Battery voltage from the ECU

[aceventura]

I am just trying to understand a discrepancy between the battery voltage I measure with a multimeter, and that read from the ECU.

While running, the ECU reports a battery voltage of 13.6V and an injector voltage of 14.2V.

If I use a multimeter to measure the voltage between the battery posts, I get 14.2V. I also get the same result if I measure between the alternator and the negative terminal or the positive terminal and 3 different earth points (on the body near the battery, plus the two earth points on the engine).

Is this normal? Does anyone know how the ECU determines the voltage? Any comments or suggestions welcome.

[bpefi]

First you need a high impedance multimeter to use on EFI systems - your usual el cheapo wont be good enough - injector power comes from a different source that the Pcm or ECU - the injectors will have power all the time & it will be grounded by the PCM to make it fire. Should have same voltage with KOEO - with engine running it will use some & the voltage may drop - though I would check back through your electrical system for faults.

[The1]

on VS V6 Battery to Engine Fuse to Red & Black wire B8 & A8 at ecu connectors. Same wire powers the EFI Relay which goes to the injectors, Oil Pressure Switch and DFI Module.

It depends how the ecu reads the voltage, it may go through a series of circuits before it reads voltage which is causing the slight drop, nothing to worry about with 13.6v

[aceventura]

Thanks guys for the replies. I only know enough about this stuff to know that I don't know much at all.

First you need a high impedance multimeter to use on EFI systems - your usual el cheapo wont be good enough - injector power comes from a different source that the Pcm or ECU - the injectors will have power all the time & it will be grounded by the PCM to make it fire. Should have same voltage with KOEO - with engine running it will use some & the voltage may drop - though I would check back through your electrical system for faults.

I was only measuring the battery voltage with the multimeter (14.2V), and I don't think I need a high impedance multimeter to do this(?). I noticed that the ECU was giving different readings for the injectors (14.2V) and the battery (13.6V). It just struck me as odd that these two numbers differed, as I a friend has a manual that shows the expected values for these two readings should be the same. On your suggestion I checked the ECU output with KOEO, and the two numbers were: Battery voltage = 11.8V, Injector voltage = 12.2V. So I will do some checking.

on VS V6 Battery to Engine Fuse to Red & Black wire B8 & A8 at ecu connectors. Same wire powers the EFI Relay which goes to the injectors, Oil Pressure Switch and DFI Module.

It depends how the ecu reads the voltage, it may go through a series of circuits before it reads voltage which is causing the slight drop, nothing to worry about with 13.6v

Yeah 13.6V is no problem, the car is running ok. Was just cleaning the earth points in the engine bay and checked all the voltages afterwards. I might check the voltage at the ECU at the connectors you mentioned, might point me in the right direction.

Is it conceivable that one of the circuits is using an earth point I don't know about (that needs cleaning)???

[The1]

the ecu has an earth point near itself from memory.