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Bench-to-Bucket Seat Swap: Low/No Cost Solution to Fix SRS Light Found!

23K views 57 replies 18 participants last post by  OutOfTheNorth 
#1 · (Edited)
First, the good stuff...

Center seat connector resistances [brackets include tested ranges]:
  • Occupancy Detection (white plug): 390 Ohm [360-400]
  • Seat Belt Pretensioner (half of yellow plug): 2.5 Ohm [2.3 - 3.2]
  • Seat Belt Buckle (half of yellow plug): 2300 Ohm [1900-4500] ***
    ***Edit: newer model PMs (~2017+?) seem to do better with a ~460 Ohm resistor in place of the 2200 Ohm I used for the Seat Belt Buckle
    ***Edit 2: it seems the wiring for the 2.5Ohm and 2200 Ohm pairs is sometimes swapped from what is shown.
    ***Edit 3: AlfaOBD has a BCM configuration option that appears to allow reconfiguring the seating layout to not include the center seat, potentially making this thread obsolete.
59828


The longer story...

This has been beat to death on this forum, but as far as I can tell, there are no reported successes without purchasing another seatbelt buckle assembly ($280 MOPAR), or in rare cases, the "ebay dongle". Took me awhile to notice that. With countless hours dumped into this, I was at my wits end about to cave and purchase the MOPAR part or maybe hit up a junkyard for late-model FCA belt buckles.

The primary gotchya is that the seat belt switch is a Hall Effect sensor. *** Chrysler/FCA.
The Occupational Restraint Controller (OCR) applies a voltage to it every 100ms and reads the output current. If it is outside the expected range, your SRS light pops on after 2.5 seconds.

Notably, when you measure the switch resistance with a multimeter, you will find an open circuit whether the belt is buckled or unbuckled.
This is because the sensor requires a voltage across its leads to operate.

Couldn't find any spec on the Promaster Hall-Effect sensor but I found a datasheet for another hall-effect seat belt switch (attached).
Wondering if the switch could be spoofed with a resistor for a particular state, I used Ohms Law to calculate a substitution resistance. I assumed 12V "operating voltage" and 5mA output current:
12V/0.005A = 2400 Ohm

I was ready to test with a bunch of resistances and had set up a potentiometer to do so (pic attached). Miraculously, the first attempt nailed it.

Cycled ignition 3 times and the SRS light was off for the first time (10,000miles, 6 months) since removing the bench seat . Did not have to start engine. Dunno if I actually had to cycle ignition.
EDIT: There are some reports of needing to start the vehicle for the change to take effect.

I did not unplug the battery for this. I used to, but came to the conclusion that the only thing we need to avoid is applying a voltage to an existing squib. Latent charge in the OCR could possibly do that so I would still recommend disconnection of battery before unplugging connectors from any seat. We are working only with resistors.


The occupant detection thing (white plug) measures 400-Ohm empty, begins registering the seat belt warning around 350 Ohm and goes down to 20 Ohm with a heavy person in the seat. I went with 390 Ohm as I did not want to risk a fault by going over 400 Ohm.
I have no code reader that can read SRS codes.

Other notes pertaining to misconceptions seen elsewhere in the forum:
  • The OCR system is "floating" as to prevent accidental discharge, so the ground is isolated from the chassis ground.
  • The short circuiting shunt is only applied on the seat-side of the yellow connector when it is disconnected. Not our problem here.
  • The Airbag system is almost certainly entirely disabled if the SRS light is on. If OCR is getting funky measurements, it does not want to risk accidental deployment.
OCR seat belt switch operation - From stolen Chrysler tech doc (posted elsewhere on forum)
59825


Big thanks to all the others who paved the way in other forum posts.

Good luck, hope this helps!

Edit: @smdub reverse engineered the whole system, worth taking a look at his thread if on the same topic you have trouble.
 

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#37 ·
I believe post #28 is referring to the airbag pre-tensioner, part of the yellow clip that connects to the seatbelt. A seat with an airbag installed in the seat has the yellow clip for the seatbelt, the passenger has the occupancy sensor and both the driver and passenger seats have an orange connector that attaches to the internal airbag. I have searched the webs and cannot find a specific post for a promaster in seat airbag ohm rating.
 
