Voltage at OBDII port and Gauge don't match battery or alternator.

[Shadow_Death]

Edit: I'm pretty sure I've solved this. I was just chasing what turned out to be a garbage gauge problem.



Just as the title says and I'm chasing my tail over this one.

This is where my voltage gauge usually sits (for the year that I've owned it):


As of today it's been sitting like this:



When I check Torque Pro via OBDII port I get this:



And the kicker is when I check voltages at the battery and the alternator I get this:


Another note, this is ONLY at idle. The second RPMs go up everything goes back to normal. It stays normal while driving as well as of this moment. Another side note is that my headlights have recently begun to dim at idle and brighten when I give it gas. I was leaving towards alternator but that wouldn't explain the 13.5v at the battery and 13.6 at the alternator.

Any ideas?

 

[Shadow_Death]

Walked outside to do some more testing and on a cold start everything reads normal. So now I'm letting my truck warm up to see if it's heat related.


Edit: after more testing I believe this may just be a wild goose chase and now I just don't trust that gauge in the dash as it wants to report the actual voltage instead of 14.7 all the time. As for the OBDII port I suddenly think that it is supposed to look like that. I think I'm going to install an actual voltage gauge and stop trusting the one in the dash.

 

[WilliamS]

Are you checking the voltage at the port, as that is only to power devices plugged into it. The voltage torque reads is from the PCM telling the app what the current voltage is, not the voltage at the port.

If this is on the 1996 I would check the alternator voltage regulator or the battery cables. To me I would start on the battery to ground and motor to ground strap but thats just me.

 

[Shadow_Death]

Are you checking the voltage at the port, as that is only to power devices plugged into it. The voltage torque reads is from the PCM telling the app what the current voltage is, not the voltage at the port.

If this is on the 1996 I would check the alternator voltage regulator or the battery cables. To me I would start on the battery to ground and motor to ground strap but thats just me.

At this point I've ruled it down to a faulty gauge in the dash. I checked power at both the battery and the alternator using the ground to the battery and I get 13.5 at the battery and 13.6 at the back of the alternator. The two smaller bolts on the back of the alternator for the voltage regulator show 13.6v at one and anywhere from 0.002ish to 3v that one is super small voltage that I managed to read from it. I even checked voltage that the cigarette lighter in the dash and read 13.5v. I will continue to drive it and see what she does. I drive about 5 miles to work, so if the alternator or voltage regulator go out it won't be that big of an issue to get her home. I think what threw me off is that it went from reading over 14v to under and it fluctuates with engine rpms while the alternator output doesn't. This is where I figured it's just the gauge being dumb.

 

[dudeman2009]

Glad to hear you got it solved. But yeah, its a common issue on older vehicles, you have rather small wires on the OBD port, and if there is another slight load on them, that voltage will drop considerably, the pins on that also like to corrode heavily in northern climates, not sure if your truck was ever up north.

Those gauges are dumb gauges, they read relatively accurately as long as everything is doing alright but aren't for precise measure, they basically tell you if the alternator is fried or if your battery is shorted. I've got a custom center console with a voltmeter in my 01 and it will sometimes read 1V off what the gauge says. But the PCM reads from the alternator itself, not sure where the gauge reads from on the 96, the 01 is bus controlled from the PCM.

 

[Shadow_Death]

Everything I've read said PCM on my 96 since the Voltage regulator is controlled by the PCM. I figured that the gauge is just going out because the system is charging just like it should according to 2 different multimeters. So that tells me that the PCM isn't dead yet. Lol