[TNEngineer]

I will share whatever I learn.

The wiring harness in question is the one running under the seat(s). I don't know how that (dis)connects to other harnesses in the vehicle, but I presume it's not like the entire dash/body harness.

I suspect after the Lotus is out of warranty I'll be remaking that harness. I had to do that on my diesel truck. Manufacturers foolishly use stiff, low strand count wiring in these harnesses and they work-harden and then fail as they get flexed. In the case of the truck, the driver's side rear door - where my then-toddler son's carseat was located which meant it got opened and closed a lot - frequently broke at least one of the many wires in the harness that passes from body to door. The electric lock would stop working, or the window, or the speaker. I finally cut out a foot or so of every wire in the harness and replaced them all with a section of high strand count, silicone insulation test lead wire which is designed and intended for flexing. This meant the length of that harness which did all the flexing was built with wire designed to flex a lot. Presto, no more failures.

The Lotus driver seat harness gets flexed every time I get in or out, since I must retract it all the way to get in/out and scoot forward it to within 3-4 notches to reach the pedals properly. Hence why I suspect the harness will "need work" again, and if that happens I know how to fix it properly and permanently. Not until out of warranty, though!!!

 

[TNEngineer]

I promised to post the followup on this topic. I got the Evora GT back on Monday after the nearest (though not the selling) dealer handled warranty repairs on the airbag and the driver's window regulator. On the trip over I noticed that the passenger window rattled when partially down, and that is addressed in the shop notes too.

Here is the shop report from this visit, with unimportant and privacy details omitted:

Item "A" is the driver's window regulator. They swapped that out with no issues. I was impressed that the Technician noted a squeak in the seat, but found it interesting that despite having the car an entire extra week they didn't resolve it. They said they'll do so at my next visit in the Spring, which will happen for item "C" below.

Item "B" is the airbag system. During the previous visit the Tech noticed that the harness went from shorted to open as he moved the harness around (which would happen every time the seat was moved forward and back, of course). Unlike other airbag "events" which have been reported here, in this case the wires had actually broken! I suspect Lotus doesn't use high strand count wire here, which would be a good idea since lots of movement is almost a given. I'm thankful the Tech made the airbag harness longer, which should minimize the likelihood of this recurring. Once the harness was reconnected the fault cleared and has stayed cleared. Hooray! Definitely not a fault that would have been fixable with a OBD2 tool, even if we had one that knows the Lotus proprietary codes.

Item "C" is the passenger window rattle. Interesting that the "guides" in question appear to be part of the window glass itself, and not just some standalone clips (as I've seen mentioned here before). They have no idea how long it will take to get a replacement window shipped in from the UK so they're going to get it and sit on it until Spring when the mountain passes clear enough for me to take the car back over to them.

Now we get into the more interesting stuff.

Item "D" originates from the CEL being on when I pulled the car from winter storage last Spring. It showed the traction control system being disabled, and cruise control was inoperative. The CEL cleared itself after a few engine starts but I wanted them to check for stored codes. Odd that the code was for the ambient air temp sensor.

Item "E" was related to item "D" as mentioned. For some reason this revealed a low voltage code stored in the ECU. They asked me if I keep the car on its battery tender: "Yes, virtually all of the time unless we're on an overnight trip. The longest it is ever NOT on the tender is when it's here at the dealership." And it's on the tender 100% of the time it's in winter storage, AND I go check on it periodically to make certain the tender is powered up. Very odd.

Item "F" was an unannounced firmware upgrade to the ECU. Hadn't heard of a "recall" for this and don't know what it is supposed to address. I was a little nervous when I read this because I hadn't noticed anything "wrong" with the drivetrain, but the behavior of the engine hasn't noticeably changed since the reflash.

Item "G" was me having them do a LOF since they were already into the car anyway, and a brake fluid replacement too given the Evora's history of overheating the fluid due to brake line routing. I meant to ask them if they offered a higher-temp brake fluid (so I can minimize how often I need to do this) but forgot in the midst of everything else we were discussing. I may end up doing that myself next time it's needed.

