VR circuit components??

[Mr Gadgets]

Is there a recent drawing of the VR circuit with proper component values, per the lastest BOM? I am having problems with melding the MSD 6A and the MSII ver 3.0. Something is causing noise to get into my MSD and it fires intermittently 90* off. Any help is appreciated.

Dick

[Mr Gadgets]

I found where it says in the manual to disregard the print and look at the BOM.. Changes in the VR circuit have not been good for my setup. I will play with them till I can get it to run right, then draw my own prints. So far this new MSII I build does not like the MSD6A ignition. The last MSII I built worked great. I get enough noise in the DC to make any ignition I run on the motor to miss fire, 90 degrees advanced.

Thanks
Dick

[Mr Gadgets]

I would like to know why the components in the VR circuit have changed. I have an older ECU with much different values and my new one built to the BOM.. and it gives me very unstable timing.. and it will drop the VR signal every 20 seconds or so.. I have moved the ground on the VR sensor to the Base of Q23.. adjusted pots till from end to end where the motor reacts unfavorably..

I am temped to build it like the previous one, but alot of work, just because I am guesing..

Thanks
Dick

[Mr Gadgets]

Well I finally modified my old MSII to work on the new wiring harness. The timing it rock solid with it. So unless someone can tell my why the components in the VR circuit have changed, I will have to modify my new MSII to mirror the old one and see if that is the issue..

Anyone, with any ideas on the subject.. This is very frustating!!

[Mr Gadgets]

Here is an update on my engine. After running it on my old MSII v3.0 with 2.35 and running solid ignition timing, I decided to change the components in the VR circuit to match my old unit. After scoping the whole circuit it seems like everything in the circuit was working properly. I started with U7. Figured the active componet would be the most likely one. Well that fixed it!! I swapped in the same number op amp that was in my older MSII and the timing is now rock solid above 1200rpm. Below that, it does wiggle some, but not like it had.

Another problem I had was the timing jumping 90*. I did it on all cylinders and with the crank trigger connected directly to the MSD 6AL box. I did it with an HEI distributor in the motor, and the MSII was running off the crank trigger. I tried at least 100 different combinations.. When the HEI did it, the only thing left was plugs and plug wires. The last thing I swapped out was the plug wires and the problem disappeared. Now whether I was seeing the plug fire with that much advance or maybe the timing light (one of five diferent lights) was sensing a pulse large enough to fire the light, I dont know. The motor now runs like it is an 8 cylinder motor, before it didnt.
So 4 months after I first started my new motor, I am finally on track to head to the dyno. It has been a very discouraging road. I am not sure how many others are using the MSD crank trigger and having luck, not too many people spoke up when I was tossing out questions. I did read the manual and the answers were not there.
Here I am ready to move forward. I have waveforms from the whole circuit with a million different variations. The latest after switching U7 is on the bench with MSD crank trigger, the 6AL, and coil and a plug to simulate the motor. After I did that, I put the MSII on the motor and fired it. It ran fine after that. And as soon as I get my MSD 6AL back from upgrading, I will scope out the end setup, on the running motor. I did get a lot of help with the first problem from another source that will be rebuilding my 6AL.
If anyone would like what I found, please request it, or send me a pm and I will post it here.

I would still like to know what the reason behind changing the BOM for this circuit. I did not search out a new original U7 to try. I may just have a defective one, but I now have 10 of the old part number, so I am done chasing that..

Thanks for listening.
Dick

[Matt Cramer]

The original BOM worked well for old school distributors with 4 to 8 teeth on their wheel. It didn't work well when people started using it with things like BMW 60 tooth crank triggers and other setups with high tooth counts - it had too much high frequency filtering, I believe.