05 Ford F-350 Dual Alternator Low Voltage On Start-Up

Kaimana Mechanic Fairfield, California Posted   Latest  
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Electrical
2005 Ford F-350 Super Duty XLT 6.0L (P) 5-spd (5R110W)
Low Voltage From Upper Alternator

Am I missing something? I have an F-350 PowerStroke 6.0L Dual alternator setup, with what appears to be an upper alternator failure. Before condemning the replacement rebuilt alternator for the second time, I wanted to make sure I have covered all my bases. The vehicle came in for a no charge condition on start-up. After doing a little research, I've learned that the lower alternator is powered on by the pcm after the glow plug control module shuts off. I've verified that the upper alternator is commanded on immediately after start-up and the lower alternator is commanded on approximately 45 seconds after. I've attached the scope data showing the pcm commanding the upper alternator on start-up (red), battery voltage (blue), pcm command lower alternator (green), upper alternator amperage (yellow). I'm assuming that the upper alternator is sort of working because it is providing just over 100 amps without the lower alternator on. I am not familiar with dual alternator setups and any insight will be greatly appreciated.

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Troy Owner/Technician
Longbranch, Washington
Troy Default
 

I have never had good luck with AM alternators on fords well on any really it usually takes 2 or 3 to get a good one that the computer likes

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Kaimana Technician
Fairfield, California
Kaimana Default
 

Thank you for your response! I’ve considered that, just want to go through this learning process since this is the first time I’ve come across this scenario.

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Albin Diagnostician
Leavenworth, Washington
Albin Resolution
 

Where did you take your voltage reading? I'm not all that familiar with the dual generator setups on that vehicle, but I do see a lot of the single generator setups, and when those engines start up cold, the voltage at the battery doesn't come up very fast, since the electrical loads are huge. What is the output rating of the upper generator on your vehicle? If the generator is putting out 100…

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Eli Owner/Technician
Toronto, Ontario
Eli Default
 

What you’re seeing is a low voltage condition rather than a no charge problem. The intake plenum heater is drawing approx 130A following a cold startup which is more than the alternator can produce hence the low voltage observation. This is totally normal for this truck.

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Chase Owner/Technician
West Salem, Illinois
Chase Default
 

6.0 does not have an intake heater, that is done with the glow plugs on a 6.0.

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Kaimana Technician
Fairfield, California
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Albin, Thank you for your response! I’m taking the voltage reading off of the right hand battery after verifying that the charging circuit is operational. As far as it working good, I thought that also and I want to make sure it’s a normal thing for this vehicle before I ship it.

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Eric Owner/Technician
Grand Rapids, Michigan
Eric Default
 

I would assume you have bi-directional control through your scan tool, with an current clamp around output of upper alternator, what is the output? It looks from your post it is over 100 amps. Sounds like it is charging fine. Is it because the light is on? Usually need an OE alternator on Fords for this problem.

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Al Instructor
Hamilton, Ontario
Al Default
 

The upper alternator is rated for 140 amps. The lower alternator is 120 amps. The PCM turns off the lower alternator when the glow plugs are commanded on to prevent over voltage to the glow plugs. Do you have Bi-direction controls were you can command the alternators on to 95% and monitor your output and see if the alternator has the ability to produce the amperage. You would then know if the…

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Mohammad Owner/Technician
Champaign, Illinois
Mohammad Default
 

"You would then know if the alternator will output the full amperage of 140 Amps upper and 120 amps lower. Is your AC ripple within specs (.30-.05 AC volts) on both alternators?" I am wondering based on what System Information and application catalog you got the above specs about the alternator output and the Ripple voltage...?! I do realize you meant (0.3-0.5 VAC). and not (.30-.05 AC…

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Al Technician
Hamilton, Ontario
Al Default
 

Output specs are from Mitchel,l AC ripple is a generic spec. I was not aware there are no bidirectional controls on that vehicle to turn the lower alternator on or off. you could also put a load on the electrical system and see if output increases.

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Kaimana Technician
Fairfield, California
Kaimana Update
   

This site is amazing!!! Thank you for all of your replies. I'll go ahead and take a measurement of the alternator output while commanding the duty cycle with a bi-directional scanner and update. update I was not able to command the duty cycle of the alternators. I'm wondering if it's just an on/off switch controlled by the pcm. I've taken an amperage measurement from the GPCM circuit when it was…

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Glen Owner/Technician
Arthur, Illinois
Glen Default
 

I my experience on Ford dual alt system the PCM will set a DTC or battery light if there is a problem.

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Marlin Technician
Estacada, Oregon
Marlin Default
   

I have not tested that version of dual-alternators, but the single systems delay alternator output after startup. It can really throw you off if you expect to see full output right away.

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George Owner/Technician
Mobile, Alabama
George Default
   

The top unit is a 135 amp....The Bottom is a 110 amp....I would unplug each alternator 1 at a time and see if they each put out there rated voltage ( 14.4v) and current output.A low regulator setting or whatever could prevent the upper alternator from only putting out the 100 amps you posted All the computer does is cut the alternators off or on as far as control is concerned​.​At start up 1…

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Kaimana Technician
Fairfield, California
Kaimana Default
 

Thank you for your response George! What is an LRC delay? And how do you test for a system drain? And battery discharge into the other battery after shut down?

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Allan Instructor
Winnipeg, Manitoba
Allan Resolution
 

The charging system on that truck is designed to slowly bring up voltage after start up due to the high current draw of the glow plugs. It is normal for the voltage to be low at start up and slowly ramp up. With the dual alternator setup the secondary alternator is not activated unless needed, so what you are seeing is normal operation.

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George Owner/Technician
Mobile, Alabama
George Default
 

Allan..... Also according to Ford.......If removing one of the alternator for whatever reason the system will need to be rewired The 6.0L Diesel engine Powertrain Control Module (PCM) will RANDOMLY select only one of the alternators to be active when the glow plugs are active. There is no way to tell the PCM that one of the alternators has been removed, so the removed alternator may be…

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