Low Battery ECU voltage

[mattlovo]

Hi guys,
Got a CA18DET S13 that ran with 13.6 to 13.8V for batt.

But recently, it has been showing 11.8V for a long time.

Swapped in 2 alternators and rebuilt one but not affecting the readings either.

Now with car running, multimeter to battery shows 13.8V.
However, nistune shows 11.8V.

I also noticed I have a weird 0.6V coming from the b/w wire at eccs relay. Red wires are showing 12V from battery as per normal.

I checked the entire harness but did not find any short either.

With the drop in voltage .. afr are screwed up as well.

Need some expertise on this matter.
battery in car is new as well.

[Matt]

Battery at the ECU shows what is measured at its power pins. IF you are measuring anywhere else, then it will be different

So measure from the two end power terminals at the ECU plug (paperclips on probes helps to access)

[mattlovo]

Hello guys,
so I jumped the egi or eccs relay for the S13 CA engine.

Got 14.2V to the ECU on Nistune.

Tested 3 relays, 1 brand new too.
All clicks when tested with a car battery.
Just not sure why it doesn't engage when it is back in the relay socket. Car will start but down to 12V.

Anyone got any idea ?
Wiring checked out ok for all the pins in the ECU too.

[Matt]

I also noticed I have a weird 0.6V coming from the b/w wire at eccs relay

The ECCS relay input (normally orange but my diagram shows green/yellow) will switch from floating to near 0V when it activates

The other lines to the relay are normally battery power (check your diagram). You should hear the relay 'click' and pass through 12V through to the ECU when the key is activated

So ECU has continuous battery power (12V in) on pin 58

So ECU 12V ignition key line on pin 45 will tell ECU to power on ECCS relay line pin 6

Pin 6 will change from floating to ground, and power ECCS relay

Power to relay then passed through to ECU pins 59/49 (blk/white) which should be 12-14V

When you say 14.2V I'm not sure which pin you are measuring above, but check all three (45, 58 and 59/49) and they should all measure the same

I'm not sure which one the ECU reports it is seeing, but would be one of those three