I need an electrical expert

[petersonj]

hat the batteries need to recharge for a bit after being depleted.

The only way to really know is for @GAmes to duplicate the symptoms by leaving his grid heater switch on with the engine idling until the "GEN" light comes on. Turn of the grid heater switch and leave the engine running. Using a good voltmeter (not the one he used to previously to check his batteries) and check voltage at the batteries for a few minutes. If the voltage stabilizes well below 13 volts, then the alternator is not charging. If voltage continues to slowly rise above 13 volts, then the alternator is charging. That should offer a solution to this debate.

@GAmes , it sounds like your grid heater switch is a maintained switch. Why not use a momentary switch? Mine has been wired with a momentary switch since the truck was new. Can't accidentally be left on.

If it is decided to do the test, my bet is the alternator will be still charging. But who knows - I think I was wrong about something once before.

- John

 

[GAmes]

That the batteries need to recharge for a bit after being depleted. The recharge would take some time after 10 minutes of grid heater operation.

There is likely a delay built into the GEN light in the PCM so that normal grid heater operation doesn't light that light every time the post-start grid heater cycles, and that timer would be reset if you cycle the ignition.

The GEN light goes out immediately after the engine is restarted and the needle goes to 14+. So the batteries are not only depleted from the grid heaters but also by the start sequence. I've had to drive quite some distance to pull off safely to restart the engine, with the switch off, yet there isn't any change in the voltmeter or the GEN light. That is the reason why I have to shut off and restart the engine.

There isn't a normal grid heater operation because I've bypassed the PCM. If you believe there could be a delay built in why is it so hard to envision an overload protection?

 

[AH64ID]

If you believe there could be a delay built in why is it so hard to envision an overload protection?

Because there is no shut to monitor amp draw. Without a way to measure current you cannot have overcurrent protection (overcurrent protection would shut down the alternator, not overload), it's as simple as that. Overload protection is there, but it's in the form of a 120A fuse.

Without a delay you would get a GEN light every time the grid heaters cycled post startup since the grids draw a lot more power than the alternator can put out, plus any other electrical load.

 

[GAmes]

The only way to really know is for @GAmes to duplicate the symptoms by leaving his grid heater switch on with the engine idling until the "GEN" light comes on. Turn of the grid heater switch and leave the engine running. Using a good voltmeter (not the one he used to previously to check his batteries) and check voltage at the batteries for a few minutes. If the voltage stabilizes well below 13 volts, then the alternator is not charging. If voltage continues to slowly rise above 13 volts, then the alternator is charging. That should offer a solution to this debate.

@GAmes , it sounds like your grid heater switch is a maintained switch. Why not use a momentary switch? Mine has been wired with a momentary switch since the truck was new. Can't accidentally be left on.

If it is decided to do the test, my bet is the alternator will be still charging. But who knows - I think I was wrong about something once before.

- John

I hate to abuse the PCM by purposely leaving the switch on but for testing I'll do it after I install the temp sensor.

I have a toggle switch panel for my exhaust brake, trailer lights and aux backup lights. A momentary push button wasn't considered.

 

[AH64ID]

I hate to abuse the PCM by purposely leaving the switch on but for testing I'll do it after I install the temp sensor.

The abuse is to the alternator and it's brushes, not the PCM. Excessive amp draw causes premature wear of the brushes, but one test shouldn't be detremental.

 

[petersonj]

I am sure there is programming in the PCM that is never shown in the wiring diagram. So, it is possible for the PCM to be programmed to shut down the alternator based on a voltage parameter instead of current draw. There is just no way to know for sure. Just because the FSM doesn't mention it, doesn't mean it doesn't exist.

@GAmes , I understand why you may not want to do the test. It was just an idea.

- John

 

[GAmes]

The abuse is to the alternator and it's brushes, not the PCM. Excessive amp draw causes premature wear of the brushes, but one test shouldn't be detremental.

That would be true if your theory is correct.

BTW, the 120 amp protection fuse isn't in the grid heater circuit. They each have fusible links. Stand by for the grid heater left on test. I have a hunch the PCM needs to be replaced anyway.

