Request for help with TR ignition problem (I think?)

Wondering (and hoping) if someone out there with a TR or a 412 could make some AC voltage measurement coming from their Flywheel RPM Sensor (white connector) for comparison to my results. My "symptom" is:

1. At cold start-up, I get ~2.1 VAC at idle, and things run OK (the "spec" is 2.0 VAC minimum),

2. When the coolant heats up to 150~160 deg F, the system transitions to running closed-loop with the O2 sensors, but the Flywheel RPM Sensor voltage has drooped down to 1.8~1.9 VAC at idle; however, things still run OKish,

3. After a few more minutes of warm-running, the Flywheel RPM Sensor voltage droops down further to ~1.7 VAC at idle, and it starts to get small, erratically-timed "misses" on both banks (although all plugs in both banks seem to be firing).

4. In all cases, as the RPM is increased, the Flywheel RPM Sensor output increases by something like 0.2~0.5 VAC.

I replaced the RPM Sensor, but get the same results. I also measured the spacing from the RPM Sensor to the Flywheel teeth, and it is a little large (0.8~0.9 mm on both the new and old RPM Sensor -- spec is 0.5~0.7 mm), but this is not so easy to decrease (and it hasn’t changed so it was working for 16 years at that spacing). So before trying to decrease the spacing, I wanted to see if I was even in the ballpark – i.e., if a “normal” output is something like 5 VAC or more, I would be less hopeful that a 0.1~0.2 mm change would make a real difference.

The first jpeg below shows how to make this measurement (on a TR) at the white 2-pin connector for the Flywheel RPM Sensor -- of course, you need to add some sort of “jumpers” to have a place to attach the voltmeter leads but still keep the connections working. The second jpeg shows how to make this measurement using the 25-pin factory breakout box on a TR – you would measure between pins 3 & 16 (on either a TR or 412 -- the voltmeter lead positions in the jpeg don’t necessarily show this).

Alternatively, if someone in the US has a used/working MED 120B or MED 120B/D ignition ECU, I’d be glad to “rent” it for $100 (and pay for the shipping both ways). I’m not even sure that this Flywheel RPM Sensor stuff is the real problem, but it’s the only thing that I can find wrong with the system (all other tests on Diagnosis Sheet N. 1 are OK) – and I don’t want to needlessly waste a few thousand $ on a new ignition ECU.

Please let me know if you can help.
Image Unavailable, Please Login
Image Unavailable, Please Login

 

Glad you can commisserate with me -- but I still want TR help

By "OL" above do you mean that sometimes you get an infinite resistance measurement of the new (unplugged) Flywheel RPM sensor or that you are making an operational AC voltage measurement of the Flywheel RPM Sensor and sometimes your multimeter flashes "OL"? One thing you might try (rather than swapping Digiplex units) is to just unplug each of the Digiplex units separately and see if it will run smoothly on 4 cylinders. Since they share the Flywheel RPM signal, if one of the Digiplex units is doing a bad thing on the RPM line, swapping will really have no effect -- just a thought.

(Even though I'm not sure if they use the exact same sensor) What is the AC voltage reading from that Mondial8 Flywheel RPM sensor at idle (when it's working)?

 

Thanks for the response (my first message was getting sort of lengthy, but I've got a lot more horrible details if you are interested ). The slight erratic miss is evident as "puffing" at the tailpipes (I have a Tubi silencer so the two bank's exhaust streams are completely separated) and a short sudden drop (that recovers) in the O2 sensor output voltages when at idle (super-imposed over the normal closed-loop wandering). The more serious operational problems are:

1. pulling away from a stop in a normal manner has some slight hesitation and just sounds "flat" and feels "off",

2. higher RPM feels somewhat better, but power seems off (maybe 10~20%?), and if pushed, even normally/moderately, will overheat the cats.

Your point about x-talk from the secondary is well-taken -- I even have to unclip the longer spark plug wires from the routing looms to prevent multiple triggering when using my timing light on those wires. I do have an oscilloscope -- the jpeg was more trying to illustrate how to make the connections. I didn't notice any significant noise or instabilities in the signal when using either the oscilloscope or multimeter, and they seemed to agree -- i.e., the oscilloscope showed roughly a 4~5V p-p sine wave and the multimeter ~2 VAC (and they both "drooped" together, and I think some drooping makes sense -- alternator voltage being highest after cold start-up, clutch housing expanding away from Flywheel, etc.). I also looked at the primary coil waveform and it seemed like, although cyl #1 firing was consistent wrt the flywheel, other firing events in the sequence had more jitter than was physically possible (and more jitter corresponded to the audible puffing at idle).

Your 328 uses a very similar ignition architechture to the TR/412 -- could you measure your Flywheel RPM sensor output? One thing I'm starting to wonder is if the Flywheel RPM Sensor output waveform should be more of a square wave rather than a sine wave -- TIA...

 

Yes, I've already thrown everything but the kitchen sink at the fuel side -- rebuild FDs, new injector set, all pressure/flow measurements OK -- I even measured the actual fuel volume coming from the individual injectors at several airflow plate openings. On the ignition side -- fresh plugs, new extenders, new caps/rotors, repierced the wires (which had been previously replaced and measured OK), and confirmed each coil/power module is firing the appropriate number of times (just some not at consistent times).

It's not really a "glitch" in the O2 sensor signal -- it's more of a (lean) "dip" -- like a (single) slug of the unburned, or partially burned, intake charge passing by the O2 sensor.

By "more jitter than physically possible" I mean the time jitter in some of the primary side firing events corresponds to an angular amount that the flywheel couldn't really be varying by for each subsequent revolution (based on the audible quasi-steady idle).

You said it well with "some type of reference waveform to compare it to". I'll expand my request -- can anyone with a TR/412/328/308QV/308i/Mondial please measure their Flywheel RPM sensor voltage at idle and confirm if sine wave or square wave and give p-p amplitude (or amplitude VAC reading with a multimeter)?
Image Unavailable, Please Login