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?

This is a fantastic fix! I traced every wire in that circuit trying to figure our why the tach in my 300 SD worked roughly a third of the time. All to no avail......until I read this thread & wandered out to my car, pulled the cylinder, scraped off the silicone, tidied things up at the circuit board in the cylinder, stuck three used cigarette butts in there and put it all back together again. BOOM! The damn thing works like a champ. Unbelievable. Thanks sixto!

__________________
Ich muss geisteskrank sein.

The 1983 300 SD: "Old Smokey" (Hilda) Quarter Million Mile Club

The 1986 420 SEL EURO: "¡El Conqístador!" (Helmut) Trunk Triangle Club

The 1995 Cadillac Coupe deVille: "Schlag Kaltluft" (Shaft)

I would like to report that I was able to fix my tach too, by following sixto's advice. After 4 years of owning the SD with a non functioning tach, it now works great. Thanks.

__________________
http://i136.photobucket.com/albums/q.../ForumSIg2.jpg

I have an 1983 300D and am trying to fix the tach which seems dead. I will try the silicon removal and cigarette butt fix, but one other concern is that someone cut the wires to the tach amp, butt spliced red wire to wires and shoved exposed ends into the female receptacle for the amp - I assume to make a good connection. The wire are shoved into pin holes # 2,3, and 6. Is this right? Can anyone point me in the direction of a wiring diagram of this device or know the pinouts?

Thanks
Doug

I have a wiring diagram for the 300SD. This area of the schematic should be the same for the 300D. PM me with an email address and I'll forward it to you, since it's too large to post.

__________________
Bob
'82 300D Petrol B-G Metallic

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.)

__________________
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

__________________
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???