 

[BigPapa]

The momentary switch doesn't have to be push button. I guarantee the manufacturer of you toggle switches offers a momentary toggle switch, too.

 

[AH64ID]

I am sure there is programming in the PCM that is never shown in the wiring diagram. So, it is possible for the PCM to be programmed to shut down the alternator based on a voltage parameter instead of current draw. There is just no way to know for sure. Just because the FSM doesn't mention it, doesn't mean it doesn't exist.

- John

I would agree about all the programing not being listed, especially since the wiring diagram just shows physical wiring and not programming. The PCM is certainly programmed to effect voltage output, but only based on voltage and temperature. All the trouble codes are associated with the generator field control, voltage sensing, or overall performance. None of them deal with current because the system doesn't know the current. In the troubleshooting procedures it requires diagnostic tools to measure current to verify fault.

Voltage and current are two different things thou, and you cannot measure one with the other. In order for the PCM to know the current draw it needs to have some sort of current measuring device, which would absolutely be shown in the wiring diagram. I am not aware of one showing up on Dodge/Ram trucks until the IBS in 2013.

BTW, the 120 amp protection fuse isn't in the grid heater circuit. They each have fusible links. Stand by for the grid heater left on test. I have a hunch the PCM needs to be replaced anyway.

I didn't say it was in the grid heater circuit, it's in the PCM. The fuesable links at the grid heater are just for the grid heater.

 

[GAmes]

The momentary switch doesn't have to be push button. I guarantee the manufacturer of you toggle switches offers a momentary toggle switch, too.

I know, but those are a matched set, and not the easiest to switch out.

 

[GAmes]

I took the back off my multimeter and adjusted the needle to match the readings from my digital battery charger. The screw is as sensitive as a cyclic on the first day of flight school. Both batteries, after sitting all night were at 12.8 volts. I installed the temp sensor, then started the engine with only the passenger side battery connected. Voltage went to 13.9. I disconnected it's ground and installed the driver side battery with the same results. After reconnecting the pass side ground I again started the engine and flipped on the grid heater switch. Voltage dropped to 11.9 at the battery, the truck voltmeter was at the bottom of the arc. It took awhile but the GEN light finally came on. I turned the grid heaters off and much to my surprise the voltmeter needle came back to the 14 volt mark. Battery voltage was 14.2 The GEN light stayed on. I let it run while I reset the time and date on my radio, came inside the house and wrote a check to Cabelas, put it into an envelope and walked to the mailbox and back. Opened a beer, went outside and the GEN light was still on.

To conclude, here is what I learned. More than likely the temp sensor was the culprit. With a new one the voltage is a steady 13.9. If the alternator voltage exceeds a certain parameter (15 volts?) the gen light will illuminate, then go out when the voltage is back within it's range. If the GEN light illuminates from low voltage it will stay on until the engine is restarted. IOW, a GEN light does not tell you if the alternator is charging. I guess it will remain a mystery why the alternator would be regulated after cycling the grid heaters.

 

[AH64ID]

I took the back off my multimeter and adjusted the needle to match the readings from my digital battery charger. The screw is as sensitive as a cyclic on the first day of flight school. Both batteries, after sitting all night were at 12.8 volts. I installed the temp sensor, then started the engine with only the passenger side battery connected. Voltage went to 13.9. I disconnected it's ground and installed the driver side battery with the same results. After reconnecting the pass side ground I again started the engine and flipped on the grid heater switch. Voltage dropped to 11.9 at the battery, the truck voltmeter was at the bottom of the arc. It took awhile but the GEN light finally came on. I turned the grid heaters off and much to my surprise the voltmeter needle came back to the 14 volt mark. Battery voltage was 14.2 The GEN light stayed on. I let it run while I reset the time and date on my radio, came inside the house and wrote a check to Cabelas, put it into an envelope and walked to the mailbox and back. Opened a beer, went outside and the GEN light was still on.

