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Old 09-08-2020, 06:57 PM   #1
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Fluctuating system voltage after starting engine

Hello-

2013 Freightliner XCS chassis. The RV is not plugged in and the generator is off.

Recently, immediately after I start the engine, I started getting system voltage fluctuations that cycle over a 10 second time period for about 2 minutes before returning to normal. The system voltage on the cockpit gauge will show 12 volts for around 10 seconds, then 14 volts for around 10 seconds, then 12 volts, then 14 volts, etc., etc. Sometimes I get a message on the console that says "Low system voltage" depending upon how low voltage gets on the downswing.

I tried increasing idle RPM with the gas pedal thinking idle RPM wasn't producing enough rotation for the alternator to put out minimum output, but that has no effect on system voltage or the fluctuations. And as I mentioned, after a couple of minutes, the system voltage returns to 14.1 volts per the digital readout, and everything is normal.

What's going on?
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Old 09-08-2020, 07:25 PM   #2
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This is normal. Its the intake grid heater kicking on and off. It cycles off and on until a certain intake temp is reached. Heater grid pulls voltage way down when on.
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Old 09-08-2020, 07:28 PM   #3
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Battery could be on the weak side or wiring needs some freshening up.

Assuming you have an emergency start button that combines your batteries try holding that down for the first minute or so. If that does not make any change, try running the generator and hold the same button down.

Often generators do not charge engine batteries when the alternator is running and the generator at the same time so you need to hold the switch to bypass that.
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Old 09-08-2020, 07:30 PM   #4
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It's the grid heater.....
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Old 09-08-2020, 07:53 PM   #5
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Quote:
Originally Posted by rte154 View Post
This is normal. Its the intake grid heater kicking on and off. It cycles off and on until a certain intake temp is reached. Heater grid pulls voltage way down when on.
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Originally Posted by bob caldwell View Post
It's the grid heater.....
OK thanks. I know nothing about intake grid heaters other than what I just Googled. Is there any particular reason why these voltage fluctuations are so noticeable now vs. in the past months/years? I've had this rig for a couple of years now and have never seen the fluctuations nor received a low voltage warning. Also, when I first noticed it, the outside air temp was in the lower 80s, although today it was in the 50s. Does that have an affect?

Thanks!
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Old 09-08-2020, 07:55 PM   #6
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Yes....all about temp.....
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Old 09-08-2020, 08:03 PM   #7
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It’s about the block temperature when you turn the key. If the engine has been sitting overnight on a mild night, even if the outside temperature is warming up, the intake heater may activate to warm up the engine promptly.
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Old 09-08-2020, 08:03 PM   #8
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Yes....all about temp.....
Crap. Well I just dropped off my rig at the shop to have that looked at (among some other stuff). Hopefully they'll know about this heater!
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Old 09-08-2020, 08:04 PM   #9
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It’s about the block temperature when you turn the key. If the engine has been sitting overnight on a mild night, even if the outside temperature is warming up, the intake heater may activate to warm up the engine promptly.
Thanks Dean.
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Old 09-08-2020, 08:05 PM   #10
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Battery could be on the weak side or wiring needs some freshening up.

Assuming you have an emergency start button that combines your batteries try holding that down for the first minute or so. If that does not make any change, try running the generator and hold the same button down.

Often generators do not charge engine batteries when the alternator is running and the generator at the same time so you need to hold the switch to bypass that.
thanks YC1
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Old 09-08-2020, 08:09 PM   #11
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My truck has the same type system and Wait to Start light also.....I can plug my block heater in for when it's cold out, and not have the light come on and will start right up....then when you achieve 15 mph, the grid heater turns itself off and voltmeter is back showing charging....
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Old 09-08-2020, 08:13 PM   #12
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My truck has the same type system and Wait to Start light also.....I can plug my block heater in for when it's cold out, and not have the light come on and will start right up....then when you achieve 15 mph, the grid heater turns itself off and voltmeter is back showing charging....
Interesting. You learn something every day with these diesel rigs.

I'm just surprised I never have noticed it before, and I start the RV up all year round and in extremely cold temps in January and February. I can't believe I've missed it these past few years.
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Old 09-08-2020, 08:27 PM   #13
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I would not count in it. I've seen posts where freightliner worked it as an issue.
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Crap. Well I just dropped off my rig at the shop to have that looked at (among some other stuff). Hopefully they'll know about this heater!
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Old 09-08-2020, 08:48 PM   #14
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I would not count in it. I've seen posts where freightliner worked it as an issue.
OK, thanks. I'll call them tomorrow and let them know it might have been operator observational error and mention the grid heater.
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