Power drop

[turboman93]

Hey guys,

First, I want to thank everyone for the forums as I've been able to search and find all the little crap that's wrong with my 'Burb and fix them.

I've done a search on the forums and I'm having issues with the power on the truck dropping. In the mornings, I'll show the Ammeter charging at 14V but as the day goes on the gauge will drop to between 11V and 12V. Based on what I read on the forum, I've tested the alternator with a meter. It's showing 13.6 volts with the headlights on, both AC units on and at full fan speed, radio on and right turn signal going. I figured that's the most load I would ever put on the system. I've also checked the battery at this same point and it's showing 13.6V as well. I just put a new battery in the truck on Sunday and it's a full 800 CCA battery, so that's working as should.

Can someone help explain why the meter in the dash is so far off? Also, I start to lose power to the 12V outlet in the ashtray when it starts to show closer to 10V. Is this a ground issue in the wiring?

Thanks for looking and the help.

-Shawn

 

[RawbDidIt]

Ok, how are you testing alternator output? You should be reading output voltage for the alternator by placing the positive lead on the bolt with the charging cable attached, the negative lead should be placed on the body of the alternator. That should be regulated somewhere between 14V and 14.8V (14-14.5V is much more common for OEM or OEM replacements, my alternator is regulated to 14.8V but it isn't common to be this high). This reading should be slightly higher than the voltage across the battery terminals. If you're reading 13.6V with the truck running, it's likely the alternator is not charging enough to fully charge the battery. If the alternator output is exactly the same as your voltage across the battery terminals, your alternator isn't charging the battery at all. Are you noticing large voltage drops after making frequent stops and starts (not stop and go traffic, but like running errands, and turning the engine off for frequent stops)? Is your battery dying, and you need to charge it every couple/few days? Is your battery light on on the dashboard? Just because the alternator isn't charging the battery does not mean it needs to be replaced. If your alternator and your battery read the same voltage while running even when the voltage is low (around 12V) and your battery light isn't on on the dash, pull the dash and check to see if the battery light is working. you don't have to pull the light to check this, if you have a test light you should be able to touch the leads to each of the copper contacts with the lightbulb in. If it doesn't light up, pull the bulb. test the bulb the same way, if the contacts are placed on each of the bulb leads and the light doesn't turn on, the bulb needs to be replaced. The dash light will keep the alternator from charging the battery, so I would check this first as it's the cheapest solution. You can also pull the alternator and test it at a parts store.

 

[east302]

Can someone help explain why the meter in the dash is so far off? Also, I start to lose power to the 12V outlet in the ashtray when it starts to show closer to 10V. Is this a ground issue in the wiring?

Thanks for looking and the help.

-Shawn

As for the dash gauge, it’s not too far from being for decorative use only. The tick marks are not linear...or, if they are, they’re not numbered to allow for a quick read.

For example...see the tick mark halfway between 9 and 14? Without thinking, what is it’s value?

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Doing the math, it’s 11.5V, which is dumb as hell and probably a typo on GM’s part that no one ever noticed. The temperature gauge is just as pointless and not really meant to be an exact indication.

But to answer your question, GM’s manual says that if the dash gauge isn’t an “approximate” reading of what the battery voltage reads, then it’s an issue with the cluster. I’d guess it would be the stepper motor, even the newest ones are now twenty years old.

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[turboman93]

Thanks for the replies, guys. I was measuring the voltage from the alternator with the positive lead on the charging screws and the negative lead on the body of the truck. I've just done the measurement from the charging screw to the case of the alternator and it's 13.9V and I measured the battery and it was 13.7V. The truck was running with only the AC turned on with both fans at low speed and no headlights or markers on. The battery light on the dash comes on when the key is in the ON position but engine not started so the light bulb is good.

I have a 94 'Burb C1500 with the 5.7L and I've attched a pic of the gauge cluster with the engine running and headlights on.

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[RawbDidIt]

I'm actually jealous of your green lit gauges, I'll have to get some LEDs for mine. That yellow orange color is terrible.

As for your problem what is the low end voltage reading, and does your multimeter confirm that reading, or is the dash not reading reliably? I know that there are some alternators that won't turn on until voltage reads below a certain point, or until there's a load against the system (someone with more experience with our trucks feel free to correct me if this piece doesn't apply). Is this a new problem, or did you inherit the truck with this issue? Either way, I'd pull the alternator and take it to the parts store to have it tested. It only takes 10 minutes to pull it out, another 10 to install it. It's worth it just for the piece of mind, and gives you the opportunity to check a couple of other causes as well. Check the belt and the belt tensioner while you're there, either can cause unreliable alternator operation, especially after warming up and the belt expands a tad. Tensioner should move smoothly and shouldn't be resting against the stop; if it is, replace the belt. If it doesn't move smoothly, replace the tensioner. If you have any doubts, replace both. they're cheap, and that kind of stuff will leave you stranded.

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[turboman93]

This was my Dad's truck and he bought it in Jan 95. He gave me the truck last Sept and it's been like this since I got it. The truck has had weird electrical issues that I've been trying to hunt down. I replaced the alternator with a unit from O'Reilly's last year and replaced the belt and tensioner at that time. I just replaced the battery on Sunday as the issues were getting worse. The truck will always start in the morning and I've never had to jump start the truck to get going in the morning and I drive it almost every day.

Thanks for the help!

 

[RawbDidIt]

Is the voltage dropping over consistent short trips, then recovering after running the truck? Could be a sign of a starter going out, or increased current needed to start the engine. With the engine off, key out how does it feel when you rotate the crank manually? Should be reasonably easy to turn with a standard ratchet, but it the resistance should be rhythmic as you build pressure in one cylinder and release pressure in another. If that checks out I'd listen to the starter as you turn it over, should be able to diagnose if it's weak pretty easily, it won't give consistent rotations like you'd expect, it'll sound labored. If all of that checks out, I'd start a parasitic draw test.

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[turboman93]

Truck is never hard to start and the charge doesn't recover after start. It will slowly drop as the truck runs for longer periods of time. It would seemed like a parasitic draw but it seems to drop to 12V and stays there. Adding a full load on it will see hard drops on the gauge like when the turn signal flashes, you'll see the gauge dip.

 

[RawbDidIt]

There may be a spot in your alternator harness for an external voltage read, you could try going that to the input for your distribution block. Others on this site have had similar problems with unreliable voltage clearing up by doing this. I'm planning on doing this myself when I redo my writing in the next couple of months. I'll post a link to the write up in a few hours when I get off work. Do yupi have the newer style alternator with the oval pigtail, or do you have the rectangle plug for the back of the alternator. If you're not sure, post a picture of the harness when you get a moment.

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[RawbDidIt]

The "s" slot on the harness goes to an external sensing wire. You can connect that to either your positive wire or your distribution block, but it will likely not fix the problem, though it is an upgrade. The more I think about it the more I think it's the alternator. The only other thing I can think of is bad alternator connections. Could also be a bad ground to your engine block, have you checked that?

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