I need an electrical expert

[GAmes]

Last week while crossing southern ID I noticed that my volt gauge was reading higher than normal. Outside air temp was in the single digits. I reasoned that the cold air was raising the voltage demand on the alternator which is normal. Then my GEN light came on. I have my grid heaters on a toggle switch so I flipped them on which brought my gage down to the bottom of the arc (13 volts?). After a minute or so I turned the grid heaters off, the needle went to it's normal range and the GEN light went out. The next day, in OR with the temps in the teens it happened again. Same procedure, same results. Two days later, temps in the teens, same problem, same procedure, same results. I decided that my battery temp sensor had failed and the cold weather was the catalyst. However, back in TX the temps were in the 40s and it happened again. Same story. Today I removed the temp sensor and it tests good. Before I spend nearly $50 for a new sensor I'm looking for another explanation for the spike in voltage that was corrected by putting a large load on the alternator.

 

[petersonj]

Not sure how the '97 truck charging system is set up. Does the battery temperature sensor sense the driver side battery temperature? Which battery does the alternator output connect to? Last question, which battery is sensed by the voltage regulator (inside the PCM, I think)?

Just wondering if it is a battery condition / connection issue. When the symptoms occur, can you pull off of the road and put a voltmeter on each battery? Might get some good information there.

I know that you are pretty savvy on this subject, so just throwing some things out there.

- John

 

[GAmes]

Since the batteries are connected the voltage is the same to all the accessories. To get a separate reading you would have to disconnect one battery. If it is disconnected it obviously it wouldn't get a charge. I know the temp sensor affects charging because I had a no charge issue one time and replacing the sensor fixed it. The sensor on a '97 is under the driver's side battery but on a '94 it is on the firewall. The batteries are both the same voltage when separated, engine off, 11.98 and 11.99 with my multimeter. I discovered today that the temp sensor has a lifetime warranty so I'm going to replace it and see what happens. I'm still curious why the charge rate settled back to normal after loading the alternator.

 

[petersonj]

Since the batteries are connected the voltage is the same to all the accessories. To get a separate reading you would have to disconnect one battery. If it is disconnected it obviously it wouldn't get a charge.

In a good operating charging system assuming all electrical connections are sound, yes. But, in an electrical system where there is a poor connection somewhere - not necessarily. This is why I ask the questions regarding which battery the alternator output goes to, which battery is sensed by the voltage regulator. And, one more question - which battery does the dash voltage gauge read?

Not knowing how your charging system is wired, I can't really offer another direction to go. But, it is conceivable to me, for example, if there was a voltage drop issue with the crossover cable, then it could be possible to experience your symptoms under a specific charging system setup.

I'm still curious why the charge rate settled back to normal after loading the alternator.

Could be that the PCM may take the alternator out of the circuit when a certain high voltage parameter is met. Then, when you activated the grid heaters, the system voltage dropped quickly and the alternator began charging again. Just guessing here - again not knowing how your charging system is wired.

The batteries are both the same voltage when separated, engine off, 11.98 and 11.99 with my multimeter.

Why so low?

- John

 

[GAmes]

The system charges both at the same time and draws power from each. As it is sitting right now, the driver's side battery is removed and the truck starts and charges normally yet all the wires are connected to the driver's side positive terminal. There isn't a one battery or the other getting charged or being sensed by the voltage regulator when they are in parallel.

That's the problem. The PCM did not take the alternator off line. The GEN light was on but the voltmeter was (guessing here since the gauge only shows a 14)) 15 or higher volts. It took the high amperage demand to drop the volt meter back down to 12 or 13 volts. I have inadvertently caused the alternator to go off line by leaving the grid heaters on too long in the past. In those cases the GEN light came on and the voltage dropped to about 11. The fix was to shut the grid heaters off. Shut the engine down, wait a couple seconds for the PCM to reset, then start the engine back up.

I don't have a high dollar multimeter. That is why I specified "my multimeter".

 

[BigPapa]

Charging System

Starting System

 

[AH64ID]

The system charges both at the same time and draws power from each. As it is sitting right now, the driver's side battery is removed and the truck starts and charges normally yet all the wires are connected to the driver's side positive terminal. There isn't a one battery or the other getting charged or being sensed by the voltage regulator when they are in parallel.

When everything is working normal this is correct.

A bad crossover cable is the most common issue when there are voltage issues. I also can’t speak for a 2nd Gen, but on a 3rd Gen the ECM controls the dash voltmeter and voltage regulator but the alternator connects to the passenger battery. When the crossover cable is loose, or failing, the ECM sees low voltage can increase the voltage output on the alternator which overcharges the passenger battery.

Just one example of how you can get overcharging on a dual battery setup even thou they should be in parallel.

I would check all your connections and look for any corrosion.

