Used to work, now dies in a few seconds

[dtl1982]

Hi,

I'm running MSII V3.0 with B&G code 2.89, controlling fuel and spark.

I used to run the ignition with a HEI 7-pin module for both rpm input and drive the coil. Worked for many years in this configuration as long as I replaced the module once a year

I attempted to use a MSD 6A digital to drive the coil, but that never worked out (long painful story of working in the garage during 105 deg days)

I've now reconfigured the setup to take the VR signal straight off the sensor and directly to the MS VR circuit. I am now also using the VB921 to drive the coil. I have gotten it to fire successfully, but after a few seconds it mysteriously dies off. The setup is nearly identical to the working setup, just the addition of the VR adjustment pots in the mix. Otherwise, I imagined it should be just plug-n-play...but apparently not.

Does anybody have any suggestions? The datalog looks pretty clean to me, and similar to when it was working, aside from dying for no reason.

Thanks!

[Matt Cramer]

Is anything in the ignition setup getting hot?

[dtl1982]

I felt the VB921 and the coil after a couple, few second long spins and neither felt any warmer than anything else.

[Matt Cramer]

Any chance the VB921 is damaged? They are easily damaged from too much dwell or looking at them cross-eyed.

[dtl1982]

Anything is possible. Can't think of what would cause it, but if they are as sensitive as you suggest...I am a little rough on my toys.

Quick google search didn't turn up an easy way to test it with a multi-meter. My stim got...um...cannibalized a few years ago for another project. If it's possible it would show up on the stim then I guess I could get another one. Without running the actual current through it, though, I'm concerned the chip may not show the symptom under that test.

I'll hold a timing light on it later tonight and see if I can see anything weird.

[dtl1982]

Timing light test came back clean. It started suffering while the light was still strong, and the light didn't die until maybe half second before engine was done. I'll poke fuel tomorrow, even though I didn't even touch that system.

If anybody has any other ideas, keep them coming please. Ignition isn't ruled out, just less likely than others at this point.

Thanks!

[Matt Cramer]

Do you still have spark when it dies?

[dtl1982]

Update: (Sorry for the delay, had to buy a new car)

I have used a timing light to verify that the spark is strong right up to when it drops off. It's not definitive that the lack of spark is causing the stall, but I can't think what else it might be.

I checked to ensure the fuel pump wasn't cutting off for some reason, and it is on the whole time.

Not sure if it makes a difference, but I've installed a '55,000 volt performance' coil (it was in when the car worked for years). The ignition module used to drive it okay. Is the VB921 capable of driving such a coil? Everything else is essentially stock-equivalent (magnetic pickup, cap/rotor, wires, plugs, etc.).

[trakkies]

Don't think a timing light tells you anything about the quality of the spark - merely when it occurs. IIRC, you'd need a 'scope with a special probe to measure the spark size and duration.

[dtl1982]

Well blast it. Cheaper to replace the ignition system than pick up a scope...plus I wouldn't have much else to use it for. I think I'll get a few resistors and see if I can utilize the laptop's sound card as some sort of scope (not on the coil secondary of course, but would be easy to pick up 12v signals).

I'm tempted to replace the VB921 with the new Bosch piece just because it's the cheapest/most likely place to start.

Priority for this has taken a bit of a hit since I got another vehicle to drive. I will be sure to report back after I replace the VB921.

Thanks for you help guys!

[trakkies]

Sorry if my comment about the timing light confused things. If the timing light is still showing it sparking after it cuts out until it stops I'd be suspecting something other than ignition.