How does your Voltmeter read under load?

[shkira]

I have been working through charging issues for a while. I had both alternators rebuilt. A few months back I noticed my volt meter down a bit from where it normally is. I sent it back to the shop that rebuilt it (Gen1) and they said the regulator went bad. They swapped it out and said everything is fine now.

I am not convinced. I believe my volt meter used to sit squarely in the middle of the white tick mark in the middle of the green zone no matter what. Now after start up and stabilization it sits on the left side of the white tick in the green. Still ok, maybe?

when I load up the system; headlights, max vent fan, aux cooling fan, voltage needle moves out of white tick toward yellow zone, no longer touching the white tick mark and half way between the white mid mark in the green and yellow. (1/4 way down in green zone).

This truck does not have much of the typical aged rusting and corrosion as some of these vehicles do. With both alternators rebuilt, extra ground wires, etc I would not expect twin 100A alternators not to be able to keep up.

I have checked the voltage output on both alternators unloaded and loaded.

Gen 1 unloaded 14.3V, Loaded 13.6V
Gen 2 unloaded 28.2V, Loaded 27.1V

I know these are in the acceptable range but certainly doesn't make me feel comfortable that something still is not perfect with these alternators.

Can someone give me your thoughts? How does your voltmeter read when you load up the system? What kind of voltage readings do you get when you load up the system. Does it stay perfectly centered no matter what the load? I don't feel that I am putting any load out of the ordinary on the system. Gee Whiz this supposed to be a command and control vehicle and have tons of comm equipment. I suspect that if I did mount some off road ligths, major comm gear or other loads the system would not be able to keep up the way it is acting now.

I would not expect Gen 1 Voltage to drop that low after I have added load. If anything I would expect it to drop and then come back up to at least the 14.3V.

Your opinions would be very appreciated. Should I pull both alternators off and have them both checked? Wish I could find a better alternator guy but I am already in for two alternator rebuilds with this guy!

Thanks!

 

[cranetruck]

How are your batteries? Check the specific gravity of all cells.

 

[that1028guy]

I have the same kind of issue, Both alts completely gone through, snd the gauge barely sits in the green.

 

[Marschmallow]

I recently replaced my volt gauge with one purchased on e-bay. New gauge has numbers. NEW Stewart-Warner Model 82347 2-1/16" 24V Voltmeter
I see about 24-25v with engine running high idle, glow plugs energized, when glo plugs cycle off volts go to 27. then 27 volts with engine warm at idle, headlights on, heater fan on high. 28 volts while driving with just a set of auxillary lights on (low electrical load).
hope this helps.

 

[dstang97]

if your getting 14v from both alternators with the truck running, you are fine. It could be the gauge. IIRC the gauge is 12v with a resistor to cut the power in half for the gauge reading. I don't really trust any of the gauges in old school chevys.

 

[shkira]

Cranetruck,
Thanks. I checked both battery SG and all cells are 3/4 or higher. 4/6 and 4/6 are 3/4 charged and 2/3 and 2/3 are fully charged.

I would say that the batteries are in good shape. I just would be happier if they were getting higher juice from the Alternator. This CUCV is my daily driver and I never any issues that would make me suspect batt condition.

Batt Voltage 12.6V Batt 1, 12.8V Batt 2


What difference would it make on the voltages I am reading at the alternator output terminals?

 

[shkira]

Marschmallow
Thanks...well that tracks with the numbers I am getting on number two alternator output and at batt 2 terminal.

Any idea what voltage are you getting off the Gen 1 Driver side alternator at the terminal?

I guess the gauge is tipping me off that it is not as strong as I remember it being before I rebuilt these alternators. While you are showing 27 at idle so I am I. Difference is that my gauge doesn't come up higher when revs increase. I would guess my voltage stays down too. I.e. if I were to load up with more demand I suspect the gauge and voltage output would continue to decline.

Something I can't say with certainty but I believe that the batt continues to discharge the longer you run. My trips are usually on average 30 min. The gauge always stays in green but I would expect the longer I run even under load that over time it would recenter to the 28V reading (I am assuming that is the solid white tick in the middle of the green zone?)

I recently replaced my volt gauge with one purchased on e-bay. New gauge has numbers. NEW Stewart-Warner Model 82347 2-1/16" 24V Voltmeter
I see about 24-25v with engine running high idle, glow plugs energized, when glo plugs cycle off volts go to 27. then 27 volts with engine warm at idle, headlights on, heater fan on high. 28 volts while driving with just a set of auxillary lights on (low electrical load).
hope this helps.

