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Thread: Retrofit 24x reluctor to early V8

  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

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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.

  7. #7
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    Quote Originally Posted by kur4o View Post
    Now how the sensor aligns with the slots on the wheel is unknown and you have to verify it manually.
    I'm not sure what you are saying here but we could be in agreement for the most part.
    I think there is very little chance that chart is an actual scope trace. Have you put a scope on it and checked it against a timing light in some way?(And have you ever seen a scope trace that clean?) I'm not entirely sure how you would even do that. How were you able to check the offset?

    Go back and read my first post. It is very unlikely that the scope trace from that pickup would be anything but a series of 48 equal negative pulses unevenly spaced. Until someone actually puts a scope on it and checks it we have no basis to claim it is something else based on a published timing graph. Even if it is, that graph only tells us the pulse train beginning at the zero reference hole in the tone wheel. It does NOT tell us if that hole lines up with the pickup when the crank is at TDC. That is the information that is critical for installing an external crank trigger.

    Jim
    Last edited by Jim Blackwood; 02-23-2020 at 07:27 PM.

  8. #8
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    It is very unlikely that the scope trace from that pickup would be anything but a series of 48 equal negative pulses unevenly spaced.
    It is. 48/2=x24 full events the name says it all. I wouldn`t question official GM published data. No need to manually reinvent the wheel.

    By looking at the width of the wheel and the sensor, I can guess the switching point is at the center of the sensor. The edge of the empty slot needs to get to the center of the sensor to trigger state change of the signal to the pcm. You will have to manually verify it.

    Even if it is, that graph only tells us the pulse train beginning at the zero reference hole in the tone wheel
    It says it. It have a degrees reference. 0 degrees always refers to #1 or #6TDC. 1 full rotation of crank is 360*. The signal at the scope chart represent crank rotation of 360* starting at 0 degrees or #1 TDC.

  9. #9
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    Since it has been asked several times. Here is a real scope chart and it has the cam position also. Good luck with the project.
    Now you have to need to confirm the switching point on the sensor.
    Attached Images Attached Images

  10. #10
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    Thanks for posting that. Hard to argue against an actual scope trace. However as I said, the actual pulse train is a secondary concern. The primary and original question remains:

    How did you verify that the zero reference hole actually lines up with the pickup when the crank is at TDC?

    A reference line on a trace is just that, a reference line. Unless you have some way to generate a trigger from that line and then use it to trigger a timing light, and then use that timing light to flash on the damper or degree wheel and let you confirm that the timing mark lines up with the zero, then you haven't proven anything. Or it could be done by marking the tone wheel as I said before.

    Has anyone done either of those things? Without that it is impossible to set up the external sensor position.

    Jim

  11. #11
    Super Moderator dave w's Avatar
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    Attached is the data I have from my "Google Research" or "Personal Research".

    dave w
    Attached Images Attached Images

  12. #12
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    Forget about timing light tune up. What you need to do is mount the sensor to your preferred location. Bring the engine at #1 TDC. Rotate the wheel till the #1 tdc mark on the wheel is aligned with the sensor. Than you have your starting position for the wheel and very small adjustments will be needed from there.
    Did you thought about the cam sensor. It must be aligned with crank too.

  13. #13
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    As much as I appreciate the suggestions, you are avoiding the question.

    Until this sensor/tone ring alignment question is answered it's nothing but recycling old web info that isn't on point. Simply assuming that the sensor lines up with the zero reference hole at TDC just isn't enough. Maybe it does, but maybe it doesn't. So far I have not heard from anyone who can actually confirm that it does. Based on my own testing results I'm inclined to think that maybe it does not, but again that is not definitive because failure to achieve an expected result does not disprove the existence of a relationship.

    So, we are still at square one. Just so you know, I did align the sensor with the reference hole and there was no spark. This could be from other causes but the first step is to eliminate the obvious. Make sure the sensor is aligned properly so the ECM gets the right signal. At this point it is still not possible to do that.

    Perchance in the next few weeks I can visit my machinist and he might have a LS block and crank and be willing to let me visually confirm the relationship. A few moments with a sharpie are all that are required. However, that again delays the operation. I was hoping someone here might be able to help.

    Jim

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    Make a drawing what exact measurements you need and I will try to get them in a day or two.

  15. #15
    Super Moderator dave w's Avatar
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    Quote Originally Posted by kur4o View Post
    Forget about timing light tune up. What you need to do is mount the sensor to your preferred location. Bring the engine at #1 TDC. Rotate the wheel till the #1 tdc mark on the wheel is aligned with the sensor. Than you have your starting position for the wheel and very small adjustments will be needed from there.
    Did you thought about the cam sensor. It must be aligned with crank too.
    Good point on the Cam / Crank Alignment. My own personal research showed the 24x Cam / Crank Alignment is less sensitive than the 4x '0411 low resolution system. Pic below is from my R&D development of a low resolution dual trigger '0411 distributor (based on the L31 engine). The reason for 8 teeth on the dual trigger distributor is because 2 crank rotations = 1 cam rotation. Theoretically, a dual trigger distributor with 48x crank trigger to 1x cam trigger would provide coil on plug capability. Theoretically, a Large Cap HEI distributor diameter is needed to provide enough resolution for the 48x trigger wheel, pic below. I use CREO Elements Direct https://www.ptc.com/en/products/cad/...irect/modeling for my CAD drawings.

    I think it's possible to CAD a Coil-On-Plug dual trigger distributor for virtually any domestic V8 engine manufactured after 1955 that used a distributor. Once the CAD is complete the CNC machine will do the rest of the work. Would it be practical .... that depends on the Budget. Would it be cool .... likely the answer is Yes

    My 48x dual trigger distributor design would be based on the very successful aftermarket trigger wheel.

    dave w

    CIMG2743.jpg

    144mm_Crank_Trigger_Wheel_48X.JPG
    Last edited by dave w; 02-24-2020 at 01:39 AM.

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