To me, the most interesting parts of this service were the airbag system (I haven't read of others have actual physical harness breakages) and the ECU reflash.

 

[agentdr8]

Item "C" is the passenger window rattle. Interesting that the "guides" in question appear to be part of the window glass itself, and not just some standalone clips (as I've seen mentioned here before).

The guides are glued to the window. It's possible they were glued in the wrong spot on yours. They'll also wear over time, so having alternatives that you can glue on is a nice option, since Lotus doesn't sell the guides separately.

 

[Champale]

I promised to post the followup on this topic. I got the Evora GT back on Monday after the nearest (though not the selling) dealer handled warranty repairs on the airbag and the driver's window regulator. On the trip over I noticed that the passenger window rattled when partially down, and that is addressed in the shop notes too.

Here is the shop report from this visit, with unimportant and privacy details omitted:

View attachment 1325671

Item "A" is the driver's window regulator. They swapped that out with no issues. I was impressed that the Technician noted a squeak in the seat, but found it interesting that despite having the car an entire extra week they didn't resolve it. They said they'll do so at my next visit in the Spring, which will happen for item "C" below.

Item "B" is the airbag system. During the previous visit the Tech noticed that the harness went from shorted to open as he moved the harness around (which would happen every time the seat was moved forward and back, of course). Unlike other airbag "events" which have been reported here, in this case the wires had actually broken! I suspect Lotus doesn't use high strand count wire here, which would be a good idea since lots of movement is almost a given. I'm thankful the Tech made the airbag harness longer, which should minimize the likelihood of this recurring. Once the harness was reconnected the fault cleared and has stayed cleared. Hooray! Definitely not a fault that would have been fixable with a OBD2 tool, even if we had one that knows the Lotus proprietary codes.

Item "C" is the passenger window rattle. Interesting that the "guides" in question appear to be part of the window glass itself, and not just some standalone clips (as I've seen mentioned here before). They have no idea how long it will take to get a replacement window shipped in from the UK so they're going to get it and sit on it until Spring when the mountain passes clear enough for me to take the car back over to them.

Now we get into the more interesting stuff.

Item "D" originates from the CEL being on when I pulled the car from winter storage last Spring. It showed the traction control system being disabled, and cruise control was inoperative. The CEL cleared itself after a few engine starts but I wanted them to check for stored codes. Odd that the code was for the ambient air temp sensor.

Item "E" was related to item "D" as mentioned. For some reason this revealed a low voltage code stored in the ECU. They asked me if I keep the car on its battery tender: "Yes, virtually all of the time unless we're on an overnight trip. The longest it is ever NOT on the tender is when it's here at the dealership." And it's on the tender 100% of the time it's in winter storage, AND I go check on it periodically to make certain the tender is powered up. Very odd.

Item "F" was an unannounced firmware upgrade to the ECU. Hadn't heard of a "recall" for this and don't know what it is supposed to address. I was a little nervous when I read this because I hadn't noticed anything "wrong" with the drivetrain, but the behavior of the engine hasn't noticeably changed since the reflash.

Item "G" was me having them do a LOF since they were already into the car anyway, and a brake fluid replacement too given the Evora's history of overheating the fluid due to brake line routing. I meant to ask them if they offered a higher-temp brake fluid (so I can minimize how often I need to do this) but forgot in the midst of everything else we were discussing. I may end up doing that myself next time it's needed.

To me, the most interesting parts of this service were the airbag system (I haven't read of others have actual physical harness breakages) and the ECU reflash.

IDEngineer - thank you so much for this incredible thread and all of your work! I know it's winter time up there, but did the light stay off? Did I see somewhere else a while back that Lotus actually has an updated harness or wiring for under the seats?

 

[TNEngineer]

Yes, the fix appears to be permanent (at least so far!). The technician did install a "new" harness, but I don't know if it's a factory supplied harness or a technician repaired one.

