GM 4 pin HEI to 8 Pin Hei conversion possible

[lani16]

i think i'm more clear on this ,i just need to swap modules wire them as per megamanual and i should be set . thanx a lot guys and beware i'm always on the look for a dumber question to ask and i'll be haunting ya'll with them

[MegaScott]

Lance,

Does the MSD dist. not have a reluctor type sensor in it? Why can't he just wire that to the V3.00 VR input, and use the VB921 to trigger the HEI coil? (of course after fixing the mechanical advance)

[Bernard Fife]

Scott,

If it has a 4 pin module, it will have a VR sensor, I believe. The user certainly could use the VR input circuit and the VB921 to drive the coil directly. But he asked about using the 7 or 8 pin HEI module, so that's what I have concentrated on.

Lance.

[lani16]

using that setupp u just mentioned will i stoll be able to control my timing ; also i built the MSII with the gm 7 pin in mind so i'm using the jumpers for a hall setupp

[MegaScott]

The method I mentioned is just another way of doing it, like all things DIY, there is usually more than one way, since you are already setup for the 7pin with the Hall effect jumper configuration you can use that, it's a tried and true working method. The other way requires you to install the VR circuit and re-jumper for that which is probably more work for you now anyway.

Better to just keep on with the 7pin setup, forget what I said, it will just be more confusing. Nevermind.

[lani16]

thanx a lot to all of you guys that's one less thing i have to worry about i'll go with the 7 pin setupp

[ronsonol]

lani16,

In addition to Scott's advice, a few notes:

- the 4 pin, 8-pin and 7-pin modules have slightly different mounting patterns, which would require some drilling and tapping in your distributor,

- the 8-pin module isn't the best choice for a 4-pin distributor, it has a completely different case design that is partially inside and partially outside the distributor cap,

- the 7-pin module is the closest fit for the 4-pin distributor. The VR sensor plug fits, but may have to be turned upside down (some 4-pin distributors use the oppposite VR sense compared to 7-pin modules),

- you can use small female spade connectors for the 7-pin module (rather than the specialized connectors in the 8-pin module).

Lance.

I am going through this change right now. I have not been able to get the car started yet properly.

Using MSII, v3.
I had a old GM HEI 4-pin in a sbc 350 (2 VR sensor wires are yellowish and green). Drilled and tapped new hole to fit in 7-pin hei module, and wired as per HEI instructions.

I noticed a problem, as you mentioned, about opposite polarity. BTW, the plug will not fit on mine, I had to use little spade conenctors after cutting the plug. So, it seems the 4-pin does yellow positive first wire, and then green wire is negative second. Also confirmed in a diagram i saw about 4-pin HEI setup, but that diagram mentioned a white wires first, then a green wire from the VR sensor to the module. (eg terminals W and G)

Do I just hook the first wire yellow to Hei 7-pin on N, and the second wire green to pin P. Will this cause problems? I mean does the actual sensor send negative and positive voltages. Or does the module simply look for a signal to N first then to P second?

I may also have troubles may also be due to incorrect base timing and trigger angle. I have locked all advances, and am trying to get the rotor phasing and the tigger angle done.

The reading I've done says;
1- move engine to 30degreens BTDC, then align rotor to center of #1 terminal, then lock it down. This sets rotor phasing.

2- move engine over again to 10 degrees BTDC, and make the teeth, line up exactly to the magentic pick-up terminals. This becomes base timing/trigger offset.

I've spent hours this past weekend with little results, and I would greatly appreciate any advice anyone may have.

Many thanks,

[Bernard Fife]

Do I just hook the first wire yellow to Hei 7-pin on N, and the second wire green to pin P. Will this cause problems? I mean does the actual sensor send negative and positive voltages.

ronsonol,

Yes, the sensor actually sends positive and negative voltages, and these are measured across both wires.

The best way to check the 'polarity' of the sensor is with an oscilloscope. You want to arrange it so that the signal across the wires goes negative first, then positive (with the scope probe on the signal wire, and the ground clip on the wire you use to ground the sensor).

If you don't have a scope, you can try the sensor one way. With the timing stable (the bypass voltage disconnected), the timing should be stable as seen on a timing light. If it varies with rpm (or doesn't work at all), then it is likely the sensor is the wrong way around.

Your set-up procedure sounds right to me. Be sure you have the dwell set to 3.5 millseconds.

Some people have apparently had trouble picking up the signal from the ECU. To check this, disconnect the bypass voltage and crank the engine. It should start and run, though perhaps not great. If it doesn't, check the wiring inside the distributor.

If it does start without the bypass votage, you might have trouble with the signal. You can add a transistor 'booster' to the signal (see this: http://www.msefi.com/viewtopic.php?p=11 ... tor#118873).

Something you could try instead is using the VR input circuit for the input. I am doing this with my 8-pin set-up, and it works great. The VR circuit is generally more sensitive to low voltage signals than the Hall circuit. You do have to adjust the R56 pot a dozen or so turns clockwise from the fully counter-clockwise position (see: http://www.megamanual.com/ms2/vradjust.htm)

Lance.

[ronsonol]

Thank you Lance,

I do not have a scope. What confuses me, if my VR sensor sends + first, then - second, so it goes (+ 0 -) 0 being the tooth contact.

If I flip/change the wires, then I am sending a + to the (N)egative pin, and a - to the (P)ositive pin. The VR sensor still goes + to -, thats how its designed. Now, if I flipped the VR sensor over, not flip the wires, I mean actually place it upside down, then it would go - to +. I was going to try that, but it wont fit upside down in the shaft. I've tried both ways on the wires, something seems very wrong, even though I get spark, as seen using a spark plug conencted outside to one of the plug wires.

I have tried starting w/o bypass pin - eg disconnect the relay entirely so 5volts does not get passed, I still have trouble getting it started and running at any kind of reasonable timing. And my dwell setting are textbook, as are all other ingition settings. But the problem at this point seems unrelated to MSII. ITs in the VR sensor, module or trigger.

The VR input option of MS sounds interesting, but I have already come so far as is, wired and ready to go, I'll continue if I can with the current setup.

[Bernard Fife]

ronsonol,

The sensor doesn't inherently go + to - or vice versa. It depends on which wire you use as a signal, and which you use as ground.

For example, with a 4-pin HEI distributor (the one I ran in my Corvette before MS-II), if the green wire is used for the signal and the white wire is grounded, the signal goes positive first, and then negative. If I ground the green wire and use the white wire as a signal, the signal is negative first, and then positive (as required for the 8-pin module).

There's no need to flip the sensor over. Another thing to note is that it doesn't make any difference which direction the distribuotr is turned, if it's + to -, then it will be in reverse as well (and vice versa).

In the 4-pin distributor, the white wire goes to the W pin, the green wire goes to the G pin. But with the 7 or 8 pin modules, I believe the white wire should go to the P pin, and the green wire to the N pin.

Lance.