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Ignition Fault

Discussion in '308/328' started by Dorsetgem, Dec 24, 2020.

  1. Dorsetgem

    Dorsetgem Karting

    Nov 15, 2014
    109
    Verwood UK
    Full Name:
    Glen
    Merry Christmas everyone, let's hope next year is a better one.
    I drove to a funeral today of my 32 year old cousin and as he was a petrolhead his wife (just married) asked all of us to turn up in our classic cars.
    After the service we left the Crematorium and she wasn't idling very well and the rev counter was not working. I pulled over and switched off the engine to check leads & coils were all tight as I thought it was only running on one bank (big smell of fuel).
    Anyway I called out the AA who when trying to diagnose the fault used a length of wire from the one of the coils and shorted it out whilst I was cranking the engine (after checking for voltage both the coils).
    He thought it could be the hall sensor so ordered a truck to take us home.
    When the truck arrived I tried to drive onto the bed but it would not even start on the other bank, and hasn't started since. Before the AA guy did anything she would always start but only on one bank.
    My question is do you think he has fried the Digiplexes as I checked when we got home with a strobe light and I am not getting a spark on either bank, and I have swapped over the Digiplexes.
     
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  3. miketuason

    miketuason F1 World Champ
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    To me, it sounds like the sensor on the bell housing, but to verify if it’s the Digiplex, try switching them and see if the problem moved over to the other bank.
     
  4. johnk...

    johnk... F1 Veteran
    Owner

    Jun 11, 2004
    8,467
    New England/FL
    If you are talking about the primary leads to the coil and one being shorted while cranking, one side it 12v, unfused. Not good to short. The other is a ground controlled by the Digiplex. Grounding either side should not effect the digiplex but it would create a no start condition. However, the digiplexs get their power from the same source as the coils. So if he shorted the hot side of the coil he was also shorting the poswer supply to the digiplexs. Effect unknow.
     
  5. Dorsetgem

    Dorsetgem Karting

    Nov 15, 2014
    109
    Verwood UK
    Full Name:
    Glen
    Thanks for your replies. I will test at the Digiplexes with a scope to check hall sensors are working, and test coils.
     
  6. johnk...

    johnk... F1 Veteran
    Owner

    Jun 11, 2004
    8,467
    New England/FL
    If you check the signal from the Digi to the coil (coil -, green or yellow/black wire on a QV) it should look like this if good. It sits at 12V (left part of trace) then drops to ground for a short period, then spikes and ultimately returns to 12v (right).
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  8. Dorsetgem

    Dorsetgem Karting

    Nov 15, 2014
    109
    Verwood UK
    Full Name:
    Glen
    Thanks John. After testing the at the Digipexes yesterday I found one dead sensor (bank 1-4) but I am confused about the signal from the Bank 5-8 and the replaced 1-4 sensor. The speed sensor gave me a uniformed sine wave , but on the 5-8 bank it did not (1-4 no signal and infinite ohms with multimeter) I after tested today again the speed gave a uniformed sine wave but both 1-4 & 5-8 signal were similar to each other but not the same as the speed sensor. (I am still getting to grips with the scope as I was given a second hand one Vellman HPS 10).
    I have tested the coils and both are the same reading. Primary to Primary 0.3 Ohms. Primary to Secondary 3.7K Ohms which seem good to me.
    I put the scope on the coils earlier but did not get the same sine wave that you had on the picture, what was the time you had it set at please ?.
    So I now know why it was only firing on one bank (before the AA man used a long H.T test cable) but have a horrible feeling that he has taken both Digiplexes out when trying to fault find by grounding out the coils.
    I will carry on testing tomorrow and practise a bit more with my scope.
    Thanks again.
    Glen
     
  9. Steve Magnusson

    Steve Magnusson F1 World Champ
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    #7 Steve Magnusson, Dec 28, 2020
    Last edited: Dec 28, 2020
    This is how it should be. The RPM (speed/tachimetrico) sensor measures the teeth on the ring gear passing by = many, many pulses one after another (so it sort of looks like a sine wave). The 1-4 and 5-8 sensors measure two pins (labeled "1" in the figure) passing by each revolution so only two pulses per crankshaft revolution.
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    Here are some stylized drawings of what the waveforms should look like (but for a different model so ignore the pin callouts):

    RPM (speed/tachimetrico) sensor sensing ring gear teeth)
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    1-4 or 5-8 sensor (sensing pins)
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  10. Dorsetgem

    Dorsetgem Karting

    Nov 15, 2014
    109
    Verwood UK
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    Glen
    Thanks again Steve. Do you know how easy it is to damage these Digiplexes ?
     
