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Thread: 7.4 454 won't start

  1. #1
    Carb and Points!
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    Exclamation 7.4 454 won't start

    I have a 94 winnebego motorhome with a chevy 454 TBI ECM 16196395 PCM. I drove it up to my house and it ran fine. Then later in the day I went to start it and it cranks but won't start. I popped the engine cover off and I can see the injectors are not firing. I sprayed carb cleaner in the throttle body and I can get it to start.

    I found the pinouts for the different ecm that says it works 16197427 on this forum. I checked for power and ground on all wires I could identify with power or ignition. I checked all fuses and pulled fuse box out and checked all connections behind with a power probe. (History of poor connection at ecm fuse).
    I cracked the fuel line loose and turned the key to on. Fuel does spray out. (updated: checked f15, e15 ignition feeds, e16 battery, a1, a2, b4, b3 grounds all good.)

    I bought a new ignition control module since it was cheap, no change.
    I still have spark. No fuel injectors. I verified power at the supply wires on the injectors. I do have constant power.

    I replaced the ECM because it was cheap and swapped the prom over. I was told by one of those online help justanswersrv the ecm was probably bad/injector driver failed.

    I'm at a loss where to test next. I don't know what order to test anything else in. I'm wondering if I should try a pinout of the 16196395 ecm since I just found it. But I believe it is the same.

    Please help! It is a 34foot motorhome blocking the driveway. lol
    Last edited by Isaabalot; 01-20-2021 at 09:54 PM.

  2. #2
    Carb and Points!
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    Still checking, stumped...

    Ok so I went through the proper pinouts for 16196395 like I had suggested. I looked at every pin that I could guess would effect starting.
    E15, F15 are ignition and they do come on.
    E16 is battery and it has power.
    A1, A2 are ground and have no resistance to ground.
    B4 map ground is good.
    B3 Ect ground is good.

    F6 says fuel relay control. I am assuming it grounds the low current side on the relay when triggered? Not sure how to test this.
    B12 fuel pump signal. Not sure how to test this.
    A16 Injector control 1. I assume it pulses ground to the injector 1.
    A9 Injector control 2. Also assume it pulses ground to injector 2. Is there a way to test this?
    F11 IC signal from ignition control module.

    So I'm still at a loss but not giving up. Would I still have spark if the pickup coil was NOT working?

  3. #3
    Fuel Injected!
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    wasn't this exact issue just posted like a week ago?

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  5. #5
    Super Moderator dave w's Avatar
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    I wonder what a NOID light test would show?

    dave w

  6. #6
    Fuel Injected! ralmo94's Avatar
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    Quote Originally Posted by dave w View Post
    I wonder what a NOID light test would show?

    dave w
    He might need more help, as to what you want him to put the NOID light on.

    If you have spark, and can start with priming the TB with fuel or starter fluid as you said, I would look somewhere besides the distributor.

    Pinout should be the same between 6395 and 7427.

    If you have the dog house in an RV that you can watch the TBI from the driver's seat, I would test to see if you get the key on prime pulse. When keyed on injectors should fire a priming pulse. You should be able to hear them if you listen, and should be able to see fuel squirt.
    If I remember correctly, the fuel pump relay is only used during cranking, and PCM takes over after that. You could jumper the relay to verify you hear the fuel pump running, might require a helper with a 30+ft RV

    Without being hands on, that's the best I'm able to offer hope it helps. I would suspect a connection somewhere.

    Sometimes trouble shooting is the hardest part.

  7. #7
    Super Moderator dave w's Avatar
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    How to test fuel injectors with a Noid Light: https://dannysengineportal.com/noid-...-a-noid-light/

    Keep in mind that when you are testing with a noid light; this test will not tell you if your injector has gone bad. But, it will tell you if your (ECU) or (PCM) is sending the signals; needed for your injector to work correctly.


    dave w
    Last edited by dave w; 01-21-2021 at 04:22 PM.

  8. #8
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    Lets get a good basis for diagnosing this. Engine requires fuel, spark, and compression to run. By starting the engine and running it briefly on carburetor cleaner there's a very high probability the issue is not spark or compression. My next step would be confirming the pump is operating. This can be as simple as having a helper turn the key on while listening at the filler neck with the cap off for a brief, telltale "whirrr" of the pump. Or it could get more involved, possibly removing the fuel pump relay and jumpering pins 30 and 87 to see if (1) a small spark occurs when the jumper is inserted and 2) the pump begins to operate. If the pump is running then diagnosis gets more involved. I do not like using the "fuel sprayed out" test as any pressure which develops in the tank from fuel expansion, from vapor buildup, or from gravity if the test point is lower than the tank can cause fuel spray. I will leave it to the person doing the testing to determine whether or not it has been conclusively proven the pump is working.

