83 300 SD.. where does the tach get it's signal from ?

I have an intermittent problem where if I hit a series of bumps in the road or when it's raining, my tach either goes goofy or dead.

So I'm suspecting that there's either a bad plug connector or sending unit/switch involved somewhere on the engine/trans, as most of the time the gauge works fine except under those conditions.

I've heard something about a tachometer amp ? What and where is it ?

Thanks..

Behind the driver side headlight there's a 1.5" diameter cylinder. Spin off the tethered cap. A piece inside with electrical connectors is free to slide in the cap. Pull it out -- pull it to the end of its travel then rock it out. Carefully scrape off as much of the silicone goop as you can. Don't cut any wires, solder or traces. Use your imagination to figure out what insulating material you can fit between the cap and the sliding piece to put pressure on the solder joints. I used a couple of furniture pads. Some folks use a foam packing peanut or a cigarette butt. Put it back together and tighten it just enough to get the tach to work and maybe a little more. I owe you a worn silver rack damper pin if that doesn't do it.

Oh, also clean the magnetic sensor at the 1 o'clock position over the crank pulley.

Sixto
91 300SE
87 300SDL
83 300SD ... any takers at $1100?

Sixto,

Thanks for the info... and you comment.. "I owe you a worn silver rack damper pin if that doesn't do it", has me baffled.

A joke I didn't catch or are you the person that sold me the car ??

It was my was of betting something of no value to anyone that the suggestion would fix your problem.

I haven't sold my 83 so you couldn't have bought it.

Sixto
91 300SE
87 300SDL
83 300SD

Duh... obviously I was a bit tired when I read your reply and completely missed the humor.... furniture pads, packing peanut and cigarette butt.. good ones.

That was on par with some advice I got about how to move my drivers seat (that was struck in the rearward position) enough to gain access to the rear bolts to remove the seat to change the cables... among the suggestions, wrap a pad around the end of a 2X4 and beat the hell out of the seat frame until the seat moved forward. The solution was to have someone energize the motor while I reached under and I wiggled the cable around until the rounded end enagaged enough to finally nudge the seat forward where I could get to the bolts.

While working on that I also discovered way to move the seat had the problem been a dead motor instead of a slipping cable, and posted that in the Tech forum.

By the way.. I found the mag sensor... completely caked up with road grime and oil, will be doing some serious clean up tonight.

Thanks....

Ahhh, actually I think sixto was serious about the cigarette butt, foam peanut, etc. That's a pretty well known fix for that tach problem. Try your cleanup first but if that doesn't work, and I'm guessing it won't, follow sixto's advice (except about the rack damper pin, that WAS a joke.)

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold

I guess Conrad hasn't heard of peanut butter to refinish 126 door handles either.

I was serious about the cigarette butt, packing peanut and furniture pad. Basically you fill the gap between the electrical piece and the cap with something springy yet insulative.

Sixto
91 300SE
87 300SDL
83 300SD

Caquino, "The solution " turned out to be different from what someone suggested that time... but remember that suggestions come from people not at the scene of the problem.. and are seeing the physical situation only through the eyes of the person describing it... often hazy at best, downright misdirected at worst...
TxBill told me about the peanut butter on the the fiberglass handles... I immediately had a squirrel chew off all the handles from my wagon.... they really like peanut butter.....

Everyone, thank you for your replies... this is all so new to me working on A) a diesel and B) a Mercedes. I'm quickly finding that there is a wrong way and a "Mercedes way" in doing repairs.

Peanut butter on the door handles.... that actually makes sense as the bonding resin in that handle is basically a plasticized hydrocarbon, the peanut oil would add back oil that was in the compound that has been lost, giving it that faded/dull look. I've had success using WD-40 to fix solvent damage to polystyrene plastic.

leathermang.. point well taken about suggestions made in replies. Guess I'm a bit paranoid after an experience on another board, we had an individual that maliciously posted plausable sounding advice that led to people ruining what they were asking for help on.

Being a newwbie the wacking the seat frame suggestion to motivate it to move sounded like it would have damaged, if not broken the gear mechanism, so I though it was a real off-the-wall suggestion.

By no means do I want to offend anyone on this forum, especially being a newbie.

Anyway... I've obviously have much to learn with the 126 chassis and 617.95 diesels.

You are almost right....there's the "Mercedes Way" and then there are all other methods of repair, right or wrong.

The only way to become enlightened to the true path (Mercedes Way) is to blunder along the dark side for a while. Hopefully, the Teutonic Gods of Daimler will smile upon you, so that your learning curve will be a gentle slope and not immediately involve injection pumps, ALDAs, timing chain stretch, radiator necks or B2 valves.

But, there are certain caveats. Frequent tithes to the Visgoth's dieties of Hazet and Stahlwille are mandatory as well as regular pilgrimage to the on-line temple of eBay to seek the rare and sacred holy texts of 123 Chassis Manual Vol 1 and Vol 2, or the often read tract: OM617.952 Repair Manual.

These texts will often carry the marks of the true believers and will have been anointed in the holy oil of Rudolph. Be not deterred, for the true message is contained within the cryptic translations and dark photographs. It is up to you to correctly intrepret their messages and shape your course accordingly.

Good luck,
R Leo

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Never a dull moment at Berry Hill Farm.

Well, I pulled my tach amp and it looked brand new! Notwithstanding, I went into the street and picked up a butt, stuck it under the cap, and...voila! It works!!!
Now my question is, who was sitting there with a broken tach and thought, mmm...I wonder if I stick a cigarette butt under the cap if that will fix it???

I just got a 83 300sd with a tach problem (not working) the techguy tach amp (great unit! and guy!) didn't fix it...so I was sent hunting for the "crank sender"...I looked and looked and asked for directions and was told where to go....well, I found it! (sort of ) chewed up and forgotton...now, my question became the tab or dowel like piece of metal that is in the flywheel timing pulley...does it usually stick out a 1/8" or so? mine had a collision with the old sender/sensor unit and looks like neither actually won...Is there a photo or diagram where I might see an original position etc? thanks for any help/info/pics! mike

Sixto,

YOU ARE THE MAN!!!

Reading this thread today. I read the whole thread andyour post from 2003 I think, about scraping the goop(silicone) from the top of the Tach Amp and putting something in there that puts pressure on the solder joints and is non conductive.

I thought. Yeah, Sure, that's going to work.

Well I owe You. I tried the fix just now(8:15 P.M.) and my dead tach came to life. Such a simple fix. I used part of a new sponge(plastic) and I have a lot left, so I can repack the amp as needed.

FANTASTIC!!! YOU really are the Man.

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1986 Euro 500SEC "RUF" 9:1CR, "Rose"
1985 Euro 500SEC Cabriolet AMG Widebody
1982 Euro 500SEL "Blue"
2001 Texas Heeler (Aussie/Queensland X) "Sulphie",
2012 Queensland Red Heeler "Squeak"
Best dogs I've ever had.

Just bought another MB today, '82 300D Turbodiesel, knew I read the solution here in the past, thanks for answer without having to use the search.

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2 (TWO) 1979 MB 300SDs FOR SALE
WHITEy aka Rusty - parts car - 149.5K
TANya - was daily driver - 160.8K

1982 MB 300D - White

1985 MB 300D - Gray

1985 Chevy Suburban 6.2 Diesel Veggysub - running on WVO/RUG/D2 Blend

In the first pic, you may be able to see the stud.
In the second pic, I painted the end with white paint, right on the mark.
Hope this helps.

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83 SD

84 CD