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Thread: 97 M3 wiring harness horror story

  1. #1
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    97 M3 wiring harness horror story

    97 M3 sedan 5spd, not my daily, but kept on a battery tender and driven once a month or so with no issues whatsoever. Last time I went to start it though, it absolutely would not turn over. I pulled the air filter and sprayed starter fluid up the intake, and she would cough through that for a second or two, and then right back to no-start. Fuel level was very low, but even after adding about 4 gallons, I still couldn't get the car to start. I assumed fuel issue and immediately went to the fuel pump relay. I pulled it and it looked a little dark where the prongs met the casing:

    20190424_183255.jpg

    I figured replacing it would be the easiest first step, and here's my first mistake: I went to AutoZone, and trusted they had given me the correct replacement relay. And even though the colors didn't match (mint green vs. original purple) I figured the internals were the same. The pin-out matched on the outside, at least (pic shows them turned to show their circuit diagrams):

    20190424_183401.jpg

    Box new relay came in:

    20190424_183423.jpg

    Commence horror show. I get home and plug-in the new relay. I don't even pull away from under the hood before smoke starts pouring out from the tiny vent hole in the wiring harness housing. Had not even tried to start the car, and key wasn't even in it. I had simply pulled out the old relay and replaced it with new "thought-to-be-correct" one. And the smoke started pouring out within 4-5 seconds. I stood there frozen for what seemed like an eternity, but finally yanked the new relay out of the fuse box--and it was HOT. Nearly too hot to touch. The smoke was that very obvious "burnt rubber/plastic" smell. I knew things were going to be very bad...

    I open up the wiring harness housing, and see this:

    wiring2.jpg20190306_212902.jpg

    I can only add 5 pics, but from what anyone can see here, any idea what part of the wiring exactly has burned? I am guessing the fuel relay was obviously not the right part and it poured unchecked voltage downstream, but I have no idea what I am looking at with that harness. How screwed am I here exactly? I appreciate any thoughts in advance!

    Thanks!

  2. #2
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    I figured out this was the relay they sold me and was installed in the fuel pump socket. It's for the AC control. This is so, so frustrating...
    https://shop.advanceautoparts.com/p/...B&gclsrc=aw.ds
    Last edited by HotWaterMusic; 04-25-2019 at 12:21 AM.

  3. #3
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    I just had the same issue with my 323is it melted a brown wire all the way across to the ecu side does the car run now

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    Quote Originally Posted by ebmwe36 View Post
    I just had the same issue with my 323is it melted a brown wire all the way across to the ecu side does the car run now
    No, my car doesn't run, I was posting here in hopes someone could give me some insight on what should be the next step I take. What caused your brown wire to melt?

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  6. #6
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    Unpleasant - yes, but that damage doesn't look terrible, thanks to you pulling that rogue relay out quickly. Seems like the most damage is done to the brown (always ground in bmw) wire. I would be thorough with repair of that area.

  7. #7
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    Fix the melted wires. The original relay doesn't look that bad, they all look like that, the car is nearly 25 years old.

    You probably need to replace a dead fuel pump.

  8. #8
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    The relay diagram looks correct, so unless it's internally broken/shorted/incorrectly assembled, I don't see how that could cause it. To get the magic smoke, you obviously have a short ground to +12 somewhere. Fix the wiring in the box, then do some continutiy and ground/12+ short tests at the relay block before you plug a new relay in. Also agree the old relay looks fine. Easy to test, jut put a battery across 86 and 85 pins and see if it clicks.
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  9. #9
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    I did that once. Tried testing with a standard Bosch relay. Yes, the standard numbers may be used for terminals on some of the bmw relays but the location on the terminals differs. You put power to ground and smoked your harness.

    You can carefully wire in new sections and use heat shrink, but I recommend buying a used harness and swapping it in. A used harness might cost $100 and take a day to swap for a first timer. I have done the job enough that I don’t even have to look at a wiring diagram to figure out where stuff goes.

  10. #10
    NeilM is offline Member BMW E36 M3 Expert
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    Been there, done that, same way (relay substitution fail), many years ago. It’s possible to do a wire by wire repair if you’re handy with a soldering iron, heat shrink, and multimeter, but much better simply to replace the engine harness. Source one either from a scrapped/parts car or from a racer who has built a custom minimal harness and still has the old one.

    R&R is not quite as horrible as you might imagine. Pro tip: label every single connector on the old harness as you remove it. Have a Bentley shop manual on hand for wire color ID. You must use an OBD-2 harness, but there can be minor harness differences between model years. I believe you can use one from an automatic if necessary.

    Neil

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    Swapping the harness is very easy. Few hour job once you are familiar with the car.

