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Old 01-07-2014, 08:11 AM   #1
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Sensing wires

Does anyone know what sensing wires on the altenator are for. Recently had to have altenator changed out on the road and found the sensing wires were not put back on, called the road service that did the repair and was told didn't need them. Everything seems to be working fine without them, wondering if I should reattach. It is a 2004 freightliner chassis.
Rick
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Old 01-07-2014, 10:19 AM   #2
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call freighliner and ask them. It might have been sensing wires for external voltage regulator for instance. If your new alternator has an internal regulator you would not need them and they might damage the alternator if you put them back on. This is just an example of what they might do. Freightliner can tell you for sure.
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Old 01-07-2014, 11:03 AM   #3
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Originally Posted by gemini5362 View Post
call freighliner and ask them. It might have been sensing wires for external voltage regulator for instance. If your new alternator has an internal regulator you would not need them and they might damage the alternator if you put them back on. This is just an example of what they might do. Freightliner can tell you for sure.
The alternators in motorhomes are fairly specific. The guys who replaced just slapped an off the shelf unit on there and it may or may not be ok. Freightliner can tell you for sure. If you kept the old alternator, take it to a rebuild shop and have them rebuild it to OEM specs.
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Old 01-07-2014, 12:10 PM   #4
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Some alternators are controlled by the igintion switch, Field power (Well rotor power) comes from one of those "Sense" wires.

Some alternators are "Self Enegerzing" and thus do not need external power to energize.

Some alternators have an external voltage regulator and thus the wire just mentioned is fed via that.> Some have internal regulators.

The other wire can be either a remote voltage sense or the idiot lamp.
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Old 01-07-2014, 04:12 PM   #5
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Last year my alternator went out so I removed it and had it rebuilt and my next trip out it started outputting 13-16.7 volts just up and down whole trip. When i got home I took back to the rebuild shop and they checked it out and said it was ok and that I should check my sensing wires. I found a connection on sensing wire corroded. I repaired it and have had no problems since. So having said all that you might want to check alternator output is within limits, because if it is outputting 16 volts it will boil batteries very fast.
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Old 01-07-2014, 08:34 PM   #6
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Originally Posted by brookra1 View Post
Does anyone know what sensing wires on the altenator are for. Recently had to have altenator changed out on the road and found the sensing wires were not put back on, called the road service that did the repair and was told didn't need them. Everything seems to be working fine without them, wondering if I should reattach. It is a 2004 freightliner chassis.
Rick
I think it depends upon the type of alternator that was installed. Sounds like the original had an external sense wire. The external sense wire is used to monitor the voltage actually getting to the battery so that the internal voltage regulator in the alternator can adjust accordingly. It cranks up the output voltage to overcome voltage loss due to wiring resistance.

I copied the below text from
Alternators Installation Note | Documents | McLaren Applied Technologies

Quote:
The Sense (S) connection, where fitted, should be connected to the vehicle battery via a dedicated wire which is not connected to any other circuit. This ensures that the regulator, inside the alternator, monitors the voltage at the battery and continually adjusts to keep it constant. If the sense wire is connected to the alternator B+ terminal, the battery voltage will be less than 13.5V because the alternator will not compensate for the voltage drop caused by the resistance of the wires joining the alternator to the battery.
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Old 01-08-2014, 07:23 AM   #7
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I was standing with the mech. when he called Freightliner to find out if the alternator he had was ok to use and Freightliner approved it. Must have an internal regulator on it and not need the sensing wires. Watched voltage for the rest of the 1200mi. we traveled and it ran from 13.6-14.0V and never varied from that. Will call freightliner thou and make sure everything is right with this alternator on my coach.
Thanks, Rick
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