Ignition Diagnosis for CB/CM 400/450 and CB450SC manual trans 1978-1986
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Thread: Ignition Diagnosis for CB/CM 400/450 and CB450SC manual trans 1978-1986

  1. #1
    Super Moderator longdistancerider's Avatar
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    Ignition Diagnosis for CB/CM 400/450 and CB450SC manual trans 1978-1986

    These tests are used to check the CDI system. There are no valid tests for the CDI unit so this is an elimination process. If all these tests are good then the CDI is at fault. Before replacing a CDI all these components have to test good.
    Use a multi-meter and set it to measure electrical resistance in Ohms.

    Coil Test:Primary Side.Measure the resistance between the Yellow and Green wires.
    The Yellow and Green wires being measured will be coming out of the spark plug coil.
    The resistance should be less than 1 Ohm. Spec is .35 to .55 ohms
    Secondary Side: Measure the resistance of the spark plug wires with the plug end caps removed, they unscrew. Insert a probe into each wire. Spec is 7.2K ohms to 8.8K ohms
    Plug End Caps: Measure the resistance of the plug end caps. Spec is 5K ohms. Replacement part number is NGK XD05F

    Stator Tests:
    The connectors used are found under the left side cover
    Measure the resistance of the Pink and Green wires.
    The Pink and Green wires to measure will be going into the bundle that goes along the frame and eventually to the stator. This is the advancer pickup sensor
    The resistance should be about 135 Ohms. New Spec 129 Ohms

    Measure the resistance between the White and Blue wires.
    These wires to measure will be going along the frame and eventually to the stator also. This is the CDI power source
    The resistance should be about 85 Ohms. New Spec 84.2 Ohms

    Measure the resistance between the Brown and Light blue wires.
    These wires to measure will be going along the frame and eventually to the stator also. This is the primary pickup coil, the aluminum piece outside the rotor
    The resistance should be about 207 ohms. New Spec 203 Ohms

    The stator ohm readings listed can exceed the old spec by no more than 1 ohm, more than that will have a negative effect on the timing advance.
    The New Spec was obtained by measuring 2 NOS stators, fresh unused old stock early model with the bullet Blue and White wire connectors.

    Run/Kill and Ignition switch Tests:
    Connect one probe to the Black w/White tracer pigtail wire coming out of the main wiring harness that connects to the CDI unit. Connect the other probe to the negative battery terminal. The results should be:
    Ignition switch on, Run/Kill switch in Run position: Infinite Ohms reading meaning an open circuit
    Ignition switch on, Run/Kill switch in Off position: Zero ohms or close to that showing the circuit is closed/completed to ground
    Ignition switch OFF, Run/Kill in any position: Zero ohms or close to that showing the circuit is closed/completed to ground

    Alternator Tests: Select/connect to one Yellow wire with a probe. Connect the other probe to one of the remaining 2 Yellow wires, resistance should be below 1 ohm. Repeat twice using a different wire each time. If the readings are good then test each of the Yellow wires with one probe connected to ground. There should be an Infinite ohms reading meaning none of the charging coils is shorted to ground.

    NOTE: ​Some VOM's are auto-ranging meaning they switch ohm scales as needed and some are range selectible. If you have to select the range be sure to do that matching the expected ohm range.
    Last edited by longdistancerider; 07-22-2017 at 10:28 PM. Reason: editting
    Jim O'Brien
    1979 CM400T "road bike" modified for travelling
    1978 CB400T1 restored
    1972 CL350 nuts and bolts restore in progress


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  2. #2
    Senior Member dtsmjr8dan's Avatar
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    This is what the group has needed for a while, Jim. It's been meted out in bits and pieces, but, having it all in one sticky is awesome.
    jbc2408 likes this.
    1977 CB750 (acquired project)
    1979 CM400T (rider),
    1979 CM400T (resting)
    1980 CM400T (just picked up in pieces)
    1981 CM400T (in surgery receiving transplant)
    1982 CM450 (donor engine for 81)
    1982 CB450 (parts bike)
    1985 VT1100C (rider)

    Past rides: Vincents, Triumph, BSA's, Suzuki's, Twingle, Corgy, others.

    Remember, today is the tomorrow we worried about yesterday.
    My chest fell, but, fortunately, my belt caught it.
    I don't know everything, nor, have I met the one who does.


  3. #3
    Super Moderator longdistancerider's Avatar
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    I've been meaning to do this for awhile, finally got around to it.
    Jim O'Brien
    1979 CM400T "road bike" modified for travelling
    1978 CB400T1 restored
    1972 CL350 nuts and bolts restore in progress


    https://www.hondatwins.net/forums/22...d-attempt.html and https://www.hondatwins.net/forums/1-...r-attempt.html
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  5. #4
    Junior Member turfnsurf4ever's Avatar
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    Hi Jim,

    Brought my 84 CB450sc out of storage a few days ago. There is a 20 seconds delay between hitting the start button and starter starts cranking on the 1st try and around 10 seconds delay on subsequent starts. Tested the battery its fully charged and stator seems charging fine. Thanks...
    If I can't take it with me, then I am not interested...

  6. #5
    Super Moderator longdistancerider's Avatar
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    This could be the starter button or the solenoid. Put a test light to the Yellow w/Red wire are the solenoid, push the starter button and it should light up immediately. If not then the button switch is the source, if it does then either the solenoid is lazy or there's a ground problem for the solenoid
    Jim O'Brien
    1979 CM400T "road bike" modified for travelling
    1978 CB400T1 restored
    1972 CL350 nuts and bolts restore in progress


    https://www.hondatwins.net/forums/22...d-attempt.html and https://www.hondatwins.net/forums/1-...r-attempt.html
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  7. #6
    Junior Member turfnsurf4ever's Avatar
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    Thanks Jim, for the clear instructions. So its the solenoid and not the switch. A follow up quick question, the positive post on the solenoid is always live regardless if the bike ignition and kill switch are off. Is that normal?
    If I can't take it with me, then I am not interested...

  8. #7
    Super Moderator J-T's Avatar
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    Yes, the positive post on the solenoid is connected directly to the battery and should always be hot. The small yellow/red wire supplies power to energize the solenoid when the start button is pressed. When the start button energizes the solenoid it should "click" and you should then have power to the opposite large post on the solenoid. The cable on this post supplies power directly to the starter.
    JT
    81 CM400T

  9. #8
    Super Moderator longdistancerider's Avatar
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    If the solenoid is getting instant power from the Yellow w/Red wire when the starter button is pushed that's good. If the solenoid/starter is still delaying then find the Lt. Green w/Red wire coming from the solenoid and connect that directly to the negative battery terminal. If the solenoid/starter works immediately then the problem is in the neutral safety wiring or switches. If it still has a delay then the solenoid is bad.
    Jim O'Brien
    1979 CM400T "road bike" modified for travelling
    1978 CB400T1 restored
    1972 CL350 nuts and bolts restore in progress


    https://www.hondatwins.net/forums/22...d-attempt.html and https://www.hondatwins.net/forums/1-...r-attempt.html
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  10. #9
    Junior Member turfnsurf4ever's Avatar
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    Thanks JT. I didn't hear any click when I press the starter so the plunger must be sticking. I will order a replacement asap. In the mean time, can I hot wire the bike in a pinch by connecting the relay's 2 posts with a screw drive or an Allen key?
    Last edited by turfnsurf4ever; 03-30-2017 at 08:28 PM.
    If I can't take it with me, then I am not interested...

  11. #10
    Super Moderator longdistancerider's Avatar
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    Yes, you can jumper the 2 battery cable terminals to jump start it. Pliers seem to work easiest
    Jim O'Brien
    1979 CM400T "road bike" modified for travelling
    1978 CB400T1 restored
    1972 CL350 nuts and bolts restore in progress


    https://www.hondatwins.net/forums/22...d-attempt.html and https://www.hondatwins.net/forums/1-...r-attempt.html
    Road Trip https://www.hondatwins.net/forums/11...here-i-go.html or "where's Jim now?"
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