View Full Version : Auxilary battery install triggered LHM?

09-15-2013, 10:42 PM
Hello all, I apologize if this is a stupid question... I did try to search.

I just installed an auxiliary battery in a buddy's 2012, with a Battery Doctor automatic disconnect. Once I connected everything back up, it started and idled fine, the auxiliary battery charges fine when the engine is running, and the automatic disconnect does disconnect properly shortly after the engine is switched off.

However, I've got a CEL and traction control warning light, the shifter is locked (automatic, used the bypass button to get it in gear) and it's in limp home mode. Any ideas? I did nothing at all to it beyond installing the auxiliary battery stuff, and had the main battery disconnected while I did the work. Brake lights work fine, van only has 12k miles. No problems whatsoever before the battery install.

I gather it will need the proper scan tool to reset LHM, but any insight on what caused this and what the proper fix is?

Thanks very much for any help.

09-16-2013, 03:17 PM
Oh dear.
Are you familiar with working on Sprinter electrical systems?
Did you use a factory installed auxiliary battery connector to hook up to or just make a new tap from the PDC?
These vans have an onboard computer system that monitors all sorts of unexpected electrical systems over the CANBUS network in the vehicle.
Others have had problems with hooking up devices, not sure about batteries but I would expect them to be problematic if it didn't at least have the wiring installed at the factory. Even then you might need a diagnostic tool to re-program it to accept the second battery.

You should ask in the NCV3 subforum (T1N model stopped in 2006), and RV subforum, almost everyone in the RV subforum has an auxiliary battery setup, some of them quite complex.

Unfortunately I can't offer any more help, not only is my Sprinter a T1N it is earlier than any of the US models and doesn't have CANBUS so is tolerant of running traditional taps off the battery or fuse box, I just know from reading the forums for over a year that the newer ones are not :-(

Good luck.

09-16-2013, 03:30 PM
Thanks Boater... 'Oh dear' is right. My buddy asked me to help with auxiliary battery install, and being somewhat 'old-school' I assumed it would be just as straightforward as it was 15 years ago. Silly me. Apparently if you burp without excusing yourself these things throw a code and lock you in second gear. :rolleyes:

Anyway, reading here has certainly convinced me that I don't need to be fooling with his van anymore. Should have read first and wired later, but never would have thought a simple second battery install would lead to such a catastrophe. :frown:

09-16-2013, 05:14 PM
If you pulled a new wire from the battery back (6 guage) for the Aux Battery, and have a relay that connects the Aux Battery only when the engine is running (rated at 200 amps is suggested), you are probably ok (there is a channel between the seats in the T1N's to allow you to pull a 6 gauge battery cable between the front seats). If you wired it any other way, you are probably going to damage your Sprinters Electrical system.

NOTE: There are fuses on the + battery connection - this is where you would tie into - with a 150 to 200 amp fuse.
I have sine wave inverters and my subwoofer tied into the electrical system this way. Other than tapping into a switched power circuit to drive the relay that connects the battery to the devices I mentioned, I don't tap into any power wiring in the Sprinter.

You need about 12 to 15 feet of 6 gauge wire - 8 feet from the Battery to down thru the engine compartment and up thru the rubber opening (where the wiring harness comes thru the floor into the area below the drivers seat). Add the relay here. Then pull a 6 gauge wire under the floor to the passenger seat to your Aux Battery. If your Aux Battery is some place else, then you may need more wire. The Body is the - (ground) connection (altho you can pull a 6 guage wire for ground if you want; however, you won't be able to get 2 6 gauge wires thru to the passenger seat thru the floor channel).

I have no idea about how to do this other than for the T1N's .

09-16-2013, 07:37 PM
Thanks... per Boater, the van is actually a NCV3 so I guess this is the wrong spot.

Anyway, I hooked it up just as you say with an isolation relay that is only on with engine running, and I even initially fused the connection to the main battery at 30 amps just to help ensure I wouldn't blow anything up.

On startup we got the codes and LHM. Isolation relay works fine, battery charges, fuse is fine. Buddy took it to a dealership this morning and they told him the transmission control module was fried. I'm afraid I cost him a lot of money trying to help. It blows my mind that merely hooking up a second battery would be enough to destroy an $800 control module. What do you guys do if your battery dies... buy a new van??? :laughing:

Anyway, I understand that perhaps all the sensors and computers and so-forth could note a change in the electrical system and throw a warning or whatever, but to have this actually destroy a major component seems odd.

09-16-2013, 08:26 PM
Even for a finicky Sprinter electrical system that sounds odd!

09-16-2013, 09:04 PM
It blows my mind that merely hooking up a second battery would be enough to destroy an $800 control module.

Correlation does not equal causation.

I have my doubts that you damaged anything if you proceeded as you describe. Dealerships are quick to say module problems since they are a simple and profitable fix. There are at least three possible scenarios:

1. You did nothing and some other reason caused LHM.

2. In the process of installing the battery some other wiring was disturbed. In this case it is possible that the transmission control module is still fine.

3. While installing the battery you arced/sparked/reversed a connection and damaged the module. I'd consider this as somewhat unlikely. If the only connections you made are a ground connection and a isolation switch connection between the main and aux battery there is little way for the system to "know" that there have been any changes. That said I cannot find a description of a "Battery Doctor automatic disconnect". If this device has a relay without a suppression diode it could kick back a damaging voltage spike into the electrical system every time the relay coil magnetic field collapses.

I guess a good question is did simply replacing the module fix the problem?

09-16-2013, 11:31 PM
Thanks Wires. I definitely don't think anything I did was really seriously wrong... I design and build control systems for industrial automation for a living, so while I'm pretty clueless about CAN bus and these vans in particular, I do not do silly stuff like hook batteries up backwards, make/break connections under load, etc.

I really want to think that I did not smoke the module with my work, but while correlation /= causation it seemed like an awful big coincidence. Unfortunately my buddy went straight from 'having the codes read' to 'van on the wrecker going to dealership' before I had any chance to look at it more. I'm almost wondering if we just bumped loose the connector to the TCM while working on it.

We will not know if module replacement fixes it, and at this point they could very well wiggle the connector, 'fix' it, and charge my friend... I have no way of knowing now. Anyway, this is the isolator we used. It's obviously not the slickest piece of kit, but I'd think it would do the job -


So now the big question is once it's fixed, do we try it again at risk of killing it again... or do we pay megabucks to have it factory installed. Hmmmm.

Thanks to all for trying to help! :cheers: