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Thread: Retrofit 24x reluctor to early V8

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  1. #1
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    Retrofit 24x reluctor to early V8

    Hey guys. Here's one that needs to be cleared up. Now I know lots of us have thought about adapting the '411 along with port injectors and COP to an earlier engine. Obviously the Gen 1 SBC comes to mind but although what will work for that will work for any of the GM v8's from that era and others besides, the specific aftermarket hardware available is not interchangeable. In my case a 1967 Buick 340. Same firing order as the early SBC and a few other similarities, nothing much to make it a difficult task once the injectors are in place. Before you ask, that's all done. The engine is in the car and ready to be started. If it would just start. So back to basics, starting with no spark.

    Looking at the 24T tone wheel, in this case an OEM part bought new and mounted to the rear of the crank damper. Two things are needed with the wheel itself. First it has to have the correct side facing to the rear. Based on photos from the web it appears the two chevron shaped holes are to be towards the flywheel. Can anyone dispute this?

    Next is the indexing relative to the VR sensor and to #1 TDC. This is where we run into trouble. After all these years 95% of the references simply say to use the fixture which only applies to the LS crankshaft. Very few even mention the actual alignment and among those there is no real clarity that I've seen. So I'm hoping we can answer this question in a situation where the fixture clearly cannot be used.

    These things need to be verified:

    1) The small round hole represents #1 TDC (zero reference) on the 24T tone wheel. According to the Holden document, the pulse pattern used does correspond to the teeth in the front half of the wheel going from that hole around in a counter clockwise direction. The teeth in the back half do not match the pattern at all. So far as I know, we as a collective have no idea how the logic of the two patterns is combined, other than to be combined in the sensor itself and come out on a single wire (the other two are +12v and ground according to the Holden document). We apparently do not know what that resulting pulse train looks like or how it is generated. If it matches the front plate what is the function of the rear one? The one thing I noticed is that the rear plate "closes up" the gaps in the front plate so that at each leading and trailing edge you get a short duration gap. If the VR sensor simply adds the pulses, then the rear ones would fill the gaps in the front ones and you end up with 48 individual negative pulses of perhaps a degree or so in duration irregularly spaced either about 3 or 12 degrees apart (numbers roughly based on Holden). In other words, every place the published pulse train shows an excursion going positive OR negative the actual signal would show a negative pulse. The resulting spacing looks like the following starting from but not including the zero pulse:
    L-S-S-L-S-L-S-L-S-L-S-L-L-S-S-L-S-L-S-L-L-S-L-S-S-L-S-L-L-S-L-S-L-S-S-L-L-S-S-L-L-S-L-S-L-S-L-S There is likely some digital programming reason for this pattern.

    Can anyone confirm any part of the above, particularly the location of the index hole at TDC?

    2) Is there any built-in offset in the LS between the zero reference and the location of the VR sensor mounted in the block? In other words, with #1 at TDC is the sensor aligned with the hole in the tone ring or is it offset? If so, by how much and in what direction? This is required in order to know the correct position of the externally mounted sensor. To simplify, with the crank at TDC does the sensor line up with the small round hole, or is it offset? This needs to be confirmed by someone who has actually gotten it to run and checked time with a timing light, and/or by someone who has positioned the crank in the block and marked the tone ring through the sensor hole.

    3) The VR sensor. Presumably it is installed with the connector opening facing forward, centered on the tone wheel and with minimum safe clearance. But this needs to be verified.

    So. Is there someone out there who can say yea or nay with some authority?

    Thanks,
    Jim

  2. #2
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    Look at this chart.chart this is how the signal looks like at the pcm. When you draw a line at the middle of the small hole on the reluctor wheel and you are at #1 tdc.

    Pcm gets one signal of the wheel. It has 2 parts stamped due to very small 3 degrees openings. When they overlap you got solid piece resistant to damage.
    I can only guess that at the center of the sensor the signal gets switched.

  3. #3
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    That's the same chart shown in the Holden document I referred to above, just not as sharp. If you are saying #1 TDC corresponds with "0" on the chart I would tend to agree with you but that is still only one of several secondary questions. Even if true it does not answer whether or not there is any offset, which is the primary question.

    Jim

  4. #4
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    There isn`t any offsets. When the pcm reads position at 0 degrees the engine should have just reached #1 TDC. You can trace the slots on the reluctor and linked them to the chart. That way you will know exactly where #1 tdc is. You can wire the sensor to the pcm and measure when the signal is switched.

  5. #5
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    Have you physically checked that by putting the crank to TDC and marking the tone ring through the sensor hole, then checking the mark to see if it lines up with the zero reference hole? If you have not, how have you verified that what you are saying is correct?

    Jim

  6. #6
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    You are over thinking it. The chart is not just some drawing. It is taken by an oscilloscope on a running engine and you have the absolute relation between reluctor wheel position and sensor at #1 TDC which equals to 0 degrees. There is no need to check it unless you don`t trust GM engineers and a scope chart. If there is any offsets how PCM interprets signal it is already there on the chart and there is no way you will know it. The small holes lines exactly with the slots on the wheel linked with the signal generated in the chart. That is how I know it. It is also used as a reference with the stock GM tool used to install the wheel.

    Of course you turn the wheel any position if you keep the relations between #1 TDC, wheel position and crank sensor mounting position.

    Now how the sensor aligns with the slots on the wheel is unknown and you have to verify it manually.

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