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Thread: Airbox temperature/pressure sensors

  1. #1
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    Airbox temperature/pressure sensors

    When I pulled everything apart, I labeled these "airbox", but for the life of me, I can't figure out where they go in. Yellow one is drivers side under the headlight area, circular one is passenger (US spec 96 840 Ci). Anyone know where these go in?
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  2. #2
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    There is a square duct that goes back towards the wheel opening. The driver side sensor mounts in the small hole on the underside of this duct. The passenger side disc mounts in the passenger side duct.
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  3. #3
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    Perfect, Thanks! I didn't have those ducts back in yet, hence nowhere to put the sensors. I'm now down to some sheet metal plates as the only pieces I can't yet figure out where to reattach. Not too bad considering had it down to an empty engine bay and transmission tunnel. You just don't know how many pictures you should be taking at the beginning, and memory just isn't good enough at the end.

  4. #4
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    Quote Originally Posted by Spandrel View Post
    Perfect, Thanks! I didn't have those ducts back in yet, hence nowhere to put the sensors. I'm now down to some sheet metal plates as the only pieces I can't yet figure out where to reattach. Not too bad considering had it down to an empty engine bay and transmission tunnel. You just don't know how many pictures you should be taking at the beginning, and memory just isn't good enough at the end.
    I just posted a thread asking the EXACT same question. I feel stupid.
    Spandrel, can u help me out and take a picture of where the flat, round temp sensor connects to? I found mine was cut and need to reconnected but can't see where?


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  5. #5
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    where the sensor goes? or where the cable runs? here's a picture of it installed - it goes on the bottom off the square duct that runs from the front grille back to the passenger wheel well to cool the brakes (ignore the gooped epoxy that nobody was supposed to see that's holding my airbox together). If you mean where does the cable run? I'll have to take a few things out tomorrow evening and show you...

    Quote Originally Posted by nrp22101 View Post
    I just posted a thread asking the EXACT same question. I feel stupid.
    Spandrel, can u help me out and take a picture of where the flat, round temp sensor connects to? I found mine was cut and need to reconnected but can't see where?


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  6. #6
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    Quote Originally Posted by Spandrel View Post
    where the sensor goes? or where the cable runs? here's a picture of it installed - it goes on the bottom off the square duct that runs from the front grille back to the passenger wheel well to cool the brakes (ignore the gooped epoxy that nobody was supposed to see that's holding my airbox together). If you mean where does the cable run? I'll have to take a few things out tomorrow evening and show you...
    At least the white goo matches the car. Lol.
    Thanks for the pic but what I'm trying to figure out is what the sensor is wired to. I know the sensor itself is connected to the underside of the air box but I can't find the other end of the connection. Any direction you could provide wouId be much appreciated.

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  7. #7
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    Here's the view from the top with the headlight removed - cable goes over the receiver/dryer for the A/C and I think merges in with the larger harness there. Front of the car is to the left of the image
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  8. #8
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    Sprandrel...you're the man. Thank you so much, that helps a bunch.

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    What are those sensors responsible for?

  10. #10
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    Quote Originally Posted by dragon850 View Post
    What are those sensors responsible for?
    Wondered the same myself. I'm guessing to get a temperature reading prior to the air filter. On the E38, I believe the temperature sensor is on the air filter box itself (V8).

    Spandrel - if you want a recommendation for good black epoxy resin, I have had great results without any prep greater than an alcohol bath with this:

    https://www.amazon.com/gp/aw/d/B01IB...MQCM9VG3Z9FEXP

    It is cheap as well and has just enough flex to not crack. I wouldn't have bought new airboxes had I used this beforehand. Oddly, it seams I often get the beat results with the lower end stuff.
    Last edited by toomanyparts; 02-13-2018 at 03:10 AM.
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  11. #11
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    Quote Originally Posted by toomanyparts View Post
    Wondered the same myself. I'm guessing to get a temperature reading prior to the air filter. On the E38, I believe the temperature sensor is on the air filter box itself (V8).

    Spandrel - if you want a recommendation for good black epoxy resin, I have had great results without any prep greater than an alcohol bath with this:

    https://www.amazon.com/gp/aw/d/B01IB...MQCM9VG3Z9FEXP

    It is cheap as well and has just enough flex to not crack. I wouldn't have bought new airboxes had I used this beforehand. Oddly, it seams I often get the beat results with the lower end stuff.
    I finally figured out what that sensor is. Can't believe it was so elusive. It's the windshield wiper air temp sensor that heats up the water in case temps drop below freezing. I'll post realoem reference.

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  12. #12
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    See #24

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  13. #13
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    Wow. Good info.
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  14. #14
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    That's some good detective work!

