Creating a 12v signal from 0.3v input - transistor? -help!

Discussion in 'The Projects Forum' started by Dom1, Jul 1, 2010.

  1. Dom1

    Thread Starter New Member

    Jul 1, 2010
    Hello all,

    I hope you can help. Firstly I would like to state that I am no expert on electronics!

    I am hoping to be able to use a transitor circuit to be able fix a fault on the ignition circuit of my car. From the engine management computor there should be a 12v signal going to a coil at each spark plug (measured using a volt meter from the battery negative and the output from the computor. When the 12volt signal is switched off, controlled by the timing in the computor. i assume that high tension spark is created by coil as the field collapses. This is true for 3 of the 4 spark plugs but 1 of them is only showing a signal of 0.3volts and i am not getting a spark at the plug. I suspect that a transistor has broken in the computor. I am not able to get inside the computer to fix the actual transistor. Replacing the computor costs more than I am prepared to spend on the car.

    *What I would like to do, is take the signal that is coming from the wire coming out of the computor (0.3 volts) and use this to switch in a 12volt feed. So, when the 0.3volts is present i want a single wire coming out of my new circuit to have a 12 volt signal that I can connect back to the coil and when the 0.3 volt signal does not exist I would like no output from the circuit to switch off. I believe that current required on the output side it about 5 amps. I seem to recall that transitors can be used as switches in this way, but in the diagrams I have seen for NPN transistors the load sits between the 12volt supply and one of the pins of the transistor. This would be fine if I had the two ends of the load component to connect, such as the two connecters for a bulb. But I don't I only one signal cable?

    Is any one able to suggest what circuit I need to achieve my goal? I just can't get my head round it! Any assistance greatly appreciated.

    Kind regards

  2. retched

    AAC Fanatic!

    Dec 5, 2009
    You have 3 pins on a transistor. One is where your (power) is coming from(battery 12v).
    One is your control (.3v from computer) and one in where you want the power to go to. (coil)

    When the transistor receives the signal, .3v in your case, it will open the gateway between the battery and the coil.

    Spend a few minutes reading up on transistors:

    And then you will be on the lookout for a small signal transistor that can handle MORE than 5A.
  3. John P

    AAC Fanatic!

    Oct 14, 2008
    Before you rush into anything--have you tried this with no spark plug in the circuit? Maybe you've done the obvious diagnostics, but you didn't say what you've tried. Suppose the spark plug was bad--could it cause the same effects?
  4. Papabravo


    Feb 24, 2006
    Well 0.3 volts will not turn most transistors on. You need like 0.7 volts and enough current to get the transistor to supply enough current from the 12V supply.
  5. Bernard

    AAC Fanatic!

    Aug 7, 2008
    Just an outline. PNP transistor needs to be high voltage with enough snubbing to protect but low enough to not squelch spark.
  6. SgtWookie


    Jul 17, 2007
    Why can't you open the ECU?

    When you were measuring the 0.3v pulse, was that with the coil connected or disconnected?

    Are you certain that the primary of the coil is not shorted?

    Did you check to make sure that the wiring from the coil to the ECU is OK? ie: not open and not shorted to ground, and the connectors are clean and corrosion free?
    Last edited: Jul 1, 2010
  7. Potato Pudding

    Well-Known Member

    Jun 11, 2010
    Sounds like you have an older car.

    Start calling local salvage yards and checking internet boards where people buy and sell auto parts. You might find an old ECU for closer to what you would be happy to pay. Some salvage yards will test the part first and or allow you to return it for only a small charge if it doesn't fix your problem. They have to deal with people being afraid of buying used parts so the smarter ones will do what they can to keep you from feeling worried, and they will eat most of the risk.

    If you are not certain you know enough to do the repair try the local community colleges for an automotive repair technician program as they will fix vehicles for practice and this type of problem is one they would love to practice on. (Darn! They are all probably all on work terms or graduating now!)

    You will want to look into how your ECU is set up. For the ignition coils it might use separate driver transistors or an IC with multiple switching transistors. Failure prone parts like transistors that switch a coil should be allowed to be replaced without forcing the replacement of a whole computer.
  8. Potato Pudding

    Well-Known Member

    Jun 11, 2010
    Depending on the vehicle type there is a big market in aftermarket "Tuner" computers. The tuner computer market is big enough that for some of the popular boy racer model cars there is probably a low end computer that is priced at the same level as a used OEM part from a salvage yard.