#38 ·
Are these actual OEM promaster seats? Do they have the matching plugs? If they don't, the above fix will fool the ECU. If you are just trying to put a resistor where the airbag is supposed to be (blown airbag?) I believe it's 2.5 ohm. Everything I've read suggests 2.5 is the common resistance for airbags.
 
#40 ·
I have read this thread 3x now and am more confused than ever. Can someone please do a summary of the process and what resistors go where? I really want to swap out the bench seat but it seems so intimidating. If I can solve the issue without buying Mopar parts or taking apart the bench seat to salvage sensors that would be awesome. What about going to a place that modifies vans to make them wheelchair accessible? I need help!!!
 
#41 ·
I feel your pain.I tried a few methods on this site with no results. This method does work. Refer to the pic in post #1.

My final setup was 2.5 ohms on the airbag connection, 460 ohms (in place of the 2300 in the pic) on the seat belt connection and 400 ohms for the occupancy sensor. Just cycling the ignition didn't clear the SRS light for me. After starting the van and running for a minute or so, the lights cleared.
 
#44 ·
Wish I had found this post earlier! The dealer charged me $140 to tell me the airbag light was on because the sensors for the middle sweat were unplugged. Ordered a range of resistors and connectors to replace the sensors for $26 shipped from Elliot Electronics. Will report on results. Looks simple enough, thanks for the info!
Good luck my dude. Mind the note regarding different resistance for the seat belt buckle required for 2017+ PMs!
 
#46 ·
Tackled this today in my 2017 with the following resistors:
  • Occupancy Detection (white plug): 380 Ohm
  • Seat Belt Pretensioner (half of yellow plug): 2.4 Ohm
  • Seat Belt Buckle (half of yellow plug): 470 ohm
Worked the first time, and there's no way I could have done this without this (and other) threads on this subject. Thank you very much @scribblersix for all of your diligence!!!
 
#47 ·
Thank you for the excellent information guys
Success on my 2017
Occupancy 390 Ohm
Pretensioner 2.7 Ohm
Buckle 470 Ohm

I actually tore the wires off the center seat connector when I lifted out the bench and they got caught, but will end up with a clean look when directly soldered, the bundle will be heat shrunk once more then It will be put back under the floor where it came from and zip tied along itself
72266
 
#49 ·
I could use a bit of input from all you clever people. I'm trying to walk in your footsteps with:
Occupancy 390 Ohm
Pretensioner 2.7 Ohm
Buckle 464 Ohm

I don't get a seatbelt alarm, just the "check airbag system" message. I cycled the engine, let it run a bit, disconnected and reconnected the negative battery terminal and still can't clear that **** message. My next step is to solder all the resistor/pin connections to make sure everything is tight. But does anyone have guidance?
75385
 
#51 ·
I could use a bit of input from all you clever people. I'm trying to walk in your footsteps with:
Occupancy 390 Ohm
Pretensioner 2.7 Ohm
Buckle 464 Ohm

I don't get a seatbelt alarm, just the "check airbag system" message. I cycled the engine, let it run a bit, disconnected and reconnected the negative battery terminal and still can't clear that **** message. My next step is to solder all the resistor/pin connections to make sure everything is tight. But does anyone have guidance?
Where did you get your new single seat from that your replaced your bench seat with? If the new single passenger seat has a bad occupancy weight sensor or some other airbag wiring issue it could be the reason your still getting the check air bag alarm because it looks to me like you have the right resistors in place for the now missing middle seat. That said, make sure you have the right resistors in the right half of the yellow connector, its easy to get those ones flipped around depending on how your looking at the connector.

If all else fails, bring the bench seat back into the van through the sliding door. lay it on its back and re-connect the bench seat to the vehicle and see if that clears the check air bag system. If you get it to clear, then one by one disconnect a connector from the bench seat and replace it with a resistor (or plug it into the new single passenger seat) and see if you are able to keep that checked air bag warning off. This trial and error system is time consuming but it will systematically show you what connector has the issue that triggers that air bag warning!

Best of luck!
 
