Low Voltage (Warm Engine) By Scangauge Indication

[gathermewool]

2008 Civic LX, 108k miles.

Finally hooked up scangauge for continuous monitoring: noted 11.9-12.1VDC indicated, which also correlates with when my engine vibrates moderately while stopped in gear.

Also noted that it only happens when the engine I warm, and regardless of whether at highway RPM (1800-3k) or at idle, though vibration is only noted at idle.

Never happens at night, with lights on.

Never happens in open-loop, such as when letting off the gas on the highway, engine-braking.

Today, while idling at a light with vibration and 12.1VDC indicated, I turned the parking lights on and the voltage almost immediately shot back up to 13.7VDC indicated.

Haven't been able to verify low voltage with Fluke at same time as scangauge indication, but intend to.

//

I'm on-shift, between briefs, so may not be able to respond until around midnight. Thanks in advance for any insight or recommendations/experiences.

 

[NHGUY]

Has the battery been disconnected recently? If so,bad connection between post and clamp.

 

[Trav]

How old is the battery? These cars apparently use something called an ELD (Electrical Load Detector), that from what i read can do some flaky things when the battery is on its way out
I really have no idea but its something to research.

 

[gathermewool]

The vibration issue has been around for a while, even after replacing the failure-prone hydraulic engine mount (which fixed another issue,) so it may be that the low voltage issue has been around for a while, as well.

The battery is ~ 2 yrs old and starts the car without issue and has never been drained, lowered, etc (no lights ever left on and I never leave the radio or any accessories on if the engine is off, for reference.) It was last disconnected this past fall, when I cleaned the clamps and battery area, charged the battery via battery minder for a day (went to trickle charge (whichever mode its called) within an hour or so,) and then re-installed it, with fresh dielectric grease.

I've got one of those cheap 12VDC plug-in voltage indicators on-order from Amazon, that should be here Monday. I'll compare what the battery is putting out to the socket and what the ECU is reading. I'm wondering if the ECU could have an issue with voltage, even if the alternator is putting out good voltage.

On my way home tonight, the voltage indication on the Scangauge read 13.7-14 VDC, with the lights on. On a back-road with decent lighting most of the way home I flicked the lights off and the voltage indication immediately lowered to 12.1 VDC; when I turned the lights back on, the voltage indication immediately rose back to 13.7VDC.

 

[HangFire]

Originally Posted By: gathermewool
It was last disconnected this past fall, when I cleaned the clamps and battery area, charged the battery via battery minder for a day (went to trickle charge (whichever mode its called) within an hour or so,) and then re-installed it, with fresh dielectric grease.


Dielectric grease on the battery terminal? Been there, done that. That might be the problem right there. It's an insulator, and if it covers enough of the terminal, can lead to problems.

 

[gathermewool]

Originally Posted By: HangFire
Originally Posted By: gathermewool
It was last disconnected this past fall, when I cleaned the clamps and battery area, charged the battery via battery minder for a day (went to trickle charge (whichever mode its called) within an hour or so,) and then re-installed it, with fresh dielectric grease.


Dielectric grease on the battery terminal? Been there, done that. That might be the problem right there. It's an insulator, and if it covers enough of the terminal, can lead to problems.


That's not how it works. The use of dielectric grease between the terminal and post will cause zero issues.

Now that I'm actually paying attention (my pregnant wife now has the Subaru full time and I have the Civic,) I'm figuring things out that can only be figured out after driving for a bit, not just for the occasional trip to the hobby shop for an oil or ATF change.

After reading about the ELD, I think the indications I'm seeing may be a normal, albeit unintended side effect. Over the past few days, my observations have been very consistent. It's still annoying that the low voltage causes the increased vibration, but a simple flip of the switch to turn the running lights on solves it.

It may be that something isn't operating properly, but the indications say otherwise. I wonder if it's solely for fuel economy, or whether there's some other reason to have the voltage shift like this. I can't imagine that a constant 13.7-14VDC would cause a battery to over-charge. What do you guys think?

 

[Trav]

Originally Posted By: gathermewool
After reading about the ELD, I think the indications I'm seeing may be a normal, albeit unintended side effect. Over the past few days, my observations have been very consistent. It's still annoying that the low voltage causes the increased vibration, but a simple flip of the switch to turn the running lights on solves it.


That's what i am thinking after reading more about it, i just don't know enough about it to say 100%.
I'm interested in learning more about it, please share whatever you find.
From your description it sounds like everything is working.

 

[Merkava_4]

Originally Posted By: gathermewool
Today, while idling at a light with vibration and 12.1VDC indicated, I turned the parking lights on and the voltage almost immediately shot back up to 13.7VDC indicated.


When the voltage shot up, did the idle smooth out?

 

[The_Eric]

Originally Posted By: gathermewool


It may be that something isn't operating properly, but the indications say otherwise. I wonder if it's solely for fuel economy, or whether there's some other reason to have the voltage shift like this. I can't imagine that a constant 13.7-14VDC would cause a battery to over-charge. What do you guys think?


It's not unheard of for manufacturers to turn off charging in certain conditions- and yes it's for mileage purposes.

Like Trav, I don't know enough to say for sure if your particular case is 100% normal.

 

[HangFire]

Originally Posted By: gathermewool
That's not how it works. The use of dielectric grease between the terminal and post will cause zero issues

The way it's supposed to work, if the metal to metal contact pressure exceeds a certain PSI, conductivity is assured.

The way it worked for me is I had ever-worsening charging and high demand issues until I cleaned out all dielectric grease between the post and the cable terminal. Now I use non-flowing corrosion inhibitor spray (like NoCo NCP-2) on exterior surfaces after mating degreased, clean bare metal to metal.

