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03-06-2015, 04:40 PM | #1 |
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DIY 6MT engine start without clutch pedal modification
Background
All modern manual transmission cars apparently have a clutch interlock safety feature which prevents the engine from starting unless the clutch is fully depressed. On other vehicles I have owned, repeated starts with a fully depressed clutch causes unnecessary crank/bearing wear since there's essentially no oil pressure under such conditions, especially with aftermarket clutches equipped with high-pressure pressure plates (which exert more lateral pressure on the crank). Such unnecessary mechanical wear leads to the so-called "crank walk" (lateral crank movement). Objective of the modification Since I'm a careful driver and I always check that my 6MT transmission is in neutral before starting the car, I want to prevent unnecessary engine wear by bypassing the clutch interlock feature WHEN I CHOOSE TO. I also want to retain the factory clutch interlock feature (if it's possible), while keeping the mod stealthy or "invisible" if possible. After much research and testing, I think I've found the perfect solution to accomplish my objectives on my 08 6MT E90 M3. Theory of operation While older cars have mechanical clutch switches: typically one at the fully-depressed position for clutch interlock, and one at the slightly-depressed position to disengage cruise control, etc., BMW has gone to a single, more advanced magnetic solid-state clutch switch that replaces the two separate mechanical switches. A magnet is embedded in the clutch master cylinder that causes the clutch switch (which is clipped on the master cylinder) to trigger the signals needed for clutch interlock. You can see the circuit diagram of the BMW clutch switch and an illustrated picture of the actual clutch switch connector below. Signal 1 (brown/black): Ground Signal 2 (blue/brown): This signal is triggered when the clutch pedal is depressed just slightly--it goes to DME for cruise control, idle-up such as when you push in the clutch to prepare for take-off, etc. Signal 3 (purple/yellow): This is +12V supply for the clutch switch Signal 4 (blue/black): This signal is triggered and goes to +12V only when the clutch pedal is fully depressed--it goes to CAS for clutch interlock. Basically, what my mod does is that I use an additional momentary-on tactile switch to simulate signal 4 of the clutch switch; this is done by using the tactile switch to short signal 4 to signal 3 (+12V) as long as the switch is depressed. Simple, right? Instead of pushing in the clutch pedal, I simply depress my switch AND the engine-start switch to start the car. The factory clutch interlock feature is fully retained: meaning a valet, who is unaware of my modification, still has to push in the clutch to start the vehicle. Other features like cruise control and automatic idle-up are unaffected since they use a different signal on the clutch switch. Since all modern manual BMW's (and Mini's) since E46 seem to use the same exact clutch switch, I suspect the same mod will work on all other BMW's but of course you'll be responsible to verify this yourself if you don't happen to have an E9x. And, perform the mod and accept the consequences at your own risk, yada yada. To keep the mod stealthy, on my E90 M3 since there is no factory front fog light, I decided to use the blank fog light switch as my "invisible switch." Instead of modifying the headlight switch circuitry (which is possible), I added a tactile switch behind the blank foglight button and added some wires which you can see in my description later. Parts--and talent--needed - Basic electrical/soldering skills - Momentary tactile switch (Digikey part #CKN1860CT-ND): this one can handle more than an amp of current @32V so it should last the life of the car. Of course, you're free to choose whatever momentary-on switch you want and you can install it anywhere you want. - Miscellaneous wires, connectors, and heat shrink wraps to keep your work neat and tidy Step 1 You'll need to remove the driver-side lower dash cover and remove the clutch switch from the clutch master cylinder. Pelican parts happens to have an excellent step-by-step tutorial with pictures here. The clutch switch easily unclips from the master cylinder. Step 2 Identify wires #3 and #4 on the clutch switch, and solder on 2 wires that will lead to the additional tactile switch. The tactile switch will short signal 4 to signal 3 (+12V) to simulate clutch pedal being fully depressed. Sorry, I didn't take pictures here but it's easy enough. Step 3 (Only do this if you choose to implement the "stealth button" like mine. Otherwise you can put the tactile switch of your choice anywhere you want) Remove the headlight switch and open it up. There's plenty of DIY out there, such as this excellent one. I used a piece of double-sided tape to affix my tactile switch on top of the LED under the factory foglight button and soldered 2 wires to it. Then I routed the wires through a small hole I drilled behind the headlight switch body. Similar to the el-cheapo rear foglight mod, I used a few Lego pieces to allow the blank foglight button to actuate my tactile switch as you'll see later. Finally, connect the tactile switch wires with the wires that come from step 2 and voila, you're done! You can see a close-up picture of my tactile switch in the picture below. Notice it is affixed on the little silicone plateau on top of the LED for the factory foglight: In the picture below you can see the lego pieces I used on the left. These are el-cheapo Chinese knock-off Legos so I don't feel bad raiding my kids' drawer. The socket on the bottom lego piece fits perfectly over the tactile switch I chose (by design). I also did the poorman's rear foglight mod while I opened up the headlight switch, and you can see the lego pieces I used for that on the right.. It takes a little patience to put the headlight switch back together but it's possible. I would take pictures of the finished product but you know what? It just looks like a factory E9x M3 switch (as that's my intention) so there's not much to look at. Enjoy!
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03-09-2015, 02:13 PM | #2 |
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I am surprised this DIY hasn't gotten any love in three days, as it's written very well and illustrates something I had been wanting to investigate for some time.
Thank you for investing the time to put this together man, great work!
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03-09-2015, 03:16 PM | #3 |
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Dude, as a DIYer, this is pretty awesome, especially when considering adding remote start.
