voltage gage goes to 8 volt mark

[brucejohnson]

First off, 1. set ohm meter to 200ohms and checked pins on alt.=no reading.
2. no voltage read on Dark blue wire on connector with Ig. on.
3. PCM is buried behind Fuel filter, can't determine how connector comes off without removing fuel filter.(may wait till tomorrow to do that)
4. since fuse replacement volt gage has not went below the ~12.5v mark on the gage(Batt. Level) I.E.- Fuse R&R made gage show actual voltage.

Just to understand:
#1. "No reading" means what? Shows blank, open circuit (usually displayed as OL), or 0 ohms?
#2. Voltage from DB wire to ground with ignition on shows 0 volts? Same with DG wire?
#3. I see BigPapa and John assisted there.

#4. Did the Check Gauges light come on in the dash on the initial low voltage report? That should occur IF the detected voltage went under 10.8 vdc.
#5. How about wiggling the ignition switch with engine running and low voltage
#6. What is the indication of No Charge after fueling up and restarting? Did a light come on saying check gauges, or just not displaying a high voltage after startup? Did the voltage change any after leaving the fuel stop?
THEN:
Have you checked for any DTC codes (Diagnostic Trouble Codes) in the dash readout?

 

[petersonj]

All excellent questions that need detailed answers.

- John

 

[BigPapa]

Minor error here. The ECM is on the driver side of the engine block.

- John

Yep, you’re right. DRIVER side.

I’m SO confused……

 

[petersonj]

Fortunately for me, my memory is perfect. I am just having more difficulty getting access to it.

- John

 

[RBeard]

1. meter shows 0L
2. Yes
4. Yes
5. No Change with wiggling all directions.
6. Showed Batt. voltage at and after start up, no change during 5 mile drive home. DTC= PCU-1682. ECU-1693

P.S.- Dark Blue wire shows continuity from alt. plug to just below PCU connector, didn't have alligator clip and straight pin to stick in small holes to check female PCU connector.

 

[brucejohnson]

1. meter shows 0L
2. Yes
4. Yes
5. No Change with wiggling all directions.
6. Showed Batt. voltage at and after start up, no change during 5 mile drive home. DTC= PCU-1682. ECU-1693

P.S.- Dark Blue wire shows continuity from alt. plug to just below PCU connector, didn't have alligator clip and straight pin to stick in small holes to check female PCU connector.

1. Likely an open field coil in the generator. Intermittent, or finally toasted? Temperature affecting intermittent condition? Try to start it up later after it cools to see what happens.
2 & 4. PCM = no field output? PCM faulty? If PCM missing main power input other circuits would also be affected.
5. Exercise and break to clear mind... No, really done to see if an intermittent worn connection in switch. That would also have involved numerous other circuits, which was not evident in observable problems, so to bopttom of list for now.
6. So, no dash light illumination - Check Gauges or MIL? EDIT: Upon a reading review I saw the word "ding" in the first post. I would assume that caught your attention, therefore a notification was possibly given. If steady battery voltage present won't trip Low Volts comparator for Info or Warning until a drop below 10.8 vdc. The open field should trip something I would have thought.

Without getting caught up in moving parts, can you check the voltage from the back side of the alternator field plug to a ground while it is running?

Suspects so far:
Alternator field coil open.
No field coil equals no alternator output.
No output equals no variation in voltage to be sensed by the PCM Voltage Regulator equals PCM VR wanting to increase charge rate equals (when battery voltage decreases to PCM preset low voltage to begin charging) no field coil to feed the Y-winding of the alternator equals no output equals...back to beginning, or endless loop.

Anyone have a suggestion to determine the best next action without having to access a DRB tool?
Remove Alternator and have it rechecked? How many miles on the alternator?
Parts toss - Alternator is less expensive than a PCM.
Load the batteries by turning on the headlights and heater fan to bring the B+ (battery voltage) down to the level where the voltage regulator should kick in and check to see if the PCM field voltage kicks in.

Next suggested step please...


Codes listed in the FSM:

(G)* - Generator Light Illuminated

• P1682 - Charging System Voltage Too Low - Charging System Output Too Low
• P1682(G)* - Charging System Voltage Too Low - Battery Voltage Sense Input Below Target Charging Voltage During Engine Operation and No Significant Change In Voltage Detected During Active Test of Generator Output Circuit.

