Please sticky this thread so we can avoid any more tragedies. I had no idea about this until after my car was ablaze
Last weekend I went to a scrapyard and picked up a few diagnostic covers. According to realoem.com all cars from E28 all the way up to E39 use the same part. Unfortunately, that's not true. There is a third version that's being used in cars with ODB II. In our old cars the plug connects the RXD line to +12V and grounds the TXD line. In the ODB II cars, however, both are connected to +12V via a resistor that probably limits the current that can flow.
Would anyone know what would happen in our cars if we'd pull the TX line up? Would the resistor prevent smoldering wires just as well as a fuse?
I want to add something into this...Took my car our for a short drive yesterday. Not even 5 minutes into being on the smoke started to come out from were the fresh air vents/windshield defroster vents on top of the dash. Of course I got freaked out, and pulled over and checked everything. I thought it had something to do with this thread so I removed my diagnostic cap and tried to start the car. My 7 won't even engage the starter without it plugged in. Turn the key and nothing but, put it back on and it'll crank.
I don't think the smoke is related now (here's to ripping out the dash now) but looks like I'm one of the few that have to have it plugged in. Going to be doing the modification still.
My E34 544iT (M60 DME, Motronic 3.3 without EWS) had this happen two years ago. I ended up having to take apart the main fusebox and half the interior to repair collateral damage. I disabled and removed the diagnostic cap entirely and noticed nothing changed - no warning lights, everything ran fine. Skimming this thread, it looks like EML is the only system to require the cap in place?
I stopped for maybe 45 minutes to try to figure out what happened but couldn't find anything. Was only about a quarter of a mile from my house so I just drove it back and nothing. Last thing I did was replace the cabin filter like a month ago so, I went and checked everything there but still couldn't find anything. Found some confidence and drove for about an hour and still no smoke. Haven't really drove it since but still gonna pull the dash. No clue what it could be..
Hi,
Sorry to revive an old thread but it is the best I could find.
I have been working 2 hours a day, for 10 days on my 1996 E34 525i automatic, weather permitting.
It's my daily driver for the past 5.5 years.
The trouble started with a sudden dead battery after parking for the weekend. 0 voltage.
I brought a replacement battery (under warranty), went ahead to connect it and got a large spark when attaching the second battery terminal. I didn't connect the battery of course.
There is a short between the positive terminal and the car body. I was surprised the car didn't burn down.
Ever since I have been trying to locate the short, tearing up my beautiful car in the process..
Several times I thought I found the culprit, wrongly suspecting old aftermarket alarm systems installed above the pedal area. Took apart the fuse box as well.
When this didn't help I switched to a more sensitive multimeter.
Now I have located 2 sources for the short:
1) When the diagnostic cap is closed, I get a reading of 4KOhms resistance.
This could be explained by what is described in this thread and can at least be temporarily fixed my removing the cap or isolating pins 14 and 15 on it. I don't really understand why you suggest to fuse it, it could just be disconnected. I have not yet found where the diagnostic cable shorts to the ground but it could be anywhere in the car.
2) The DME, between terminals 26 and 28 (B+ and ground) gives a resistance of 30KOhms. I don't know if this is normal, as I don't have a replacement DME to compare.
I am a little afraid to connect everything back together, put the DME in place, leave the diagnostic cap out and connect the battery.
Does anybody know what the normal resistance of the DME should be?
I just don't think it is likely I have to 2 problems at the same time, but 30KOhms seems to me too low.
If I would need a replacement DME, can I just buy one with the same code or will I run into problems with EWS? What do I do then?
All help will be appreciated.
Amos,
Jerusalem, Israel
original_c5f85da0-99d5-4e93-badc-4397603fd519_PXL_20210126_135415194.jpgPXL_20210121_091321850.jpgPXL_20210126_133125856.jpg
Hi Amos,
You do not need a sensitive current meter.
Just connect a small 12V lamp (maybe a 6W dome light) between your + battery post and its cable. The lamp will burn bright until your short is cleared. Then it will go dim or dark. No drama, no sparks, no smoke, just light.
