With the sensor (aneroid) disconnected, the engine will "over-run" when the throttle is quickly closed at high RPM (i.e., fast lift-off) causing the mixture to be over rich. This could fail an emissions test, depending on where / how the test is being run. Colorado DEQ performs a "rapid throttle down test" which would cause a problem in that case.

 

On a 328, the vacuum sensor is only on the ignition (it's built into the Marelli box), and doesn't control mixture. The lamda computer uses the throttle position switch to detect idle and WOT.

Without the vacuum sensor, your ignition advance will be a bit off, but the mixture will be unchanged.

The lamda computer only controls the pulses to the frequency valve, to tweak the control pressure in the CIS fuel distributor. WOT and idle override the closed loop for fixed open loop settings. It's only a slight variation on the open loop CIS system, and the 328 will pass emissions in some states even with the O2 sensor disconnected.

 

DGS ---

I'm grateful for your oversight --- seriously!

It reminds me that I really need to pay more attention and should be more precise in my descriptions and information... I must be getting a bit lazy.

Everything you say is 100% accurate. What I mean by saying "over rich mixture" is that when the ignition timing is far enough off ---- such as during throttle cut-off with the aneroid not functioning ---- this will result in incomplete / poor combustion efficiency, which in turn, will make the exhaust gases measure higher in levels of un-burnt fuel components.

So, "rich" in terms of more raw fuel-like compounds exiting the tail pipes (which of course is what our friends at the DEQ are sniffing for). Not "rich" in terms of the pre-combusted air / fuel ratio.

I should have said, "rich exhaust" rather than, "rich mixture".

An important distinction --- and again, thanks for pointing that out...

Cheers

 

By the way, how do you like the EVO?

I had a chance to test drive one a few years back (not sure which iteration --- might have been an EVO7???), and I was blown away by how quick that car was! Nice brakes and suspension package, too -- talk about a lot of bang-for-the-buck!!!

 

Why not open up the ignition module and see how the sensor looks. I would think there could only be two reasons for it not to hold vacuum. Either the plastic body has a crack somewhere or the diaphram or whatever is inside it has a tear or other defect. If you find a crack on the plastic body you could probably seal it with a dab of silicone or something like that. If it's internal there's probably nothing you can do other than split it open, take a photo and show the rest of us what it looks like inside

 

I had to go through this in detail, a few years back, after an indie shop in MA completely messed up my engine setup at a regular service. (Ferrari only used CIS for 10 years, compared to decades with Webers.) The mixture was so far off, it was setting off the smoke alarm in the stairway next to the open garage. (Yes, you have to calibrate exhaust gas meters from time to time.)

What floors me is that after I "ballparked" the mix using just MP and throttle response (two full turns on the mix screw!), I took it to a dealer who declared it "spot on". And it passed NoVA's rolling road test. (Better lucky than good, eh?)

Probably not a 7, unless you were lucky -- the 7 wasn't US legal, because the lightweight glass wasn't "DOT certified" and a 5 mph impact would crush the front mounted intercooler. The 8 was the first US model -- with a rounded nose to protect the FMIC ... but which gives the EVO as much front overhang as the 328. (It doesn't look it, because the Ferrari's overhang is all the way across, while the EVO's is rounded -- but at the middle, it extends as much as the 328.)

The massive Brembos are impressive, and I need to bungee cord down anything in the trunk to keep things from thumping against the walls. But I'm an old RWD guy, so I'm still more comfortable pushing the 328 in corners. I'm also not a big fan of turbo lag. You really have to get hard on the binders and then power hard through turns. More "brute force" than the Ferrari, but it gets there. Where it shines is in seven inches of snow, shod with Nokians. (But it's not really much better at that than the old Celica AllTrac -- but the EVO is still supported by the dealers. )

But I had the Ferrari in MA for five years, and never got pulled over. The first year I had the EVO, I got pulled over three times, once for just "looking fast". (No tickets from any of the stops -- it was new to the US at the time. I wouldn't have minded if they just *said* they wanted a closer look.) And all the cops were stunned to see an old guy driving it.

In the original post, he mentioned a third reason it wouldn't hold vacuum: it's not there.

From the description, I'm guessing a previous owner removed the sensor component to try to find a replacement, and never followed up.

 

Given that there are three "select" pins, grounded (or not) by the frame of the car when installed, I gather that the Marelli was built for multiple applications. I wonder what other cars used them.

 

On finding the equivalent aneroid device in another Marelli ECU... well, good luck.

Marelli is a "deeply vertical" manufacturer, just like Bosch. They make nearly all of their own components in-house. The good news then is that the identical aneroid will definitely be found in some other (non-Ferrari application) Marelli ECU ---- and, they are / were the OEM supplier for many auto makes. So, it's out there somewhere and available for cheap.

The bad news is that is nearly impossible to find out from Marelli what the p/n is and where else it was used. Sounds stupid, right?!

About 10 years ago I got the bright idea that I was going to start reconditioning DigiPlexes for people as the units were crapping out all over the place, and were hard to find and very spendy. I had a contact in the Engineering department at Marelli, so I asked him for the application info and the operating characteristics (in case I wanted to use an alternative manufacturer's part) on the aneroid in Digi's...

When I approached him as a "private citizen" he pretty much farted me off. So, I pulled out my business card and inquired as a professional Engineer... This got me all the way up to the "Manager of Products" who informed me that Marelli does not maintain design / application records for their components past 10 years of life!! Which meant that no one at Marelli even knew this information anymore!!! Now, there is where Marelli is not at all like Bosch, whom are very diligent (hey, they're German, after all) about their records going back as far as you would want to go...

Long story short ---- I've got a box full of about a dozen "fried" DigiPlexes collecting dust in my basement --- one is employed as a paperweight on my desk at work...

I wish you better luck than I had ---- and, I hope your threshold for frustration is greater than mine was!

 

I believe the different grounding pins were to select different tables depending on which market the 328 was sold in. That's not to say the Marelli could not have been used by other cars.

 

Oh, right. Now I remember: this came up when someone was parting out a euro 328 in Canada -- the Marelli was the same, but the intake manifolds weren't.

If the 806A was ever used anywhere else, Google can't seem to find it.

I got about the same results as Dave (finnerty) when I went looking for information on the Marelli -- they don't make them anymore, it's not the same divisions, they don't have those records -- this is an ex-item. (It's a stiff, bereft of ... etc.)

Somebody needs to find a working one and scope it big time.

 

Well, I was lucky enough to buy a working MED 806A today for a whopping $300 .....

For what it's worth, I unscrewed the back panel to peek inside, here's what ancient technology looks like I didn't feel like taking the printboard out to see the vacuum port, after all it's still working now

I'd go for the Jag box, or any other cheap microplex (Fiat Uno, ...) floating around on E-bay and rip it apart. Might just work!
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You might want to look over this thread http://www.ferrarichat.com/forum/showthread.php?t=36012 or do a search on COP or coil-on-plug conversions.

Those plug extenders in the 308/328 seems to beg for individual plug-mounted stick coils instead.

 

Yes I was lucky, found it locally at a classic parts dealer who has left-overs from a parted out 328.