direct ignition/relay board/coil wiring

[c2ron]

V3 Board, 2.889 code

1985 CRX, using direct coil control with locked dizzy. MSD blaster coil.

The answer to this may be obvious (or not), but I wanted to throw the question out there because the last time I went along on my merrily own way I fried a trace on my relay board.

Per Megamanual:

Negative coil terminal to S5: check

VR sensor positive lead to to pin 15: check

The question is, negative coil terminal to share pin 15 with the VR lead?

With just the VR lead to Pin 15, I'm not getting a tach signal. Well, duh one might say, the VR isn't an ignition signal, and the manual says "Tach/Ignition & VR sensor positive lead" but I don't understand why the positive coil terminal would provide both an input and an output for the MS.

We have had the car running (not well- weird tach signal ) using a GM 4 terminal HEI because we hadn't built the board for direct coil control, but rather than troubleshoot something we were going to move away from anyway we added the direct coil stuff to the board.

So, should I run another lead from the negative coil to pin 15? Any other words of wisdom, besides don't let the smoke out again?

[Bernard Fife]

c2ron,

Pin 15 on the relay board is connected to DB37 pin 24 (http://www.bgsoflex.com/mspower/mspower_ShemV1.2.pdf). This is the tach input. It is where you would wire a VR sensor to, or the coil negative terminal if you were triggering off the coil (in which case you couldn't use the direct coil control).

The output for the direct coil control is on DB37 pin 36 (which connects to pin 11 on the relay board) and this is where you wire a ground wire for the coil to (http://www.megamanual.com/ms2/vb921.htm). The coil is grounded by the VB921 to charge the coil, and 'ungrounded' to create a spark. If this was mine, I would run the coil wire directly to the DB37 and avoid the relay board altogether because of the noise that might be induced in other circuits (and the relay board trace might not be up to the current demand of your particular coil, either).

So the coil negative terminal might be connected to either DB37 pin 24 or 36, but only under different setups, never at the same time.

Lance.

[c2ron]

Thanks, Lance.

Wired as instructed. Still no rpm. Checked the jumpers on the bottom of the board, and all the components in step 51 of the assembly guide, and every thing is as it should be.

Any further suggestions?

[Matt Cramer]

Where is the negative VR lead grounded? It should be at one of the return terminals.

Also, is the MS able to get RPM on a Stim?

[c2ron]

Thanks for taking the time to brainstorm this with me!

RPM works fine on the Stim, and the VR is grounded at the TPS return. The TPS is functional.

The coil was slightly out of range on resistance, so I tried a different one with the same result.

I guess the next step would be to check the VR itself, plus any further suggestions someone might have.

[Matt Cramer]

Also try adjusting the VR trim pots.

[c2ron]

Spun those little pots counterclockwise until we were both dizzy (sorry, bad play on words), and still no spark. Resistance on the TDC sensor wires is within spec.

I have a theory...

The TDC sensor output goes to the stock PCM for fuel timing. Perhaps it sends just a wimpy little signal that's beneath the notice of the high current driver circuit. I could use the other VR output from the distributor, the igniter signal, and send what is a presumably nice robust signal to the MS. I'm guessing it would be out of phase with actual TDC by the amount of static advance, so I'd have to account for that in my tables.

Stupid? Brilliant? You mean you haven't done that already?

[Matt Cramer]

I've personally scoped a Honda TDC sensor. The amplitude is the same as the CKP and CYP sensor signals. Its angle before TDC usually matches the cranking timing.