The Evora GT is indeed sleeping peacefully until Spring. Makes me very sad, I'm having withdrawals and shopping for an Elise or Exige as therapy in the meantime. {grin}

 

[Champale]

Hah good for you! I understand the desire to get additional Lotuses!

Thank you for the reply!

 

[Jedifar]

Hey ID, renewing this again.
I have some beautiful Elige carbon buckets on the way from the UK which I will be installing in my 400.
I have also ordered some 2.2 ohm resistors for the airbag light. Ideally, I would like to do this right the first time and not trigger the light at all and have it recorded in history, afterall, I'm not doing anything bad here by installing OEM seats
So had 2 questions with follow ons:
For installing the resistor, should be straightforward and just plug in to the SRS harness? Assuming its just 2 pins for a ground and power? No specific order/direction the resistor needs to be plugged in?
Does the passenger sparco have a passenger occupancy sensor in it that will trigger a seat belt light if not buckled? If so, will i need to transfer that?

 

[TNEngineer]

Theoretically, the resistor would simply go across the two wires from the underseat harness. The SRS system monitors that resistance to confirm no shorts or breaks exist.

Standard resistors are not polarized so there's no "direction" to worry about.

Yes, the driver's seat belt in my Evora GT definitely knows if it's buckled but I don't know where that wiring ties into the underseat harness and I ALSO don't know which system monitors that. However, looking at the Evora schematics just now, I see that the Instrument Cluster module has the "DRV SEAT BELT SWITCH" coming into input A12. There are lots of passenger inputs too but none that refer to the seat belt. From this I deduce that the seat belt is not directly monitored by the SRS.

Thinking about it, there's got to be a pressure sensor in each seat that gets logically AND'd with the seat belt switch. Otherwise there'd be no seat belt warning. In the SRS section of the schematics there are two things labeled "LH G-SAT" and "RH G-SAT" and two more things labeled "LH P-SAT" and "RH P-SAT". All four are two terminal devices so I really want to interpret one set as seat pressure switches. It's tempting to presume the other two are the seat belt switches but then what connects to the very obvious "DRV SEAT BELT SWITCH" above? There's no breakout for that one signal to go two places.

Anyway, if you get a passenger seat belt light after the seat swap, just measure whether the (removed) seat belt switch is normally open or normally closed and simulate the occupied condition in the harness that will then be unconnected under the seat.

Amazingly, it appears the SRS communicates not via CAN but via K-Line!!! The only data connection appears to be a K-Line connection to the OBD connector which is paralleled with the ABS and ICM. That suggests the only way to interact with (read: clear codes on) the SRS and ABS is via K-Line, unless the ICM does some sort of passthrough from CAN to K-Line. That in turn may explain why so many OBD tools cannot interact with Lotus SRS and ABS systems... they may not support K-Line. Fascinating.

 

[agentdr8]

AFAIK, none of the Evoras have pressure sensors in either seat. The seat belt tell-tale is only linked to the driver's seat belt switch, which the car assumes there should always be a driver in the seat, and buckled, if under way.

The non-Federal cars have a PAB switch to disarm the passenger airbag in case you're putting a small person or rearward-facing child seat there. But in the US, that practice has been frowned upon (and in some states, against vehicle code).

 

[TNEngineer]

AFAIK, none of the Evoras have pressure sensors in either seat.

Then I wonder what the "LH G-SAT", "RH G-SAT", "LH P-SAT", and "RH P-SAT" devices are in the schematics. They are solely connected to the SRS module. I would expect they suppress airbag deployment unless the respective seat is occupied. It's really tempting to interpret "G" as gravity and/or "P" as pressure, but that's just a wild guess.

There's also generally a pressure sensor to defeat the seat heat unless the seat is occupied, which is why most car seats cannot be preheated {grin}. However, I notice there are temperature sensors in the seat heaters so perhaps Lotus DOES allow their seats to preheat with no thermal load in place, and just rely on the thermal sensors to prevent runaway. Our Evora GT is in storage when seat heat would be useful so I've never played around with how these are implemented.

