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Old 04-16-2004, 08:02 AM
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Join Date: Apr 2004
Location: winston-salem, nc
Posts: 37
300 sd tach sending unit?

Tach, clock, odomoter do not work on my 83 300 sd. Went to junkyard yesterday and picked up two instrument clusters. Checked both on car. Now have working odometer and clock, but no tach response on any unit. Coincidence? Three bad tach units? Only one wire connection for tach-so must have electronic
sending unit of some sort.

Where is tach sending unit on 300 sd? Are these things particularly quirky, or am I just unlucky enough to have gotten 3 bad clusters?

No real need for tach, so I can live with this. But it would be nice to see that needle moving in a purposeful way.

Help anyone?

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Old 04-16-2004, 11:24 AM
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Join Date: Oct 2001
Location: Atlanta Ga
Posts: 281
I picked up an 83 300sd from the original owner that was hit on the driver's side fender who says hers worked fine until the accident. It is located on the driver's side fender (a little black cylinder that's a b-tch to unscrew) and sometimes you can just give it a couple of taps to see if its getting a connection. I did this and it worked for a bit although I just figured the jolt of the accident knocked it out. I also went to a junkyard and removed one from another 300 for $1.00 and replaced and she works fine now.
1997 E320
1986 560SEL
1983 300SD
1972 220D
1970 250C SOLD
1971 250C SOLD
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Old 04-16-2004, 05:32 PM
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Join Date: Apr 2004
Location: winston-salem, nc
Posts: 37
Thanks. I'll check it out.
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Old 04-17-2004, 01:19 PM
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Join Date: Apr 2004
Location: winston-salem, nc
Posts: 37
Question tach sending unit location

Looked around on driver's side fender and didn't see unit.
Or at least didn't recognize it...a more plausible explanation.

Any chance of posting a digital photo of the thing?
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Old 04-17-2004, 08:23 PM
Registered User
Join Date: Mar 2004
Location: ILLINOIS
Posts: 348
Tach sending unit

Sending unit is located on the driver sider inner fender,it looks
like a black plastic cylinder about 2 in. tall and has electrical wires
under.It is right behind the glow plug relay(square plastic black box).Quick fix for the problem:when you locate the unit,remove
the cap(like any bottle cap),place a piece of cardboard inside the
cap and reinstall the cap,if you feel pressure when you put it
back on its OK,otherwise the cardboard piece is to small it has to
create pressure so the electrical connectors are tight.Tach should
start working again.Problem is caused by the old glue that is
supporting the connector pins,it gets old and the connectors are
"sinking in",so,by placing the cardboard you create the suport
for the will see that when you open the sending unit.
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Old 04-17-2004, 11:39 PM
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Join Date: Apr 2004
Location: winston-salem, nc
Posts: 37
sending unit

OK. Thanks for dialing it in. Turns out, I did see both
the square and round doohickeys in my initial under the
hood gropings.

Rapped both a couple of times with a screwdriver handle.

Weird thing. Drove 200 miles this afternoon and evening.
About 30 miles into the trip, the tach started working.
Worked all the way to my destination. It was nice to see
how many rpm's I was turning while running 70.

However, after dinner and a movie, the darn thing
persistently refused to work again on the trip home.

I can't have this. Cannot allow a device to tease me
with intermittant functioning. Will do some fooling
around with this thing tomorrow.

Update will follow.

Appreciate the help.
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Old 04-18-2004, 02:44 AM
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Join Date: Jan 2001
Location: oregon
Posts: 2,013
There have been thread after thread about ways to fix SD and D tach sending unit from cigarette butts to plactic peanuts. But from my experence these trick only work for a time ,I bought a new unit from FastLane and now enjoy a smooth tach.........

William Rogers.......

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