712 Relay configuration help

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wgstatenj asked Feb 13, '24

712 Relay configuration help

Need help trying to config the 712 to auto start my Sprinter engine based on SOC or Low Voltage of my Lithium batteries. Installed Mid City Engineering RV Remote/Auto Start module. If I touch the 2 wires they supply from their module engine starts. When I Connect to NO, Com does not work. I tried default, Charging, etc with no luck. Relay appears to work, but no start on engine. Any help would be appreciated. Notes from Mid City below

Optional start / stop / status out triggers for use with external input (eg: battery monitor with start/stop relay, compressor monitor, temperature monitor & more)

Note: when using this auto start with external 1-wire trigger (latched ground to start and run) the system will re-start if ground is still applied to start wire after run-time expiration. The system will continue running and re-starting until external trigger removes ground signal.

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Answer 1 · 2024-02-13T22:41:23Z

Justin Cook answered · Feb 13 at 10:41 PM

What do you mean "when I connect to NO, COM does not work"? COM is where you connect IN to the relay, it's not a port that performs work. So in your case (and to be clear, I strongly disapprove of those MidCity Engineering modules) what you'd do is run a simple wire from chassis ground in to COM, and out of NO to your module, since the module is looking for a switched ground wire.

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wgstatenj commented · Feb 13 at 11:01 PM

Appreciate the information. When they installed the module they supplied the pig tail connected from their module. No information provided by Mid City or could I find in their manual. My inexperience, but maybe the wire is coming from a chassis ground but will verify. Any more information on disapproval of Mid City module? Thank you, Wayne

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Justin Cook ♦♦ wgstatenj commented · Feb 14 at 12:39 AM

Hi Wayne,

My understanding of their module is that -when using the autostart function- it's just looking for a switched ground, that's why I say to run a ground wire into the COM port of the BMV and back out of the NO port... not sure how their two wires would hook up to that setup though.

With regard to disapproval of the system as a whole, well two main things:

First I absolutely loathe that their system uses an OEM fob with a button pusher left inside the vehicle at all times, since that defeats the OEM immobilizer system and obviously causes me some security concerns;

Second though, keep in mind that if you ever take the rig in for service or an update you absolutely must ensure that you either bypass their system using the (as I understand it) bypass jumper harness they should have given you, or uninstall the system, as noted in their user manual. The system is a piggyback on the ECU, so if you don't bypass or uninstall it before you take it in for service, the dealer may not be able to accurately diagnose any issues (best case) or, if the dealer tries to update the ECU, the update can fail and completely brick the ECU, which can cost between two and four thousand dollars and up to around 6 months to get a new ECU and a new OEM tune on the ECU.

I only know that last bit because one of my commercial installer clients recently dealt with this situation on a van that an end-user brought to him to have repaired, and the end-user had just gone through this process because the original vanbuilder hadn't advised the end-user of any of the dangers of the MidCity system, so the end-user took the van in to Mercedes for some routine maintenance, the dealer tried to perform a couple of ECU updates (since Mercedes is regularly releasing updates and recalls) without, obviously, bypassing or uninstalling the MidCity system since they didn't know it was there, and yeah... bricked the ECU and took 4 months and a little over $2500 to get the van back.

Bottom line, use with caution. It seems kinda neat (other than the janky button-pusher setup) but there are definite risks.

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Answer 2 · 2024-02-16T15:34:56Z

wgstatenj answered · Feb 16 at 03:34 PM

Followed your advice on running wire from chassis ground then NO to Mid City. SOC setting when reached, woke up dash as if hitting the push button once but engine failed to start no crank. Drone responded that engine started. What settings/configuration should be on 712? default or Charged, invert, closed time etc. Thank you, Wayne

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Answer 3 · 2024-02-23T00:19:20Z

wgstatenj answered · Feb 23 at 12:19 AM

Issued solved. After getting advice from Sunlight Conversions, ignoring Mid City, simply set voltage to desire setting, using default on relay with the 712, 0 closed time (did not work with 1 minute) it worked. Previous advice about Mid City from Justin or any device that connects to ECU no matter what vendor says will void warranty. Remove entirely, not just valet mode as stated by Mid City. Was at dealer today for another matter and was verified.

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