Next time it occurs I’d pull over and see if the voltage is the same on both batteries, or if they are different.

Lastly, why are the batteries below 12V? They should be 12.7V if fully charged.

You shouldn’t be able to shut an alternator off by drawing voltage down.

 

[petersonj]

@GAmes , after reviewing the wiring diagram provided by @BigPapa , it appears that the passenger side battery has only one wire connected to the positive post and that wire is the heavy crossover wire. So, I think that this configuration would probably rule out any concerns that I had with alternator output connection location and voltage sense connection location for the voltage regulator attributing to your problem

That's the problem. The PCM did not take the alternator off line. The GEN light was on but the voltmeter was (guessing here since the gauge only shows a 14)) 15 or higher volts.

I jumped to a conclusion when I read that the "Gen Light" came on. I made the assumption that the alternator stopped charging. As you pointed out, this was not the case.

I do recommend getting a reliable multi-meter for checking battery voltage. You may have a cheap multi-meter, but if it is reading accurately, then you could have a battery / charging system problem.

I would also check all major ground points. Electrical resistance is not a constant when a poor connection is in the loop. Varying electrical loads can improve or worsen a high resistance electrical connection instantly. Poor electrical connection performance is not reliable. It can go from good to bad in a couple seconds, or vice versa. This could explain why activating the grid heaters is masking the problem.

- John

 

[GAmes]

I appreciate the comments but the issue only happened once a day, four times, on a 4000 mile trip. My digital battery charger shows the batteries to have 12.9 volts after being at rest for over 24 hours. The engine starts the same using either battery with the other disconnected. The terminals are clean & secure. Just prior to the trip all the clamps were removed, cleaned and reinstalled. The batteries are about a year old. With the temp sensor removed the alternator still charges a bit below the 14 mark on the gauge. My plan, after I install the new one, is to measure the charge rate for each battery independently by disconnecting the ground wire of the opposite battery.

 

[GAmes]

Next time it occurs I’d pull over and see if the voltage is the same on both batteries, or if they are different.

You shouldn’t be able to shut an alternator off by drawing voltage down.

When the temp is 5f I wasn't thinking about stopping, finding a 1/2 inch wrench to disconnect the crossover and a multimeter to measure voltage. If it occurs while I'm on the way to the lake on Sunday I will.

The PCM is supposed to shut down the alternator if the amperage draw is too high. Apparently it does not when the voltage is too high.

 

[AH64ID]

When the temp is 5f I wasn't thinking about stopping, finding a 1/2 inch wrench to disconnect the crossover and a multimeter to measure voltage. If it occurs while I'm on the way to the lake on Sunday I will.

The PCM is supposed to shut down the alternator if the amperage draw is too high. Apparently it does not when the voltage is too high.

No need to disconnect the crossover for this, in fact you want it connected so you can verify the system is working and you have one big battery.

Shutting down the generator with high amp draw is counterintuitive. What if you’re winching? What if you have weak batteries and a the grid heater is on post-heat?

I don’t see a shunt on that diagram, and am not aware of one on a 2nd Gen, so how does the PCM know the amp draw?

 

[GAmes]

No need to disconnect the crossover for this, in fact you want it connected so you can verify the system is working and you have one big battery.

Shutting down the generator with high amp draw is counterintuitive. What if you’re winching? What if you have weak batteries and a the grid heater is on post-heat?

I don’t see a shunt on that diagram, and am not aware of one on a 2nd Gen, so how does the PCM know the amp draw?

Disconnecting them would tell me if one battery is faulty. It would also isolate the crossover cable if it is faulty. I already know what the battery voltage is with them connected, it is on the gauge.

I can only tell you that the PCM only sends a signal to the alternator when it knows the engine is running, that is why we have an engine speed sensor. I also know that high amperage draw, like when a grid heater relay sticks closed, will shut off the alternator. I have my grid heaters on a toggle switch. Normally the PCM provides the ground signal to activate them. The reason I use the switch is twofold. The only warranty issue I had when the truck was new was a relay that failed. Both batteries cooked while my wife was shopping in the commissary. The scumbag Dodge dealer in Newport News, VA billed her for the batteries because "the batteries have no warranty". They didn't wash the battery acid out of the fender wells either. I was thousands of miles away at the time. By the time I learned about being ripped off it was too late to do anything about it. The second reason is I don't like having the grids activated after the engine is running, especially while driving in the dark.

 

[Timd32]

My thinking, toss that sensor in the freezer and give it a test, simulate that 5f or colder deal. Maybe even keep it wrapped up somehow and plug it into the truck see what happens.

That sounds like the only thing changing was the ambient.

If it was the IGN switch and say a high resistance it would possibly show up at other times.

Could always have been a 5V reference power supply internal to the PCM was not liking the cold, not going to recommend freezing the PCM!

