Hi everyone, I am new to the 300D autos as I have my first and am loving it! I have an 83 300D with 245,000 miles. Bought it from the original owners family. The Tach does not work most of the time....and once in a while will bounce and work for a short time. The cylinder shaped device on the right fenderwell seems to have something to to with the sensor, however I am not sure what to check?
Also, just want to mention to those of you that have over heating problems. I made sure my thermostate was good and the fan clutch. I bypassed the airconditioning circuit and basically added a switch to the electric fan on the front of the condenser/radiator. I can now turn on the electric support fan by flipping a switch...works great in traffic and hot weather!
Nathan

The cylinder on the fender well is the diagnostic socket, sometimes referred to as the tach amp, there is a lead that runs from it to the tach sensor just as you observed. There are some tricks you can try to get the tach to operate. 1. Sometimes an engine thats almost twenty years old has alot of grease and grime and in the case of the tach sensor it can get really dirty at its pickup point and even though its pickup is magnetic I think it helps if the sensor is clean. 2. Unscrew the cap on the tach amp and look at the pins and mating sockets for corrosion, you can clean if necessary. 3. Separate the cap from the male portion of the pin assy, it will take a little pulling but it will seperate, on the back side of the male pin assy is a potting material (silicon rubber) check to see if it is intact and not allowing the pins to float excessively. 4. Cut out a piece of standard 1/8" thick cardboard in a circular shape the same diameter as the inside of the tach amp cap, position it inside the cap. 5. Install the male pin assy back into the cap. 6. Screw the male pin assy with cap to the female socketed base. 7. Start the car, the tach should now work, you may have to make adjustments as to how tight the cap is screwed on so that the cap with cardboard spacer applies the correct amount of pressure on the pins for good contact. If you are more of a craftsman you can make a spacer out of 1/8" fiberglass sheet as I have done, I have not had any tach issues since. BTW the internet "trick" is to use a cigarette butt as the spacer, I do not think it would provide even pressure but people claims it works. Good Luck

Thanks RSH;
I have cleaned the sensor and I have unplugged and replugged in the Tach Amp (thank you for the correct name) and the tach does work for a short time.
I will try the spacer idea, thanks.
Nathan

The "spacer" or cigarette butt trick may get you by for a while. Those tach amps are notorious for going bad, often only after a few years, and they are a high-$$$ part. I just gave up on mine...not important enough to worry about on an automatic.

Mike

__________________
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1979 300 SD
350,000 miles
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1982 300D-gone---sold to a buddy
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1985 300TD
270,000 miles
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1994 E320
not my favorite, but the wife wanted it

www.myspace.com/mikemover
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I believe you can purchase a new tach amp from the manufacturer that makes them for MB for about $60 or so. You can also get one from some of the used Mercedes parts places for about $25 or so. If you can't get yours to work you might want to explore one of those options.

Thanks guys;
The spacer (pressure device) is working and the tach has worked all day...
I run a computer repair business and have dabbled in electronics for years....if anyone has and old tach amp that I can take apart, please let me know.....I used to repair VW electronic fuel injection control units and Onan generator control boards for a hobby...
Nathan

I have a tach amp that I have taken apart and removed all the potting compound. Being that we are in the same city, we should get together and see what we can do about drawing a schematic and making a test jig to check these out. I was in the electronics industry for 30+ yrs as a field engineer for an aircraft electronics company. My tach works on a random and unusable basis. I tried the spacer trick with absolutely no success. The pick-up point seems to be a long way from any spot on the harmonic balancer but I have nothing to compare it with.


'83 300D 361,000 mi. and going strong

To Nathan and Lee,
Yall have enough electronic's experimenter knowlege and experience to make a much simpler and more reliable tach in less time than you can fix that factory unit...think about it... a couple of magnets glued somewhere, a sensor, a counter, a clamping device... and a few filters.... yall can do this... since my car does not have a stock tach I will be thrilled when you design one which I can replicate... Radio Shack here we come...

No tach? How do you drive....lol?

Got a question for you? Does your harmonic balancer have the post sticking out and the bracket for the pickup?

Nathan

I have a 240d... but will look to see if I have that.... when I figure out exactly what to look for... but aren't some tachs mechanical , coming from the back of the ip ?

Hmmmm.....you are asking an electronics guy that knows very little about Mercedes diesels....about a mechanical tach......lol.....my old bicycle had a mechanical tach, my brothers tractor has a mechanical tach......
Nathan

LOL, sorry,,,,
I went out in the rain and looked at the front of the engine... at about the 12:30 position there is a flat bracket with a round hole which looks directly at the harmonic balancer... and a screw post.. is that what you are referring to ?

HA!
Electronic tach....here we come.....yep...sounds like you are tach ready.......could I interest you in used 300d tach components?

I will let you know what we discover...
Nathan

UH... Did you read my first post in this thread ? I did read yall's posts about the problems figuring out the factory system....I only look dull.... Yall could whip this out in a couple of evenings... while it would take all the remaining brain power I own for months to get it right... it is time to design a new system for tach challenged people...

LOL....yes I read your post.....I was joking....! I may put an aftermarket tach in my car....then I would sell you my parts....cheap!
Nathan