I think my version of the document does not have enough resolution to see the details on that. Is there a higher resolution version of the doc anywhere? Image Unavailable, Please Login
I've only got that same poor-quality pdf file of the 248/82 OM from the Ferrari website. Getting a real print copy of the 248/82 OM would be a good investment for you IMO that would never lose value. Hopefully, someone with a print copy of the 248/82 OM can post a better scan of that page 74. Does seem clear enough to see that the RPM sensor signals are pins 2 (black) and 3 (red) at the Digiplex ECUs.
Here are the connections to the diagnostic plug. The speed sensor is not present. Only the 2 TDC sensors. Image Unavailable, Please Login Here is a better shop of the ignition Image Unavailable, Please Login
My mistake -- you'll have to measure at the Digiplex ECU(s) or the RPM sensor connector (while still plugged in).
Just realized why I had this wrong in my mind -- the RPM sensor signal is connected to the Diagnostic Socket in the Microplex ignition system (but not in the Digiplex ignition system)!
Hey guys. I tested the RPM sensor and had 1.6VAC at the digiplex while cranking. Also, while I was cranking it a few times, I did eventually smell gas, so I think there is no fuel issue. Purely ignition I just noticed something though. The diagram that I have in my manual is identical to the one JohnK posted, except for one thing. John’s diagram has a switch/relay in the bottom left corner labeled 12. Can anyone tell me what or where that is? That would potentially be a component I have not tested yet. EDIT: I just enlarged it so I could read it and it says it is US 2 valve cars only. I have a Euro 4v so probably doesn’t apply.
That's correct -- it's a thermoswitch mounted in the coolant expansion tank used on US version Digiplex models. It retards the ignition timing by ~5 deg IIRC when the engine is cold, but, even if not functioning correctly, both banks would always still have spark. There isn't much else common to both banks -- have you measured for +12V power at the Digiplex ECUs (pins 8 & 9)? Otherwise, you'd have to have simultaneous failures of something on each bank, and that seems very improbable (but not impossible).
I checked for 12v with the ignition on…. But I did not check for 12v while cranking. I wonder if the 12v is cutting out during cranking? I will check it out.
I checked the voltage at the digiplex. It is 12.7 v with the key on. While cranking, it drops to 10.7v. Doesn’t that seem like a pretty significant drop? I think it seems like a lot.
No, that's normal -- when the starter motor is drawing 150~200A to crank the engine, the battery voltage "droops" down some due to the internal resistance of the battery. Are you measuring from pin 8 to pin 9 (this is more correct) at the Digiplex ECU, or from pin 8 to another ground (this won't detect a problem in the ground connection to pin 9).
Well, unfortunately, that all seems good, and can't think of anything else that would take out both banks of ignition -- unless something went wrong with your alternator (overvoltage), and it blew up both of your Digiplex ECUs . This is where a dedicated F shop has an advantage over a DIYer where they could borrow another Digiplex ECU for a quicky test. About the only thing that you haven't done is measure the signals from the TDC sensors during starter cranking, but don't see how both would've simultaneously failed (or if both metal pins fell out of the flywheel).
Thanks Steve. The next things I am going to try: 1. Test the crank position sensors at the digiplex. Just do I know what is going on. 2. Test the coil trigger wires at the digiplex and at the coils to see if the trigger is leaving the digiplex and if it is getting to the coil. 3. I think there is a big plug next to the coils. I am going to unplug that, clean the connections and reconnect I have also thought about digiplex failure. I think these tests will show if that is possibly the case. There has to be a fault somewhere. There is no way things just stop working with proper signals everywhere.
Does anyone know how to test the coil trigger out of a digiplex? I think it’s a pulsed ground on pin 11. When I put my meter on it while cranking I get a very slight voltage, like .02v, but I am wondering if that is because the pulsing is throwing off my meter. I am not sure how to definitively say it is putting out a good signal. Also…. What is “normal behavior “ for the crank position sensors. Are they AC voltage as well, or pulsed DC?
I tested continuity from both digiplexes pin 11 to the coil, both tested good. Both crankshaft position sensors tested at about 720 ohms across pins 1 and 5 at the digiplex. … so that seemed ok as well. The only thing I can think of is that both digiplex units failed, or both coils failed. Or…. For some reason the timing is so far off they are firing at the wrong time and not connecting in the distributor cap…. Although I tried to get spark directly from the coil wire and could not do so. Is there a relay the digiplex units and coils go through? I want to replace it just for kicks in case its an amperage problem.
If you want to do a functional measurement, they should be something like 0.1~0.2V AC during starter motor cranking IIRC (they are similar to the RPM sensor, but just have many fewer pulse per revolution). No, but you do want to make all these voltage measurements under load (i.e., with everything connected and trying to work). Measuring a voltage at an unplugged connector can detect an open bad connection (i.e., infinite Ohms), but it can't detect a bad connection that has a few tens-hundreds-thousands Ohms of unwanted resistance.
I may have just found a clue. I was testing for spark, and had no spark when cranking…. Then, as I released the key and it went from crank to “key on” there was one single spark. I checked both the voltage at the digiplex and at the coil, and neither of them seem to lose voltage during cranking other than the standard voltage drop while the starter motor cranks. Does this behavior help identify the issue at all? Video below. I have heard people say that you need to run a better ground for the digiplex. I think I am going to try that next. Or at least see if I can clean up the existing ground. https://drive.google.com/file/d/1ip0t7EGWJQIJg_CHqkVqot8KpHtnNYxD/view?usp=drivesdk
That is a huge clue; in lieu of other information it would make me want to test the key switch. You should be able to jumper across to the ignition hot wire (ie, switch output) but still be able to use the key for starting. As I recall, they are flat spade connectors there so you can probably fabricate a pretty simple testing harness to make it all work.
coil triggers at the diagnostic port, you need a scope for that. You should see 12V, then a momentary drop to ground. Then if there is spark a big, sharp spike. Image Unavailable, Please Login If no spark you still see 12 then drop to ground.
I would think if that was the problem that voltage would go away somewhere while cranking, and I don’t think that is the case. I will try to see how hard it is to pull the switch.
One thing I had been wondering earlier and removed it from my post because I thought it had been checked... I do not know the physical configuration of the contacts in the ignition switch but it MIGHT be possible for the power from the switch to the digiplexes to be interrupted between ignition "on'/'start" if an internal wire/connection/trace is corroded/burnt out, etc.
Switches (on many makes/models of cars) fail in all sorts of ways like this. And to see the correlation with video here is good data. Not conclusive but still very good correlation. On other cars I have worked on, the switch could be controlling 3 or more separate +12V feeds in the crank position. Usually house/acc power, ignition hot, and starter at a minimum. I don't know enough about these cars but I imagine the wiring schematics would tell the story.
