voltage gage goes to 8 volt mark

[brucejohnson]

I assume that you have reinstalled your alternator and associated wire connections. If you have, do not install the fuse. Instead, using a test lamp, clamp one end to the positive post of a battery and touch other end to the alternator output terminal. The test lamp should NOT glow. If it does, then there is likely a diode shorted to ground in the alternator, or the alternator output wire is shorted to ground (not as likely).

If the test lamp doesn't glow, all is well. Install the new 140 amp fuse. That fuse is not there to protect the alternator. It is there in case something shorts to ground between the battery and the alternator. If the fuse wasn't there and a short to ground occurred, an engine fire could easily be started as you have two large batteries there to provide the necessary high current flow for quite awhile.

If there doesn't appear to be a direct cause for the fuse to blow, I don't think I would be too concerned. It could be that the fuse is tired because it is over 20 years old. It could that the alternator output was accidentally shorted to ground during an oil filter change..., it would take much a direct short to ground to blow the fuse.

- John

John, you beat me to it...
I was going to suggest removing the power output wire from the alternator or use the fuse socket that leads to the alternator, and take an ohmmeter (Diode check function) to check the diodes by putting the positive meter lead on the output terminal and negative on the case to see what reading shows. Then reverse the leads (positive on case and negative on output terminal}. The meter "should read an open, or very high impedance in one direction, and a few ohms in the other lead position as the diodes get forward biased. If there are shorts it will show up there, too.
Then there's the windings and brushes. Should show up something. Might account for the 79VAC reading earlier. (?? But it did check OK by a parts store)
As mentioned, maybe it was old and ready to give up after years of hot and cold cycles helping the batteries in feeding the grid heaters and other stuff.
Years ago, I was driving on I-5 in an 83 Monte Carlo when the car just up and quit. Pulled over to the side of the freeway and could not find anything wrong. Had to have it towed to a nearby dealer. Took them a day, but they found a fusible link blown. No evidence of any shorts, overheated wiring, etc. They replaced it and the car ran great for many more years. My luck with Chevies.

 

[petersonj]

Might account for the 79VAC reading earlier. (?? But it did check OK by a parts store)

I don't put a lot weight for drawing conclusions when someone reports that kind of a reading. Too many variables - could have misread the meter, the meter could have been on the floating scale and the reading jumping along with the scale jumping and it wasn't noticed. Not even sure why he was checking for AC voltage. With the fuse being blown, I would expect the AC voltage to be wild, and maybe the DC voltage as well, so no value in the reading. Also, I am not sure why he didn't check the DC voltage at the alternator output when he checked the battery voltage with the engine off. That would have been more relevant. He would have had his answer then - there would have been no voltage present at the alternator output terminal.

Chances are that his alternator is still good. I have known several people who had that very fuse blow and in every case it did no apparent harm to the alternator.

There is value in using multi-meters for electrical troubleshooting, but there is just as much value in using a simple test lamp for troubleshooting as well. Just have to know when and which tool to use to give you the best and most reliable results.

- John

 

[Humblecowboy]

some automatic transmission functions

Ah, yes, the infamous "Torque Converter Lockout" syndrome where you first think your auto trans is going south when it's actually the ground wires to the PCM that are corroded and causing stray voltages to mess with the PCM's little mind. It's very sensitive to voltage fluctuations.

 

[brucejohnson]

I don't put a lot weight for drawing conclusions when someone reports that kind of a reading. Too many variables - could have misread the meter, the meter could have been on the floating scale and the reading jumping along with the scale jumping and it wasn't noticed. Not even sure why he was checking for AC voltage. With the fuse being blown, I would expect the AC voltage to be wild, and maybe the DC voltage as well, so no value in the reading. Also, I am not sure why he didn't check the DC voltage at the alternator output when he checked the battery voltage with the engine off. That would have been more relevant. He would have had his answer then - there would have been no voltage present at the alternator output terminal.

Chances are that his alternator is still good. I have known several people who had that very fuse blow and in every case it did no apparent harm to the alternator.

There is value in using multi-meters for electrical troubleshooting, but there is just as much value in using a simple test lamp for troubleshooting as well. Just have to know when and which tool to use to give you the best and most reliable results.

- John

Totally agree on all points.
Speaking of points, my test light has been a constant imp for the past 55 years, always hiding when I need it most, and poking me with its ice pick point when I dig in the toolbox trying to locate it or another tool in that box. It has been a faithful tool, IF and when found. My Fluke meter is very obedient and stays in its place and is less painful when found. Just have to feed it a 9-volt battery every 6 to 8 years, and one set of display conductor-strips a few years back.

 

[RBeard]

Points well taken John and Bruce. I guess I was kinda overloaded with where to start diagnosis and just trying to test everywhere without really knowing where and how to try to find the weak point. I'll R&R the fuse today and report back.