 

[agentdr8]

Then I wonder what the "LH G-SAT", "RH G-SAT", "LH P-SAT", and "RH P-SAT" devices are in the schematics.

In the US, those are the required door pressure sensor and side accelerometer modules, for side impact detection and SAB deployment:

EDIT: More fun details from the SNs:

 

[TNEngineer]

Great data, thank you!

I wonder how it knows not to flag an unbelted passenger when the seat is unoccupied.

 

[agentdr8]

I wonder how it knows not to flag an unbelted passenger when the seat is unoccupied.

Your car has a tell-tale for the passenger seat belt too? The schematics only show an input for the driver's buckle switch in the cluster:

 

[TNEngineer]

Right, that was the signal I mentioned earlier in the thread. I have not specifically tested how our Evora GT handles the right seat belt but I thought US regs require passenger warning about the seat belt. If true, there has to be a way to sense that 1) someone is in the seat (to sound the alarm), and 2) their belt is buckled (to turn off the alarm).

 

[agentdr8]

Right, that was the signal I mentioned earlier in the thread. I have not specifically tested how our Evora GT handles the right seat belt but I thought US regs require passenger warning about the seat belt. If true, there has to be a way to sense that 1) someone is in the seat (to sound the alarm), and 2) their belt is buckled (to turn off the alarm).

Hmm, I know that a passenger can sit in any S1 and never buckle, and the car isn't the wiser or care. I assumed the S2s were the same way, since they don't include a passenger seat belt switch or seat pressure sensor. The passenger airbags (dash & side) will effectively fire with appropriate impact, regardless of an actual passenger if I'm not mistaken (assuming no other SRS faults, like the airbag light). And I believe the seat belt pre-tensioners will also fire, for both sides, regardless of occupancy, which is why the service notes recommend replacing them both after any impact.

EDIT: FMVSS 208 surprisingly does not include the front outboard passenger seat, only driver and rear seats:

 

[Jedifar]

If it helps, theres a set of sparcos on ebay with the passenger seat deployed and driver intact...so its confirmed that there is something that triggers one seat vs both

 

[agentdr8]

If it helps, theres a set of sparcos on ebay with the passenger seat deployed and driver intact...so its confirmed that there is something that triggers one seat vs both

Yep, the side-impact sensors can likely trigger one or both. But I'm fairly positive it would do so even if that particular seat is unoccupied.

 

[Jedifar]

Yep, the side-impact sensors can likely trigger one or both. But I'm fairly positive it would do so even if that particular seat is unoccupied.

Regardless, I'm glad that I don't have to tear my sparco apart for an occupancy sensor

 

[gaye.binns27]

Just to add my bit of good news in.

I chased the light for 6 months having dis then reconnected battery overnight. Followed all the OBD stuff etc etc, no luck. Went to Lotus Brisbane they hooked up to Lotus tech, (they were awesome BTW) and after about 3 hours of fiddling decided it was "something in the drivers airbag area." Took her home, CAREFULLY! disconnected and inspected airbag, found a prior "specialist" had used pliers to disconnect it - crimpled the wire, and had bent two tiny connecters - fixed them with tweezers put back together and the good news........

After 5 mins of running the system self checked itself and the light went out and stayed off! In future I have got a small 9v battery with croc clips and I will keep the electrics powered up even while the battery is disconnected.

I take the challenging issue eventually as a tribute to the Lotus tech guys, plus an awesome car that reset and saved me another 4 hour journey to tech's.

 

[TNEngineer]

In future I have got a small 9v battery with croc clips and I will keep the electrics powered up even while the battery is disconnected.

Not sure what you intend here, but be DARNED CAREFUL about connecting any voltage source to any SRS device. It doesn't take much to set them off, and it's both potentially very damaging and not inexpensive to replace the deployed module. Speaking only for myself, when I've disconnected them I keep a jumper across the leads going to the SRS device to avoid the buildup of static electricity while the leads are disconnected from the harness.