  12. #12
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    I a similar issue I fixed the burnt harness got some relays think there the right ones turn key dash comes on and fuel primes tuen key to start and nothing no sound does anyone kno if my 323is has a starter relay

    - - - Updated - - -

    I pulled my engine and harness planning a m50 swap decided to go back to stock put everything together put relays in and melted the wire then tore apart the harness and fixed it still nothing happens after I turn the key

  13. #13
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    Quote Originally Posted by NeilM View Post
    Been there, done that, same way (relay substitution fail), many years ago. It’s possible to do a wire by wire repair if you’re handy with a soldering iron, heat shrink, and multimeter, but much better simply to replace the engine harness. Source one either from a scrapped/parts car or from a racer who has built a custom minimal harness and still has the old one.

    R&R is not quite as horrible as you might imagine. Pro tip: label every single connector on the old harness as you remove it. Have a Bentley shop manual on hand for wire color ID. You must use an OBD-2 harness, but there can be minor harness differences between model years. I believe you can use one from an automatic if necessary.

    Neil
    Based on what I am reading, it seems like an entire harness-swap would be the way to go. Before I went that route though, what are the chances the only damaged wiring I need to fix is right there in front of me in the box? Is it likely there is ground-wire damage some where else up or down stream of that burned area? That was the only area I noticed smoke before pulling the relay, and the only area of the visible harness that's burnt. If completely replacing the harness is the ultimate outcome of this, I wouldn't mind taking a couple hours the day before and just see, like you said, if I can repair/splice the burned-out parts first. Doesn't seem like I have much too lose but the time it takes. But if the damage is in other, covered, parts of the harness...I am guessing a complete R/R is pretty much the only answer.

  14. #14
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    Quote Originally Posted by Braymond141 View Post
    Swapping the harness is very easy. Few hour job once you are familiar with the car.
    Are there any walk-through guides online for the process? I imagine it's 50% removing of things in the way and 50% labeling/matching. That's not completely above my paygrade but not something I've done before. And what's the most reliable source for a harness? I can't trust my local yard to have one intact. Local Craigslist (Austin) doesn't have one either, which obviously would have been a miracle if it did, so NBD.

  15. #15
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    are slow.
    Jesus.

    Wouldn't hurt to see what exactly is burnt, and check from there. It looks like there are only 3-4 wires heated up, so possible to fix. The issue is you don't know if anything else happened along the harness.


    Should also look under the fuse box to see if anything is burned.
    Last edited by Hova; 04-26-2019 at 10:27 PM.
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  16. #16
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    For me it was just the engine harness. The connections are fairly simple but it helps to have a Bentley manual to identify similar looking terminal ends by pulling back the bolts to see the wire colors. It’s not rocket science. If you can pull an intake manifold you can change a wiring harness.

  17. #17
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    Quote Originally Posted by pbonsalb View Post
    I did that once. Tried testing with a standard Bosch relay. Yes, the standard numbers may be used for terminals on some of the bmw relays but the location on the terminals differs. You put power to ground and smoked your harness.

    You can carefully wire in new sections and use heat shrink, but I recommend buying a used harness and swapping it in. A used harness might cost $100 and take a day to swap for a first timer. I have done the job enough that I don’t even have to look at a wiring diagram to figure out where stuff goes.
    Almost seems like a right of passage....I did the same, thought I had the correct relay, swapped it in and went to turn the key and the entire wire in the harness melted...
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  18. #18
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    Your starter is 22 years old, and your brushes are probably worn. Rap the starter with wooden end of a shovel of broom, and it will probably start.

    Also, check your battery connections and the connections at the starter.

    What is your battery voltage before you try and start the car, and what is the voltage while your trying to start the car? 12 and 10 would be close to being correct, but try on your other cars as some meters don't always read correctly.

  19. #19
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    Quote Originally Posted by marvinstockman View Post
    Your starter is 22 years old, and your brushes are probably worn. Rap the starter with wooden end of a shovel of broom, and it will probably start.

    Also, check your battery connections and the connections at the starter.

    What is your battery voltage before you try and start the car, and what is the voltage while your trying to start the car? 12 and 10 would be close to being correct, but try on your other cars as some meters don't always read correctly.
    The starter was definitely engaging and turning the car over, but it was as if it had no fuel to actually start-up. I sprayed starter fluid into the intake and the car "ran" for about 2 seconds but would die. That's what lead me to the fuel pump relay, as I wanted to check it before to the fuel pump itself. Alas, I didn't even get to move on to the fuel pump.

  20. #20
    NeilM is offline Member BMW E36 M3 Expert
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    Quote Originally Posted by HotWaterMusic View Post
    Are there any walk-through guides online for the process? I imagine it's 50% removing of things in the way and 50% labeling/matching. That's not completely above my paygrade but not something I've done before. And what's the most reliable source for a harness? I can't trust my local yard to have one intact. Local Craigslist (Austin) doesn't have one either, which obviously would have been a miracle if it did, so NBD.
    Used harnesses appear on eBay from time to time, although I find none on offer currently. ECS will sell you a brand new one, but for $1100, so that doesn't seem like a desirable option. I'd suggest posting a request for a used one in the Track, Auto-X & Drag Racing forum. You may be able to use a 328 harness as long as the donor car had ASC+T — anyone know if that was standard or an option?

