Rotary coupler airbag circuit continuity question!

To hell with my "ongoing problems" thread for this one - this is just a simple question!

On a rotary coupler there is a yellow "conduit" carrying two airbag wires (red and yellow). There is a red plug on each end.

If I put the multimeter on "Pin A" at one end, I get a circuit on both pins at the other end with equal resistance.

If I put the multimeter on "Pin B" at one end, I get a circuit on both pins at the other end with equal resistance.

Basically, both pins at one end connect to both pins at the other end.

Does this mean it's broken or is this built into the design as a fail-safe and the airbag is earthed elsewhere?

That's not good... in this case i suppose u have closed circuit between A and B at the same side too which is classic short circuit ... anyway they are separate circuits( example is from D2 diagram but the coupler circuit is the same).

bugger... so the first one from the scrappy doesn't work.

The second one had a melted plug despite being "tested and from a working vehicle".

The third one looks fine, but has the problem described above.

Three times unlucky... am I ever likely to get a working rotary coupler from a scrappy or should I just fork out for a brand new one, given the MOT is looming??

Only trouble is... scrappy - £37.00, brand new - £200.00.

rotary coupling is no more than a tape of wires (quite a few feet of it)

yeah, i know what's inside, but given that the scrappy has sent me 3 bad ones, I'm tempted to just spend £200 on a brand new one... or do you think i'm just very unlucky and the fourth one will almost certainly work fine?!?

i have no experience of buying these damn things!

give me the number ill price one for you

Insist with the ohmmeter... it must open circuit between 2 pins on the same side with the other side "in air" then bridge the pins one side and it must be closed circuit on the other side

the part number? I *think* it's YRC100350...

pricey £168 including vat

Tempting... trouble is, my local garage said both pins are supposed to have continuity with the other two pins.

He says the coupler is fine and this is how they're designed.

I don't know what to believe now! This one could be fine, but maybe not.

The circuit diagram suggests they're two completely separate wires, but my garage says both pins are supposed to be connected to the other two pins...

Oh, and Marshall's Land Rover gave me a very vague answer basically saying "some are like that, others aren't. Depends on your model". Useless.

I didnt check myself that but as graduate in avionics engineering, according to the diagram i can tell without fear that they're full of sh*t. It has no logic to be like that cos in this case it's no need for two pins at one side. Moreover based on the electrical circuit description from RAVE(for D2):
Driver airbag
The driver airbag is connected, via the rotary coupler, to the airbag DCU on an
RY wire and an R wire. The RY wire supplies a negative digital output and the
R wire supplies a positive digital output for airbag operation.

If it's like they said there's no difference between the RY wire output and the R wire output cos on the other side they mix themselves...which is a classic short circuit example

I can't believe that on D1 is different.

Or that system is above my knowledge

sierrafery said:

I didnt check myself that but as graduate in avionics engineering, according to the diagram i can tell without fear that they're full of sh*t. It has no logic to be like that cos in this case it's no need for two pins at one side.

Click to expand...

Well, the only reason I could think of having two pins at each end, with them both combined into one single "wire" is that if one splits or breaks, there's another available, so a bit like a failsafe.

The diagram suggests this is rubbish though, and so do you, so I think this is another faulty coupler. I think it's rather unbelievable that the scrapyard has sent me THREE faulty couplers now though... they happily replace them if i'm not happy, but this means i'm about to get my fourth one sent through! Is it worth it or is this a common fault I'm 99% likely to see again?

sierrafery said:

Moreover based on the electrical circuit description from RAVE(for D2):
Driver airbag
The driver airbag is connected, via the rotary coupler, to the airbag DCU on an
RY wire and an R wire. The RY wire supplies a negative digital output and the
R wire supplies a positive digital output for airbag operation.

If it's like they said there's no difference between the RY wire output and the R wire output cos on the other side they mix themselves...which is a classic short circuit example

I can't believe that on D1 is different.

Or that system is above my knowledge

Click to expand...

Is the RY wire the yellow one and the R wire the red one? I don't think I have a wire that is both red *and* yellow.

As for there being "no difference between the RY wire output and the R wire output cos on the other side they mix themselves" you say this is a classic short circuit example... do you mean this is a normal short circuit, or that this particular fault is a regular fault that happens to a lot of people?