You might have another problem entirely, or more than one problem...

 

[SHOZ]

Most OBDII voltage monitoring is from the ECU regulated value I think. Not the battery voltage.

 

[zzyzzx]

I don't see any problem here. It's not charging much at idle when it's warmed up because alternators don't always charge much at idle, and when it's just off a cold start, it does need to charge the battery more.

 

[gathermewool]

Originally Posted By: Trav
Originally Posted By: gathermewool
After reading about the ELD, I think the indications I'm seeing may be a normal, albeit unintended side effect. Over the past few days, my observations have been very consistent. It's still annoying that the low voltage causes the increased vibration, but a simple flip of the switch to turn the running lights on solves it.


That's what i am thinking after reading more about it, i just don't know enough about it to say 100%.
I'm interested in learning more about it, please share whatever you find.
From your description it sounds like everything is working.


I agree, that everything may be normal. My plug-in volt-meter just came in, so I'll be able to monitor bus voltage and ECU voltage at the same time.

Originally Posted By: Merkava_4
Originally Posted By: gathermewool
Today, while idling at a light with vibration and 12.1VDC indicated, I turned the parking lights on and the voltage almost immediately shot back up to 13.7VDC indicated.


When the voltage shot up, did the idle smooth out?


Originally Posted By: The_Eric
Originally Posted By: gathermewool


It may be that something isn't operating properly, but the indications say otherwise. I wonder if it's solely for fuel economy, or whether there's some other reason to have the voltage shift like this. I can't imagine that a constant 13.7-14VDC would cause a battery to over-charge. What do you guys think?


It's not unheard of for manufacturers to turn off charging in certain conditions- and yes it's for mileage purposes.

Like Trav, I don't know enough to say for sure if your particular case is 100% normal.


Yes, it did. Turn the lights off, voltage drops, vibration returns.

Originally Posted By: HangFire
Originally Posted By: gathermewool
That's not how it works. The use of dielectric grease between the terminal and post will cause zero issues

The way it's supposed to work, if the metal to metal contact pressure exceeds a certain PSI, conductivity is assured.

The way it worked for me is I had ever-worsening charging and high demand issues until I cleaned out all dielectric grease between the post and the cable terminal. Now I use non-flowing corrosion inhibitor spray (like NoCo NCP-2) on exterior surfaces after mating degreased, clean bare metal to metal.

You might have another problem entirely, or more than one problem...


Dielectric grease is easily penetrated and displaced by connector contact pressure, IME. It's used pretty widely, and in many applications where water exclusion is a must, or where corrosion is likely, such as copper connections and contacts. If you're not adding dielectric grease to your marine connectors, you're making a mistake - it's a must. But that's neither here nor there...As much as a weirdo maintains his cars, dielectric grease may not be all that important, but it's for sure not causing any issues.

Originally Posted By: SHOZ
Most OBDII voltage monitoring is from the ECU regulated value I think. Not the battery voltage.


Right, and that's my concern, more so than low actual voltage; because, that would be indicative of a possible ECU issue, such as a short. Based on what others have said and my continued observations, it's looking like the former is true, and that's it's normal.

Originally Posted By: zzyzzx
I don't see any problem here. It's not charging much at idle when it's warmed up because alternators don't always charge much at idle, and when it's just off a cold start, it does need to charge the battery more.


This isn't just at idle, it occurs at highway speed, and while accelerating lightly. It seems to shift back to full voltage while accelerating at higher RPM, but shifts back down to lower voltage when I let off and the AT upshifts.

 

[SHOZ]

Newer cars with ECU controlled alternators will reduce the alternator output voltage when no really needed.

 

[gathermewool]

With the cheap $10 voltage detector plugged into the 12VDC jack, I noted a consistently 0.6 VDC higher than what the scangauge indicated.

Hmmm

 

[Kiwi_ME]

Well, those could be different circuits with different loads and consequential voltage drops. The battery + terminal is the only reliable place to measure consistently.

The behavior sounds just like an ECU-controlled regulator, as mentioned.

 

[gathermewool]

Originally Posted By: Kiwi_ME
Well, those could be different circuits with different loads and consequential voltage drops. The battery + terminal is the only reliable place to measure consistently.

The behavior sounds just like an ECU-controlled regulator, as mentioned.


Right, they track and trend together, which is the data I was looking for.

 

[armos]

Voltage affects the operation of the fuel injectors. The ECM is programmed to take this into account, and that's an important reason that it needs to know the voltage in the first place.
Perhaps for some reason (general age and wear?) the injectors are exhibiting significantly different flow characteristics as a function of voltage than what was programmed into the ECM, so this is causing it to get confused and run badly when the voltage changes to 12.1v.

ECMs do have some learning behavior, but it has significant limitations. On the much older ECMs that I'm familiar with, the injector response curve vs voltage is not something they can learn on the fly. I don't know if modern ECMs are capable of that, but I really doubt it.

I don't know how likely an injector issue is on your car though, and I don't know anything about ECMs made in this century. I wouldn't want to intentionally swing the voltage around on an older ECM but maybe yours deals with it better - or maybe it doesn't, hence the issue. I don't know.

Another theory would be that the fuel pressure is shifting when the voltage changes. It shouldn't, but if it did, that would confuse the fuel calculations also.

Last theory I can think of is that maybe the ignition system isn't happy working at lower supply voltage.

I don't think the ECM electronics should be affected. It should have an onboard voltage regulator that regulates down to a much lower voltage. Whether the input is 12v or 14v shouldn't bother it.

 

[SHOZ]

If it is an older car the grounds are always subject to question.