Obd voltage reading vs on the electric bus

[JHZR2]

Does anyone know if the voltage read off obdII, like off a scangauge, is a reported numerical value, or a sensed value that is part of the circuit inside the obd reader?

The reason I ask is because I get a different reading (0.2-0.3V higher) from the power socket than from the reading on the obd scangauge.

The issue is that this is my pickup truck, and with a brand new battery, I'm seeing 12.2V after resting a while. Yet it charges at 14.4V.

So not sure if I have a fluke reading or what, but I'm hoping that somehow there is a bad reading...

Need to do some probing with a multimeter and an ammeter, but just curious about this first...

 

[wirelessF]

I get the same result too. Using a multimeter reading off the battery I would get 14.4v. The scanguage would report 14.0v.

I guess the scanguage reads off the voltage from the OBDII port.

 

[JHZR2]

Hmmm, so maybe there is a voltage drop off the obdII port...

Anyone know which pins I'd hit with a multimeter to read voltage?

 

[friendly_jacek]

Originally Posted By: JHZR2
I'm seeing 12.2V after resting a while.


way too low. Did you check for a parasitic draw?

 

[rationull]

I too can recall seeing lower voltage readings on my SG2 vs a multimeter on the better terminals or the cig lighter. I haven't put a multimeter on the OBD2 port though, I figured the SG2 was just less accurate in sensing voltage.

 

[JHZR2]

Originally Posted By: friendly_jacek
Originally Posted By: JHZR2
I'm seeing 12.2V after resting a while.


way too low. Did you check for a parasitic draw?


Agree. Its on my list...

 

[JHZR2]

Actually it rested at 12.41 overnight, do I was off. It may have been as low as 30 F last night, making accuracy of readings at any morning condition unreliable...

This was on a brand new wal mart battery first put into use on Friday, seeing >14V over 200+ miles of driving this weekend...

On those GM battery connections (screw in), should the battery wire be able to rotate when the screw is torqued to the battery?

 

[George7941]

Originally Posted By: JHZR2
Hmmm, so maybe there is a voltage drop off the obdII port...

Anyone know which pins I'd hit with a multimeter to read voltage?


Pin 4 is ground and pin 16 is B+

My guess is that the Scangauge does not measure any voltages directly but just reports the voltage measurement it receives from the ECM thru the data bus.

 

[George7941]

Originally Posted By: JHZR2

On those GM battery connections (screw in), should the battery wire be able to rotate when the screw is torqued to the battery?


Assuming you are referring to side terminals, I don't see any need for the cable to be able to rotate. What I am trying to figure out is why this is even a concern. Are you perhaps considering the prongs on the flat face of the battery terminals which bite into the lead pads on the battery?

 

[JHZR2]

Asking because since my truck sits, I bought a battery disconnect thing that I screw into the terminal on the battery, and then screw the battery cable to. Even with a washer on there, the battery cable rotated when I torqued the screw to the right amount.

NOTE: I have seen these low battery readings WITHOUT use of that disconnect switch in there. I bought it thinking it was good to have for when the truck sits in situations where I cant get a float charger on it. But Im still seeing only 12.41V outside right now, when it is 56 degrees F out. It does seem stable there though, which is why this all comes full circle... If the ECU is seeing a different voltage than the system, could something not be right, causing the battery to not properly charge?

 

[George7941]

Sorry, misunderstood your question. Certainly, the cable terminal pad should be clamped tightly against the battery side post with the prongs on the cable terminal pad digging into the lead of the battery post. No relative movement is possible when clamped down properly. You definitely have a problem there.

The 12.41v -- are you seeing that on the Scangauge or are you mkeasuring it with a DMM? ECUs typically see a slightly lower voltage than the battery voltage because of voltage drops across wiring and the ignition switch. This is why the Scangauge reports a slightly lower voltage. I don't think you have an issue there.

What you really need to do is to monitor the charging voltage, at the battery, with a DMM and see if you have an issue.

 

[eljefino]

There's a generic OBDII PID for "control module voltage". It's plausible either way. I doubt the scanguage would pursue a non-generic PID for this when it could fake it with its own voltage sensing circuitry... which it needs to shut itself off when the car is shut off.

The +12v on the OBDII connector is a (relatively) straight shot to the battery; we used it to backfeed when changing batteries to keep all the memory settings.

Since it's hot with key off, your scanguage (and progressive dongle etc) need a reliable self-sensing shut-downing if you leave them plugged in.

 

[mechtech2]

Even if the scan gauge and your multimeter were calibrated exactly to each other, there are different voltages in different circuits due to resistance from various wiring and connections.

 

[JHZR2]

Originally Posted By: mechtech2
Even if the scan gauge and your multimeter were calibrated exactly to each other, there are different voltages in different circuits due to resistance from various wiring and connections.


Absolutely, but I was suprised to see it be that substantial a difference!