This company provides remote start BMWs, incorporating it into your OEM keyfob....just not for 6MTs. Seems like your mod would allow this to work for those of us w/a manual. www.m3post.com/forums/showthread.php?t=869327 Last edited by whats77inaname; 03-09-2015 at 03:31 PM.. |
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03-09-2015, 06:50 PM | #4 |
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Awesome write up, this is going to help a lot of people
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03-10-2015, 01:00 AM | #5 |
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Thanks guys. I actually bought a spare clutch switch so I could play with it and verify its operation so that I know for sure the modification is safe and effective, and I thought others would benefit from this information. As @whats77inaname pointed out, a remote starter that can use a relay to temporarily "short" pins 3 & 4 of the clutch switch can remote start a 6MT that wouldn't have otherwise been possible. I had contemplated the remote starter at one point and that was indeed one of my motivations for performing this mod.
I'm not sure how easy it is for thieves to hotwire and steal these modern cars with smart keys, but another idea for a theft-deterrent feature is to use a toggle switch to turn off the 12V (signal 3) going into the clutch switch. Without +12V, the clutch switch cannot send the "clutch depressed" signal 4 to the CAS, and therefore the engine cannot be started.
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03-10-2015, 09:46 AM | #7 | |
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We have the same car ('08/6MT) so if someone wanted it, all they have to do is break the glass, pull up the hand brake and roll it onto a truck. I don't know if you have the OEM alarm, but if you don't, your car comes pre-wired for it (only the '08s do), and you just have to buy the OEM alarm kit, install it, and have it coded in, which I did. The OEM alarm does have a tilt sensor, which is pretty nifty, but no shock sensor, so I followed this DIY to detect glass breakage and did a little something extra to go along with it. Cool. Handle that and report back to us, as I haven't seen anyone do that across any of the other BMW forums. I'm always interested in seeing/learning about new options Last edited by whats77inaname; 03-10-2015 at 11:06 AM.. |
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03-10-2015, 10:19 AM | #9 |
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03-10-2015, 02:08 PM | #10 |
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Thanks for the OP for this for the idea of a hidden switch to prevent the car from being started. I did a hidden starter bypass switch before on my integra. This is the first I heard about the wear on the lateral support bearing for the crank-walk. But I guess it make sense. I think I found the 'option' that can be coded in NCS. I will switch it to the brake pedal instead of the clutch and see how it goes and report back here.
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03-10-2015, 05:09 PM | #11 |
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@Craigy ....haha my expert knowledge of "crank walk" gives away my DSM roots eh? My first performance car was indeed a 2GT "wrong wheel drive" DSM which I had modded to hell and back. No crank walk with 230k miles on the original bottom end though!
@whats77inaname....yup, I'm also a 2JZ-GTE guy. In fact, my M3's stable mate is a hardtop 6MT TT Poopra. |
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03-10-2015, 05:45 PM | #12 | |
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To give you an idea of how long I've enjoyed them, that poster you have on the left in your garage? I have that in a shirt that has been unworn for about 15 yrs now and my MKIV model collection numbered well over 200 pieces. I've been trimming it lately |
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07-15-2015, 04:27 PM | #13 |
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I was thinking you just wire the entire pedal portion to a switch so that's the only way it can be operated. Just turn the switch off when you go into a sketchy area and a thief won't be able to start your car! anti theft device.
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07-15-2015, 10:14 PM | #14 | ||
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07-15-2015, 10:15 PM | #15 | ||
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07-16-2015, 08:03 AM | #16 | |
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If you set it to nicht_activ you should be able to then start the car without having to depress the clutch pedal. For DCT cars, the option is ANLASSSPERRE_BLS. Setting this to nicht_activ will allow a DCT car to be started without depressing the brake pedal. |
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07-16-2015, 04:06 PM | #17 |
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I won't act like I know, but I find it super hard to imagine this is a problem... Like the engineers at bmw didn't consider this, or consider that it didn't matter?
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08-21-2015, 03:40 AM | #18 |
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You mean the same BMW engineers who designed the E46 M3 (or our E9x M3) rod bearings? Sarcasm aside, why subject the crank to unnecessary lateral load on a cold start if there's no absolute necessity? I'm an engineer, and I wanted a clean solution to disable the clutch interlock, so figuring it out was satisfying to me in and of itself I guess.
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11-24-2017, 08:35 PM | #19 | |
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03-29-2020, 11:19 PM | #20 |
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If anyone could confirm, i am going to use this guide to wire a clutch module after a manual engine swap and a manual transmission swap. I currently have harness from both engine and the AT. I could just code in for the MT correct? If anyone is willing to help i can offer something look for some really good help!
I also want to add that mechanically everything is set and done i'm just on this wiring so i'm already past the "oh so expensive no point yadada" phase. Last edited by solace25; 03-29-2020 at 11:21 PM.. Reason: Adding information |
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03-30-2020, 12:07 PM | #21 |
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clutch activation actually goes to the CAS, you will have to FA write the cas for manual transmission zmt. after u FA write the cas update the frm.
im pretty sure the clutch switch is part of the body harness so unless you change that you may be better off coding off the depress clutch to start
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07-02-2024, 11:13 PM | #22 |
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How does one remove the connector to the clutch switch? I can pop the sucker off the pedal, but BMW plastic electrical connectors are always a pain to disconnect. I tried using a pry tool against the ridge but the thing was not budging (and actually starting deforming the plastic). Do I have to depress some tab and then pull down?
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