    As suggested, cranking the engine while watching for pulses is a valid diagnostic step. The ecm will pulse an injector each time it sees a reference pulse from the ignition module. TBI systems should deliver pulses of fuel even if fuel pressure is below specification. If there are no pulses the possibilities are no fuel pressure, no reference pulses, no power to the injector, ecm not providing ground to trigger the injector. Typically a technician would next attempt to determine whether the injectors are beign triggered using a noid light. A noid light is a special test light designed to plug into the injector connector to visually indicate the ecm is attempting to turn the injector on and off. The noid light can be simulated by substituting an LED. Voltmeters can also be used to indicate some activity is present at the injector control circuit during cranking. If testing indicates there is no pulsing at the injector the next steps would be to attempt to determine if power is present at the injector connector. A test light or voltmeter will be helpful for this. With power present it becomes important to look for reference pulses from the ignition module. A scantool could be used to watch rpm during cranking. An oscilloscope could be used to show waveform and voltage at the reference circuit. Early efi diagnosis manuals would use a DVOM to look for 0 volts with key on, engine off and a non-zero reading while the engine is cranking.

    I have a few thoughts and questions.

    The first is that TBI systems are fairly reliable. IME TBI no-start is often related to fuel pump failure and/or filter clogging as the pump fails. Because the engine started with fuel added, I would definitely be looking at the pump and fuel system. While the description indicates this may have been done, I'm simply pointing out that more testing may be required before concluding the fuel supply system is operating correctly.

    The fuel pump control circuit is probably one of the most highly misunderstood circuits in an EFI equipped GM vehicle. The fuel pump is actually powered by two circuits. The fist is ecm controlled through a relay. The ecm has the ability to power the pump when the key is turned on and again whenever it is receiving reference pulses from the ignition module. This circuit alone is enough to operate the fuel pump. The second circuit is through an oil pressure switch and bypasses the fuel pump relay completely. This circuit provides power to the pump when engine oil pressure is at or above about 2 psi. This circuit alone is enough to operate the pump.

    Do not ground a fuel injector manually to see if it works. You will ruin the injector. The ECM is able to monitor and limit current through the injector so it does not fail.

    Good luck!

  9. #9
    Carb and Points!
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    I appreciate the help and pointing out the other article. I will do more searching next time. I am purchasing a noid light and doing a continuity test on the wires tomorrow. As far as the prime pulse there isn't any clicking noise or pulse coming from the injectors when I turn the key to on. However I can hear the fuel running through the line and I do get a 2 second burst of fuel if i crack the line loose off the throttle body. I thought about manually activating the injectors during that prime to see if they fire but you said not to do that. So I won't.

    As far as where to test the noid light, I was going to use that on the two ground/signal wires on the injectors. I know two of them have constant power and have tested ok as such. I too suspect a poor connection but if I can't find one. My goal is to verify every connection going into and from the computer.

    Time will tell. Thanks for all the comments.

  10. #10
    Fuel Injected! sturgillbd's Avatar
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    I have seen the ignition module fail while still producing spark. It may not be sending reference pulses back to the PCM and will not fire the injectors. If you have a spare ignition module, you could try swapping it and see if it runs.

  11. #11
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    As stated previously I did swap the ignition module. Both the old and new module produce spark and have a clean crank signal. I just tested the NOID light and as I thought the injectors are not getting a pulse from the ecu. Next I plan on running a continuity test with all four wires running from the ignition module to the ecu.

  12. #12
    Super Moderator dave w's Avatar
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    I like your plan to verify the distributor wring with an OHMS meter.

    As a reference, I'm posting a picture of a GM Ignition Module. I don't know if your Ignition module is faulty or not. Maybe you have access to an oscilloscope to verify the distributor reference signal or distributor reference pulse (DRP) is good? I'm thinking garbage in = garbage out. Maybe the DRP is garbage going into the computer so the computer signal going to the injectors is garbage? Maybe the DRP is rock solid good going into the computer, so possibly the injector driver inside the computer is garbage?

    I've been know to "borrow" a known good distributor and "Simulate" the known good distributor is installed into the engine. Ground the "borrowed" known good distributor body to battery negative, plug in the wiring from distributor that's in the engine into the "borrowed" known good distributor, including the coil wire, but not the spark plug wires. With the ignition key "On" spin the "borrowed" known good distributor by hand and watch the NOID light. If the NOID light flashes the no start is a faulty distributor. If the NOID light does NOT flash, try a "borrowed" computer and repeat spinning the "borrowed" known good distributor test again.

    dave w


    GM Module.jpg

  13. #13
    Fuel Injected!
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    have you tested the pickup coil? your symptoms sound like classic intermittent pickup coil

  14. #14
    Fuel Injected!
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    Aldo what is ypur definition of spark? is it orange or blue?

  15. #15
    Carb and Points!
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    Quote Originally Posted by tayto View Post
    Aldo what is ypur definition of spark? is it orange or blue?
    Not sure I understand the question. I used a spark tester off each spark plugs wire. The light flashed. Also the engine runs with carb cleaner sprayed into intake. As far as I know that means the coil is working ok?

    Update on my latest testing. The continuity between the injectors and the four from the icm to the ecm are perfect.

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