GM.Battery & GM.Volt PIDs

[bimbleuk]

Hopefully a simple question but should the values read via these PIDs match the battery voltage i.e. are these PIDs just a measure of battery supply?

On my setup the voltage is dropping with RPM by about a 1V (13.5V - 12.5V). The log is consistent and repeatable each time.

I've measured at the battery and it's stable at ~14V (Alt charging) so if the above PIDs should match this or at least stay the same then I need to the check the PCM battery feeds.

I have a converted RX7 so it's possible there's a wiring issue or I just need to bypass the Mazda wiring for a more consistent feed.

[joecar]

There will be some drop between the battery and the PCM's measurement circuit... 1V drop seems a bit high, but it may be ok;

but do check the wiring anyway just in case... if you can, using a DMM measure the external voltage drop between the battery and the PCM (with ignition key on), do this for all power feeds to the PCM... if the external voltage drop is more than 0.3V then there is a physical problem with wiring or connectors.

[bimbleuk]

I may not have explained the situation 100% clearly but the voltage read at the PCM is for example 13.5V at idle and drops to 12.5V at 6000 RPM.

The voltage has a linear relation to RPM (or maybe load?) so when accelerating at a constant rate the voltage drops in a linear (but opposite) relation to RPM.

Anyway I bought a decent DMM today to check over the wiring. I suspect there's either something in the Mazda wiring which is affecting the voltage or it may even be the immobiliser which has burnt out a track in the circuit board previously. The cars 19 years old, the immobiliser may be over 10 so I'll revamp the PCM power feeds and bypass the old wiring.

[joecar]

Ah, I see... at high rpm... where the current demand is higher so the alternator is outputting more current...

higher current causes any "non-zero" resistance across any wiring/connections to show up as an appreciable voltage drop...

so, as you said, yes check/clean wiring/connections and see how it goes from there

( I dab a small amount of dielectric grease on pins/connections to prevent later corrosion )