 

[shkira]

Dstang97
Less so am I putting faith in the gauge. I just am trying to nail down if the rebuild guys still need to keep working on it.

If I was getting 14V and 28V at Gen 1 and Gen2 regardless of what load I put on it then I would be happy. That's my issue. Why am I getting a drop to 13.6V at Gen 1 and it doesn't come back up?

Based on the readings I am getting I think the gauge is doing its job. Informing operator that there may be an issue. I am just pretty sure that prebuild when you loaded up the system it would stay pegged at the 28V?

Thanks for the gouge on the 12V resistor. Good to know.

if your getting 14v from both alternators with the truck running, you are fine. It could be the gauge. IIRC the gauge is 12v with a resistor to cut the power in half for the gauge reading. I don't really trust any of the gauges in old school chevys.

 

[Michael]

This has been covered before many times, but I will summarize it again.

The gauge on the dash is just a ballpark guide and is little better than the idiot lights.

You need to get a digital voltmeter reading across each alternator. The TM recommends testing across the ground terminal and the output terminal on the back of each alternator. I like to test across each battery post myself. I like to see 14 to 14.7 across each battery.

If you do the math on your readings you get 14.3 and 13.9 unloaded and 13.6 and 13.5 loaded. I would be unhappy with those numbers.

How are you loading the system and is the truck at idle?

 

[shkira]

Michael --
Appreciate your reply. >>>I searched and searched but either no one ever posted output numbers or discussed how the system reacts when placed under electrical load.

This has been covered before many times, but I will summarize it again.

The gauge on the dash is just a ballpark guide and is little better than the idiot lights.
>>>And grateful to have it.

You need to get a digital voltmeter reading across each alternator. The TM recommends testing across the ground terminal and the output terminal on the back of each alternator. I like to test across each battery post myself. I like to see 14 to 14.7 across each battery.

>>>Check! VOM read off back of each alternator, ground to body and double check against negative batt term. Both sets of readings check. I would be happy if I could get 14/28 at each battery under load. That is not the case presently.

If you do the math on your readings you get 14.3 and 13.9 unloaded and 13.6 and 13.5 loaded. I would be unhappy with those numbers.

>>>Roger! Exactly why I don't want to let this rest. The shop has once before told me reg went bad and swapped it out. Suspecting something may have gone again.

How are you loading the system and is the truck at idle?

>>>Hi beam lights, fan on high, aux cooling fan, AM/FM radio (negligible)

Not much more to add. Could take and rig up an off road light too but that would just be jumpered to batt (Haven't done that though). Point is -- not a tremendous load for twin 100A alts!

Yup truck is at idle but the loaded gauge doesn't improve when I am rolling at speed. The gauge reacts as I would expect if system wasn't keeping up.

My gut tells me its not right -- I just want to hear how other peoples Voltmeter acts when they turn on all systems.

If I added load I would expect the Alternator to kick in more juice and therefore reading to stay stable (drop when you first kick on the equipment and then rise back up to normal reading)

Thanks again

 

[Michael]

Michael --
Appreciate your reply.
How are you loading the system and is the truck at idle?

>>>Hi beam lights, fan on high, aux cooling fan, AM/FM radio (negligible)

Thanks again

That is what I usually do but that only loads the front battery, shouldn't effect the back one much.

Most of the auto parts stores can load test these alternators if they allow for the isolated ground if you want to get a second opinion.

There is a good write up in Military Vehicle Magazine this month on the CUCV charging system.

 

[shkira]

Check. I will check out the MV write up. Good timing -- but I guess CUCV charging systems are one of those things where you can count on issues.

 

[Michael]

I have been searching for some of the good old posts on this subject and I can't find them either. I did find one that cautioned about getting alternators tested at the parts store. They said that if its someone who doesn't know what he is doing that he can damage the alternator. I wouldn't think there would be many working there that couldn't handle it, but I have heard that in some places they can't do anything but run a computer.

 

[shkira]

Thanks for taking time to look.

You think they know how to run a computer?! Unfortunately that is the sad truth. Whenever you go to the stores and if they can't use a code reader they can't explain or figure out anything. In this case they have their hand held alt testers but I am certain other then pushing the start button forget about them adjusting setup of the tester.

