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Thread: Pickup coil wires p & n

  1. #1
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    Pickup coil wires p & n

    After much searching and reading I've found a fair bit of info on this.looking at new large cap hei distributors I see which color wire goes to which pin on the 7 pin icm.my understanding of the pickup coil is that the rotation direction affects the output signal for triggering a spark event.it puts out a wave that is converted to square wave and the trigger should come from the later side just before it drops again.and having the p and n wires backwards would mean it gets the trigger signal from the beginning of the square wave rather than the end where it's desired.I hope I understand this correctly. My question is looking at a corvette large cap hei the icm pickup is wired the same as my cadillac 4.5 distributor but they rotate in opposite directions. Which has left me confused. Is it the ecm that configuration that decides which signal or at which part of the square wave is the trigger. I also found a diagram for megasquirt and it shows the pickup coil wired opposite of the factory wiring.so more confusion. So which way should my set up be wired.my distributor rotates ccw and is wired like factory.but my ecm came from a sbc that rotated cw.the symptom I've had since day one has felt like the timing just isn't right and no matter where it's adjusted or how well it runs it just feels like it has this miss to it.in checking plugs I don't have any that stand out looking different from each other.if anyone with knowledge of this can help that would be great.
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    87 sjimmy 4.5L /nv1500/ 8.8 3.55 rear/disc brakes.

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    20171109_210509.jpgew7hei.gif the pic is wired same as factory.second pic is wired opposite
    Last edited by S.O.B.B.; 06-11-2018 at 10:20 AM.
    Jeeper
    87 sjimmy 4.5L /nv1500/ 8.8 3.55 rear/disc brakes.

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    Can someone point me in the right direction.my engine call for 10 initial advance.at what degree of Crank rotation should the reluctor line up with the tabs on the pick up coil.should it be at 10btdc that they line up?or more like 30btdc?as in initial set up of a new to me distributor.any help would be greatly appreciated.thanks
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    87 sjimmy 4.5L /nv1500/ 8.8 3.55 rear/disc brakes.

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    Well today I played with my distributor. I tried setting the distributor to 0TDC and changed it in my bin aswell.felt same.I also swapped back to smaller injectors and did a few logs to get the fuel close again.I also tried a chip with all spark latency set to Zero and entire timing table to 20 of advance.I went through the rpm range in the latency table and marked what I got.it said I was getting 30and the timing light confirmed it.I was trying to figure out if the latency table I'm using is way off.the laptop agreed with the data.but it's not what was requested in my bin.I was getting lots of knock count at higher rpm and map.seems I also get a count when I shift. Anyway it still has a felling of a slight miss.like something not firing at the right time and this has bend a problem since day one of this conversion.so I looked at a few pics I took b4 of the small cap and large cap distributors.I think I'm onto the problem but just need to rest my brain and bit and figure out how to rectify this.small cap dizzy the rotor does not point at and plug terminal when the pickup coil is aligned.it's kinda in the middle of two plug terminals.on the large cap the rotor is aligned with a plug terminal when the pickup coil is aligned. So I'm figuring this is the cause of my miss feeling. Any input would be good input.
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    87 sjimmy 4.5L /nv1500/ 8.8 3.55 rear/disc brakes.

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    If you have the positive and negative backwards it will retard timing as rpm rises and will misfire and carry on no matter what you do tuning wise

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    Thanks.I neglected to mention.I did try to reverse the wires this morning but it would hardly run so i put it back the way it was.same as is wired in a pic of a new distibutor.I had the dizzy at 10advance and my bin b4 i started messing with it as thats the cadillac spec.put the crank at 0TDC on #1 compression stroke and aligned the reluctor tabs on the pickup coil.changed initial advance to zero in my bin.then started it with bypass disconnected.it was at about 2 or 3 atdc.so I advanced distributor till timing tab at zero.reconnect bypass.unplug ecm to clear codes.start it and ran Decent.I then did the testing to try and figure what spark latency should be which is when I saw that at the crank I was getting about 30 and tprt sa hack says same.but that was with a chip with entire timing table at 20.so I'm kinda stumped.went back to the garage to compare distributor to pics.still unsure if the difference I see makes much difference.I did the zero setting to see if that would change the miss feel.I did end up changing my main spark bias from 0 to 10 and that changed what tprt reports for timing.less that ten. I didn't get the chance to look again with the light to see if it was 10 less at the crank but id bet so because i could feel it and hardly any knock count.as compared to b4 the main sa bias change.
    Last edited by S.O.B.B.; 06-15-2018 at 11:03 PM.
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    87 sjimmy 4.5L /nv1500/ 8.8 3.55 rear/disc brakes.

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    From some bins I looked at from f body and y body tpi cars with large cap hei none are initial sa zero.they have 6 advance for most.one f body had 10.so I'm thinking a reason for this.I don't have 2 distributors on hand to verify if there is any phasing difference. Anyone know for sure? Or why the difference in initial advance from large and small cap?back to reading and searching. No adjustable rotor for hei that I see.only way I can think of getting phasing adjustment is to use a hei vacuum advance style housing with all the parts from cc hei and make an adjuster for the pickup where the vacuum was.I don't know if that could work.
    Jeeper
    87 sjimmy 4.5L /nv1500/ 8.8 3.55 rear/disc brakes.

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    Found some cam specs for 4.9 which I believe is identical to 4.5.Screenshot_2018-05-29-02-29-28.jpgScreenshot_2018-05-29-02-29-08.jpg can anyone tell me how different this is compared to a stock 305 tbi
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    87 sjimmy 4.5L /nv1500/ 8.8 3.55 rear/disc brakes.

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    If you swap the wires round it shifts the timing a long way will need to reset base timing for each wiring polarity

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    Delco wizard you sound familiar with this.the top of my cadillac distributor is identical to corvette large cap.same part numbers for pickup coil and icm.its wired the Same.only it spins ccw.I'm using a 7747 which the distributor spun cw from.does it sound like this could be my miss feeling problem.I don't know if it could even be set up and able to run if it wired opposite. Been driving it 8+ months.when I did reverse the wires I did it with it set on #1tdc and could not see the timing mark.hence I didn't even try to advance it to see if it would run better.I thought if the p and n is wired correctly then when u set to Zero and start it that it's supposed to be within a few degrees and if it's far off its backwards.this leaves me confused.should I have it backwards so ecm gets the correct signal for drp. Any advice is appreciated.
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    87 sjimmy 4.5L /nv1500/ 8.8 3.55 rear/disc brakes.

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    The pickup coil could be wound different which would negate having to swap wires around but if its the same part number as a coil to suit the opposite rotation I would try swap it arse about colours wise and reset base timing and see how it runs.ive had Chinese distributors with the coil wound backwards or the coloured wires soldered on wrong that run not right till you flip the wiring .same with msd dizzys they say they can be wired either way but in practise its wrong the signal shape is totally different and things like rev limit dials on some msd boxes do nothing when the pickup is backwards
    I just fixed this exact problem in a 351 Clevo wouldn't rev out sputtered up top low power on the dyno everyone the guy took it too did stuff one said lifters one said valvesprings so they just fitted new lifters nope fitted new heads with heavy springs nope tried other dizzies and mad boxes nope .I turned up reversed the wires for backwards spinning dizzy and set timing its an animal gained 2 seconds on the 1/4 and no more eratic timing above 4500rpm and low and behold his rpm limiter now works lol

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    Thank you.that helps a lot.I don't know if my timing is iratic at higher rpm as about 4300rpm is as high as I've had it.from the first time I got this to fire it's felt like I'm not getting the power it should have.I started thinking the same things.valve spring or lifter.cause this engine sat in a bagged crate for near 20 years.I thought about it but all my spark plugs look about even.the distributor is in factory form running factory direction.but the ecm I'm using gets the pickup coil signal the opposite of what its now getting.I found running the advance much higher it seemed more responsive.but to much ping. I think I'm gonna try swapping the wires and play with it to see if it resolves this.I've been playing almost daily with this tune for 8 months. I didn't realize I was having fuel issues again till I couldn't Rev it.put in new tank and new e2000 inline pump.since then I've still played daily with tuning but far less correction and more consistent data.thanks for your input.that gave me something to try.I wanted to have it running properly b4 attempting 7427 pcm swap.thanks again
    Jeeper
    87 sjimmy 4.5L /nv1500/ 8.8 3.55 rear/disc brakes.

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    I would say it is correct if you align the reluctor points at TDC and get close to 0* spark when you start it with the bypass wire disconnected.

    The ECM does not receive the signal from the pickup coil. The ignition module receives the pickup coil signal and conditions it for the ECM. So, the ECM knows nothing about the coil connection direction. The pickup coil connection will not have any effect on the electrical signals that pass between the ECM and the ignition module.

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    Thanks for more input.so reversing the distributor rotation would not change the signal from the icm to ecm. That leaves me a little confused.from what I had read about the variable reluctor and how the gm set up works I was understanding that reversed rotation is equivalent to reversing the pickup leads and does make a difference in the signal output.or is it just the time of the output signal.something along the lines of reluctor puts out a zig zag wave of current .rise/fall.icm converts that to square wave form.halfway it inverts the voltage so it continues to rise.so u get the strongest signal from the end of the square wave.if wires backwards or reverse rotation it gets the signal at the beginning of the square wave.something like that.I don't have it here to quote.I hope I said that right. I'd like to understand why it does or doesn't make a difference. Is there a lot off disagreement on this I'm unaware of.thanks for any enlightenment.
    Jeeper
    87 sjimmy 4.5L /nv1500/ 8.8 3.55 rear/disc brakes.

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