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Thread: Trans Failsafe Mode Cog-of-Death! The sky is falling!

  1. #1
    Join Date
    Jul 2008
    Location
    Lake Oroville, CA
    Posts
    13
    My Cars
    352Ci

    Trans Failsafe Mode Cog-of-Death! The sky is falling!

    Or maybe it's not! I've been experiencing the cog of death trans failsafe mode on my 325Ci for quite some time. It started several thousand miles back and the first time I trailered it home so I could take a look at it. I believe my first "reported" failure was a speed or gear sensor. It just did failsafe and then lost the first couple gears and would wind up like crazy with no power. Standard deal. It did smell hot though.

    I thought the first thing to try would be to just replace the fluid/filter and inspect for chunkiness in the pan. Just fine black fuzz on the magnets and nothing that seemed alarming, though the fluid was obviously burnt. I also got the tools/interface off eBay and cleared the errors and adaptation. Also noticed that the heat shrink tubing on the wiring harness had seemed melted. Odd, right?

    So, after sealing it all up it seemed fine for a while and didn't grenade or turn into a pumpkin like folks had made me scared it would after changing fluid. But a little while down the road, it went into failsafe and threw a code again. I used a cheapo code reader to clear the codes. It took several attempts and it did, and I could then restart the car and drive again. This cycle repeated a few times and I found it would actually go a lot longer if I hooked up the tools and cleared the errors and reset adaptation. If I didn't reset the adaptation, it would show up quicker and if I checked the errors the common one present in almost all failsafe mode instances was P0734 (gear 4 incorrect ratio). By driving gently one could go weeks or even months before the failsafe would come up. In addition, sometimes the failsafe required immediate clearing with the laptop (meaning it wouldn't go away on key cycle), and sometimes I could clear it by cycling the key.

    I was just buying time until I bought a new wiring harness and sensor and just got lazier about getting around to buying the stuff and undertaking the task of putting it back up on jackstands and doing the refill process. After all it didn't happen all the time and when it did, five minutes with the laptop it was going again (and I have several cars so I wasn't relying on this vehicle.) After doing the clear/reset a couple times I just wasn't paying attention to the errors but then I noticed something very odd one time when I chose to review it closer. The temp when considered in Celsius was pretty flipping hot on one of the reports (134 celsius in the last one)! Not to mention, this always seemed to happen after running hard than coming to a slower roll to roll into my parking spot at home which is uphill, or slowing to a roll after heavy traffic and more often in hotter temps. Oddly it rarely happened to my wife (who drives it way more than me but drives it like she drives her Honda Odyssey) but it would happen to me much more often (as I drive it like I ride my two BMW bikes.) She just must not be running it as hard as I was.

    So I started thinking in terms of autos I worked with in the past and figured maybe something was obstructing flow to the cooler. Of course I learned that these use an exchanger instead and found one for rather cheap on eBay and figured I'd give it a try. I also bought the trans cooler thermostat too just in case (both together were $40ish). So, I swapped out the heat exchanger (not hard if you bought the tool, though one of my lines had the retainer crack and crumble to I cut line and securely clamped until I buy a new line to replace it when I service serp belt, etc.) I thought it seemed so simple that how could that be an issue, stuff flows through it right? Well after I pulled the old one out I found out that stuff was not flowing through it. It was clogged up!

    Well, after it was changed I hooked up the laptop I keep in the car with the BMW tools on it and proceeded to drive the heck out of the car trying to make it go into failsafe again while having the wife read out the temps when I asked. I could never get it to happen and the temps stayed in line with the radiator temps. I was BRUTAL with it too. I also let my wife and college aged son drive it numerous times too and I even took a couple good trips in it. So, I'm to the point where I think my issues were in fact all along heat issues and I failed to notice that. I wanted to wait awhile before adding this to the collective cog-of-death info out here to make sure I wasn't wrong.

    Moral of the story? Make sure and check your temps before thinking the sky is falling. It might be something this simple! But, I can't also help but think that if I would have swapped out the tranny or some other drastic measure that this still would have popped up if the exchanger was not changed.

    I will once again do a couple drains and refills to get the potentially temp damaged fluids out of there, but I once again finally have faith in this car again as I was even thinking that when I changed the sensor, that probably wasn't going to be it as it only occasionally happened.

    BTW, the details on this car is it's a 2004 (2003_03) 325Ci with ZF 5HP19 and now about 85k miles on it.

  2. #2
    Join Date
    Jul 2008
    Location
    Lake Oroville, CA
    Posts
    13
    My Cars
    352Ci
    And of course I am not saying this is *the* solution, I'm just sharing this as I had never seen anything about this when searching before. It may be overlooked by others and is just something for future seekers of knowledge to consider.

  3. #3
    Join Date
    Mar 2019
    Location
    Chattanooga
    Posts
    1
    My Cars
    BMW 2003 330i
    Tim, What you described as your vehicle's problem is similar to what I've been experiencing with mine. Could you post the actual part name "exchanger"? Is this the same as mass air flow sensor?
    Thanks

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