    - - - Updated - - -

    I may try to use that that to patch the holes that are on one side from previous owner scraping. To be honest, i just wanted the thing back in one piece, and didn't worry about aesthetics that nobody would ever see. Thanks for the link

    Quote Originally Posted by toomanyparts View Post
    Wondered the same myself. I'm guessing to get a temperature reading prior to the air filter. On the E38, I believe the temperature sensor is on the air filter box itself (V8).

    Spandrel - if you want a recommendation for good black epoxy resin, I have had great results without any prep greater than an alcohol bath with this:

    https://www.amazon.com/gp/aw/d/B01IB...MQCM9VG3Z9FEXP

    It is cheap as well and has just enough flex to not crack. I wouldn't have bought new airboxes had I used this beforehand. Oddly, it seams I often get the beat results with the lower end stuff.

  15. #15
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    You definitely want the lower airboxes to be without rips and holes - this is all part of the cold air induction on these cars. Another part to look at is the rubber boot directly under the headlight assembly. Once I fixed all of those problems on my car years ago, it did run smoother and better.
    '93 850Ci - Mineralweiss Metallic
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  16. #16
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    Spandrel - if you want a recommendation for good black epoxy resin, I have had great results without any prep greater than an alcohol bath with this:

    https://www.amazon.com/gp/aw/d/B01IB...MQCM9VG3Z9FEXP

    It is cheap as well and has just enough flex to not crack. I wouldn't have bought new airboxes had I used this beforehand. Oddly, it seams I often get the beat results with the lower end stuff.[/QUOTE]

    I need to repair the bottom of both air boxes but I have not tested any adhesives, like epoxy, to see if it will stick to the plastic. I have a large hole, apprx 3" one or both boxes so I need a larger quantity of glue that I can use with some glass or CF cloth.

    Looks like the JB Weld Plastic Bonder is some type of urethane glue that may be the best stuff to use if I can find larger bottles of it:

    J-B Weld Black Plastic Bonder Syringe
    Plastic Bonder is a quick-setting, two-part, urethane adhesive system that provides strong and lasting repairs and works on thermoset, carbon fiber composites, thermoplastics, coated metals, concrete and more. Plastic Bonder has a 1:1 mixing ratio, sets in 15 minutes and can be sanded after 30 minutes. With a tensile strength of 3770 PSI, Plastic Bonder sets black in color and cures to a hard, permanent bond.

  17. #17
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    I think if you sand lightly and drill some holes you will get something just as sturdy as new set of boxes. Probably better, as the boxes aren't all that sturdy to begin with.

    For the hole, I would get a piece of plastic and fix it from inside, then use the bonder over it.
    Last edited by toomanyparts; 02-16-2018 at 12:35 AM.
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  18. #18
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    Quote Originally Posted by toomanyparts View Post
    Wondered the same myself. I'm guessing to get a temperature reading prior to the air filter. On the E38, I believe the temperature sensor is on the air filter box itself (V8).

    Spandrel - if you want a recommendation for good black epoxy resin, I have had great results without any prep greater than an alcohol bath with this:

    https://www.amazon.com/gp/aw/d/B01IB...MQCM9VG3Z9FEXP

    It is cheap as well and has just enough flex to not crack. I wouldn't have bought new airboxes had I used this beforehand. Oddly, it seams I often get the beat results with the lower end stuff.
    I mixed up some epoxy Friday and applied it to a broken piece of plastic from one of the broken covers from under the hood of my car. I think its the same plastic that is used for air boxes, and the epoxy did not stick to it. I was able to peel it away quite easily. So I will use the J-B Weld Plastic Bonder to make repairs. I would like to find a suitable thinner, maybe lacquer thinner, that I can mix with the plastic bonder so I can apply it to some fiberglass or CF cloth to make repairs.

  19. #19
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    I did a patch job this afternoon on the bottom of air boxes using some JB Weld Plastic Bonder and carbon fiber cloth. May not look pretty but boxes are sealed now. I dont think its worth the time to finish them for appearance since they are behind the bumper.

    IMG_20180228_140129434_TOP.jpg

    IMG_20180228_161638746_HDR.jpg

  20. #20
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    So I drove out of town about 30 miles to speak with a shop that does custom exhaust work. I think my car does run better now with the air boxes sealed up. Engine pulls even harder and smoother from about 4500 rpm to red line. And, my fuel mileage went up about .4 mpg on MID compared to where its been since I started driving it.

  21. #21
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    Glad to here it worked. I know some of the PET and ABS + PC plastics can be troublesome when trying to repair.
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  22. #22
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    Quote Originally Posted by toomanyparts View Post
    Glad to here it worked. I know some of the PET and ABS + PC plastics can be troublesome when trying to repair.

    The plastic bonder does seem to adhere very well. I was shopping for some gear oil at wallyworld and found the JB Weld available there in the auto department. I may just take a DA or sanding block and clean up the edges. I used some leftover CF cloth to make repairs to each side using three layers of cloth.

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