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  #1  
Old 01-28-2003, 04:34 PM
hotskillet
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Question phantom tachometer

For the longest time, the tach on my 300sd has simply just not worked. I've read the threads on tweaking the tach amplifier and such. My tachometer has begun to work sporadically without even touching anything to do with the tach at all. It will go to 4000k under hard accellleration while shifting and then drop down to zero. Sometimes it will hover between 500k and 1500k while cruising. It obviously isn't very accurate. I'm not really interested in fixing the tach immediately, I was just wondering if anyone els has experienced this with their cars.

Thanks
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  #2  
Old 01-28-2003, 05:19 PM
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Location: Puyallup, Washington
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The best thing you can do is bite the bullet and get a new/used amplifier. I got mine at a "European Car" junk yard in Seattle (Aurora Auto Wrecking) for about $45. My tach did exacatly the same thing.

Good Luck
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1977 240D Hell Blau (Verkauft)
1981 240D Weiss (Gestorben)
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1982 240D Dunkel Blau (225k) "Panzer Wagen"
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1981 300SD Beige (205K) "Panzerkampwagen"

Last edited by Elliott Fager; 01-28-2003 at 05:52 PM.
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  #3  
Old 01-28-2003, 06:25 PM
surfblau's Avatar
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Location: san francisco - immer kalt, immer windig, I want to move
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provided that you have an amp...

It depends on your model, but 1984 and up do not have an amp. There are lots of articles on the "cigarette butt" trick and such.

Only applicable to the pre 1984s (at least on the 123 chasis, probably on the 126 chasis also).

My intermittant tach probs are similar to yours, and in my case are due to a dirty/malfunctioning/poorly attached pickup.

best of luck (but it would help to know the model and year of your car).

alec
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  #4  
Old 02-01-2003, 10:14 PM
Hamgunner
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Cool tach sensor location

Where is the tach sensor located?

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  #5  
Old 02-02-2003, 05:11 AM
hotskillet
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Talking

I'm embarrassed to say that I'm not exactly sur wher the sensor is located. I assume it has some sort of crank trigger. I'm mostly just curious how the tach just decided to become semi-functional all the sudden.
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  #6  
Old 02-02-2003, 10:01 PM
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Join Date: Jul 2001
Location: Olathe, KS
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The tach in my '85 300sd was doing the same thing for nearly two years. It finally gave out completely. I replaced the speed sensor and it works fine now. The sensor is located on the top of the bell housing just slightly toward the drivers side of the engine. It is held in place with a single allen bolt. If I remember correctly the replacement is available on FastLane for about $50.00

Good Luck.

Bill
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  #7  
Old 02-03-2003, 05:36 AM
hotskillet
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Thumbs up Thanks amptique!

How much of a PITA is it to reach the sensor. I"m not sure how useful the tach is on an automatic> however I find it pretty interesting to see how diesels chug along effortlessly at such low rpms compared to their gas brethren> Anyhow< I will eventually change my sensor as per your recommendation and i"" let you know how it works out>

thanks
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  #8  
Old 02-03-2003, 10:28 AM
NEVREM
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tachometer on 82 and 84

Does anyone know if the tach guage on an 82 and 84 are interchangeable. My 84 (no amplifier on the wheel well and rear engine pickup) works fine. My 82 (amplifier on wheel well and front engine p/u) doesn't work properly.

In trying to diagnose, I intend to swap guages (after checking front pickup condition) but since the 84 hs no separate amp in the circuit, I'm thinking it may not work with the 82 input circuit.

I'e tried the cig butt trick (and used foam etc also) in the amp. Haven't tried resoldering anything in the amp.

thanks
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  #9  
Old 02-03-2003, 12:49 PM
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Location: Olathe, KS
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hotskillet,

The sensor is pretty easy to get to. You have to reach around the oil filter housing to get to the lone screw that holds it on. It is a black plastic housing about 1" long and 1/2" wide with a wire coming out of one end. The other end of the wire hooks to a connector inside the first firewall. If you haven't tried to run your air conditioner with the tach dead you probably haven't noticed that the A/C compressor clutch will not engage when the tach isn't working. This was my main reason for fixing the tach. It can get pretty hot in Kansas without A/C.

Bill
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  #10  
Old 02-05-2003, 12:49 PM
hotskillet
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That's great to know!

I had no idea that the HVAC and tachometer were at all related. This provides much greater incentive toget the job done.
Thanks again.
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