E46 M3 highway shutdown: tach/speedo drop, DME unreachable afterward

[OldRanger]

Need help diagnosing an intermittent shutdown/no-start on my 2002 E46 M3, ~182k miles.

I’ve now had two highway failures, but the second one gave much better data, so I’m focusing on that.

### Second failure:
- Engine shut off suddenly while driving
- Tach dropped to zero
- Speedometer dropped to zero (not 100% sure about this one)
- Other gauges still worked
- Climate control stayed on
- Car would crank normally, but would not start

Clue:
Right after the failure, my Foxwell NT510 could still communicate with other modules, but could not communicate with the DME at all for about 30–45 minutes. Later the DME came back.

Other notes:
- Both failures happened approximately one hour into a highway drive
- Dyno tuning session w/four or five flashes two weeks prior
- Battery was reading 12.3 V
- Charging voltage was reading 14.1 V
- I’ve already replaced the blue DME relay and ignition switch speculating it was a DME power issue
- E-box relay/fuse area does not show obvious heat damage or corrosion

My current thinking:
This seems less like a whole-car power failure and more like the DME itself or its power/ground/feed path dropping offline temporarily, since:
- Climate control stayed on
- other modules remained reachable with code reader
- DME became unreachable

Questions:
- What are the common causes of temporary DME dropout?
- What DME power supply fuses, grounds, or E-box connections should I target first?
- Has anyone seen an MSS54 go offline for 30–45 minutes and then recover, especially after about an hour of highway driving?

Appreciate any insight.

[karter16]

Any codes on the DME? Any codes on other modules like the instrument cluster?

[OldRanger]

Yes. On the first occasion, we had a DME 01 Fuel Pump Relay. Because of that, I swapped out the fuel pump relay with a spare that I had - as part of that other work I did between the first occurrence and the second.

We had that same fuel pump relay code again on this second occurrence tonight and there was also a code 90 oxygen sensor control cyl 1-3.

The car has that same fuel pump relay code before the dyno session two weeks ago. But I think that could be a trace of the DME dropping out based on my research

[OldRanger]

This was odd. Right after the failure (<10 minutes)

the Quick Scan worked, but didn’t see the DME.

About 30 minutes later I could read the codes

[sapote]

This seems less like a whole-car power failure and more like the DME itself or its power/ground/feed path dropping offline temporarily, since:
- A/C stayed on E46 M3 highway shutdown: tach/speedo drop

I'm not clear. Engine shutdown and you still have AC working blowing cold air?

Tach and odometer = 0 with car still moving? Manual box or SMG?

[OldRanger]

Manual box. Clutch in.

After 1 hour-ish: car starts to stumble. Almost feels like a fueling issue. That lasts about 2 minutes

Next, the engine shuts down. Car is moving from momentum. RPM gauge at 0.

Instrument Cluster is illuminated. So is the AC button and climate control system, and the fan is running.

(But the engine is no longer running)

[terra]

\
- Has anyone seen an MSS54 go offline for 30–45 minutes and then recover, especially after about an hour of highway driving?
.

I haven't seen such a thing, but sounds like potentially like something is overheating. Perhaps some component with an internal short leading to such.

Too bad MSS54s are so expensive these days, I think it'd definitely be worth a shot to replace

[oceansize]

Thinking along the lines of heat, perhaps the DME fan has quit?

[OldRanger]

Thinking along the lines of heat, perhaps the DME fan has quit?

I didn't check that, I will figure out how to do that. (I didn't have a code for it). I see there is an Ebox temperature switch that relies on.

[oceansize]

I didn't check that, I will figure out how to do that. (I didn't have a code for it). I see there is an Ebox temperature switch that relies on.

I never had to delve that far since mine was audibly failing with the typical chirping noises. I have seen other reports the fan simply dies, or perhaps it wasn't making enough noise to be noticeable.

[Cubieman]

Holy shit, a new ebox fan is $360..

[OldRanger]

Interesting input from the tuner, when I asked if a DME could/would shut down from excessive (external) temperatures in the Ebox: "It shouldn’t. (Their) Race cars never have the ECU fans and we never see them have overheating issues."

[oceansize]

Interesting input from the tuner, when I asked if a DME could/would shut down from excessive (external) temperatures in the Ebox: "It shouldn’t. (Their) Race cars never have the ECU fans and we never see them have overheating issues."

Seems like a fair assessment. Mine didn't get to the point of failure as I replaced due to chirps, which means I likely wasted my time and money. I could see over prolonged mileage/time then maybe the lack of one comes into play.

[heinzboehmer]

Holy shit, a new ebox fan is $360..

You can swap just the motor: https://www.e46fanatics.com/threads/...71019.1285892/

[OldRanger]

So…does anyone know of a MSS54 DME failure? What caused it?