  11. Steve Magnusson

    Steve Magnusson F1 World Champ
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    #9 Steve Magnusson, Dec 28, 2020
    Last edited: Dec 28, 2020
    Don't have anything too specific on the "damagability" of the Digiplex ECUs but things like using a "dirty" jump starter (giving an over-voltage spike) might be unhealthy. Can you clarify this statement:

    "Anyway I called out the AA who when trying to diagnose the fault used a length of wire from the one of the coils and shorted it out whilst I was cranking the engine (after checking for voltage both the coils)."

    Does that mean you confirmed there was +12V (meaning maybe about +10V or whatever the battery droops down to when the starter is operating) on terminal 15 of each coil during starter motor cranking, and then did something else? I don't follow the "shorted it out" thing. Would surprise me that you could blow up both Digiplex ECUs by mucking around at the primary terminals of one coil.
     
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  13. Dorsetgem

    Dorsetgem Karting

    Nov 15, 2014
    109
    Verwood UK
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    Glen
    I saw him pull a HT lead from one of the coils & connect a longer HT lead into a coil and then he asked me to crank the engine. I think he held the end of the HT cable on the engine to see a spark.
    The reason I called them out was I had lost a bank and didn't want to drive it home, I thought it would be best to have the car transported home. So before he tried this the car would fire up instantly albeit on one bank. After he tried this I did not try to start it again until the recovery truck arrived (ordered by the AA guy).It didn't start so we had to push it onto the flatbed truck. I said to the recovery guy (2nd guy) at the time I think the AA guy has damaged the ECU's, and he said that he himself wouldn't of tried anything like that, incase anything was damaged. I reckon the AA have damaged both of the Digiplexes.
     
  14. Steve Magnusson

    Steve Magnusson F1 World Champ
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    That's not a good thing to do on a Digiplex ECU, and can blow-up the built-in power transistor if the wire he used had zero impedance (This is why your model uses both coil/spark plug wires and spark plug extenders that do have some impedance). However, it couldn't blow up both Digiplex ECUs (unless he did it on both coils, nor does it prove that his actions did blow it up). If you can confirm:

    1. You do have ~10V on terminal 15 of each coil during starter motor cranking,

    2. The three flywheel sensors have the proper waveforms going to the Digiplex ECUs during starter motor cranking, and

    3. Have had a look inside the dist caps to confirm if the caps and rotors seem OK (and they do),

    then, IMO, you have amply reason to send your Digiplex ECUs to someone like https://www.sarasotaitaliangarage.com/ferrari-parts for testing/repair or getting another known-good Digiplex ECU to swap in and see if at least one bank will spark. Good Hunting!
     
  15. Dorsetgem

    Dorsetgem Karting

    Nov 15, 2014
    109
    Verwood UK
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    Glen
    Once again thank you, will have another look tomorrow & let you know how I get on. I really appreciate your time and knowledge.
    Glen
     
  16. johnk...

    johnk... F1 Veteran
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    Jun 11, 2004
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    New England/FL
    The time base was 5 ms I believe.
     
  17. MFlanagan

    MFlanagan Karting

    Dec 21, 2016
    88
    Please enlighten the uninformed: Which models of 308-328 came equipped with "Digiplex?" When did it start?
    Mike Flanagan
     
  18. Steve Magnusson

    Steve Magnusson F1 World Champ
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    308i 2-Valve and 308QV use Digiplex

    328 uses Microplex
     
  19. MFlanagan

    MFlanagan Karting

    Dec 21, 2016
    88
    So, no Digiplex with Carburetors? It started with Injection?
     
  20. Steve Magnusson

    Steve Magnusson F1 World Champ
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    Correct. Late euro carbed 308 could be equipped with an electronic ignition, but it's not Digiplex.
     
  21. Dorsetgem

    Dorsetgem Karting

    Nov 15, 2014
    109
    Verwood UK
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    Glen
    Ok I have completed my testing and here are the results

    Crank Sensor Banks 1-4 TDC 665 Ohms
    5-8 TDC 649 Ohms
    Speed. 642 Ohms
    Tested at the Digiplex
    (Replaced 5-8 sensor as it was very high impedance and tacho was not working when the bank was not firing before the AA tech shorted the HT lead)

    Both 1-4 & 5-8 giving similar signal on the scope
    Speed gives constant ripple signal.

