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GM 7-pin HEI and zxt Optical CAS Distributor


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I'm doing some research on using a GM 7-pin HEI module with MS2 and have reached a dead end on one point -- can I (or does it make sense to) use the stock ZXT Optical distributor with the 7-pin HEI? The megamanual (http://www.megamanual.com/ms2/GM_7pinHEI.htm) shows the setup with a VR distributor and explains how the HEI module conditions the VR "AC" output into a "square wave" signal that MS can use as a trigger input on pin #24. Can the ZXT Optical distributor output, which is a "square wave" be used as an "AC" input to the HEI? Seems to me that would not work properly, if at all. Or could the Optical output signal be routed directly to MS pin#24, skipping the VR conditioning of the HEI module? In that case, is there any advantage to the 7-pin HEI over a 4-pin HEI?

 

Whoops - one question turned into several. Any thoughts, conjectures, ACTUAL EXPERIENCE with this setup appreciated.

 

post-196-0-59239900-1375208456_thumb.gif

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There's no reason to use a GM HEI module (any flavor) with any MS2 build  (or even MS1 on a v3+ board). The HEI modules are a leftover from when the MS hardware didn't support as many features as it does now. Not to mention you can now get a rock solid ignitor (with thermal cutoff) in the BIP373 and I personally never had any issues with using the older VB921 ignitor.

 

All this talk about VR is mostly irrelevant though. You also are using an L28et distributor which is an optical/hall type sensor and already outputs a square wave signal that MS can use. Just wire it up as shown here

 

 

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Thanks Metro. What you said makes perfect sense. In my case, I'm trying to help a friend complete an MS install that was started by someone else and the 7-pin HEI is already in the circuit. I have no information as to "why" things are set up as they are, so I am trying to figure out if using this module with this distributor is a good idea, or even possible.

 

I think one of the biggest hurdles to a MS conversion is figuring out the "why". The "how" is pretty well documented, although spread out in multiple places with multiple MS versions, but rarely have I found good explainations of "why". When you are trying to decide which combination of bits to use for your specific build, understanding "why" you want to use a specific component (or MS version or firmware etc.) vs. another is extremely valuable.

 

So in this case, I don't think there is a good reason why we should stick with the 7-pin HEI, other than the fact that we already have it. The reasons why not would be that 1) I still don't know if it will work, even if we bypass the VR input to the module, and 2) It seems unnecessarily complicated vs. direct coil control. I will have to check and see if he has the BIP373 on his board.

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If you wired it up like in the picture, but ignoring the VR inputs and not connecting R, it probably would work as a dumb ignitor where MS controlls the dwell. I'm not sure if it's like the 4pin HEI module where it's a pulsed +5/12v signal to the ignitor. You could test it by powering the coil and ignitor, making sure everything is properly grounded, and then tapping 12v to E and see if it fires the coil.

 

Years ago I used a 4pin HEI module with mixed results. There always was a slight miss at idle (tick.. tick.. ticktick) that drove me mad. I tried multiple different modules, new cap, rotors, plugs, and wires, always with the same result. Once I finally understood how to properly wire the VB921, I switched over to it and all my problems went away.

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I'm going to try that the next time I get out there. The car is not ready to drive but at least if I can get the engine to fire it might motivate him to do all the other things that need to be done (I hope you're reading this Lee!). Then I'll look into the BIP373. We need another zt on the road in central NC :icon7:

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  • 2 weeks later...
  • 1 month later...

I opened up the megasquirt. it is a V3.0 board, and there are three pin transistors along the heat sink with mica insulators. None of them are marked as BIB373, though. 

 

The ignitor is on the heatsink located at the top right on the very end next to the DB37 connector. You might have a VB921 if it's an old kit, either one works. The BIP373 is a little nicer because of the thermal shutdown.

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Unfortunately I don't have access to either ignitors (they're hidden away on my car) but it's the transistor pictured here:

 

post-563-0-54165100-1380251423_thumb.gif

 

I don't know if it's one based off of the markings and I seem to recall neither will say exactly what they are.

 

If it's missing, you can buy a new one from DIYautotune.com http://www.diyautotune.com/catalog/bosch-bip373-coil-driver-mod-kit-p-230.html

Edited by Metro
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