Any insights on specific and strange electrical gremlin?

[Dicoray]

I drive a 1996 750il with a C23 radio unit and no navigation system or modifications of any kind.

After travelling about a mile at a sustained speed of around 40mph the car's stereo system will completely shut off and the dash indicator lights (high beams, turn signals, etc.) will stop working.

After slowing down to a speed below around 35mph the dash lights will resume normal function.

When the car falls below 10mph the radio/MID back light and clock display will turn back on. The DSP unit will sometimes resume normal menu function, but since there is no audio there I do not know if the module is actually having any effect. The radio and MID units remain unresponsive however, none of the buttons have any function, the audio remains completely off, and the clock's displayed time stays at the time it was when the MID back light came back on.

The radio will not resume normal function unless the car is turned completely off and back on again. If the radio is not turned on within the first minute or so of the car being started it will not immediately go into it's unresponsive state.

Stereo volume, ambient temperature, and recency of the last drive have no effect on when these systems fail.

Over the past year I have replaced the ignition switch, battery, and any fuse that could possibly effect these systems. My next plan is to send the radio off to be repaired, but am hesitant to do so as there is no indication the radio is the sole cause of the problem and it otherwise works great.

If anyone has resolved the same issue or has any insight to give into the issue it would be greatly appreciated. Thanks.

[Timm]

This is the most likely cause:

https://www.meeknet.co.uk/E38/BMW_E3...l_Gremlins.htm

[Dicoray]

That could be it, the first thing I tried was a replacement ignition switch, but there is a chance that it's also bogus since it was non OEM. I probably should have bought an OEM one in the first place. I'll try installing OEM ignition switch and post an update afterwards. Also, I want to thank you for your website, it has been unbelievably helpful and useful more times than I can count during the ownership of my 7 series.

[AHolland]

For whatever reason the Meeknet site appears to be down when I went looking for it. I will check it later. I have an electrical gremlin of my own to put out there as it hit me tonight. In short it seems to effect battery charging. I have a new alternator that was double checked, New battery with fluid levels checked. Checked all grounding wires, battery posts, the usual things. This has hit on and off over the last two years but tonight left me stranded and for a guy in a wheelchair in the winter that is not a good thing.

It seems that periodically there is either a large parasitic drain on the battery or the battery is simply not telling the alternator to turn on. The only thing I can think of is that on some BMW's 740/750 there is a cold weather circuit whereby if the battery voltage reads too low, the alternator does not turn on so the engine can be spun over without the drag of a charging alternator. Once the engine starts up the circuit slowly ramps up the charging rate of the alternator until it is at full power.

Now I can tell that the previous owner of the car might have fiddled with a few things because I found an ABS pump module on the car that was not working and where the idiot had cross threaded the mounting bolt to the pump assembly which is already a red flag for someone who mucks with cars but may not know what they are doing. You have to be pretty dumb to badly cross thread a bolt and not recognise that you are doing it. It was visually noticeable. I think when he could not fix the trifecta of lights he tried changing the abs module but the real problem was the control module. Even though I have changed the control module I am sure the problem is not there because I had noticed the problem well before the module was changed out. I do need to reprogram the control module with some software so it matches my 740 as I took it out of the same year of 540.

Any suggestions where this problem might lie? I am fairly new to BMW's and I do not know their electrical systems yet and what might trigger either a parasitic draw or failure for the alternator to kick on at times.

Tonight I drove to the local university to pick up the daughter and waited, listening to the radio for maybe 15 minutes. The car started badly and then drove about 20 feet bucking and with the trifecta lights blinking on and off. I stopped for a minute to see if I could figure out what was going on and then it stopped and was dead to the world. I could tell there was a little power but not enough to start the car. Even jump starting it was quite difficult so I presume the battery was draining while I drove and the radio for 15 minutes pretty much killed it off.

Is there anything outside of the alternator which might signal (or not signal) the alternator to turn on like it should when the heater/radio or other electricals are used. I do wonder if it is a bad parasitic draw because about a year ago the car had problems starting a number of times and then the problem went away not to be seen for about 6 months. Again was great all summer and then hit with it today again.

[Timm]

There's no cold-weather circuit - but the alternator does not charge at start-up for a few seconds (10 seconds or so) and then ramps up current by 10A per second after that.

https://www.meeknet.co.uk/e38/E38_Alternator_System.pdf

But, if you ever get the 'battery' warning lamp then the alternator is either not spinning or is broken. Here's more information regarding battery drain:

https://www.meeknet.co.uk/e38/E38_Battery_Drain.htm

From your post above I would hazard a guess that your battery has failed as explained in the document above.

[AHolland]

This is the page I had on the alternator ramp up that we are both talking about. Somewhere else I have a BMW factory notice which does indicate that the car picks up the outside temp and based on that changes how the alternator works. If I can find it again I will post it here but I did read it and save it in my mess of computer hard drives. I recently lost aobut 5TB of data and a lot of it was the car stuff I had collected over the last year or two. In any case that explains how the alternator will work, regardless if there is a cold weather circuit or not. It can throw a mechanic into thinking the alternator is bad when they really just need to wait a bit for the alternator to ramp up. Damn but I want to find that other article now.

I am still searching for my electrical gremlin. I am tempted to just change the ignition switch (not key switch) and change out the heater module in the front consol and see if that helps me. Unfortunately mine goes away for 6 months and then suddenly comes up again so I suspect mine is a loose wire or switch that does not quite connect at times and allows the car to continue to drain power even when it has been off for more than 15 mins. I will let you know if I find my problem and what it was.