ECU Voltage lower than Battery

Hello,

I am still in the early stages of my tuning and trying to make sure everything is functioning properly before I fully tune my fuel map. I currently have the car idle and cruise areas safely tuned.

I am running a Gen2 3SGTE in an AW11 with the ST205 PNP G4+ ECU.

I was having inconsistent voltage issues that I attributed to ground issues. I added a second ground to the battery in the trunk and the voltage is holding fairly stable now but I am getting a few different readings depending on where I check,

Ecu voltage in pc link 14.10Alternator to chassis 14.58Battery positive to negative 14.58B+ backprobing to each of the 5 grounds on the ecu connectors 14.48

Is this somewhat normal? I have checked the grounds in the engine bay and they all seem secure.

I am mostly concerned about the ecu reading lower than what I'm getting at the ECU pins with a multimeter.

Hopefully someone has some experience with this. Thank you

Every part of a circuit has a different potential (relative voltage) to any other part of that circuit and as the vehicle wiring system is made up of multiple, interconnected circuits, the potentials will normally vary from place to place - some on different circuits may be the same.

For the most part, this isn't something to worry about too much except sometimes the ECU may be expecting the same 12, 5 or 0V values and power or ground loops can be a problem.

You may recall Taz mentioning ground loops, and that single ground straps should be used, a similar thing applies to the 12V supply - where practical have them coming off the same point - if he hasn't already covered this topic, I expect he will in the near future.

To give an example, if you had some components powered directly from the battery (fused) and earthed to the battery, and others taking power from the same starter terminal the power is run to, and is also ground to the engine, there may not be much difference in normal running BUT under cranking it isn't unusual to have 0.5-1.0V drop between the battery +ve and the starter terminal, and a similar 0.5-1.0V drop between the engine and the battery -ve terminal - that means there could be 1.0-2.0V difference between some components and others and that CAN cause problems.

In extreme circumstances, especially with a 'low' battery, it can cause enough voltage drop to the ECU - especially if it's wiring is less than optimal - to drop too low for it to operate.

It turns out the link ECU uses pin 71 for the main power and pin 1 for main ground. The ecu voltage reading was correct at 0.5v lower than the alternator voltage likely caused by the ecu main power feed also feeding the fuel pump, TVIS and T-VSV.

The other issues turned out to be a bad voltage regulator in my recently rebuilt alternator.

This also caused a voltage spike to 17v once I hit 6k rpm.

Thanks for your reply!