Bad Alternator? P2502, P0622 and Check Gauges light and Voltmeter reads 0

Hello, Here is my issue... As described in the thread title, Driving along minding my own business and I hear a chime "Check Gauges" light is on. Voltmeter reads 0. Everything else seems ok. Turned key off and back on... Voltmeter comes up to about 11V, then drops to 0. Cycle key again and same thing. Cycle a third time and the Voltmeter holds at somewhere between 11 and 14V. Turn motor on and start driving again... Couple miles down the road, same scenario. Chime, Check Gauges, Voltmeter at 0. Took it to a battery shop to have them check the system. Their tester shows the alternator is not charging. This all sounds like a bad alternator, but here is my question... Why does the voltmeter go to 0? Shouldn't it be showing whatever charge the batteries are holding? I was told by someone that the gauge shows the alternator output and not the battery level. If this is the case, then why does the gauge show voltage when you turn the key on, before you start the motor (the alternator isn't running at this point)? Any insight would be appreciated and hopefully helpful. Thank you, Trey

 

I had the check gauges light and 0 volts when my alternator went.

 

Thanks Mark, I picked up a new (remanufactured) one from O'Reilly Auto Parts yesterday, but haven't had a chance to swap yet. I'm hoping it's the alternator and not something more involved, like the computer not telling the alternator to charge, or something stupid like that. I guess I can't complain too much. It's the original alternator and I'm at 215k miles. The original batteries lasted 13 years and 170k miles. With hindsight, they may have actually been fine. The lift pump died shortly after I replaced them. I think it may have been the pump causing the slow starts and not the batteries after all.

 

You got better than most on both the alternator and batteries. It's not uncommon for them to die about the same time. Weak batteries are extra hard on the alternator. A weak or dying alternator is bad for the batteries. The computer monitors and controls the Alternator, likely it commands the volt guage to zero to help identify the failure.

 

Come on now... 57 views and only 1 reply? There's got to be someone out there with some expert knowledge on this. Anyone know why a voltmeter would come on and then go to 0 when the key is in the "On, but not running" position?

To reassure myself, I think I'm going to pull the old alternator and try to get it bench-checked, before committing to installing the new one. It was only ~$150.00, but if it's not broke...

Thank you, (I hope) in advance.

 

Are you saying the new alternator is installed and you are getting momentary voltmeter, then it goes to zero with key on, not started?

 

Please be a specific as possible: specific year of truck ( System changed in 2008 ).

At your mileage the brushes at a minimum are shot in that original alternator, actually surprised you got that many miles.

There were also different alternators in different years as well. I would like to help, but I'd need as much data as possible for the condition.

 

If you have a handheld voltmeter you can disconnect the batteries and test them for voltage to see if they're reading the same. If not, it's likely one of them could be bad.

I would (and did) bench test the alternator before replacing it.

 

I had this issue a long time ago and sometimes my volts would shoot up a TON. did some research and found out that my alternator was fine but my batteries have blown.... the end result i found out that is was the crossover cable from one battery to the other.

The thing i learned after some research is that the Voltage regulator is in your PCM and is not apart of the alternator itself. The alternator runs to the passenger battery and the voltage is taken from the driver side battery. so if your PCM was reading a low voltage your alternator is going to continue to push charge. this is a result of a corroded crossover cable due to the electrical resistance that takes place.

 

Update... First off, thank you for the replies. I went ahead and pulled the old alternator, took it to the parts store to have it bench checked and it was confirmed "DEAD". Returned home, installed the new alternator and all is good. How I think the system works... The voltmeter gets a signal from the computer of what the alternator is putting out, thus the 0 reading. I'm guessing this signal is held in memory, explaining why the voltmeter would go to 0 after turning the key on (not starting the motor). If I did this more than once, it would usually stay up at a voltage reading around 11V, until I started the motor, creating a "No-charge" condition from the alternator again, making the voltmeter go back to 0. I'm guessing after the memory gets dumped, the voltmeter goes back to reading battery voltage? That's the only thing I can think of that explains what was happening.

Thank you again,

Trey

 

The ECM reads the driver side battery (primary battery) voltage and determines the target charging voltage. It doesn't know/care about the alternator output or passenger side battery voltage. The alternator output can be higher than 16V when ECM sees the driver side battery voltage low, far away from the target voltage.
Yes ECM dictates the stock gauge readings. It will show Zero when the voltage is LOW. It's not showing the actual driver side battery reading.
During grid heater cycling on/off operation my stock gauge show 12V all the time until the completion of the operation, while the actual voltage jumping between 12 V and over 15V.

 

If the 2 batteries are connected in parallel how can the ECM read the voltage in only 1 of them?

 

When there is NO RESISTANCE between the connection of the two battery positive posts (NO voltage drop) the driver side battery voltage will be the same as the passenger side battery voltage. If the electrical connection is not perfect (has resistance) then there is voltage drop. The driver side battery voltage will lower than the passenger side battery voltage.
And ECM is looking at the driver side battery voltage.



When the connection is BAD the driver side battery will be undercharged (drained) and the passenger side battery will be overcharged (boiled).

 

The alternator outputs to the passenger side battery positive post.

All the electrical loads (except the grid heater) draw from the driver side battery positive post.

The bridge between the two battery positive posts is vital for the charging/battery system operation.

 

This includes the ECM.
ECM has NO direct electrical connection to passenger side battery positive post.

Passenger side battery voltage = Driver side battery voltage + Voltage Drop across the two battery positive posts (crossover cable).

 

My batteries went 14 years with one acid refresh but the water was black in the end, gravities were low too. I lost lift pump at 100k, took it apart and the failure was caused by gunk under the brushes, solution install a pre-filter on pump suction. I bought a spare pump just in case. Alternators go after about 150k or so, typically brushes and bushings, both easy to fix. When you have time take it out, take it apart, new brushes and bushings, and put it back, make sure you mark case and stator alignment. Diode stacks only go do to excessive current draw or overheating. IÂ’ve rebuilt several. Your batteries may have taken out the alternator.

goody

 

There is a thermal switch mounted in the base of the drivers-side battery tray that reads battery temperature and talks to the ECM to control charge rate based on that battery's temp. This switch, when corroded/shorted, fill cause utter mayhem to the passengers-side battery/cables/and grid heater relays. Make sure that switch is clean and is in good working order.