PSA: only you can prevent data link circuit melting

[moroza]

...because BMW evidently can't. Some verfluchter Trottel in the mid-80's at BMW decided that the diagnostic link circuit, which goes all over the car and whose splices are bundled with many, many others, should remain unfused.

See attached pictures, and these three threads.

Solution: either unscrew the diagnostic cap under the hood and leave it (bandaid fix), or disable the entire system (IMO unnecessary if it hasn't failed you yet), or put an inline fuse between the B+ terminal and pin 14 of the diagnostic cap (7.5A to be safe), or an inline switch for the rare occasion that somebody actually uses the diagnostic system.

I'm a mechanic and the repair for this circuit's failure on my car took about a week's worth of afternoons, including removing half the interior. No mechanic in their right mind would ever take on a can-o-worms job like this for a customer, so if it happens to you... have fun. Best believe that it's a lot less work to prevent than to fix.

[Racerhoze]

Thanks for the heads up... did you find out what caused the short to ground + melting?

[ross1]

"Some verfluchter Trottel "
Bist Du aus Wien?

[moroza]

No, I am not a Wiener

In my case, the short was a plug for the seat memory module, which shouldn't exist on my car as it never had memory seats. I tucked it into the seat springs in such a way that one of the wires rubbed on the seat frame and grounded out.

[Layne]

So the jumper inside the diagnostic cap is what powers these wires? What's the sense in that?

[moroza]

What's the sense in that?

Shhh... one must never utter those words in the presence of a German car...

[Layne]

Shhh... one must never utter those words in the presence of a German car...

I mean... the data link is for diagnostic use... which can only be done with the diagnostic cap off... so why is it powered when it cannot be used?

[ross1]

I mean... the data link is for diagnostic use... which can only be done with the diagnostic cap off... so why is it powered when it cannot be used?

SILENCE! Swine! You will not question our wisdom.

[Layne]

And digital data is rarely ever transmitted with enough power to melt anything. I feel like moroza's car had something strange going on, besides shorting out. I need to investigate this.

[moroza]

Actually this is a known problem, first brought to my attention by E32FAN when I PMed him. I'm just spreading the info as I feel it's important. My car is modified but the data link circuit was untouched until this happened. See wiring diagrams 0670.2-01 where it shows unfused B+ power to Pin 14 (via a fat 2.5mm wire). Then see your own car's diagnostic cap for the straight bridge from Pin 14 to Pin 15. Then see diagrams 0670.5-03 and -04 for what Pin 15 connects to.

[Layne]

On a somewhat related topic, I've been studying 535i diagrams because I'm using a M30B35 motronic system in a 3.0CS. Earlier ones have a fusible link in the power to the motronic system (60 amp I think), but later on they removed it and left the system completely unfused. Maybe because 60 amps is still too high to protect the wire at all, or maybe it caused some unreliability...? I don't know.

[Layne]

New development... I have a brand new diagnostic cap that I bought for the B35 harness. While the metal pins exist at locations 14 and 15, there is no continuity between them. Perhaps BMW learned their mistake and this was the fix. So all you really need to do is buy a new cap.

[marshallnoise]

So can you just de-pin 14 and be done with it?

[Layne]

Found the same cap on my car. No connection between pins 14 and 15. No idea if the cap is original or replaced, but the engine is a '95.

Also leaned the WS/GE wire is the 'receive' line (for the car to receive info from the diagnostic machine I assume), there's also e WS/VI 'transmit' line.

[Kodin]

Why not put a fuse at the feed point for the wires on the cap rather than in the cap itself? Where is the original battery tap point located upstream? (Yes I know this is an old thread, this is a newly-learned-about issue for me).

[shogun]

the solution https://www.bimmerforums.com/forum/s...ire+diagnostic

[eternalightwith]

I'll have to check my cap once it's back from the shop. Couldn't one solve this by just cutting link and screwing cap back on?

[m60power]

After I first found out about this issue I've unplugged the diagnostic caps on both of my E32s and haven't noticed any abnormalities to date from doing so. At some point I will get around to doing a proper modification to the cap housing.

[shogun]

I'll have to check my cap once it's back from the shop. Couldn't one solve this by just cutting link and screwing cap back on?

then you cannot rear fault codes when cutting the link.
Just do it like this, as shown on my website and here once more for download
E32 Diagnostic Cap Mod June 28, 2018.pdf

[moroza]

It's been a while since I've seen a BMW shop with a scanner that can read the body modules. In 16 years of E34 ownership I don't think I've once encountered a need to do so.