Now take a look at your "armchair dave" E34 ETM:
https://shark.armchair.mb.ca/~dave/BMW/e34/e34_88.pdf
You may say these diagrams are strange, but it's just a new kind of map reading.
On page 0670.2.00 you can see 4 wires leaving the battery positive post. One of these is shorted to your chassis.
When you disconnect the correct one, your lamp will go dark.
To create the sparks you saw, it will probably not be a fuse. It will be one of the thick cables.
I would disconnect the starter and alternator first.
Last edited by E32FAN; 01-26-2021 at 08:12 PM.
My E32 (US 1994 740iL) is currently off the road as of a couple days ago for more suspension work, but once it's driving again I'll experiment with driving for a couple weeks with the diagnostic cap removed and report back with the findings.
I have never had an issue with wires on this circuit shorting on my E32s or E34 but it worries me, and I don't want to deal with the aftermath of finding all the burnt areas of wire loom if it happens!
Last edited by m60power; 01-26-2021 at 08:27 PM.
I found another area that is easly checked and can start a fire or at least a short: the headlight plastic caps:Capac lumini.jpg
Thank you E32fan.
I know the ETM, it's been my reading material for the past week..
I also started with the starter/alternator on day 1. disconneted all 4 wires coming from the battery and investigated.
Anyway, yesterday I put the DME back in. Finally I found out, that with the diagnostic cap removed The short is definitely gone.
I still have to find where the diagnostic cable running between the car modules is damaged.
I will have to wait for a fair weather day to put the car back together..
I have to wait for a new gas pedal I ordered as well, because I broke it while working under the dash.
0670.5-04 shows X6002 Pin 15 = RxD (WS/GE)
0670.2-00 shows X6002 pin 14 = Circuit 30
Pin 14 is unfused. Our experience is that Pin 15 (RxD, WS/GE) can short to ground in some module.
In my case it was the A21 Memory Seat controller, X646 pin 5.
In some cases this melts the WS/GE conductor inside the cable looms.
I assume the exposed wire, if touched, can cause a nice short and melt the car down?
The headlight or fog light housing wires those circuits are protected by fuse that is far a way till reach the fuse wire will be melted first. It is good to put fuse near head lights somewhere in safe area.
All other items are far from the fuse box, no need to put extra fuses. The main function of a fuse is to protect your wiring. NEVER install a larger fuse than given by BMW. Fuses operate because the fuse element is the 'weak link' in the circuit, so that over current will melt it and break the circuit. The time taken for the fuse link to break the circuit (to 'blow') varies depending on the type of fuse and on the characteristic of the device. Where the current carried is very much greater than the rated value (which is usually associated with a fault rather than with an overload) operation is usually very fast. For small overloads, where the current is not much larger than the rated value, operation may take a very long time.
Fuses are critical in any electrical system and are used to protect a circuit's cabling from excessive current that could lead to damage/electrical fire. Excessive current is most likely to be caused by three things:
Incorrect wiring - wrong connections as a result of human error
Damage to the circuit - e.g. a wire working loose or insulation wearing through and causing a +ve wire to short to ground.
Overloading the circuit - e.g. connecting more pieces of equipment that draw more current than the circuit is designed or, conversely, using cable of insufficient size for the current draw of the intended equipment.
Shogun tricks and tips for the E32 series are HERE!
Yes, like shogun said, no need to put an extra fuse closer to the possible short location for an electrical circuit that is already protected by a fuse. The only case where this would be valuable is if you think that BMW's specified fuse amperage for that particular circuit is too high such that the wiring for the component on that circuit would melt before the 'weak link' of the fuse.
Regarding the diagnostic cap - I've been driving my E32 for just over 2 years now with the cap removed from the harness plug and haven't noticed any differences from when it was plugged in. One of these days I'll get around to modifying the cap so it is properly fuse protecting the rest of the car's wire harnesses...
Bookmarks