voltage gage goes to 8 volt mark

[brucejohnson]

I just finished rereading this thread from beginning to end. This refresher helps put things into perspective. Post #1, #27, and #54 are have some common elements:

Post #1 - charging system quit and then started working again on its own after restarting engine, then quit again in 5 miles.

Post #27 - After blown 140 amp fuse was replaced, charging system worked, but quit after five miles later after an engine restart (at the fueling station).

Post #54 - No voltage present with at dark blue field wire with key on. But, when plugged back in with key on 12.55 volts appear.

I think that the blown 140 amp fuse may have happened while removing and re-installing the alternator (between Post #1 and Post #27) and is not related to the problem in any way. It would have been very easy for a momentary short to ground to have occurred if both batteries were not disconnected during all of the work process. Just speculating here.

It appears that he field supply current to the alternator is random. I fully expect the problem to show again very soon. Nothing has changed.

I listed the relevant posts below.

Post 1 - "So, I'm driving along and I hear a chime, look at the dash and there's a check gages light on. Look over and voltage gage in pegged at 8 volts, pulled over and turned off engine, popped the hood and belt is still on.
restarted and after 10 seconds, gage back to 14 volts, drive 5 more miles and ding, gage at 8 again. I'm 2 miles from home, so I hooked'em to the garage, put a volt meter on battery with engine running=12.55 volts. Gage still at 8. Pulled alt. off and parts store tested it=Passed the test."

Post 27 - Okay "o learned ones". I did the test light test=Passed. R&R fuse, Battery voltage at alt. post=yes, Battery voltage pre start=12.68v, started engine and had 14.18v at batteries, 14.18v at either side of fuse. Let run for 20 min. Grid heater ops @ 51degrees=yes for 1-3 sec. every 10-15 sec. for a while, then stopped. engine temp=145 degrees. Drove 5 miles to fuel up, all good. Fueled up and restarted, no charge, Fuse good, batteries @ 12.50, each side of fuse=12.50.

Post 54 - #1-check Dark Blue wire at alt. field plug with Ig. on. Result was 0-volts from batt. ground to meter probe stuck in DB wire female plug. Dark Green wire showed 0v, but continuity to ground. (BUTTTTT, when I plugged the field connector back into the alt. and skinned the DB wire a little, it tested 12.5 v with Ig. on.) So I started the truck, Batt. at start=12.60v, after start 12.32. About 5 seconds later, the volt meter I had connected to the Batt.(I'm looking at it through the windshield) started climbing to 14.10v and stayed there for 2 minutes.

- John

Logical analysis well stated...
Intermittency, this is where the aforementioned Gremlins may be hiding a surprise...
Time will tell...
Fingers crossed...
The next bridge crossed, if/when presented...

 

[petersonj]

As we anxiously await for @RBeard to report back...,

- John

 

[brucejohnson]

And also a big, BIG shout out to Bruce and John for having the perseverance and knowledge to help me work through this miniature ordeal.
Thanks Again,
Ronny

Keep us posted if any changes occur.

 

[RBeard]

Had the Grandkids all day in Houston today, set to crank up at 0700 tomorrow morning.

 

[RBeard]

Sorry so late to post. Had to push the truck the last mile home. Just joking, So after 7-10 start/stop cycles today all is well(so far). The only thing to report is that since I'm watching the V gage intently after each start, it seems like it takes a long time(5-10 sec) for the gage to start moving from Batt. voltage point to alt. kicking in point and gage rising to the 14v+ mark after starting engine. (Note- grid heaters still disabled) Probably never really noticed before. I'll report back if anything happens in this vein. Thanks again, Ronny B. signing off

 

[petersonj]

Thanks for the report. It sounds like you haven't driven the truck yet. My concern is that after about 5 miles of driving (not pushing), the alternator will quit charging again - as it has done twice in the past. I would leave the grid heaters disabled until for certain the charging issue is resolved.

- John

 

[RBeard]

Sorry, To be clear, I drove the truck to multiple jobsites around the county today ~50 miles total.

 

[brucejohnson]

Sorry so late to post. Had to push the truck the last mile home. Just joking, So after 7-10 start/stop cycles today all is well(so far). The only thing to report is that since I'm watching the V gage intently after each start, it seems like it takes a long time(5-10 sec) for the gage to start moving from Batt. voltage point to alt. kicking in point and gage rising to the 14v+ mark after starting engine. (Note- grid heaters still disabled) Probably never really noticed before. I'll report back if anything happens in this vein. Thanks again, Ronny B. signing off

Ah, more questions for a better understanding.
Were these - - - -
ah-ah. you answered as I was responding...
So, after several starts-n-stops the charging "appears" to be working OK, with no intermittent warning dings or lights illuminating.
The short delay in voltmeter response seems to be normal and similar to what I experienced with my 2002.
How old are your batteries? (Oh, that's a SQUIRREL type question)
A couple dozen more starts-n-stops...The more the better before declaring a semblance of success.

