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Old 06-03-2002, 11:26 AM
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Join Date: Mar 2000
Location: Southampton, NJ
Posts: 126
Tach stopped on '82 300D--any ideas

My recently acquired 300D with 255K miles is running great, but the tach just stopped working this weekend. The only work I did this weekend was to replace the stereo so I am wondering if I could have done anything with that.
Where should I check to test the signal, ie. where does the signal come from at the engine, and what type of output should I be getting.


'85 300DT--215,000 miles
'82 300DT--255,000 miles
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Old 06-03-2002, 12:24 PM
Holson Adi's Avatar
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Join Date: Jul 2001
Location: Boston, MA
Posts: 2,561
if you search the words tach and cig butt then you'll finds tons of answer Anyway,

I didn't use a cig. butt, I used a cognac bottle cork.

There is this thing that looks like a cylinder on the driver's side under the hood, just above the wheel wells. It's a small black cylinder with a screw on cap. Unscrew the cap and you should see that inside the cap there is a bunch of male connectors and the cylinder that is on the wheel wells has a bunch of female connectors.

Now, Just stuff the cig. butt in between the cap and the cylinder and tighten it down.

The other thing I did was to use WD40 in the holes... and amazingly it has not failed. I don't know if it's due to the cork or the WD40.
2008 BMW 335i Coupe

Last edited by Holson Adi; 06-03-2002 at 01:12 PM.
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Old 06-03-2002, 12:35 PM
240Joe's Avatar
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Join Date: Nov 2000
Posts: 525
I tried the "butt method" but it did not work very long. Others have had better luck. I took mine out and completely resoldered it. It is potted so it's a bit of a chore to do.

It's been working for 2 or 3 months now.


PS there is a pickup down by the balancer. I believe it generates a small signal from the magnetic effect and feeds to the cylinder shaped item described earlier. I don't have a schematic and didn't put a scope on it so I don't know what signal levels we are talking about. I've been told that the sensor almost never fails... it's usually a bad solder connection in the cylinder.

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