S13 sr plug and play link ecu on/off while cranking intermittently

Working on getting this sr20det started up and the ecu keeps disconnecting every other crank and sometimes happens one after another. Eccs relays good and working and checked wiring to the ecu. Anything I should be looking for

What is the battery voltage while cranking?

When starting an engine for the first time on a new ECU, where it may not start; you usually need to have the battery on an external charger (especially between start attempts), since the engine never runs, the normal recharge never happens.

The most common problem we see on those is low voltage on the "ignition switch" pin. This pin controls a separate piece of hardware (mosfet based) on the adapter board which in turn activates the ECCS relay. If it drops below about 7.0V during cranking then the mosfet drops out and the ECCS relay will switch off. This pin is not the power supply to the ecu and is not connected to the actual ecu - it is simply an "ignition on" signal so you cant monitor it in the software etc.

The ignition switch pin is pin 45 on the 76pin ecu or pin 36 if you have the 64Pin ecu. So start here, confirm voltage is near battery voltage on this pin during cranking (multimeter may not update fast enough to show true voltage drop but usually shows the issue). This same wire/circuit powers several high current loads - solenoids, the oxy sensor, and fuel pump etc and it comes direct from the ign switch (no relay, the full load goes through the switch) so sometimes it has low voltage due to either a larger fuel pump using the original wiring or bad contacts in the ign switch.

Thirded for insufficient voltage across the ECU - don't forget, it isn't just the voltage drop to the unit, but that on the earth/ground/return side as well - it's the voltage ACROSS the unit. Most will need at least 8V, and between cranking voltage drop at the battery and wiring losses, it isn't uncommen for the ECU voltage to drop that low - especially with a partially discharged, or undersized, battery.

Something you can do to get the venicle running is to disconnect the main battery feed to the starter motor and connect the starter power to an external bettery, with the return being directly attached to the starter body or engine, leaving the rest of the vehicle electrics/electronics on the vehicle's battery, charge circuit, etc.

This will isolate them and prevent the cranking voltage drop.

Thanks for the pointers to look at. I have tested that it was sitting above 12v before cranking, and then after cranking on the battery it drops to about 11.3-11.5 volts. I checked the front fuse box for voltage drop and it drops to around 6 volts then jumps up and holds at about 8 volts. I’ll check the ignition switch wire on the ecu plug and see if it drops the same here. Would it be ideal to try an ignition switch? Also had a charged up jump pack on it while cranking and even put a running car on jumpers with it.

Thanks

"Would it be ideal to try an ignition switch?"

That is the most common cause that I have seen (R32 Skylines have the same poorly designed circuit). 30 years of full fuel pump current through the switch with a nice inductive arc over the contacts every time you turn the ign switch off probably doesn't do them a lot of favours. But I have seen various other bad connections after the switch also.

Best way to rule out the switch without buying parts is to just unplug the plug off the back of the ign switch and make a little bridge wire to bypass it (like you are "hot wiring" it).

Gonna pull one out of my other s13s and try with a volt meter to see if there is any drop still.

thank you I’ll get back to you if still having the issue