Furthermore I don;t have much confidence in this alternator shop. They will only accept the alternator off the vehicle. They rebuilt it, said it was good and then later ohhh the regulator must have gone out...they won't provide any benchtest numbers just tell me it tests fine...

Are there any good old school businesses out there anymore that know their trade?!
Do you have to do everything yourself?!

 

[OL AG '89]

Sounds like a job for the trusty STE/ICE. anybody near shkira? with one.... I'm in Texas, but your welcome to it. Would be a BIG test on the charging system......

 

[shkira]

Voltmeter should stay pegged in the middle of the green

Michael
Good article in MV mag. From what I can tell the reason why I was having trouble pinning this down is that the number 1 alternator was putting out not the right amount. From the description offered in the magazine I think what was happening was that the alternator was barely keeping up. As power depleted out of my number 1 battery the number 2 would back feed and top up the charge.

Then the number 2 alternator would top up the number two battery.

Neat set up.

So with the civilian alternator installed the voltmeter stays pegged in the middle of green zone. Now I go back to the rebuild shop and make sure they fix up my number 1 alternator so it puts out right. If the alternator was totally failed then it would be much easier to figure out.

Thanks again!

 

[SouthernSteel]

Well i see this is an old post but im gonna post My meter in the 1008 drops a little then goes right back to the white line underload also im curious what kind of aftermarket gauges are avaible for a replacement for the volt meter in The M1008

 

[shkira]

Finally Fixed

An old post but I finally got it licked! I had pretty much accepted it as it was and was concentrating on other issues. Ultimately, I never ran the batteries down and I began to believe it might be my imagination despite the numbers not being to my liking. Maybe it was a bad gauge?
I am in Alabama and the heat is intolerable so I was focusing on getting my A/C working cold. Lots of full load operation to measure vent temps and I noticed that it seemed that it the gauge was lower then the new norm.

I started measuring again and now I knew somethings was definitely wrong. I swapped in a new battery. Same thing. I was only getting just barely above batt voltage from my number 1 alt. Number two seemed fine at the output terminal but it was hard to judge without both operating perfectly.

Looks like again I need to rebuild the alternator. Since I would have to find someone to do it I ran to autozone and picked up a 27SI to temp toss in.

I was getting ready to pull the old one off and as I started to loosen wires I found it. Despite previously going thru and fixing every bad connection that had been hacked into the truck I missed one. The PO had a very nice hose running over the ground wire . This is the one wire I never checked because it was the original connection at the back of the Alterantor and the connection to the engine on the other end. As I started loosening the wire from the alt it crumbled in my hand. The insulation was cooked and flaking off. Ok no problem. I grabbed a new connection, cleaned up the end and resoldered it. But something didn't seem right. How come the insulation cooked off in the first place? As I pulled off the concealing hose that prettied up the presentation I found why there was a higher resistance in the wire. He had cut off a couple inches of a ground wire (number on both ends and connected them with prob a 12 gauge! Examining the other connection I found that it was also starting to bake. I took the alt back and bought a brand new #4 wire and voila! The truck, guage reading and all the numbers are in the 14 - 14.5 range!

The last couple years I battled with this issue, while checked and fixing every other bad connection, and joint but missed the ever important one. It all makes perfect sense now. No matter how many grounds I cleaned up on the truck or repaired the positive side on a floating ground system it is crucial to have a solid dedicated ground wire. This is not a single wire alt you will find on any standard vehicle which will get its ground from the chassis. This is why in my battles when I would have a civ 27si on temporarily I would get the correct guage and numbers outputs. So word to the wise when battling electrical gremlins don;t miss the obvious no matter how pretty the wire cover may look! These are old trucks and no telling what a PO may have butchered in there!

Although I had stopped looking to fix this issue with the high system loads the ground wire couldn't handle all the output the alt was trying to put out. It overheated the ground wire and found the problem for me. Now I feel bad as how many times I had taken the alts back to get checked the guys always told me that they bench tested perfect. As soon as it went on the truck I couldn't get the 14 - 14.5 I expected.

Hope no one else needs to deal with finding something this simple but frustrating!

 

[mistaken1]

Electrons travel from negative to positive.

Thanks for the after action report. It will help many people.

 

[M1008driver]

I was going to say check all the connections. Mine was as simple as replacing the connector on the battery. I will be checking my alt connections too now. Thanks.