Mystery sound - e-box related?

[blazenXLT]

Ok, this has been driving me crazy... quick background, I drove the M3 2 weeks ago, it started right up no problems. Drove about 20 minutes, made a stop, 20 minutes home. Again, no issues on start-up or driving. Two days later I went to get something from the trunk it the battery was dead. Strange, but not a big concern, I hooked up the charger. About two hours later I came back to the garage and there was a fan-like sound coming from engine compartment, near the driver's side by the brake fluid reservoir. Not a sound I ever recall hearing before.

I know there's an electronics box fan, but I've never heard it like this, and didn't think it should run at all when the car is totally cold. I disconnected the charger since I didn't want anything running full time. It dawned on me to check the battery and it was from 2015 - not sure how I didn't realize it was that old, but oh well, I figured that was the problem - battery is spent and maybe won't take a charge at all. A few days ago I picked up a replacement battery. As soon as it was hooked up, the fan sound started again.

I'll spare typing out everything, but I've gone through testing by pulling the SAP relay, pulled the fuse for the e-box fan, pulled the fuse of the HVAC/blower motor - sounds keeps going. What am I missing?

Here a quick clip - https://youtube.com/shorts/XS3fS5Grx...5kEafhZAbcxyk-

[sapote]

check if the heater control valve is warm or hot to the touch. Pull the cable off if valve is hot/warm. What fuse you pulled for the efan?

The mystery thing drained the batt.

[blazenXLT]

I assumed the same, that it was parasitic drain that killed the batt.

For testing, I pulled fuse #4 from the small black box that's inside the electronics box, I'm pretty sure that's the one that is the output for that fan.
I also pulled the relay for the SAP. From the glove box fuse panel, I pulled fuse 50 for the HVAC blower motor. Lastly pulled the plug for the vacuum pump.

I'm pretty good with most things on this car, but electronics always make me nervous...I'd rather be wrenching on brakes or suspension. Open to suggestions.

[sapote]

Check the heater valve?

[blazenXLT]

Not yet, been stuck in the office all day. I'll check it later today.

[sapote]

For testing, I pulled fuse #4 from the small black box that's inside the electronics box, I'm pretty sure that's the one that is the output for that fan..

Based on the wiring diag, the DME itself capable of driving the small efan alone even with 20A fuse pulled out. So it's high chance that the noise is from the running e-box fan.

[blazenXLT]

Heater valve stays cold, so for sure not that. I had read somewhere that it was possible it's powered even with the fuse pulled, so given the sound, that must be it.

What's my next step? Replace the fan unit? Looks like it's impossible from above, I'm guessing you need to go through under the dash.

[heinzboehmer]

The connector for the DME fan is clipped to the inside of the DME box (on the side closest to the windshield IIRC). Think it's worth disconnecting it while you hear the noise to make sure that's the failing component.

Edit: This one

[sapote]

Heater valve stays cold, so for sure not that. I had read somewhere that it was possible it's powered even with the fuse pulled, so given the sound, that must be it.

What's my next step? Replace the fan unit? Looks like it's impossible from above, I'm guessing you need to go through under the dash.

A bad fan cannot run itself, so I would blame the DME or an external wire ( neg side of the fan) that shorted to gnd to cause current flowing thru the fan 2 wires.

To do: pull the 24pin connector off the DME (or cut black/green wire off pin12 of 24pin connector) and fan should stop. if not check if that black/green wire has 12v or 0v; if it's 0v then the wire is shorted to gnd.

[blazenXLT]

The connector for the DME fan is clipped to the inside of the DME box (on the side closest to the windshield IIRC). Think it's worth disconnecting it while you hear the noise to make sure that's the failing component.

Edit: This one

When I pull this, the sound continues...

[sapote]

not e-box fan, not heater valve, next is the ABS pump.

[blazenXLT]

I finally had some time to really dig in here. I removed the filter housing and some trim pieces and did the old screwdriver to the ear test to find what's making noise. It's pretty clearly the brake booster. The vacuum lines look good, so i'm guessing it's the internal diaphragm that's damage and won't hold a vacuum? When I press the brake pedal the sound changes. It's the original part and I have a November '01 build date car, so it's old.

[sapote]

I finally had some time to really dig in here. I removed the filter housing and some trim pieces and did the old screwdriver to the ear test to find what's making noise. It's pretty clearly the brake booster. The vacuum lines look good, so i'm guessing it's the internal diaphragm that's damage and won't hold a vacuum? When I press the brake pedal the sound changes. It's the original part and I have a November '01 build date car, so it's old.

So the booster valve(s) leaking, but the noise should stop after some time with engine not running. Not the diaphragm leaking but the valve (activated by the pedal) is. It will be a tough job to change the booster without disconnecting the brake metal pipes, but I did this way once.