Voltage drop when I come off the throttle

[6716]

Today on the drive home I noticed that when I come off the throttle, the voltage was frequently dropping into the 12 range.

Further information is that until yesterday (when I replaced 4 coil packs) I thought the volt meter was broken because it was reading super high ... and also the oil pressure gauge the needle is buried on the high side, and for a while the speedo wasn't working either, so I figured it was all the stepper motors going bad at once.

But after I replaced the coils, the needle settled in at just over 14.

Today, I was noticing the drop after I would get to the speed I wanted and then lifted my foot off the accelerator.

The coil packs are Duralast -- I wish I had been at a store with Delco, but I wasn't, but whatever they are new.

Battery is new as of three months ago. Alternator is a reman from O'Reilly that doesn't have many miles on it. There are a couple of threads out there, one is inconclusive but describes my issues in a way that sounds familiar, and the other has to do with extra sound equipment, which I don't have.

I suppose I should break out the multimeter but I don't actually know that I know how to use it.

 

[mrrsm]

Fortunately, using the average DMM (Digital Multi-Meter) or DVOM (Digital Volt Ohm Meter) is a fairly straightforward procedure as long as the Black and Red Probes are inserted into the proper connections on the DMM-DVOM. You should know that in every case that once the pressure on the Accelerator Pedal is lifted, the Engine is no longer being prompted to create continuous power during even slight coasting periods and thus... the Alternator does not "Free-Wheel" and maintain its creation of Electric Power. ALL of the Accessories and Pulleys in that Loop suddenly become an immediate Parasitic Drag upon the Engine once your Foot is 'Off The Gas' . Naturally, the Elevated Voltage reading on the Voltimeter will drop as a result.

This video is quite an excellent DMM Primer that covers the two areas of concern that you have of (1) Knowing whether or not the Battery is sustaining the right amount of Voltage to Start and Run the vehicle. AND... (2) That the Alternator (whether Brand New, Re-manned ...or even an OEM Unit with some 'age' on it) is being properly refreshed by a fully functioning Rectifier Circuit(s) that are reliably converting Alternating Current over to Direct Current of around 13.5-14 Volts:

 

[budwich]

You are looking at an electrical problem but are "afraid" to use an "eletrical instrument" to investigate the problem.... ????
The first check would be to verify the voltage of your battery with an meter (not the dash "meter" it is not really a meter). Depending on what the reading is will determine where else you need to check as opposed to "it a new battery".... "new" do fail also or may have been installed in a less than good fashion resulting in improper charging such that it is "somewhat weak".

 

[JerryIrons]

I suppose I should break out the multimeter but I don't actually know that I know how to use it.

For measuring voltage, the multimeter is simple to use. Just set it to DC voltage, put red probe on your positive battery terminal and black probe on the negative. Make sure you get a good connection, ie stick it hard to the terminals.

If it were my vehicle, I would clean off the battery terminals. And maybe clean off some engine grounds that are visible. Measure your battery voltage, hopefully it's about 12.7 volts or higher. If your battery is 12.1 volts, it's only 50% charged. Then turn off all accessories in vehicle, and start the engine. Then measure voltage on battery, should be about 14 volts. Then start turning on your accessories and measure voltage. Maybe have a helper watch gauge to see if gauge in vehicle drops, does voltage at battery drop as well?

 

[TequilaWarrior]

If it were my vehicle, I would clean off the battery terminals. And maybe clean off some engine grounds that are visible. Measure your battery voltage, hopefully it's about 12.7 volts or higher. If your battery is 12.1 volts, it's only 50% charged. Then turn off all accessories in vehicle, and start the engine. Then measure voltage on battery, should be about 14 volts. Then start turning on your accessories and measure voltage. Maybe have a helper watch gauge to see if gauge in vehicle drops, does voltage at battery drop as well?

^^^^^^ THIS.
It's easy. It's essentially free. It ~could~ potentially (no pun intended) be the culprit.

To clean terminals, remove the cables - use a terminal cleaning tool if you have one, wire brush if you do not. Safety glasses are essential here. If you have electrical contact cleaner, use it as well. Clean the terminals as well - also use electrical contact cleaner on them. For grounds - spray with a penetrating oil before and let sit for a minute or two before unbolting the ground. Also, I like to leave the battery completely disconnected when working on underhood grounds. After unbolting use wire brush, sandpaper, angle grinder (whatever) to get clean bare metal around the bolt hole for the ground (without screwing up the bolt hole). Clean the terminal that the bolt goes through with a wire brush. Clean the bolt as well. I've used different methods to help maintain cleanliness of a ground but my most recent method is to use battery terminal spray to cover the ground area after torquing the ground/bolt. Terminal spray is great at discouraging rust and is fairly cheap. You can also use dielectric (headlight) grease. I've seen people just spray WD40 and I've seen people spray paint over the ground. The "after" is less important than getting a clean ground and clean terminals.
At worst, you'll spend some time cleaning stuff that may not be cleaned. At best, you'll solve your problem while spending little to no money.

 

[Mooseman]

What year is your truck? If it has a SARVC module on the negative battery cable, it might be acting up. If you have one, disconnect it and see if things change. Use a real meter to monitor voltages.

I did have this kind of situation where a rebuilt ACDelco was acting like this and eventually died completely. replaced with a new alternator and it went away.

PS: Unlike the oil pressure gauge which is totally fake, the volt gauge is real'ish. It is filtered by the PCM and slow reacting but it mostly reports real voltages.

 

[6716]

What year is your truck?

2005 LS.

I looked up the SARCV but there's not much on it, couldn't find a picture, don't quite know what to look for there.

I'll give the ol' multimeter a shot and see what I find. Check the terminals, look for some grounds.

Thanks all.

 

[Mooseman]

We have this thread and I just updated it with more info and pic:

Regulated Voltage Control

Found this while looking for something else, "To be clear, GM’s RVC systems use alternators with field currents that truly are controlled by logic external to the alternator. You’ll see two things right off the bat on RVC vehicles tipping you to the presence of this latest technology: First...

gmtnation.com

Your 05 might not have one. I think they started in 06 or 07.

 

[6716]

My 05 does not have one. Thanks for the pic!

 

[MRB455]

In my Pro-Street car, I totaled up ALL electrical loads and purchased a 140A alternator. First three (3) times I drove the car, I was towed home. I called the supplier of the alternator to see what RPM was required for the rated 140A, and was advised that 5,000 rpm was needed. Idling in town at a stoplight, I was draining my battery!!! Went with a MECHMAN 320A alternator, providing 180A at idle. This solved ALL electrical issues, as my fans alone draw 70A, then fuel pump, ignition, etc.