Tow Haul Mode and Trailer Batery Charging

I searched threads but couldn't locate one that covered this topic.

In a forum devoted to RV's there was a post from a usual contributor that you have to be in tow/haul mode to charge your trailer or turn your headlights on as an alternative. In response to a question from another person he said he had read this in the truck manual. I checked mine and sure enough there it was. I then did some research and found some question as to whether this only applies to gassers in light-duty (1500) pick-ups and to utility vehicles. Below is a GM Bulletin that just adds to the confusion.

The question is does this apply to my 2008 C2500 with the Duramax? I only use tow/haul in hilly areas to conserve on fuel.

Service Information 2008 Chevrolet Silverado - 2WD | Sierra, Silverado (VIN C/K) Service Manual | Document ID: 2170219
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#07-06-03-009A: Information on Voltmeter Gauge Fluctuation Due to Regulated Voltage Control System and Concerns About Camper/Trailer Battery Charging - (Jul 21, 2008)


Subject: Information On Voltmeter Gauge Fluctuation Due To Regulated Voltage Control System And Concerns About Camper/Trailer Battery Charging


Models: 2005-2009 Cadillac Escalade Models

2005-2009 Chevrolet Avalanche, Silverado, Silverado Classic, Suburban, Tahoe

2005-2009 GMC Sierra, Sierra Classic, Yukon Models




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This bulletin is being revised to add the 2009 model year. Please discard Corporate Bulletin Number 07-06-03-009 (Section 06 -- Engine/Propulsion System).


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Voltmeter Fluctuation Condition
Some customers may comment that the voltmeter is fluctuating between 12 and 14 volts on their full size pickup or utility vehicle. Starting with the 2005 model year, light duty full size pickups and utilities are equipped with a new Regulated Voltage Control (RVC) system. This system reduces the targeted output of the generator to 12.6-13.1 volts when in "Fuel Economy Mode" to improve fuel economy. The generator may exit "Fuel Economy Mode" if additional voltage is required. This will cause the voltmeter to fluctuate between 12 and 14 volts as opposed to non-regulated systems that usually maintain a more consistent reading of 14 volts. This fluctuation with the RVC system is normal system operation and NO repairs should be attempted.

Camper/Trailer Battery Charging Concerns
Some customers may comment that when towing or hauling a camper/trailer, the auxiliary battery for the camper/trailer will not stay charged. In most cases, this concern is blamed on the new RVC system. While the RVC system does reduce the generator's targeted output voltage to 12.6-13.1 volts when in "Fuel Economy Mode", this feature is bypassed if the tow/haul feature is enabled. With the tow/haul feature enabled, the RVC system will stay in "Charge Mode" and the targeted generator output voltage will be 13.9-15.5 volts, depending on the battery state of charge and the estimated battery temperature. To keep the generator in the "Charge Mode", use either of the following two methods.

• The first method is to use the tow/haul mode when towing or hauling a camper or trailer.

• The second method is to turn on the headlights, which will increase the generator's targeted output voltage to 13.9-14.5 volts.

It gets clearer but not yet certain. Check out the table on page 3. in this publication.

http://www.sandyblogs.com/TECH_CONNE...05%20Final.pdf

Perhaps this belonged in the trailer section, my mistake.

The bulletin applies to your truck, but has nothing to do with "how" the charge circuit works.

The "must be in T/H mode" comment is hogwash. The electrical system circuitry is the same, regardless. Voltage management may be effected, but nothing outside of what is stated in the bulletin. The voltage management does not turn circuits on or off. It only varies the line voltage to minimize alternator parasitic power loss at the accessory drive (belt).

On some vehicle/trailer combinations, headlamps may have to be "on" to provide power to the trailer charge circuit. HOWEVER, this only applies to trailers wired to charge from the running lamp circuit, which is almost always for trailers using connectors (4-5 pole) that don't have a dedicated charge circuit pole. As mentioned, this is common for utility trailers. If you are using your OEM, or similar, trailer connector, it has a dedicated charge circuit, and is HOT at all times.

All GM (and Ford) trucks from the 90's and up have always had proper trailer wiring. OEM trailer wiring included a 7 pole RV connector, with a dedicated charge circuit. That condition may have applied to Dodge trucks. They never seem to be consistent, from year to year.

Just a thought, when we were checking the 7 pole connecter, there was no voltage coming through the hot pole. On checking the fuses under the hood, there was no fuse in that slot from the factory. When we put in a fuse we had voltage over the hot connection on the 7 pole.

Did a lot of research. The RVC system is not installed on trucks with a GVWR above 8600#'s. Looked all along both negative battery cables and no module.