Aux battery disrupts starting system?

[Et eT]

2018 Forester, Class C, on a Chevy 4500 Express chassis had a series of failures (i.e. engine would not restart after a long drive) and fix is being blamed on auxiliary battery that Chevy installs at the factory. Tech was surprised about this 3rd battery and had to call FR tech support and even FR said they don’t know what its purpose is. Anyone know what this 3rd battery does? More importantly, what is logic of the power management system specific to having a bad battery in the circuit?

[JTseed]

Found a couple photos of the mystery chassis "hidden 3rd coach battery" the dealer didn't know about. But no written explanation about why Chevy did it on
only that chassis nor how it's connected.

[Splitshaft]

This frame mounted chassis battery is not really a mystery. It is provided with the GM RPO 9L7 option package ordered on the Chevy 4500 Express by Forest River. 2016 and later Express cutaway models with RPO 9L7 will have two 30 amp fused power feeds for upfitter use. Forest River does not appear to use the three available connections, two of which are relay controlled for a total load current of 70 amps in my 2018 Sunseeker 2250LE. The connections for these auxiliary loads can be found near the foot operated parking brake on the 4500 chassis. The chassis auxiliary battery relay is located on the inside upper right (curb) fender. The fuses (3) and upfitter relays (2) are located center/left at the top of the firewall/bulkhead.

Since Forest River does not appear to use these upfitter connections for anything, the auxiliary battery does nothing other than add weight to the coach and does not power anything at least in my 2018 Sunseeker 2250LE model. If this auxiliary battery should fail, when the ignition switch is closed, the auxiliary battery relay will close and place a large current demand on the main starting/cranking battery.

In order to make use of this manufacture supplied auxiliary battery intended to be used to power auxiliary loads while isolated from the main starting/cranking battery, I have disconnected the main battery lead supplying the chassis auxiliary battery relay and relocated it to the Forest River installed jumper from the main battery to the DC circuit breaker leading to the coach battery charge controller. I than installed another 200 amp DC circuit breaker under the coach near the auxillary battery and connected it to the 250 amp DC circuit breaker in my under step battery box for my 2000 watt PSW inverter with heavy guage welding cable. The GM positive cable is also maintained (kept active) for the upfitter circuits using a positive battery bolt as Forest River uses on the main chassis battery. Now the auxiliary battery is charged when the coach batteries are being charged from the converter or alternator. And the auxiliary battery is supplementing the coach batteries to power 12V DC loads when dry camping.

If your coach is wired similarly and the frame mounted chassis battery is not desired and the upfitter connections are not in use in your coach. It would be best to remove the auxiliary battery and disconnect the feed to the auxiliary battery from the auxiliary battery relay. In my MY 2017 Chevy 4500, the auxiliary battery lead was on the top stud of the relay and the main battery lead was on the bottom stud of the relay. Once the auxiliary battery is removed and the feed to it disconnected, the main battery feed may remain connected to the relay as long as nothing comes into contact with the now unused stud of the relay. Insulate it with a suitable insulator.

[JTseed]

Thanks Splitshaft!

Great to finally see someone who has some actual knowledge and understanding of this system! Tried to get wiring diagram from FR and Chevy but neither will release them because they are "proprietary intellectual work products".

This post is about my son's rig and I've been trying to assist him (ET) since this is his first RV and he has not been able to even use it. Every time he goes out (half dozen or so) the engine will not start and it has to towed back to the dealer (one tow was over $1,000).

There's an intermittent electrical issue which no one can identify. The dealer blames the FR wiring, FR blames the Chevy engine control system, and Chevy blames FR and the dealer for modifying their engine control system. Sadly, he making a hefty payment for an unusable RV which no one know how to fix, nor how to get him out of the unsolvable mess.

Thanks again! This the first usable information we've seen in all these months of researching the problem.

[Stearn]

Would any one have some pictures on how to remove this gm installed battery? I want to remove it completely!

[Splitshaft]

To safely remove the GM auxiliary frame mounted battery if not used and not desired, is to first disconnect from any AC power so the DC converter is off and not powered and to place the coach battery disconnect in the “OFF” position. Next, disconnect both battery ground cables from the chassis engine starting battery and the under-body frame mounted auxiliary battery. Once having accomplished those steps it should be safe to work carefully accomplishing the following steps. Be aware, if not disconnecting the battery ground cable(s) from the coach batteries “usually under the entry step,” it may be possible for the charge controller to still send 12V power to the chassis battery if the coach batteries are at or above 13.2V. If not trusting your ability to keep wrenches from hitting metal objects when working, then go ahead and disconnect the coach batteries ground cables under the steps too.

The next step is to remove the auxiliary battery from under the coach by disconnecting the positive battery lead. Side terminal battery bolts require a 5/16” wrench. Next remove the auxiliary battery retaining block bolt with a 13mm wrench and the plastic block. When the battery cable has been disconnected and the retaining bolt and block removed, the battery may be removed from the tray. I recommend wrapping some electrical tape around the unused battery cables if ever desiring to use them again, or the short ground cable could be removed or cut from the frame and discarded. And the side terminal connection of the positive cable could be cut off and discarded, wrapped the end of the positive cable again with some electrical tape to insulate it.

The final step is to disconnect the “auxiliary” battery cable from the auxiliary battery relay on the inside of the right front fender under the hood. On my GM chassis, it was the top connection of the relay. The lower cable on the auxiliary battery relay was from the main chassis battery for the engine. If a test light is handy, these cables on the auxiliary battery relay can be identified once either the chassis or the auxiliary battery has been disconnected, but not both, as the one still connected will light the test light. The last step is to insulate the unused stud of the auxiliary battery relay so nothing short circuits coming into contact with it. The disconnected cable can hang free or be cut off at any convenient length. Now all battery ground cables can be reconnected and the auxiliary battery has been eliminated completely. Keep in mind though, if anything was ever actually connected to this auxiliary battery, it will no longer function once the battery is removed.

The pictures provided are for reference only and by no means are examples of how to remove the auxiliary battery. In these pictures the auxiliary battery has been connected to the coach batteries and still capable of powering the OEM GM wiring, relays, fuses if ever those circuit would be used for something in the future. The auxiliary battery is now charged and discharged with the coach batteries. No comments from the battery aficionados on the forum please, as they are not appreciated. In the photo provided, the power cable from the chassis battery has been reattached to the input side of the circuit breaker between the chassis and coach batteries. It is a safe place to keep it in addition to supplementing power flow between coach and chassis battery as allowed by the electronic battery charge controller in the coach. A high current circuit breaker was installed at the side of the GM auxiliary battery.