 

[brucejohnson]

This is how we all gain experience...and refresher courses...through sharing. . A great bunch on the forum helping one another.

 

[RBeard]

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. I CONFESS, I didn't do Batt. temp sensor check yet. What and how should I check and read on meter when checking temp. sensor leads or should I look elsewhere.

 

[petersonj]

The battery temperature sensor is not your problem. Don't worry about that for now.

I would recheck your work. Is voltage present at the alternator output terminal with engine off? Should be reading battery voltage.

Are both field wires connected securely to the alternator terminals?

Double check any work you did just in case something was forgotten or got knocked loose, etc.

Make sure a grid heater is not stuck on. I would temporarily disconnect them at the battery until your problem has been resolved. Makes it easier to diagnose charging system problems with them out of the circuit.

- John

 

[RBeard]

140A Fuse still good, Batt. voltage present at alt. post. unplugged field wire connector, looks ok inside, replugged it in and it feels tight in socket. D/Ced grid heaters from batt. Still not charging. Is there a test result I should see when applying a meter to field wire connector(voltage or continuity). All those wires run in a loom that goes across a hot engine with Radiator heat also and I replaced my tappet cover gasket last year and had move all those looms around, could I have stressed already old/tired wires.

 

[brucejohnson]

140A Fuse still good, Batt. voltage present at alt. post. unplugged field wire connector, looks ok inside, replugged it in and it feels tight in socket. D/Ced grid heaters from batt. Still not charging. Is there a test result I should see when applying a meter to field wire connector(voltage or continuity). All those wires run in a loom that goes across a hot engine with Radiator heat also and I replaced my tappet cover gasket last year and had move all those looms around, could I have stressed already old/tired wires.

Since the alternator worked for a short time, it may be PCM related.
Easy checks in one spot is to:
1. (Power off) Pull/disconnect the alternator field plug and measure the resistance (ohms) between the two pins on the alternator. This checks continuity of the field coil in the alternator. (I suspect this will be OK, just a quick check while in the area)

2. Plug still disconnected - Check for voltage at the Dark Blue wire of field connector with the ignition on. This is the field energizing wire. The Dark Green should be the negative/ground. (doesn't hurt to see if any voltage is present or a ground presence exist on the DG either). Hard to say how it plays in the circuit as the circuitry is not displayed. Either way they are related to the PCM.
ON EDIT: The Dark Green wire is responsible for the regulation of the alternator's output via grounding the circuits controlling the output's intensity determined by the sensing of the PCM parameters.

3. Pull the PCM connectors and check the pins and cavities for evidence of corrosion or damage/cleanliness. Especially on any pins related to voltage sensing circuits. FSM wiring diagrams should show this. A little bouncing around will be required.

 

[RBeard]

0 volts DC no matter the polarity of meter.

 

[brucejohnson]

0 volts DC no matter the polarity of meter.

Is that checking the alternator or wiring connections.
The alternator pin connections should be done on the Ohms function and should read close to 0 (not sure if there is a specific resistance) indicates continuity.
The plug (wiring side) should read some value generated by the PCM.
I'll go dust off my 2002 FSM and see what magic circuits relate here.
Try jiggling the ignition switch with it running to see if the charging is affected.
I shall return...

 

[RBeard]

let me go back to the 3 step process , I missed t earlier. back soon

 

[brucejohnson]

0 volts DC no matter the polarity of meter.

Simple question to start:
When the voltage indication goes down to the 8vdc mark on the volt gauge, does the check gauges light come on?

 

[RBeard]

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.

 

[BigPapa]

The ECM is on the passenger side of the engine behind the fuel filter, above the starter. The PCM is on the firewall behind the air box.

 

[RBeard]

OK, I thought I was smarter than that. I've never needed to mess with it before, hence at least my confusion.
Thanks, Scott.
I'm about ready to remove the Radiator cap on this one and drive a new truck under it and put the cap on the new truck

 

[BigPapa]

I'm about ready to remove the Radiator cap on this one and drive a new truck under it and put the cap on the new truck

Yeah, I’ve been there, too.

 

[brucejohnson]

OK, I thought I was smarter than that. I've never needed to mess with it before, hence at least my confusion.
Thanks, Scott.
I'm about ready to remove the Radiator cap on this one and drive a new truck under it and put the cap on the new truck

Please note:
The 2002 FSM states to turn the ignition switch OFF,
Then DISCONNECT the NEGATIVE Battery Cables PRIOR to removing the PCM Connectors.
This should avoid any voltage spikes that may be detrimental to the PCM's circuitry.

 

[petersonj]

The ECM is on the passenger side of the engine

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

- John