#54 ·
After many many tries, I finally got this to work on a 2018. The problem I had was that in previous posts, the resistors were listed in a different order than the photo if reading from left to right. Here's a photo of the configuration that worked for me:

Luggage and bags Bag Purple Electrical wiring Gas


From LEFT to RIGHT, the resistor values are 390 Ohm (on the white), 470 Ohm (left side of yellow), and 2.7 Ohm (right side of yellow.)

One final thing to note is that I tried to attach the resistors to the wires directly (see the stripped parts) but the solution only worked once I pushed the resistors against the pins of the yellow dongle. This approach seemed to be fine for the white dongle though. No idea if this is real or if I just didn't connect them well the first time.
 
#56 ·
Just completed a bench-to-bucket passenger seat swap conversion on a 2021 Promaster 3500. Infinite thanks to all who have gone before, it went flawlessly. Here are the resistors I used:

Seat Belt Tensioner (1/2 of yellow connector)
Wires: Orange/Blue, Orange/White
Resistor: 2.7 Ω

Belt Buckle Switch (1/2 of yellow connector)
Wires: White/Violet, Black
Resistor: 470 Ω

Occupant Detection Sensor (white connector)
Wires: Green/Red, Black
Resistor: 390 Ω

I snipped off the connectors on the van loom side, then cut back the wrap about 6 inches. Stripped the wire ends, and soldered the resistors between them, with adhesive heat shrink tubing around everything. After testing, I wrapped everything up and zip tied the wires to the new seat. All good!
 
#57 · (Edited)
The following procedure can be used to reconfigure a bench seat to single passenger seat on a Promaster 3500 2016. This was successful first try on my PM, no issues. Maybe it will work on your year/model too.

The body computer needs to be told to stop checking the condition of the middle seat 3x endpoints, X number of times per second etc. If the endpoints do not respond due to a disconnected cable then airbag light will be on and entire airbag system will be disabled.

I have a PM2016 and just swapped my bench seat for a PM2017 single seat. I did not use any resistors or solder anything. I used OBDMX+ adapter from Amazon and the 'yellow cable' which gives access to the body computer for read/write.

I bought AlfaOBD software for my Windows laptop that connects to OBMDMX+ adapter over bluetooth. I visited my Chrysler/Ram dealer and asked for the five digit access code to the body computer via the 'DealerConnect' software that they all use in Canada. The access code is per-vehicle and based on VIN.

Attached OBDMX+ to yellow cable, attached yellow cable to OBD port.
Registered bluetooth connection to OBDMX+ adapter from Windows laptop.
Opened AlfaOBD software and connected to OBDMX+ adapter from my Windows laptop over bluetooth.

In the AlfaOBD software:
Opened a connection to the body computer and gave PIN code from Chrysler dealer.
Selected the tab for diagnostics.
Scrolled down to 'Car Configuration Change'

In the 'find setting' field I entered 'cen' to find settings concerning central seat. I changed the values to 'Not Enabled' for each of the following settings, but only one at a time. Each setting I would make the change then disconnect and shut down the connection to the body computer, let the vehicle sit for a while with engine turned off and key out. Probably unnecessary/tedious but I saw advice from others about only changing one thing at one time.

Front Central Passenger Pretensioner - Not Enabled
Front Central Passenger Presence Detection - Not Enabled
Front Central Buckle Switch - Not Enabled

To my understanding there are three CANBUS endpoints that need to be unregistered from cyclic enumeration by the body computer. The procedure for enacting this de-registration is called a 'Proxy alignment'. After the three above changes had been made, I triggered a Proxy Alignment from the diagnostics task list. The airbag light was off at next engine start.

SUGGESTIONS
  • Do this reconfiguration change in AlfaOBD prior to removing the bench seat and disconnecting any wires. Check that airbag light is off after changes in the body computer. Then disconnect the wires and confirm again that airbag light is off. This way, if you introduce a new single seat from a different year/model with different resistance ratings in the sensors, you have at least narrowed it down to the three leads in your new mismatched seat and can get busy with some resistors and soldering or parts replacement.
  • the OBDlink SX is cheaper and seems to do the same thing as the OBD MX+ on older PM's but is usb instead of bluetooth
Gas Electronic device Machine Cable Wire


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Rectangle Font Screenshot Software Parallel
 

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