454 BBC starts and idles - inconsistent tach signal

[Buick462.efi]

Could you help me with your experience and expertise? Just put a 454 into an '88 C1500. Controlling only fuel with my MSII. Distributor is older HEI with all new parts. Tach signal is inconsistent between starts. About every 3rd start it will be correct and the engine starts and idles great. Most of the time the RPM's indicated are triple the actual, causing a rich condition. I have solid grounds, am using a shielded wire for the tach signal, and the shielding is grounded, too. The C30 capacitor is in place. What else can I do? It took me a long time to get this engine going and I think it's because of the inconsistent tach signal - I tried to start it so many times that a pattern began to develop. About every 3rd attempt I could actually do some tuning. The rest were just a flooded futility. Won't attempt to drive it until I can get some consistency.

[Buick462.efi]

UPDATE: This is a 2nd hand unit - built by somebody else. So, I went through the assembly instructions step by step. Found that it was set up for a Hall effect ignition signal; D1 and D2 are jumpered. Will add the correct diodes to D1 and D2, the correct capacitors to C12 and C30 then give it another shot. More to come!

[Matt Cramer]

Yes, that could let in a lot of noise.

[Buick462.efi]

Thanks, Matt! I ordered my parts from DIY Autotune. I appreciate the service you provide.

[Buick462.efi]

Starts and idles! Tach signal is now rock solid. Fixed eight things on the 3.0 board - jumpers and diodes and such, de-soldered and re-soldered - to set it up for tach signal from negative side of the coil. This was my first time ever modifying a PCB. Easier than I thought and I highly recommend a magnifying headband,pencil soldering iron and patience. The 454 started right up and I was able to tune for a nice idle and hot start. Tomorrow we'll be out on the road tuning!

[Buick462.efi]

Thanks to all your guidance and support, I sorted out the tach signal issue and have the engine running nicely. So, I decided to upgrade the ignition to the 8 pin HEI for computer control of the timing. I made all the needed changes to the V3.0 board, added the MK-HEI kit from DIY Autotune and updated settings in Tuner Studio. Engine started right up and I set the initial timing to 15 degrees BTDC.

When I connected the tan/black reference wire the engine died.
1) Checked for spark - nothing;
2) Disconnected tan/black wire - engine fired right up;
3) Put my stim on the Megasuirt to check for an ignition out signal - it's sending a signal;
4) Checked continuity between the Ignition Out jumper on the PCB and the connector at the dizzy;
4) Swapped 8 pin HEI module to a known good module;
5) Started engine with tan/black reference wire disconnected;
6) Connected the tan/black reference wire - engine died, no spark

My log indicates that I'm still receiving a tach signal as the engine RPM drops (which I would expect since the VR is still active).

I'd appreciate any recommendations,

Mike

[Matt Cramer]

Could be a setting in the tune file; do you have a copy of your MSQ you can share?

[Buick462.efi]

Here's my current msq.

[Matt Cramer]

Can you try setting the trigger offset to 15 degrees?

[Buick462.efi]

Matt,

I tried +15, +10, +5, 0, -5, -10, -15 always the same result. Also tried Rising Edge at each offset, then Rising edge plus Going Low (spark output) at each offset. Finally, Falling Edge with Going Low. Then I set everything back to the MK-HEI recommended settings. If I put an O Scope on the output signal, what should I expect to see? What signal is the 8 pin module expecting? My stim indicates I'm getting an output signal and I have continuity on all the wires from the DB37 to the module.

I'm perplexed.

[Matt Cramer]

You should see a signal that's usually at 0 volts, goes to 5 volts at the start of coil dwell, then drops back to 0 to fire the coil.

[Buick462.efi]

Matt, thank you for your support on this. All of the hardware seems to be working so, I'm going to go over my wiring again. Like most issues it's probably an "i-d-10-t" error.