(M)* - Malfunction Indicator Lamp (MIL) illuminated during engine operation if this DTC was recorded (depending if required by CARB and/or EPA). MIL is displayed as an engine icon on instrument panel.

• P1693(M)* - DTC Detected In Companion Module - A fault hs been generated in the companion Engine Control Module.
• P1693(M)* - DTC Detected In PCM/ECM or DTC Detected In ECM - A "Companion DTC" was set in both the ECM and PCM.

 

[RBeard]

I will test alt. with engine running and then have my 325k mile alt. tested again tomorrow, maybe drop $125 on a new one.
I can't thank you enough for sticking with me for this long, and doing all the research after our back and forth Q&A.

 

[petersonj]

#2. Voltage from DB wire to ground with ignition on shows 0 volts?

@RBeard , you answered "yes" to this question. I am assuming that you had your meter set to DC volts with one probe on the dark blue wire and the other probe to ground (with ignition key switched to "Run". If this is true, then the alternator is not at fault. If there is no field current, then there is no alternator output. This can be confirmed by performing a full field current test on the alternator. It is a simple test and will prove whether or not the alternator is producing current.

Disconnect both field wires from the alternator. Leave the output wire connected to the terminal. Rig a jumper wire from the field terminal on the alternator (where the DG wire was connected) to the passenger side battery ground. Make sure these connections are reliable.

Make another jumper wire that will include a 10 amp fuse and a clamp that will fasten one end of the jumper to the positive passenger side battery terminal. Set up your multi-meter to read battery voltage and place the meter near where you are working. Turn on the display to show battery voltage. Start the engine. Touch and hold the other end of the fused jumper to the remaining field terminal on the alternator. There may be a small spark and the alternator should begin to charge. You should see battery voltage rising steadily. Do not allow voltage to climb above 15 volts. If this happens, your problem is elsewhere.

Also, make sure that your grid heaters remain disconnected.

- John

 

[brucejohnson]

I will test alt. with engine running and then have my 325k mile alt. tested again tomorrow, maybe drop $125 on a new one.
I can't thank you enough for sticking with me for this long, and doing all the research after our back and forth Q&A.

John chimed in with a great step to test. That would verify the functionality or lack thereof on the coils in question.

 

[brucejohnson]

@RBeard , you answered "yes" to this question. I am assuming that you had your meter set to DC volts with one probe on the dark blue wire and the other probe to ground (with ignition key switched to "Run". If this is true, then the alternator is not at fault. If there is no field current, then there is no alternator output. This can be confirmed by performing a full field current test on the alternator. It is a simple test and will prove whether or not the alternator is producing current.

Disconnect both field wires from the alternator. Leave the output wire connected to the terminal. Rig a jumper wire from the field terminal on the alternator (where the DG wire was connected) to the passenger side battery ground. Make sure these connections are reliable.

Make another jumper wire that will include a 10 amp fuse and a clamp that will fasten one end of the jumper to the positive passenger side battery terminal. Set up your multi-meter to read battery voltage and place the meter near where you are working. Turn on the display to show battery voltage. Start the engine. Touch and hold the other end of the fused jumper to the remaining field terminal on the alternator. There may be a small spark and the alternator should begin to charge. You should see battery voltage rising steadily. Do not allow voltage to climb above 15 volts. If this happens, your problem is elsewhere.

Also, make sure that your grid heaters remain disconnected.

- John

John,
RBeard said he had an open indication (OL on multimeter ohms check) when measuring the field coil. Your suggested check will definitely add a confirmation to that condition if it is indeed open. Thanks.

 

[petersonj]

RBeard said he had an open indication (OL on multimeter ohms check) when measuring the field coil.

I noted that as well, but since I don't know what the reading should be or where he had his meter scale set to, I dismissed that information. Also, an open field circuit wouldn't cause the lack of field voltage from the PCM via the DB wire. To me it is not likely to have a faulty alternator and a lack of field supply current at the same time. Something is amiss here.

If he confirms that test #2 was performed in the manner mentioned in my last post and states that there was no voltage present, then we will know for sure there is no power to the field terminal from the DB wire. No power - no alternator output. I am suggesting the full field test just to confirm the alternator is at least operating at some capacity.