To conclude, here is what I learned. More than likely the temp sensor was the culprit. With a new one the voltage is a steady 13.9. If the alternator voltage exceeds a certain parameter (15 volts?) the gen light will illuminate, then go out when the voltage is back within it's range. If the GEN light illuminates from low voltage it will stay on until the engine is restarted. IOW, a GEN light does not tell you if the alternator is charging. I guess it will remain a mystery why the alternator would be regulated after cycling the grid heaters.

Good deal, that's a simple fix.

I guess it will remain a mystery why the alternator would be regulated after cycling the grid heaters.

What do you mean by this?

 

[GAmes]

What do you mean by this?

The first time it happened I had been driving for quite awhile. I left Green River, UT and the light came on just south of Jerome, ID. The gauge needle had been reading about 1/8th inch above the 14 mark. Because of the cold I didn't think it was an issue. Voltage is supposed to be higher when temps are very low. When the light came on it was a good 1/4 inch above the 14 mark. When I flipped on the grid heater the needle fell to about 1/4 inch below the 14 mark, say 13 volts, well within the arc. After a time I turned the grid heaters off and the needle settled back to just above the 14 mark, a place it should have been due to ambient temp. The GEN light went out shortly after that. I drove all the way to Ontario, OR with no problem. The next day, it was similar even though slightly warmer. The light came on with a high voltage needle after a few hours of driving. After the grid heater application the same results. The mystery is why it only happened once a day, not continuously.

 

[AH64ID]

Gotcha, failing parts... who knows why they do what they do sometimes.

 

[petersonj]

@GAmes , thanks for doing the experiment. It provided some good information.

What does "IOW" mean?

- John

 

[BigPapa]

What does "IOW" mean?

- John

In Other Words

 

[fest3er]

It sounds lke the alternator is failing.

There is only one over-current protection for the alternator: the 140A fuse in the PDC; either it's good or it failed. The alternator can produce 136A. The grid heaters each draw 100A. The PCM will regulate the alternator to its maximum output; after that, the output voltage will drop. This is why the lights on the truck dim and the voltage gauge drops to 10VDC or so when the grid heaters are active. [Aside, this is exactly how computer power supplies work; if you draw more current on a 'rail' than the P/S can provide, the output voltage drops and the computer misbehaves.]

If there is a high current draw (such as with the grid heaters on) long enough (or the ambient temperature is high enough, but this doesn't sound like your problem), the alternator's diodes will heat up with all that current being pushed through them. If the diodes have become marginal (meaning they work until they get all het up), they'll stop being diodes. In this case, the AC voltage that the alternator produces is no longer properly rectified to DC voltage and the output voltage and current drop (because some of the output leaks back to B-). If enough of the diodes are flaky, it will seem like the alternator has quit. After the diodes cool down, the alternator will work again.

If you have a spare alternator, install it and bring the failing alternator to a local shop and have them rebuild it. It's served you well; might as well give it a new lease on life. The shop will thoroughly bench-test the alternator before they tear it apart.

If you are adventurous and want to perform a rudimentary bench-test of the alternator with the tool you have,

1. Wait for the alternator to be cold and disconnect all wires and cables from it.
2. Measure the resistance between its + and - posts (or + post and case) with red meter lead on +. The result should be infinite resistance (open circuit). This is normal operation; it prevents the batteries from draining down due to current flowing through the alternator. If you don't see an open circuit, diodes have failed; skip the rest of these steps and just have it rebuilt.
3. Reverse the leads and measure the resistance (with black meter lead on the + post). You should see (IIRC) zero resistance (short circuit) as this is the normal current direction (pulling electrons from B- and pushing it to B+ to charge the batteries.
4. Remove the alternator from the truck.
5. Put the alternator in a 150°F-200°F oven for an hour or so (preferably when your wife is out shopping so she doesn't see you use her oven and best roasting pan to hold the alternator). Then repeat steps one and two. If you do not see an open circuit with the red lead on the + post, the alternator needs to be rebuilt. If you do not see a short circuit with the black lead on the + post, something is definitely wrong with the alternator.