    Voltage tests at the Digiplex during cranking
    1-4 & 5-8 TDC 0.2v AC. (Pins 1 & 5)
    Speed Sensor 2v AC (Pins 1 & 2)

    12v at Digiplexes on Pins 8 & 9

    12v at Coils between Pin 15 and Chassis
    Coil resistance Pins 1 & 15 1 Ohms
    Pin 1 or 15 to Secondary (HT output) 3600 Ohms on both coils

    Coil terminal 15 to chassis 10.05v DC bank 5-8
    10.03v DC bank 1-4
    Pin 1 to Pin 15 80mV AC whilst cranking bank 5-8
    100mV AC bank 1-4

    Here is a picture of the signal when cranking, scope connected to Pin 1 & Pin 15 on coils

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    Could the transistors have been damaged when the AA engineer shorted the HT leads down to ground as the signal looks ok to me but I think the transistor in the Digiplex has blown as the signal is only 100mV when I believe it should be much higher between terminal 1 & 15 at the coil.
    When I put a timing light on the HT leads it does not light.

    Thanks again.
    Glen
     
  22. Steve Magnusson

    Steve Magnusson F1 World Champ
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    #19 Steve Magnusson, Dec 30, 2020
    Last edited: Dec 30, 2020
    Certainly possible that the power transistors have failed -- on the next generation Microplex ECU, they moved the power transistor for each bank to be a separately-replaceable unit with the coil so must have been failing more than hardly ever. Otherwise, I'd say your measurements justify further testing/repair by a Digiplex specialist (the voltage between terminal 15 and terminal 1 at the coil should be much higher when cranking) or trying to get a loaner from someone for a quicky test (don't think that your car could "hurt" it with the stock connections).

    Since you've got a scope, can't you make the same measurement that johnk... shows in post #5 (terminal 15 to ground while cranking)?
     
  23. Dorsetgem

    Dorsetgem Karting

    Nov 15, 2014
    109
    Verwood UK
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    Glen
    Thanks again Steve
    I thought John had connected his scope from Pin 1 & Pin 15 in post #5. Pin 1 coming from the Digiplex and Pin 15 from ignition switch (12+).
    I will try again tomorrow, so you are saying put probes of scope on Pin 15 of the coil (12v) and on the chassis. And I assume selecting DC ?
    Thanks
    Glen
    p.s. the AA have agreed that if the fault was due to their engineer, they would cover the cost of repairing/replacing the Digiplexes. We shall see.
     
  24. johnk...

    johnk... F1 Veteran
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    Jun 11, 2004
    8,467
    New England/FL
    My scope picture was between pin 11 on the digiplex and ground. Pin 11 (85 US QV) is the coil trigger signal. This is the ground side of the coil and is open until the digi closes to gorund to fire the spark. What you should see is that it sits at 12V until the digiplex momentarily grounds the coil. The voltage drops to close to ground with a little ramping up, then spikes way up, then sharp down, followed by some ringing, then back to 12V.

    Vertical scale is 5V/division.

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  25. Steve Magnusson

    Steve Magnusson F1 World Champ
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    My bad -- should've said terminal 1 at the coil to ground to match johnk...'s description.
     
  26. Dorsetgem

    Dorsetgem Karting

    Nov 15, 2014
    109
    Verwood UK
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    Glen
    Thanks Gents for the explanation, I will have another go tomorrow.
     
  27. Dorsetgem

    Dorsetgem Karting

    Nov 15, 2014
    109
    Verwood UK
    Full Name:
    Glen
    Ok so I have taken more readings with both the scope and multimeter.
    I took the readings from Pin 1 of each coil and chassis ground.
    Coil bank 5-8 on multimeter showed 0.2mV AC but on scope was 1.163v AC if I used the Marker Function and set Marker 1 at top of wave and Marker 2 at lower part of wave. Hopefully I am using the scope correctly. Using this method I set the Marker 1 and 2 on the Timebase at the repeating part if the signal which gave me 0.3 seconds whilst cranking .
    Coil 1-4 on Multimeter showed 0.3mV but using scope, freezing the frame using memory again and moving the Markers top and bottom of sign wave gave a voltage of 1.105 v AC.
    So I am assuming that the scope is giving the correct voltage as apposed the the multimeter due the the frequency being too high for the multimeter to capture, and that just over 1v is not high enough to fire the coils. If this is correct then I again would assume that the ECU's power transistor was blown due to being shorted by the working coil when grounded .
     
  28. Dorsetgem

    Dorsetgem Karting

    Nov 15, 2014
    109
    Verwood UK
    Full Name:
    Glen

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