Electrical Headlight Engine Cutout Problem

[EURO M3 CSL]

Hello,

An interesting problem have occurred with my 1975 530i that I would like to ask u guys. Couple months ago, I have been having problems where my engine, instrument cluster light, and dome light, would cut out when i turn on my headlights. This usually happens upon first start up in the morning when the weather is gloomy with cold temperatures. The engine would be running very smoothly idling around 1000RPM with no additional warning lights on (no battery light). If i pull the headlight switch to the parking lights only, the parking lights will turn on and engine will still be running. Once i pull the headlight switch one more click to turn on the headlights, the engine and all the electrical instrument, including dome light, would turn off. Usually before when this happens, I turn the ignition switch off.. start the car back up, drove it for a little while, flip the headlight switch and everything will be working again. This morning however was even more depressing. Same thing, again happened, but this time, no matter how i turn the ignition key off/on, and pushed the headlight switch off, everything wont turn on. Clock would stop running, dome light wouldn't turn on, radio wouldn't turn on.. everything. It took a couple minutes before it would randomly regain power on its own (I say it took ~5 minutes). I drove it around ~40 miles to warm everything up and retried it again, same problem. This time, the time it took to regain power on its own was ~ 10 minutes. I tried it again, engine running.. flip the headlight switch, and everything would cut off, but it took less time to regain power. I am really stumped on this as I never really had this happen before. Initially i thought it was a short somewhere, but if it was a short, why would it regain power without me touching anything every time? It would randomly regain power again on its own.

Heres a couple things i tested:
Battery level when charging - 13.8V
Battery level not charging - 12.5V
(battery level stayed the same when everything cut out compared to when everything was working.. same volts)

No resistance from positive battery terminal clamp to starter motor.
No resistance from engine block to negative terminal
No resistance from positive battery clamp to negative battery clamp

Now that the car is warmed up in the sun, the problem doesn't occur again which makes me believe it is temperature sensitive and maybe something to do with a sticky relay instead of a short somewhere. All fuses are fine and working.

Any ideas?

[DEMIURGE]

1. Battery level when charging - 13.8V
Battery level not charging - 12.5V
(battery level stayed the same when everything cut out compared to when everything was working.. same volts)

2. No resistance from engine block to negative terminal

1. Usually 12.5 V is not so much for a charged battery without load.
13.8 V when charging - I suppose at idle? It has to be more at higher rpms.
Firstly, check your battery, before starting the engine - without load and then with load (min. 20A for example and measure the voltage then - just to be sure the battery is OK. I know - you will say - if the starter is turning - the battery should be OK, but check it, if possible - before and after the problem occures.
When the problem occures - check in the fuse box - the voltage across all the fuses - is everything is OK. It is possible to find a fuse or group of fuses without voltage, due to bad connections (inside the fusebox). This bad connection could be a common positive or negative joint point, but where the headlights were powered. When you turn the headlights ON - the load current became too high for this joint and the voltage dropped after this point. When the voltage dropped - the ECU cuts the engine off, because the ECU or some other device - fuel pump for example, is after the abovementioned point with dramatically lowered voltage.

2. Are you sure - there is no resistance between the engine block and the negative battery terminal???

These temperature sensitive problems could be everywhere - inside the battery also and I think - bad connections are in the first place - positive and negative both. You have to check with a voltmeter where the 12V or Ground disappears (or even better with a test lamp, because of the loading current). I think - there is not a short curcuit in your case temporary or other - because some fuse would be blown.

Sorry, I'm not very helpful - just some thoughts...

[boxeswithknobs]

Dealing with a similar issue. Hit the horn one day and all electric access went wild - temp gauge wildly fluctuating, headlight cutting out. Finally realized that keeping a foot on the accel kept the access from cutting out and had to drive back to Mountain View toggling between neutral and gear so my brake and headlights wouldn't cut out - not fun!

Going thru battery/ground replace & then check ignition switch checklist!

What did you find?

[EURO M3 CSL]

O wow im sorry to hear that.

Believe it or not it was a bad low beam relay that cause the problem. I guess the insides where corroded. But yea changed out the relay and the problem havent came back up since. Cheap replacement!!

[boxeswithknobs]

Yep, always keep extra (good) relays in glovebox. My issue was battery, pretty surprised I was able to keep driving it for a week and 500 miles, but new batt, cables, cleaned connectors and all is back to normal.