    Installing a replacement harness isn't too bad, but of course doing it for the first time takes twice as long as the next one would (yes, I've done more than one). The entire harness is built around the splice box, the rectangular plastic box against the cowl right behind the head that's held on by a couple of screws. From there on the passenger side there are harness tentacles to the power terminals, the ECU, the transmission controller (if automatic), and doubtless a couple of other things I don't recall years after replacing mine. On the driver side are branches with the three big round connectors that go to the side of the fuse box, plus numerous individual branches (MAF, idle controller, purge valve, etc. etc.). Most of the connectors are unique and can only be plugged into one place — although when reassembling you still have to find that place! — but a couple of them are not. Hence the need to label.

    If you can't find a used harness then the repair it yourself option may be the only one. Unfortunately the splice box is already stuffed pretty tight, so teasing out the wires, repairing them, and getting them back in can be tricky. Also, burned wires lose their insulation color and can be hard to identify. In my experience the damage is likely confined to the engine harness itself, but checking around is still a good idea.

    Neil
    Last edited by NeilM; 04-29-2019 at 09:29 AM.

  21. #21
    NeilM is offline Member BMW E36 M3 Expert
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    Quote Originally Posted by HotWaterMusic View Post
    The starter was definitely engaging and turning the car over, but it was as if it had no fuel to actually start-up. I sprayed starter fluid into the intake and the car "ran" for about 2 seconds but would die. That's what lead me to the fuel pump relay, as I wanted to check it before to the fuel pump itself. Alas, I didn't even get to move on to the fuel pump.
    This is a tricky area on the E36. When you go to start the car the fuel pump should briefly pulse to pressurize the fuel rail, but it won't run continuously until the ECU detects that the engine is actually running. This is unlike older BMW's where you can listen for a running fuel pump with the ignition on.

    Neil

  22. #22
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    I run a 328i auto harness on my manual M3. 328i manual harness works also on manual M3. Not sure if M3 auto harness would work on a manual M3. And 328i auto harness won’t work on auto M3. Harnesses sell regularly for about $100 from people parting cars.

  23. #23
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    Quote Originally Posted by NeilM View Post
    This is a tricky area on the E36. When you go to start the car the fuel pump should briefly pulse to pressurize the fuel rail, but it won't run continuously until the ECU detects that the engine is actually running. This is unlike older BMW's where you can listen for a running fuel pump with the ignition on. Neil
    Yes, here copied from M1.1 and M1.3 function, same principle on the M3 DME: The Motronic electrical system is controlled by a main fuel injection relay and a fuel pump relay. A permanent voltage supply is made to the main relay terminals 30 and 86 from the battery positive terminal. When the ignition is switched on, the ECU earths terminal 85 through ECU terminal number 36 which energises the first relay winding (on the 1987 M1.1 system, this connection was made directly to earth). This causes the first relay contacts to close and terminal 30 is connected to the output circuit at terminal 87. A voltage supply is thus output at terminal 87. Terminal 87 supplies voltage to the injectors, ECU terminal 37, ISCV and the CFSV when fitted. In addition voltage is supplied to the fuel pump relay terminal 86. When the ignition is switched on. the ECU briefly earths fuel pump relay contact 85 at ECU terminal 3. This energises the relay winding, which closes the relay contact and connects voltage from terminal 30 to terminal 87, thereby providing voltage to the fuel pump circuit. After approximately one second, the ECU opens the circuit and the pump stops. This brief running of the fuel pump allows pressure to build within the fuel pressure lines, and provides for an easier start. The fuel pump relay circuit will then remain open until the engine is cranked or run. Once the ECU receives a speed signal from the CAS, the relay winding will again be energised by the ECU, and the fuel pump will run until 3 seconds after the engine is stopped. The 3 seconds delay in switching off the pump relay allows the fuel pump to maintain pressure on engine shut-off to avoid engine run-on. In addition, on some vehicles control of the OS heater is made through a separate OS relay. Once the ignition is switched on, a voltage supply is made to the OS heater relay terminals 30 and 86. When the engine is started, the ECU connects relay terminal 85 to earth through ECU pin 23. The relay actuates and the output voltage at terminal 87 provides voltage to the OS heater. The ECU switches off the relay under certain conditions of speed and load.
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  24. #24
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    Just an update...I traced the burned wire along the entirety of the splice box. The burning is worse on the driver's side of the box and weaker as it goes towards the pass side/ecu. After seeing the full damage though, I think a full-on harness replacement is the only way to go now. I also pulled the relay that's mounted to the ecu. How's that one look?

    20190429_193335.jpg20190429_193359.jpg 20190429_195106.jpg20190429_195052.jpg

    Just a matter of finding a wiring harness that works for this car now and doing the R/R.

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    I have never seen a relay next to the ECU ... is that an OBD2 thing?!?!
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