TFI feedback loop causing fuel pump relay cycling

[Stephen Armstrong]

Using
Megasquirt 3 with MS3X
TunerStudio MS Ultra vs 3.0.18
MS3 1.5.0 release
NOTE: datalog was running entire time of this event and now file is too big to upload into attachments.

I’m running a ford TFI distributor on turbocharged sbf 331. GM 3 bar MAP, CLT, IAT and ford TPS. I bought the megasquirt wiring harness and built the wiring harness for the engine myself. So I read all the manuals twice, wired up everything per MS wiring diagram, double checked all my settings and success. Engine fired off quickly on the first try, I let it run for 5 min cut it off to check for leaks and top off fluids. Cranked 2nd time just like the first and I let it run 5 more min before turning it off due to oil pressure sensor anomaly.

When I went to fire it back up, as soon as I turned the key to run, fuel pump cycles as expected, then turns off, then turns back on, then off, on, off, on, off and so on until I cut the key. I even tried to ignore the cycling and crank it but no luck, it will not start. So I figure out by unplugging connectors on things wired into the IAC circuit that it is being caused by the TFI module. TFI unplugged – no feedback loop.

So I remember reading in the manual that all inputs should be wired so that they don’t have 12V power when the MS doesn’t have power. So I wired the TFI 12V+ pin into the IAC 12V+ circuit which is activated by the fuel pump relay.
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What I think is happening is that de-energizing the 12V circuit which supplies the TFI with 12V is causing a false signal to go out through the PIP circuit tricking the MS into thinking that it is seeing RPM and thus causing the MS to energize the circuit again. When the MS sees no other spark events it turns the fuel pump relay back off which causes another false spark event and on and on.

So, Is there some setting in TS to filter out signals like that, or should I just re-wire the TFI to have power from the Ignition source regardless of whether the MS has power?

[Stephen Armstrong]

So, I rewired the TFI to have 12V+ coming from the Ign switch instead of being controlled by the Fuel pump relay. Problem seems to be fixed. Oh, the crazy RPMs I was seeing in Megalog was just a setting in that program where RPMx100 was selected.

On to dialing in the tune.