My theory is based on the fact that one wire outputs positive signal and the other negative at A side... so if on A side one pin is positive, the other pin is negative and each pin from A goes to both pins from B which is the negative and the positive on B side??? As i said check with ohmeter between the pins on one side...if there's closed circuit IMO there's short circuit

I know that rotary couplers used to fail and they can fail two ways:
1. interruption = open circuit
2. short circuit

RY=red/yellow , R=red

OK, here's what it's like:

PIN-A----------------------------------------PIN-A
PIN-B----------------------------------------PIN-B

Touch "Left PIN-A" and "Right PIN-A" with multimeter. Makes noise. Resistance is 0.08 ohms (I think I put the decimal in the right place).

Touch "Left PIN-A" and "Right PIN-B" with multimeter. Makes noise. Resistance is 0.08 ohms.

Touch "Left PIN-B" and "Right PIN-A" with multimeter. Makes noise. Resistance is 0.08 ohms.

Touch "Left PIN-B" and "Right PIN-B" with multimeter. Makes noise. Resistance is 0.08 ohms.

So basically it's one big wire.

This must mean there is a short circuit somewhere in there and it's knackered then...

Time to get the scrapyard to send me a *FOURTH* coupler in the hope that I get a good one this time...

fishsponge said:

OK, here's what it's like:

PIN-A----------------------------------------PIN-A
PIN-B----------------------------------------PIN-B

Touch "Left PIN-A" and "Right PIN-A" with multimeter. Makes noise. Resistance is 0.08 ohms (I think I put the decimal in the right place).

Touch "Left PIN-A" and "Right PIN-B" with multimeter. Makes noise. Resistance is 0.08 ohms.

Touch "Left PIN-B" and "Right PIN-A" with multimeter. Makes noise. Resistance is 0.08 ohms.

Touch "Left PIN-B" and "Right PIN-B" with multimeter. Makes noise. Resistance is 0.08 ohms.

So basically it's one big wire.

This must mean there is a short circuit somewhere in there and it's knackered then...

Time to get the scrapyard to send me a *FOURTH* coupler in the hope that I get a good one this time...

Click to expand...

its not unusual they are quite fragile ,and many in scrap yards broken coupler would have been one of the straws that put car there plus ,it will be ripped out not taken out

Most srs plugs use shorting bars when undone to protect from accidental setting off,if you unplug the srs dcu and check any of the connections on there you will get the same result.This is why LR set up Testbook to tell techs to folow the fault codes - with big warnings not to mess around with testlamps/multimeters etc.
All that stuff is designed for single use,so any melted stuff has been deployed.
I'll never forget the first P38 that I checked out with Testbook when the srs lamp didnt work.... Testbook simply said,"The car has crashed,replace the complete srs system"

ok, so rotary couplers are fragile, short circuits in them are frequent, but I might be seeing this result from my multimeter simply because of the shorting bars inside it?

Does this mean that the circuit diagram is correct (i.e. they are separate wires), but due to these shorting bars I am seeing it an one complete circuit?

If this is the case, then my only option is to fit it and then pay someone to try and reset the SRS light or read the error codes again... is that right?

Most srs plugs use shorting bars when undone to protect from accidental setting off

Click to expand...

Now that's a brilliant explanation.... if it's so the coupler might be good in the end. (i didnt know that)

bugger... this means I basically have to fit it and then pay my garage to try and reset the light (they charge £66 and no other garage near me is able to do it).

How can I test this coupler while it is off the vehicle?

fishsponge said:

ok, so rotary couplers are fragile, short circuits in them are frequent, but I might be seeing this result from my multimeter simply because of the shorting bars inside it?

Does this mean that the circuit diagram is correct (i.e. they are separate wires), but due to these shorting bars I am seeing it an one complete circuit?

If this is the case, then my only option is to fit it and then pay someone to try and reset the SRS light or read the error codes again... is that right?

Click to expand...

Fraid so,if its for your D1- they were still sort of playing around with SRS stuff then,often the ETM's didnt contain wiring diagrams for them.And worst of all the DCU wont reset after a fault like say DII ones will - where the fault remains logged,but if it self tests OK on boot up it will put the light out and function normally.(Hopefully to do nothing...)