 

[RBeard]

Correct, no warnings.
Batts. are 8/2019 install. I haven't had them load tested recently, I usually do it yearly around this time. (checking to see if they'll make it through winter in Texas)

 

[petersonj]

Sorry, To be clear, I drove the truck to multiple jobsites around the county today ~50 miles total.

Now that's good news!

- John

 

[petersonj]

@RBeard , should the charging system decide that it is going to work long term, there is one thing you can do for peace of mind.

Start the engine and turn on many electrical accessories - blower fan on high, headlights on high beam, emergency flashers, etc. Let the engine idle. Battery voltage should stay right at 14 volts. Although passing this test doesn't prove the alternator is in perfect condition, it does clearly show that the alternator is easily handling the electrical load at its lowest rpm.

- John

 

[RBeard]

Good Idea, I'll load it up tomorrow. If it holds up for a few days, I'll return the grid heater to service and see what happens.

 

[Ozymandias]

You might need to know that the dash gauge is not actually what the alternator puts out, it is what the vehicle wants you to see. Only a Multimeter shows the reality. The gauge is most of the time pretty accurate - but not always.

What's about the AC measuring I asked you on the previous page?

 

[RBeard]

Sorry Ozy, I forgot to respond back to you on that. I'll measure DC and AC output today and post it.

 

[RBeard]

OK Ozy, at idle, voltage at alt. post= DC=14.15, AC=29.78.
Then at idle, with every electric device that I can find to turn on Batt. voltage=14.12

 

[brucejohnson]

OK Ozy, at idle, voltage at alt. post= DC=14.15, AC=29.78.
Then at idle, with every electric device that I can find to turn on Batt. voltage=14.12

Looks like good DC regulation is occurring.
PWM (Pulse Width Modulation) voltage regulator is my guess for the AC indication.
Now I am curious. I'll have to go out to my truck and check it out...

Bing AI search results:

Pulse Width Modulation (PWM) is a technique used to control the amount of power delivered to a load by adjusting the width of the pulses of a square wave signal. PWM voltage regulators are used to regulate DC voltage output. They work by switching voltage to the load with the appropriate duty cycle, which approximates the desired voltage level 1.

PWM voltage regulators are not designed to produce AC voltage. However, if the output of a PWM voltage regulator is connected to an AC voltmeter, the voltmeter will read the average voltage of the PWM signal 1.

Please note that this is an approximation and not an accurate measurement of the AC voltage. The actual AC voltage produced by a PWM voltage regulator is dependent on the frequency of the PWM signal and the characteristics of the load 2.
​

 

[petersonj]

The DC voltage is right where it should be. The AC voltage - not so good.

Just double checking your work here. When checking for AC voltage, one lead to alternator output terminal - the other lead on a battery ground. Engine idling with a light electrical load. A good AC volt reading would be less that .5 volts AC (500 milivolts). If possible, you could try the test with a different meter.

Now the following is just my personal opinion. I have never put too much stock in this test. Too many variables - the accuracy of the test meter, actual electrical load at the moment, AC interference from an electrical motor (lift pump for example), poor electrical ground connections, state of battery charge, etc.

I am currently using the original OEM alternator on my truck with 384,000 miles. The alternator has had two brush replacements and one bearing set replacement. I have done the AC test 4 or 5 times - sometimes it passes the test, sometimes not. I am also still using the original PCM and ECM.

I have never used this test for any other vehicle I have owned. What makes this era of truck so special? Just my two cents here.

I do pay attention to ensuring clean, tight wiring connections and healthy batteries.

- John

 

[Ozymandias]

I'm very sure that you have broken diode(s) in the rectifier of your Alternator with that 29V underlying AC. Like said, should be well below 1V.
Double check it with a different multimeter to rule out false reading.

 

[brucejohnson]

29 VAC way too much. Definitely not the normal few millivolts ripple.
What scale was the meter on? Is your voltmeter auto-ranging? Did it read millivolts, or full volts?
(Do you have an o'scope?) I know, silly question... If you were local, I'd bring mine over.
Although my truck is newer, the readings should be about the same.
I tested:
39 degrees ambient temp.
Volts batteries at rest 12.51
Volts with grid heater active 11.5 vdc
Volts running after stabilized 14.2 vdc
AC volts variable between 12.8 to 15.8 millivolts AC
Frequency variable from 28hz to 178hz
I did not capture the high and low on the duty cycle figures but recall between 20 and 50 percent
Does your voltmeter have a diode test function?

 

[brucejohnson]

I will concur with Ozy on the diode issue. The voltage regulator is doing its best in maintaining the designed voltage.
The addition of the AC signal could very well be due to a faulty diode. It is not noticed directly in the DC output since the regulator is able to adjust the output voltage within the parameters of the circuit design. The erroneous diode will affect adjacent Wye-windings' outputs with current flow changes being adjusted by the regulator and show as a diminished amplitude signal for the specific coil/diode operation, therefore a bit of extra metered AC. The DC is what is being adjusted by the VR, so the AC is not noticed if the DC output remains in circuit designed specs. VR has to work a little harder.
Does your voltmeter have a duty cycle function to see the percentage of the off and on condition of the regulator circuit?