Digiplex 2, how does it work?

Hi, I have an 1988 Uno 75 SX.
I'm wondering if the function of the Digiplex 2 connection (11) between the module and the negative connection of the coil.
Does this acts as the old connection between the coil and the breaker points?

I'm wondering of connecting an extra wire to this connection together with a hidden breaker to connect this wire to ground to prevent it from being started (and stolen, again!)

You could do this with older cars that had breaker points.

Can I do this without destroying the Digiplex module or the coil?

Thanks in advance for answers!

Hello - I assume this is the same Digiplex as fitted to the Uno Turbo Mk2, even though your car is older.

I personally wouldn't suggest adding the wire you describe - it might not harm the output transistors that switch the coil to earth, but it still doesn't seem a good idea to me.

Why not interrupt the fuel pump relay supply instead? I don't know where the relay is on your Uno, but without it, the engine wouldn't start.

-Alex

Alex, I'm not sure but the Uno 75SX sounds like the 1500 version of the Uno that uses a carburettor. They did indeed have a funny little ecu type thing (mine 70 SXS does too) but I've no idea what it is!

I too could do with some enlightening...

Sorry to disagree but I think you're talking about something else
The small black module that you have somewhere near the right suspension strut, with about five wires, is actually the fuel cutoff ECU. It takes a reading from the coil to determine the engine RPM, and a reading (!) of the throttle switch, i.e. a wire is grounded when the throttle is closed. The output is to the fuel-cutoff solenoid on the carburettor. Your 45 also has a cut-off solenoid, but on the 45 it is permanently-live with the ignition.

As the Haynes manual tells us, the later Weber control module cuts off the fuel in response to differentiation/integration techniques (rate of change of engine RPM) and is preferable to the earlier unit that uses a fixed cut-off point. In other words, with the later unit the fuel gets turned back on at a higher speed if the engine RPM is dropping sharply, otherwise the engine might die when you put your foot on the clutch as you brake to a stop.

Meanwhile the Digiplex ECU is likely to be a medium-sized finned aluminium box, with a vacuum pipe attached, and located (I think) somewhere near the battery. Same-aged Tipos have a similar system, and the Uno Turbo has it on the bulkhead plus, of course, cailbration for turbo boost. Documentation in the Haynes manual (for the Turbo) is probably useful for the 75SX Digiplex as well.

Also I think the Uno 75SX is a single-point injected engine, as I think the whole reason for its existence was to provide good performance in a cat-equipped car. It could even be multipoint like the Turbo. I'd be interested to see some photos of the engine bay of an Uno 75SX. (And no, I don't mean an Uno with a 1500 conversion; I mean a factory-original setup )

-Alex

Alex, you are once again right and I'm not!

http://www.fiatforum.com/uno-technic...sx-1988-a.html

I wrongly assumed that it would use the same engine and carburettor as an X1/9. Same engine yes, but with SPi rather than a carb. It seemes to have less power than an X1/9 as I'm sure those had 85bhp as standard. I wonder what a 75SX is like to drive?

Though looking at this German website it seems that the 1.5 may even have had MPi? My German is a little rusty but I can understand what it says!

"ab 1985:

50 PS mit 1108 ccm
58 PS mit 1108 ccm
65 PS mit 1280 ccm
75 PS i.e.MPI mit 1498 ccm
105 PS i.e. Turbo mit 1301 ccm"

http://www.ciao.de/Fiat_Uno_1_5__Test_8319467

I also wonder why the fuel cut off on my 70 SX is so complicated compared to my previous 45S'? The 45 only had one wire that was live with ignition on and dead with it off. Nice and simple! Still, Fiat works in somewhat strange ways...

Interesting information there, thanks.

The rest of this post is basically like a public-viewable private message No-one else will be interested other than Chas, so please feel free to skip the rest, it's long and boring

The USA X1/9 from 1981 had (initially as an option, then standard '82-'84, I'm not sure whether the X1/9 was available in the USA after 1984) fuel injection which was multi-point and appears to use the same plenum chamber (the cylinder-shape with the central throttle body) as the much later Uno Turbo. This was the L-Jetronic system and in the X1/9, with catalytic converter and electronic ignition (yes, in 1981) it gave 75bhp (compared with 85bhp for the carburettor X1/9). Pieceing together the puzzle, we can guess that the Uno 75SX did indeed use a similar engine/injection setup to the fuel-injected X1/9, perhaps because of emissions compliance (Sweden) and availability - so you were right after all.

Ah - the idea of the fuel-cutoff-on-the-overrun for the 60 and 70 models is that, when going down a hill, or otherwise decelerating with the throttle closed, there is a poor intake of air and yet quite a lot of petrol admitted through the idle jet. Therefore, for reasons of emissions and economy, it makes sense to cut the fuel off - although the fuel must of course be reinstated when the throttle reopens or when the engine reaches idle speed, whichever happens first.

