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Tach & OBC dead. Not the SI Board?!

Posted by Earendil 
August 08, 2010 03:54PM
I have an issue that I thought I knew the answer to, but now I'm stumped.

My tach, fuel economy gauge, and OBC all went dead at the same time. When I say the OBC went dead, I mean the backlight is there, but there is no information.
I thought that my SI bored must have been going bad, either due to batteries, or battery leakage burning out parts of the board.

So I replaced the SI bored with a known good one, one that had its batteries replaced a year ago. Fire the it and I have all the same problems. Now I'm really scratching my head. The tach reading comes from the ECU, not a sensor. But with the entire OBC staying "off" it makes me think it's not getting the signal to "turn on". Since the back light is independent of its internal power, this could make sense.

Could this be anything else besides a bad ECU?

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1989 - E30 - M20 - Manual. Approximately 270,000 miles
2000 - E46 - M52TUB28 - Manual. Approximately 110,000 miles

August 08, 2010 05:06PM
The only thing a bad ECU would do to the OBC is not feed it any data to update the display. The display should be on, no matter what.

Perhaps the power return for these items has become open. Check the grounding terminal above the steering column and pedals.

Also, check the ECU connector, sometimes they come loose or just grow crappy connections.

Eliminate all the free stuff first grinning smiley
August 08, 2010 05:12PM
Quote
Archeo-peteriX
Eliminate all the free stuff first grinning smiley

Or the stupidly simple stuff? I just checked fuse 21 and it's blown. I can't believe I replaced the SI board before checking the stupid fuses. At least the SI board was free and something I had been meaning to do anyway ( because of it's new battery). I'll replace the fuse and see what that does for me.

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1989 - E30 - M20 - Manual. Approximately 270,000 miles
2000 - E46 - M52TUB28 - Manual. Approximately 110,000 miles

August 08, 2010 07:43PM
Just one of those days. I'm out running other errands and decide to make a special stop for a 7.5 amp fuse.
The place I'm at doesn't appear to sell 7.5amp fuses, and right about the time I start pitying myself, it dawns on me that 3 feet from where I park my bimmer at home is my part car bimmer. And inside that parts car is a full set of good fuses. Gah! I'll blame it on this head cold I've been fighting for 4 days now.

Came home, plugged the fuse in, and vwalla! Tach and OBC are back online. Why in the world BMW put those two on the same fuse I may never know...

Thanks for the info though, peter. It's always appreciated! smiling smiley

-----------------------------------------------------------------------------------
1989 - E30 - M20 - Manual. Approximately 270,000 miles
2000 - E46 - M52TUB28 - Manual. Approximately 110,000 miles

August 08, 2010 08:59PM
I'm always glad to hear that leading the way to the free stuff helped winking smiley
August 09, 2010 09:56AM
.......when you hear hoofbeats think horses not zebras!!!!!

alan
August 09, 2010 11:03AM
Quote
alanrw
.......when you hear hoofbeats think horses not zebras!!!!!

alan

In 4 years this is my first blown fuse! I.e. I see far more zebras than horses winking smiley

-----------------------------------------------------------------------------------
1989 - E30 - M20 - Manual. Approximately 270,000 miles
2000 - E46 - M52TUB28 - Manual. Approximately 110,000 miles

rkj
August 09, 2010 10:09PM
Quote
Earendil
Quote
Archeo-peteriX
Eliminate all the free stuff first grinning smiley

Or the stupidly simple stuff? I just checked fuse 21 and it's blown. I can't believe I replaced the SI board before checking the stupid fuses. At least the SI board was free and something I had been meaning to do anyway ( because of it's new battery). I'll replace the fuse and see what that does for me.

I did that once, now I have a new brakelite switch..... sad smiley

The learning curve, sometimes we learn simple things many times smileys with beer
August 10, 2010 12:43PM
Quote
Earendil
I just checked fuse 21 and it's blown.
Replacing the fuse is a simple fix. But the bigger question remains -- why did the fuse blow?

I had the same issue with my old 86 325e. I'd replace the fuse, then mysteriously it would be blown again a few days later.

The 7.5 amp fuse-21 powers all of the following items:

- Auto-Charging Flashlight;
- Ignition Key Warning/Seatbelt Warning {also fuse 10};
- Injection Electronics {also fuses 9, 10, 11};
- Interior Lights {also fuse 12, 27, 28};
- Trunk Light;
- Active Check Control {also fuses 6, 19, 27};
- Service Interval Indicator {also fuse 10};
- On-Board Computer {also fuses 10, 12, 23, 27};
- Tachometer/Fuel Economy Guage {also fuse 10}.

I sure wasn't looking forward to tracing the wiring for all those items. But eventually the repeatedly blown fuse became so annoying I had to do something about it. I started by checking the simplest items first.

If your car has an Auto-Charging Flashlight, check to make sure nobody has chucked loose coins into the charging socket located in your glovebox. My car didn't have the Flashlight or charging socket, so I moved on to the next easiest item to check.

Have a look at the feed wire to the trunk light. It runs through the left hinge arm of the trunk lid. Every time you open and close the trunk lid it flexes the wire where it bends over the sharp edge of that hinge arm. On my car that sharp metal edge had cut through the insulation on the wire so that the wire sometimes grounded out at that spot, blowing fuse-21.

I wrapped the wire in electrical tape and that solved the problem.
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