Control Module Voltage VS Control Module Voltage (SAE). Difference?

[TXjeepTJ]

Hi,

I noticed on my scans that I had 'Control Module Voltage (SAE)' being logged, and the value for it was all over the place. 17volts, 19 volts, one scan showed 1 volt. I started to freak out.

Then I logged the PID 'Control Module Voltage' (the NON-SAE one), and it pretty much shows what my measured voltage is, minus about .3volts whenever the engine is running. Engine not running, it's pretty much spot on 12 volts.


Do I need to worry about the SAE voltage PID being all over the place? I'm hoping to God that it isn't being used for my injector voltage offset, etc.... I attached a scan so you can see

The car runs great, I'm just curious about the voltage PIDs

[blindsquirrel]

The SAE channels often do weird shit like that, don't use them unless you have a reason to.

[slarsen47]

Older thread.................but......

Does anyone know what Control Module Voltage is?

How does it compare to battery voltage?

[blindsquirrel]

Control Module Voltage is, most likely, the voltage at the control module.

Whether that's measured the same way on a Chrysler or a BMW or a GM or a GM with a P59 or GM with an E67, and at which pin, I guess it depends.

Since we don't live in fantasy land and our electrical conductors are made of plain dumb copper and not magical room-temperature superconductors, every wire that carries current will have some amount of voltage drop between the source and the load. The smaller the wire and the higher the current, the more the voltage drop along the way. So voltage at the source (battery + post) and the load (in this case, whichever one of the ECM power pins is being monitored) will not be the same. Lots of wire and connectors and relays and ignition switch contacts getting in the way.

[slarsen47]

Control Module Voltage is, most likely, the voltage at the control module.

Whether that's measured the same way on a Chrysler or a BMW or a GM or a GM with a P59 or GM with an E67, and at which pin, I guess it depends.

Since we don't live in fantasy land and our electrical conductors are made of plain dumb copper and not magical room-temperature superconductors, every wire that carries current will have some amount of voltage drop between the source and the load. The smaller the wire and the higher the current, the more the voltage drop along the way. So voltage at the source (battery + post) and the load (in this case, whichever one of the ECM power pins is being monitored) will not be the same. Lots of wire and connectors and relays and ignition switch contacts getting in the way.

Your first line I think is an accurate description of what the voltage represents. i.e. not battery voltage

Your second paragraph clearly explains the many factors that affect voltage readings and I understand that.

I raise the question because my last data log showed 12.7 and some say that is too low. As if it is supposed to be alternator voltage!!

Could there be a voltage control circuit in the 411 pcm that limits the voltage so you don't get alternator voltage in the 13 to 14.5 range.

Furthermore...........is there a minimum Control Module Voltage where the PCM ceases to properly function?

[blindsquirrel]

What does a DVOM show when backprobed into one of the main power feeds at the PCM connector? And what's voltage drop on the PCM ground circuit? That's about as close as you can get to what's available on the PCM internal rails. If the DVOM shows the same low voltage then you have a wiring issue (too much voltage drop somewhere). Or maybe it's just normal! Be careful you don't send yourself off on a snipe hunt. Way to not do that is to verify with real diagnostic tools and strategies, not guessing.

[TheMechanic]

One of the reasons every manufacturer is using a module to control alternator voltage instead of using internal regulated control. Module control will better adapt to changing voltage/current requirements. Things like temperature, start up from a cold start, accessory load, lower current due to a slower/lower speed, etc.

[slarsen47]

What does a DVOM show when backprobed into one of the main power feeds at the PCM connector? And what's voltage drop on the PCM ground circuit? That's about as close as you can get to what's available on the PCM internal rails. If the DVOM shows the same low voltage then you have a wiring issue (too much voltage drop somewhere). Or maybe it's just normal! Be careful you don't send yourself off on a snipe hunt. Way to not do that is to verify with real diagnostic tools and strategies, not guessing.

Mystery solved!

Nothing like having an identical stock 2002 Camaro SS to do some comparisons!

DVOM showed 12.25 volts at the battery before starting it. Bit low, has been sitting for a while.

Hooked up the HP Tuner scanner and fired it up.

Scanner showed only 11.3 for the Control Module Voltage before startup.

Upon starting engine the CMV went up around 14.0 with the at battery voltage being about 14.4.


So to conclude the CMV should be up there along with the battery voltage with about a 0.4 v difference at least while recharging a low battery.

Scanner log attached.


2002 voltage.hpl

[blindsquirrel]

You're only checking half the circuit. The ground side has to carry the same amount of current as the positive side. And you should really be checking using voltage drop instead of straight voltage. You wanna know the difference in voltage from the start of a wire (or circuit) to the end (the load), or from the end back to the start (on the ground side); the overall voltage doesn't tell you nearly as much about the health of the wiring or where a problem might be.

[slarsen47]

You're only checking half the circuit. The ground side has to carry the same amount of current as the positive side. And you should really be checking using voltage drop instead of straight voltage. You wanna know the difference in voltage from the start of a wire (or circuit) to the end (the load), or from the end back to the start (on the ground side); the overall voltage doesn't tell you nearly as much about the health of the wiring or where a problem might be.

I think you might be onto something here. I did some more testing today and it appears that somewhere there is a voltage loss to the PCM.

I ran the procharged car today with the apparent alternator charging issue. Turns out I have up to 1 volt difference between the Control Module Voltage and the battery terminal voltage when at idle. When I first started the car to check trans fluid level cold, I measured 14.0 at the battery while the scanner was showing 13.0 at the Control Module Voltage. After returning from a 30 minute drive the battery voltage was 13.4 at idle. Later on after it had cooled down I fired it up to drive it into the garage. Battery was at 14.1 and Control Module was at 13.1. So there is a pretty significant difference between the battery and the Control Module Voltage.

So there would appear to be something causing a loss of voltage showing up at the 411 PCM.

Next step then is to do exactly what you are explaining, to check the difference/drop and to hopefully find the fault.

Thank you!