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Thread: Car won't run without jumping coil

  1. #1
    Rhymes with tortoise. Turbo Mopar Staff cordes's Avatar
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    Car won't run without jumping coil

    The title covers the issue pretty well. I've tried a different PM, but I'm down to one LM. I swapped a known good HEP into the car and it didn't make a difference. If I jump the coil for a bit to bring the pressure up the car will run fairly well until it runs out of fuel. Could this be another bad LM? The car started briefly the other morning when it was 17* out, but then died and hasn't run since without jumping the coil.

  2. #2
    Supporting Member II Turbo Mopar Contributor
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    Re: Car won't run without jumping coil

    Will it run if you place continuous power to the fuel pump??? Si it sounds like you think it is the ASD functionality
    Regards,
    Miles

    DD '87 Sundance T1, 12PSI, SLH with rear disks
    '87 CSX #432

  3. #3
    Rhymes with tortoise. Turbo Mopar Staff cordes's Avatar
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    Re: Car won't run without jumping coil

    I haven't jumped the coil other than to get the fuel pressure up, but it will keep running if I keep power to it. When the car runs, it runs just fine until the pressure drops. What I find interesting is that two different PMs and HEPs cause the same problem. To me, that indicates that there is somehow a problem with the LM's control of the PM?

  4. #4
    Supporting Member II Turbo Mopar Contributor
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    Re: Car won't run without jumping coil

    I agree with your assessment but fortunately I have zero real world experience on this issue. The two PMs and HEP are known and tested good previously, correct? Have you checked the LM is sending the correct signal to the PM? Red 17?
    The more I hear this stuff the more I want to change to SBECII.

    Oh, have you checked for corrosion/connection quality on the PM module? I am sure you did but
    Regards,
    Miles

    DD '87 Sundance T1, 12PSI, SLH with rear disks
    '87 CSX #432

  5. #5
    Supporting Member II Turbo Mopar Contributor rgl10's Avatar
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    Re: Car won't run without jumping coil

    Im not sure what car you are working on but if its a logic/power module car then it probably has only one wire that powers the injectors, coil and fuel pump or pumps. If it was loosing power on that Z1 circuit it would quit instantly as the coil would quit firing. Jumping power straight to the coil is probably just adding voltage to the circuit. Once you get the car started unhook the jumper and take a voltmeter and read the power at the positive side of the coil, then at fuse 10 in the fuse box and finally at the fuel pump. The fuel pump is farthest from the power module so voltage normally drops some to it. If its more than a volt or two lower than at the coil then you have a bad wire or a fuel pump that is drawing too much power.

  6. #6
    turbo addict
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    Re: Car won't run without jumping coil

    I'm going to bet there's corrosion on at least one major junction inside the harness causing a major voltage drop.

  7. #7
    Rhymes with tortoise. Turbo Mopar Staff cordes's Avatar
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    Re: Car won't run without jumping coil

    Well, I'm back in side and warming my hands up for a bit. I cleaned the connections at the coil really well, and verified that the LM seems to be functioning properly. I had the same symptoms as before. The pump did prime when flipping the key forward, and it did run for a brief time before dying. I am getting voltage out of pin 17 on the red connector at the LM, which I checked by back probing the cavity while turning the key forward. I think the next step is to do what rgl10 suggested and jump the coil, let the car run, and then remove the jumper before taking a reading.

    The car has had the battery relocated by the previous owner and I've been taking power off of the alternator, so it is entirely conceivable that the split for the alt/everything else could be in bad shape.

  8. #8
    Boost, it's what's for dinner... Turbo Mopar Staff Aries_Turbo's Avatar
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    Re: Car won't run without jumping coil

    you could also find the wires for the fuel pump in the back inside the car and check their voltage by piercing the jacket inside the car while you test and see what the fuel pump is getting while its running. then reseal your poke holes with some glue so you dont get green wire syndrome.

    Brian

    - - - Updated - - -

    fuel pump connector could be corroded too.

    Quote Originally Posted by turbovanman
    This one is easy, I have myself to blame, I rush things, don't pay attention to gauges when I should, change to much stuff at once then expect miracles, the list is endless.

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    Rhymes with tortoise. Turbo Mopar Staff cordes's Avatar
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    Re: Car won't run without jumping coil

    Alright. The car did start and run with the coil jumped to energize the pump. I measured the voltage from the coil + wire to the head and it was 14.6 with the car running/coil jumped, and about 14.4 with the car running and no jumper wire from the alt to the coil. IMO .2v through 30+ year old wiring that runs all the way to the driver's side of the car, and back across the firewall isn't bad at all.

    I have the gear to jump the pump in the car in case there is an issue. I'll try driving it to work this week if possible, and report back.

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