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DIY: Do It Yourself
Post here to share or improve your wrench turning skills! All BMW E46 DIY tips, tales, and projects discussed inside. Learn to work on your car and know the right BMW parts you will need!

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Old 09-09-2013, 01:19 AM   #1
Registered User
Join Date: May 2013
Location: Bloomington MN
Posts: 20
My Ride: 00 BMW 323i Sport
Post Racing Switches (DIY)

*Apologies if threads exist for this but I had trouble finding any*
Disclaimer: I am NO electrical Engineer. Just a fellow DIY'er who has learned with trial and error and S*** ton of reading.
So I am in the middle of installing a few SPST switches into my 323i. I would love to make you a guys a DIY but I feel as if I won't be providing ample knowledge so for now this is just an information and resource thread.

Let me start off with a little bit of knowledge on what are some different types of switches.
SPST- Single Pole Single Throw
SPDT- Single Pole Double Throw
SPMT- Single Pole Momentary Throw
SPCO- Single Pole Centre Off
DPST- Double Pole Single Throw
DPDT- Double Pole Double Throw
DPCO- Double Pole Centre Off

In my projects, these are all that are needed (for now) and are pretty simple to understand. The most common switch is a SPST and is something you use every day. Ever flip a light switch? Then you have used a SPST switch!

Now my project and my basic thought process- I am planning on removing the need for me to use my ignition barrel. Switches in its place and more for other things but we will get there down the road.

Just use one switch (SPDTM)
1. Bottom position as off, middle position as battery and accessory and momentary up position as starter.
2. Another option would be just using a starter button connected to a relay. (Going clockwise starting noon)
2a. Terminal 87 is connected from the ignition wire. (Green wire)
2b. Terminal 85 is connected to the wire from the start button (or momentary push)
2c. Terminal 30 is the wire connected from the starter. (Blue and Black Wire)
2d. Terminal 86 is actually tapped off from the ignition wire before it hits terminal 87. This wire contains a fuse (2A is good) and then wires down to terminal 86. This is use your fuses wire so incase anything goes wrong the fuse will play its part.

Resource: http://homepage.ntlworld.com/j.walkd...tartButton.pdf
The link above best explains how to wire up a start button. Now that isn't quite a switch (or what I am thinking of doing) but has great explanation of how to wire a relay and what wires are what.

2 Switches (SPDT)~(SPMT)
Switch one (SPDT) with Bottom as OFF, Middle as battery, up (non-momentary) as accessory or full car on. Switch 2 (SPST), now this could be push button (which is a type of momentary SPST) or an actual momentary SPMT which is wired to the starter.

3 Switches (SPST)~(SPST)~(SPMT)
Switch 1 is for battery (SPST), Switch 2 is accessories (SPST), Switch 3 is starter (SPMT)

4 Switches (SPST)~(SPST)~(SPST)~(SPMT)
Everything from here forward will be the same as long as you are using anything that is not momentary. If it will be a momentary for anything other than the starter the order must be re done. The first two IF they are for the ignition barrel can stay the same. It would only change the order if you were using one of the three (SPST) switches for something like a fuel pressure line. In which case I would activate that before I started everything in the car, but after the first switch. So to put it simply, use switch 2 for that and 3 for power to the car. Of course SPMT will still be the starter. You can continue to add switches and do more, just remember that everything needs to be done in an order.
It's a fairly basic idea and one that seems more simple than not. IT IS NOT. 3 switches means very carefully placing and tapping. You need to make sure that all the switches are correctly wired and in order otherwise you will end up miss-wiring and starting a fire.

The reason this can be so confusing because certain wires you will have to tap and others you will have to cut. My plan (SPST~SPST~SPMT) would require my first and second ones to be cut so that even though the key is in the ignition barrel, the key itself does not do anything for the first 2 positions. That means that the switch completes the circuit. Now for (SPMT), you should tap into and not cut because if the wire goes out or the fuse gets blown, you can still start the car (providing the first 2 work) and at least you can go home. NO ONE is trying to be stranded in a parking lot, not sure what fuse has gone out or where the problem is. Once its home at least you can deal accordingly.

Lesson 2: Use Relays for everything as that way the correct amps are controlled to power high amp things (ex: starter)

That's all I got for now. I will input more links and resources tomorrow, as those are on the tablet. Let me know if you guys have any more questions or whatever. I would love to get this up and going and see what other people with much more knowledge have to say! Thanks for your time.

Last edited by vanka472; 09-09-2013 at 01:21 AM.
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