My guess is that sensor relies on a reference voltage.

 

[GAmes]

The freezer idea would work if the issue was repeatable. Once I engaged the grid heaters to load the alternator the voltage dropped to normal readings and the rest of the day was uneventful. The single digit temps lasted until the next morning. And don't forget, it happened once when back in 40 degree weather. The new sensor if free. The only other things are the PCM and the alternator itself. I have a spare PCM if the sensor doesn't work and changing it only takes about 15 minutes. I might have a spare alternator in my stash too. Our forecast doesn't call for freezing temps in the near future and I won't be going bac to the PNW until spring. I do need to do the on-off-on key routine to see if any codes are stored.

 

[AH64ID]

Disconnecting them would tell me if one battery is faulty. It would also isolate the crossover cable if it is faulty. I already know what the battery voltage is with them connected, it is on the gauge.

Leaving it connected would also tell you this information, as there would be a difference in battery voltage. Leaving it connected with voltage issues is the first step in diagnosis on these trucks.

The dash is only telling you what the PCM is seeing, so it's only good in a perfect system. . It is very common for the dash to read differently than one of the batteries, hence the reason I am suggesting this. I know you're having a hard time with the system having different voltages when they are connected, but it happens and it's not an uncommon issue on older trucks.

I can only tell you that the PCM only sends a signal to the alternator when it knows the engine is running, that is why we have an engine speed sensor. I also know that high amperage draw, like when a grid heater relay sticks closed, will shut off the alternator. I have my grid heaters on a toggle switch. Normally the PCM provides the ground signal to activate them. The reason I use the switch is twofold. The only warranty issue I had when the truck was new was a relay that failed. Both batteries cooked while my wife was shopping in the commissary. The scumbag Dodge dealer in Newport News, VA billed her for the batteries because "the batteries have no warranty". They didn't wash the battery acid out of the fender wells either. I was thousands of miles away at the time. By the time I learned about being ripped off it was too late to do anything about it. The second reason is I don't like having the grids activated after the engine is running, especially while driving in the dark.

With what you are saying the PCM has to have a way to detect high amperage, which it does not. The PCM simply tries to achieve it's desired charge voltage based on battery temp.

According to the FSM the only thing that turns the charging system on or off is the ignition key. There is no mention of generator output being restricted if the amp draw is high. Voltage is only regulated based on batter temp and system line voltage.

 

[GAmes]

With what you are saying the PCM has to have a way to detect high amperage, which it does not. The PCM simply tries to achieve it's desired charge voltage based on battery temp.

According to the FSM the only thing that turns the charging system on or off is the ignition key. There is no mention of generator output being restricted if the amp draw is high. Voltage is only regulated based on batter temp and system line voltage.

If I forget my grid heaters are on and start driving the alternator will kick off line after about 10 minutes. Volt meter in the dash is at 12 and the GEN light is on. To get the alternator back I have to shut the engine off, wait a few moments and restart the engine (after turning the toggle switch off). That tells me the PCM has some sort of over amperage sense and a reset mechanism. The PCM also powered the GEN light when the over voltage occurred, but did not shut down the alternator.

So looking to see if the PCM outputs to the voltmeter I dug into the FSM. The only gauge it outputs to is the Tach. It does power the GEN light. However it also has outputs of generator field output (-) and generator field output (+). I may have answered my own question which I will find out for sure after I button the truck back up. The PCM might be faulty, although there isn't any clear troubleshooting evidence.

There is also a DTC code for the temp sensor, but it is the same whether it has grounded plus or minus. I've started the truck with the sensor disconnected and giving the difficulty of plugging and unplugging it I won't be reinstalling the old one. I'm sure the code will show up but it really won't tell me anything.

 

[AH64ID]

If I forget my grid heaters are on and start driving the alternator will kick off line after about 10 minutes. Volt meter in the dash is at 12 and the GEN light is on. To get the alternator back I have to shut the engine off, wait a few moments and restart the engine (after turning the toggle switch off). That tells me the PCM has some sort of over amperage sense and a reset mechanism. The PCM also powered the GEN light when the over voltage occurred, but did not shut down the alternator.

Nothing shows or discusses any sort of amperage sense. Likely what is occurring is you are drawing the batteries down and get an under-voltage light, which makes a lot of sense and is more inline with the FCM theory of operations. If you draw 200+ amps on a 120A alternator it won't take long for the dash light to come on for low voltage, that doesn't mean the alternator is offline.

 

[BigPapa]

Instrument Panel Diagram

 

[GAmes]

....... that doesn't mean the alternator is offline.

If I turn off the grid heaters and the GEN light doesn't go off and the voltmeter shows 12 volts what else could it mean?

 

[AH64ID]

If I turn off the grid heaters and the GEN light doesn't go off and the voltmeter shows 12 volts what else could it mean?

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.