- John

 

[brucejohnson]

I noted that as well, but since I don't know what the reading should be or where he had his meter scale set to, I dismissed that information. Also, an open field circuit wouldn't cause the lack of field voltage from the PCM via the DB wire. To me it is not likely to have a faulty alternator and a lack of field supply current at the same time. Something is amiss here.

If he confirms that test #2 was performed in the manner mentioned in my last post and states that there was no voltage present, then we will know for sure there is no power to the field terminal from the DB wire. No power - no alternator output. I am suggesting the full field test just to confirm the alternator is at least operating at some capacity.

- John

Great method to verify the "magic smoke" the engineers packed into the alternator wires is still in the wires.

 

[brucejohnson]

First off, 1. set ohm meter to 200ohms and checked pins on alt.=no reading.

I saw this when I read it and forgot to make a comment.
Which meter are you using.
Does it have a larger scale to set the ohms range to, or an Auto range mode? Any low setting, like the 200 ohm scale used will be OK for dead shorts, but not ok for large resistances. For an ohmmeter, especially a non-auto-ranging meter, you should always start at the highest scale and work (switch) down to get an accurate reading, paying attention to the decimal point and range indicators like K for kilo and M for mega.
Also, do you touch the meter leads together to insure a "close to 0 ohms" reading is displayed prior to taking a circuit reading?
Try taking the field resistance again by setting the range to the highest level, switching scale down to get an accurate reading, and see if you still get an OL or some numeric resistance reading.
You should have been good at the setting you used as most automotive alternator field windings have 5 to 8 ohms across the field terminals.

 

[RBeard]

OK Guys, Either I have a Gremlin or I probably don't always understand your diagnostic commands. I reread all the posts to be sure I followed your sequence of tests. I went out with 3 things I wanted to recheck. #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. I shut down for 5 min. and restarted for 2, shutdown for 5 and restarted for 2, shutdown for 5, restarted for 5 and then shutdown. Alt. readings stayed at ~14.1 each cycle.

 

[brucejohnson]

OK Guys, Either I have a Gremlin or I probably don't always understand your diagnostic commands. I reread all the posts to be sure I followed your sequence of tests. I went out with 3 things I wanted to recheck. #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. I shut down for 5 min. and restarted for 2, shutdown for 5 and restarted for 2, shutdown for 5, restarted for 5 and then shutdown. Alt. readings stayed at ~14.1 each cycle.

Magic?
There were a couple challenges presented that did not quite fit into the puzzle and meet the KISS principle. "We" overcame and collectively resolved the issue at hand. Lessons, refreshers, learning, doing, persistence, success.
Sounds like the gremlins have departed, or have prepared a surprise and hidden somewhere where they can snicker away...
Let's see what happens when the system gets put to use in the real world (thermal cycling and real-time usage).
Although right now you have saved the costs of an alternator and/or PCM or other unanticipated expenses and achieved another learning experience.
Return smile to face and enjoy the driving fun! (Fingers crossed)

 

[Ozymandias]

No AC voltage anymore? Magic.

 

[RBeard]

Ozy, In my unknowing defense, I tested the alt. post with the engine running in DC first. A .01-.02 reading showed and I was expecting higher. Without consulting my brain, my fingers rotated the meter knob to AC and my hand put the + meter probe on the alt. post. 79V AC is what my meter read. I was in full panic mode because I was trying to finish a job by Jan. 1st and I needed my truck, because it was loaded with all my tools to do so. That's my story and I'm sticking to it.

 

[RBeard]

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

 

[petersonj]

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

 

[Ozymandias]

Ozy, In my unknowing defense, I tested the alt. post with the engine running in DC first. A .01-.02 reading showed and I was expecting higher. Without consulting my brain, my fingers rotated the meter knob to AC and my hand put the + meter probe on the alt. post. 79V AC is what my meter read. I was in full panic mode because I was trying to finish a job by Jan. 1st and I needed my truck, because it was loaded with all my tools to do so. That's my story and I'm sticking to it.

No need to panic because of this.
Almost every Alternator puts out some underlying AC stray voltage, but that is usually in 0.0xV range.
What you measured sounds more like one or more burnt diodes in your Alternator.

I'd like you to redoo that AC measure test to rule that possibly out.

Just to write that out, you can measure a DC voltage just fine but with an underlying AC voltage - at the very same time.