There is a strange situation that arises if the engine is idling too fast - at around the 2000RPM typical cutoff point, or maybe if the throttle switch is mis-set. If the RPM is high and the throttle switch registers as 'closed', the ECU will cut the fuel. The RPM will drop sharply, so the ECU reinstates the fuel. The result is a once-a-second surging effect. The cure is to set the idle speed properly and/or adjust the throttle switch to be 'closed' only when the throttle is actually closed. I struck this on a Tipo but suspect it would apply to the Uno as well.

But again, there is no over-run cutoff the 45 since that only cuts-off when the key goes off. That provision on the 45 could be called an 'anti-dieseling cutoff', since it prevents the engine from running-on - self-igniting from advanced ignition timing. The FIRE runs advanced timing at idle (un-throttled vacuum advance), and fairly high compression, so I suspect that it might otherwise run-on in hot conditions. I guess that, on the 60/70, the over-run fuel cutoff also cuts the fuel when the key goes off...

Fuel injection systems usually include the fuel-cutoff function as part of the injector operation, though you never quite know when the cutoff takes place. I've often wondered whether it's above 2000RPM, for instance, so that coasting along in my Punto may still be using fuel due to the low revs that the CVT allows. Ditto with my 164 which is rather high-geared in the lower ratios, so I wonder whether the fuel would ever be cut-off around town.

I have the solution to these questions lying in my electronics drawer - I have an injector-duty-cycle meter kit - obviously I haven't got around to making it and anyway it would be quicker to hook up the handheld oscilloscope to the injectors and go for a drive

Just in case I haven't said it before, for the record, I think that fuel injection is always superior to a carburettor/twin carburettors for a daily-driver car. Fuel injection ends up being simpler than a carburettor+additions. It's strange that many fuel injection applications (in FIATs) - the X1/9, the Uno, the Tipo - have delivered less power than the carburettor equivalents, but I think the culprit for this lies in other changes to reduce emissions.

If we compare, say, a 1498cc California-spec carburettor 1979 X1/9 (65bhp) with its electronic counterpart (75bhp) then the difference is obvious, but what the figures don't show is the improved driveability (easier starting, smoother idle, better progression/no flat spots, etc.) because a lean-jetted carburettor is a horrid device. That's why I tend to drill the idle jet out slightly on the Uno 45, because it seems to be too small (on some particular examples) to allow for proper idling/progression - the engine just dies when you open the throttle unless the choke is on, and the choke defeats the purpose of having the tiny jet!

Chas if you ever get the chance to grab the fuel injection setup off a Turbo, be aware that you can bolt it all to the engine in your 70SX (minus the turbo/intercooler, obviously - just connect the airflow meter straight to the throttle body) and the ECU runs a non-turbo engine perfectly well. The airflow meter is the primary input (coolant temp sensor a second input) and with these, the ECU can respond correctly to your non-turbo engine. I think, if I had your 70SX, I would be very tempted to try this for economy/practicality - but I suppose you would have to watch the cost of getting the setup vs. the possible fuel savings. Since your 70SX has higher compression than an Uno Turbo, it should be a little more efficient (less fuel) than an Uno Turbo would be when driven off-boost. I guesstimate a round-town average of 37mpg would be achievable vs. 32mpg for a carburettor 70.

Also, since the Turbo's injection is MPi, it's pretty-much-certain to be more efficient than the cheaper SPi used on the Mk2 Uno i.e. models.

Of course, with the engine compression standard, you could also try a Saab-style low-pressure turbo installation with the Uno Turbo's standard small turbo and modified wastegate - I guess with the boost down to 0.3bar - people will think you are mad, but the aim is fuel efficiency and driveability - not top-end power

-Alex

Interesting information Alex, thank you

Had a look at my Uno and the mysterious black box. I also noticed that there are two wires on the carburettor as you pointed out, so one must be a throttle closed sensor switch. I've got no idea how to adjust it but will have a look soon.

I've attached some pictures below as to what's fitted to my 70SX.

As to fitting the MPI system to my 70SX, to be honest if I was going to go that far then I'd go for a 1242 FIRE 16v conversion instead. Just a little bit more work, I'd save weight, gain torque and have a more modern engine with easier to find parts. Such a project could well be in the pipeline, once I decide on the Cortina project and start/ finish the Uno 45S MPi 8v project

Also, I manage to pick up an absolute bargain recently. I won a rusty Uno bonnet and dented tailgate off Ebay and also 4 hub caps from a Fiat 127. Hum hum, they only went for £1 so I can't complain too much. However, when I went to pick them up the seller asked if I wanted some other spares that he was throwing out. One of the spares was a BRAND NEW WEBER TWIN CHOKE CARBURETTOR! I offered to pay him something for it but he refused. So suddenly my £1 Ebay win turned into bargain of the century

Sweet!

Fitting that will probably help your 70SX greatly, assuming that it's the right type, and I agree, it becomes less worthwhile to convert to FI if you have a new, unworn carburettor to work with.

In the photo it looks exactly like an Uno carb, has the sideways choke-linkage, though could be jetted for a 60 or a 70, so you'll no-doubt have some fun with the Haynes manual working out what it's for (and making adjustments).

-Alex