604wrx604 Scooby Newbie

Member#: 481667 Join Date: Feb 2018

Experimenting with disconnecting clutch switches



I was really interested in following this up and understand it better.



This evening I disconnected both clutch switches, 1) Starter interlock (behind blue switch) 2) Cruise control switch (blue)











Sure enough the car wouldn't start, after making sure I was in neutral, I shorted the two pins that would go from the starter interlock switch and this let me start the car.



The starter interlock switch has the following properties:



Clutch engaged (UP) = infinite resistance

Clutch open (DOWN) = x OHMS (x is unknown)



I removed the short, and drive on - knowing here that if I stalled, it would be a long time before I could restart, the stakes are high.



First thing I notice is, car idles at 800 RPM, driving in first gear and pressing the clutch down all the way at the same time as releasing the throttle, there is no rev hang, the car settles quickly back to 800 RPM! Woop, it's fixed right?



The behaviour of the car has most definitely changed, as I press the clutch in this way, the car shudders a little, this is kind of opposite of when a clutch is dropped, the disengagement between the clutch and flywheel at a high rate, I can't think of the rotational equivalent but it's like 'snap' for linear acceleration.



I feel more connected to the car, I have to 'manage' the throttle not only to go out of gear but in deciding which gear to goto - 1st to 2nd -> prepare and engage throttle to get to intended power level, 3rd to 2nd -> rev up and 'catch' second gear. It was super fun to drive - I didn't need to predict what the car is doing.



Next step, test out cruise control: it doesn't work.



So here are the properties for the cruise control switch:



Clutch engaged (UP) = x OHMS (x is unknown)

Clutch open (DOWN) = infinite resistence



With the cruise control switch being disconnected it essentially equates to having the clutch down all the time, so cruise control won't engage. Interesting the gear position light is empty during driving in gear, presumably this input is used everywhere.



This made me think, does the ECU think I'm in neutral so it's not even reaching power levels that I would in gear? It's hard to say - the car definitely wasn't 'slow', but then it's the first drive of the day.



It made me nervous, so I plugged the Cruise control switch (in blue) back in. Sure enough I got the gear position display back, didn't think to try cruise control but I suspect it will work now.



I drove back home, and plugged the starter interlock switch in as well; things were back to stock now - driving this definitely felt worse, although by this time I had personally warmed up so I wasn't as bad as say two days ago (I've had my car ~ two weeks now).



I did some data logging, it was hard to sift through the data later - gave me an appreciation for having some kind of time markers.







Red line: Throttle position (manifold) % (scaled so it appears together with RPM)

Blue line: RPM



Top graph: unplugged

Bottom graph: stock



Admittedly this is cherry picked data, but the bottom graph the RPM (blue) fall off after the throttle position (red) is release is very prolonged.







So my theory on what's happening is the switches are inverted and work in tandem, the first switch to engage when pressing the clutch (engaged to open) is the cruise control switch, it goes from x OHMS to infinite resistance; I think this is very clever and for safety.



The second switch to engage nearing the end of travel - I am guessing it's towards the end, but I have not tested it - it doesn't make much sense to allow starting the car with clutch half pressed in ... or even just enough for the cruise control switch. I think it's setup geometrically this way for a reason, otherwise you could just get away with one switch.



So, the sequence is:



Driving in gear -> Press clutch -> CC Switch -> Starter interlock switch



(Theory) The rev hang actually happens when the ECU detects these two switches activate in this sequence, if we disconnect the starter interlock switch, then the ECU has no way of knowing and thinks it is in gear - however CC will disengage, but it can't rev-hang to 'help' you since the car might still be in mid-clutch.



My belief is that this 'feature' is a hangover from an ECU that was designed for both CVT and 6MT transmissions.



Would anyone be willing to repeat the experiment of disconnecting the starter interlock switch? https://imgur.com/a/qk2hf - images hereI was really interested in following this up and understand it better.This evening I disconnected both clutch switches, 1) Starter interlock (behind blue switch) 2) Cruise control switch (blue) https://imgur.com/et07U0i https://imgur.com/MxQbaIeSure enough the car wouldn't start, after making sure I was in neutral, I shorted the two pins that would go from the starter interlock switch and this let me start the car.The starter interlock switch has the following properties:Clutch engaged (UP) = infinite resistanceClutch open (DOWN) = x OHMS (x is unknown)I removed the short, and drive on - knowing here that if I stalled, it would be a long time before I could restart, the stakes are high.First thing I notice is, car idles at 800 RPM, driving in first gear and pressing the clutch down all the way at the same time as releasing the throttle, there is no rev hang, the car settles quickly back to 800 RPM! Woop, it's fixed right?The behaviour of the car has most definitely changed, as I press the clutch in this way, the car shudders a little, this is kind of opposite of when a clutch is dropped, the disengagement between the clutch and flywheel at a high rate, I can't think of the rotational equivalent but it's like 'snap' for linear acceleration.I feel more connected to the car, I have to 'manage' the throttle not only to go out of gear but in deciding which gear to goto - 1st to 2nd -> prepare and engage throttle to get to intended power level, 3rd to 2nd -> rev up and 'catch' second gear. It was super fun to drive - I didn't need to predict what the car is doing.Next step, test out cruise control: it doesn't work.So here are the properties for the cruise control switch:Clutch engaged (UP) = x OHMS (x is unknown)Clutch open (DOWN) = infinite resistenceWith the cruise control switch being disconnected it essentially equates to having the clutch down all the time, so cruise control won't engage. Interesting the gear position light is empty during driving in gear, presumably this input is used everywhere.This made me think, does the ECU think I'm in neutral so it's not even reaching power levels that I would in gear? It's hard to say - the car definitely wasn't 'slow', but then it's the first drive of the day.It made me nervous, so I plugged the Cruise control switch (in blue) back in. Sure enough I got the gear position display back, didn't think to try cruise control but I suspect it will work now.I drove back home, and plugged the starter interlock switch in as well; things were back to stock now - driving this definitely felt worse, although by this time I had personally warmed up so I wasn't as bad as say two days ago (I've had my car ~ two weeks now).I did some data logging, it was hard to sift through the data later - gave me an appreciation for having some kind of time markers. https://imgur.com/slnhFGYRed line: Throttle position (manifold) % (scaled so it appears together with RPM)Blue line: RPMTop graph: unpluggedBottom graph: stockAdmittedly this is cherry picked data, but the bottom graph the RPM (blue) fall off after the throttle position (red) is release is very prolonged. https://imgur.com/kj4alNKSo my theory on what's happening is the switches are inverted and work in tandem, the first switch to engage when pressing the clutch (engaged to open) is the cruise control switch, it goes from x OHMS to infinite resistance; I think this is very clever and for safety.The second switch to engage nearing the end of travel - I am guessing it's towards the end, but I have not tested it - it doesn't make much sense to allow starting the car with clutch half pressed in ... or even just enough for the cruise control switch. I think it's setup geometrically this way for a reason, otherwise you could just get away with one switch.So, the sequence is:Driving in gear -> Press clutch -> CC Switch -> Starter interlock switch(Theory) The rev hang actually happens when the ECU detects these two switches activate in this sequence, if we disconnect the starter interlock switch, then the ECU has no way of knowing and thinks it is in gear - however CC will disengage, but it can't rev-hang to 'help' you since the car might still be in mid-clutch.My belief is that this 'feature' is a hangover from an ECU that was designed for both CVT and 6MT transmissions.Would anyone be willing to repeat the experiment of disconnecting the starter interlock switch? Registered users of the site do not see these ads. Last edited by 604wrx604; 02-16-2018 at 01:04 AM . Reason: add image link