    There are also some general purpose computers that work with almost "any" vehicle.

    The general purpose units typically require too much adaption to suggest them as an easy fix - with replacing connectors and other types of time consuming and skilled work involved.

    Tuner parts are meant for people that like to see how close to blowing up their engine they can get just for fun. If that sounds like your kind of fun and the car is near dead now, then if you need to buy some parts anyways the tuner computers might be an option.
  9. Externet

    AAC Fanatic!

    Nov 29, 2005
    If the on-board compfuser ECU does not get a cranking signal form a crankshaft and camshaft plus several other conditions, it will not output spark.
    You must scan the system for errors before playing with circuits. 0.3V is simply inactivity , not a bad voltage.
    Even a theft protection enabled will cut ignition, as many other conditions.

    Your intention is definitively the wrong way to get the engine to run. Troubleshoot the problem first.
  10. Dx3


    Jun 19, 2010
    The last time I bought an ECU at an Automotive Dismantling and Recycling Emporium, it cost $35 (U.S.)

    However, you have left out any information that would prove the ECU is failing instead of the coil or wiring harness. More testing, less buying, or you might spend money on parts you don't need.
  11. Dom1

    Thread Starter New Member

    Jul 1, 2010
    Hello all, thanks for your responses and help. Thanks to 'retched' for the link and the high-level summary within your post, that’s exactly what I needed!

    I should add that I have been trouble shooting this issue for the last couple of months and I have had the ECU scanned, this has led to me replacing the airflow sensor, crank angle sensor and the coil pack. I have since learnt that if the engine is miss-firing it can lead to false errors being flagged up.

    I have established that number 1 cylinder is not sparking as I noticed that when removing the coil pack and new plugs I fitted a couple of weeks ago that the number 1 plug was as new! I then checked each by earthing the plug using jump lead to the battery and turning the car over, I could then see that all but number one were sparking. I have tried one of the other plugs in number 1 coil. still not joy. I then started thinking that I could have a bad connection or broken wire, so I have replaced this part of the wiring loom. This did not work. this led me to look at the ECU where the loom connects. As I now have a connect block (intended to be temporary) at this end to connect the cables I have checked the voltage of each of the outputs. These were measured with the coil disconnected. I got 12volts (well actually 12.88, which is the same as the voltage across the battery on the working lines but 0.3v on the one that will not spark.

    ‘Papabravo’ mentioned that 0.3 volts would not be enough to enable get a transistor to switch, could I amplify the signal first? Also, will I need resistors and diodes in this kind of switching circuit or can I just connect the 3 legs of the transistor as per 'retched’s' description?

    Thanks for your help so far, more thoughts and ideas welcomed!
  12. Dom1

    Thread Starter New Member

    Jul 1, 2010

    Hi SgtWookie, just to answer your questions:

    1 - I can't open the computor as it is crimped all around and has a wax round it, I am sure I could get into it if a realy tried, but I though it best to leave alone to make sure I do not break anything else! Also, I am not sure I would know what I was looking for once inside.

    2 - The 0.3 volt was measured with the coil disconnected.

    3 - The coil pack is new and is one of the items I have replaced to try to fix the fault.

    4- I have replaced the wiring loom, this is what has led me the the ECU, basically I started at the engine end and worked my way to the ECU replacing the cables and plugs as I went!
  13. retched

    AAC Fanatic!

    Dec 5, 2009
    You may have a blown transistor in the ECU that isn't allowing it to fully switch when signaled.

    If all the others are getting 12.88 and CYL1 is only getting .3, there is a problem in the ECU.

    The only other thing I could think of is if this vehicle has a "limp home" mode that may be triggered.

    A limp-home mode is usually activated when the engine overheats. Some manufactures turn off spark and fuel injection to a cylinder or two to allow them to pump air through the block to help cool the engine enough to get to a repair shop.

    If this is the case, I do not know. But is seems more likely the problem exists in the ECU. You could use an opamp to boost the signal, but if the signal continues to degrade, or the failed part eventually shorts out, it could cause constant voltage to the coil.

    I would open this sucker up and see if you can find what is connected to cyl1's pin on the ECU out. You might just find the bad part, and a simple (yeah right) replacement is all that is needed.