R34 Seat belt Resistor

[SammyP-R34]

Any help appreciated!

I have an R34 Sedan, currently seeking any information regarding the resistor/Ohms required to disable the seat belt light. I have already installed an airbag resistor however due to having new seat belts I need to get rid of the seat belt light (On dash) for compliance. 

Unfortunately  i do not have my old seat belts to plug in and test the Ohms for purchase the correct resistor. 

Have searched before posting. Thank you! 

[GTSBoy]

Buy 1k and 10k pots and have a twiddle.  When it goes off, measure with multimeter and replace with fixed resistor to same value.

[SammyP-R34]

I like the sound of this idea, however it concerns me I could set off the airbags. I have been told the SRS system all talks together and getting it wrong could lead to potential airbag deployment. Any thoughts on this ? Appreciate the reply  

[GTSBoy]

Air bag should not go off at zero speed.  Also, anyone who designs a safety system such that an explosive part of that system will go off when a peripheral sensor is not functioning properly needs to be kicked in the nuts.

Right now, you have no resistor = open circuit = light on.  Wire in 10k pot set to 10k ohms - should be much higher than the threshold value I would expect to "close" the circuit.  But if you're paranoid, you could go all the way to 20 or 50k as a starting point.  Work your way down until the light goes out.  If it turns out to be less than 1k, then probably better to experiment with a 1k pot, just to get a better guesstimate of the resistance required. 10k pots are a bit coarse.  Oh, and use linear pots too, not logarithmic.

I would expect the target resistance to be a nice number like 500 ohms, or 2k or 5k.  We use 2k & 5k a lot in industrial sensor loops to represent a working loop.

[SammyP-R34]

Thank you again. Are you suggesting a 3 pole/pin potentiometer so it plugs directly into the seat-belt plug itself and test from there? (photos attached) 

Cheers

 

[SammyP-R34]

https://www.jaycar.co.nz/1k-ohm-linear-b-single-gang-24mm-potentiometer/p/RP3504 Something like this ? 

[GTSBoy]

Yeah.  You use one end terminal and the middle.  The middle is attached to the wiper.  The end terminals are attached to each end of the resistor.

[GTSBoy]

But.... I am not suggesting that you hook up all 3 terminals of the pot to the wiring of that plug.  You need to know which 2 wires off the loom plug that you need to put the resistor on.

[SammyP-R34]

I understand, one more question (sorry) if I track the 2 circled wires back to the control unit to find the correct color. Do you feel these are the correct wires I am looking at? Far right of pic. Your a legend!

[GTSBoy]

Now that I see the wiring diagram, you do not want to just hook up the resistor across those two pins.  If you look very carefully at the circuit in the box on the left, you will see that it is a voltage divider of some description.  12V comes in the top, earths are at the bottom.  Now there is more than one thing going on in there, and I am not sure if everything is to do with the seat belt switch, but let's just look at the switch for the moment.

What I don't know (because I've never cared) is whether that switch is open or closed when the belt is not clipped in.  I'll take a punt that pin 16 on the SRS module has high impedance, so when the switch is closed, current will flow through the seat belt warning lamp, through the switch and out to earth, giving a warning lamp, indicating that the seat belt is not clipped in - and pin 15 sees a low voltage.  If you clip the seat belt in, the switch will open, no real current will flow, and the 12V will turn up at pin 16.  The seat belt warning lamp is a globe, so it will have some resistance, so it will contribute to a small voltage drop from 12V....but how much depends on the impedance of pin 16 and the globe itself.  You'd have to measure these to be sure, but it's quite likely that it is almost all the 12V turning up on pin 16.

Pin 15 is a different story.  The SRS warning lamp is an LED, so it does not have the same V=IR behaviour as a globe.  It will have some fixed forward voltage drop, and I don't know how much.  I also do not understand the drawing of the wire that looks like a sense line reporting from the earth side of the LED that goes back around into the side of the LED, appears to go out the other side and go back to the wire that runs to pin 15 on the SRS module.  Ignoring all that, the 12V comes in the top and through the resistor then goes over to pin 15.  If the seat belt is not clipped in (using the assumption I worked through in the above paragraph) then the switch will be closed and current will flow down through the seat belt lamp as before.....but there should still be 12V at the top of that circuit, so I can't see how the seat belt switch would affect the SRS warning LED or what turns up on pin 15.  I can't see how the switch being open or closed will affect the voltages and currents around the resister and the SRS LED in that voltage divider circuit.

In short, I do not understand the circuit enough to advise you.  You should have perhaps lead with the diagram, because I just assumed you knew that you simply needed to put a resistor across the wiring - but that's not clear from the diagram at all.  It is quite likely that the SRS module is able to compare the voltages that turn up at pins 15 & 16 and make decisions based on that, but that's just a guess.  Maybe someone who is actually more familiar with that type of circuit will wade in.

[SammyP-R34]

Would I be best to measure the resistance of a R34 Skyline with factory seat-belts to find the correct one I need without playing around? The Airbag warning light is not lit. Just the seat belt dash light. I just need it gone -.- So what your saying is that it is linked to the SRS system and that is why I now shouldn't put a potentiometer in?> Uggh haha.

[GTSBoy]

8 minutes ago, SammyP-R34 said:

Would I be best to measure the resistance of a R34 Skyline with factory seat-belts to find the correct one I need without playing around?

Probably not.  If you read through what I posted above, it should be clear that I do not think that there is a "resistance" that you could measure that would tell you what you need to wire in, because I don't think that wiring in a resistor will remove the seatbelt light.

What I actually think happens is that the seatbelt switch effectively removes 12V from Pin 16.  The way to mimic that would be to simply put 12V onto Pin 16.  But I must stress - I would not trust my say so!!!!!  That is all predicated on me understanding how that seatbelt switch works to make the light come on, and what must therefor happen on the line to Pin 16.  I could be wrong.

14 minutes ago, SammyP-R34 said:

The Airbag warning light is not lit.

That kind of lines up with what I was thinking would happen.  The seatbelt switch does not appear to be able to affect that light.  Not helpful though.

15 minutes ago, SammyP-R34 said:

So what your saying is that it is linked to the SRS system and that is why I now shouldn't put a potentiometer in?

Yes, that's what I'm saying, unless you know for a fact that that is how other people have solved this problem.  In which case they know something that I don't, because I can't see how it would work.

And keep in mind, that my recommendation for a pot was only for working out what resistance you need.  Not for permanent install.  replace pot with fixed resistor for maximum happiness in these situations.  (Stops it getting accidentally maladjusted 3 years later and causing problems).