I've cobbled together a new tach sensor, a used tach amp, and a MB tachometer installed by previous owner, and with the help of people on this forum, it all works!.... most of the time. Occasionally the needle likes to flutter around before resuming normal operation. I've even tried the "cigarette butt" trick (which helped a lot). I thought I should try to find one of the OM617 Tach Amps made by techguy 512, selling for about $40. His website closed just after I started this project. If anyone has one of these amps for sale, or knows how to get in touch with techguy 512 (I've tried through this forum), please let me know by Private Message. Many thanks!

Take the tach amp apart, use a fork and scrape away the silicone potting compound till the circuit board is clear of the stuff. Resolder every joint and see if that fixes it. If not, send it to me and I can repair it.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

While we're on this discussion...

I haven't had a working tach in a coupla years now. Apart from the (obvious) potential culprit being the tach amp, what is the frequency of the crankshaft position sensor going bad?

Is there a clear diagnostic I can do on this sensor, preferably with a standard ohm/volt/amp meter?

UPDATE:
Having just tested it, I see about 70 Ohms across terminals 7 and 9 - with the engine off and the Tach Amp removed. I read ~ resistance across 7-8 and 8-9.

Oddly, see exactly the same readings with the engine running. SOOO..., unless that sensor is energized while running - I would assume it's a simple induction sensor? - it seems to be kerflooey. (Do not know how to spell 'kerflooey'...) I see no voltages, either.

Funola (anybody?), does this logic seem reasonable? Any other insights? Has anyone actually tested output of this thing with engine running?

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'83 300 SD
'68 Triumph TR 250 - The only car I ever loved more than the Mercedes; who needs electricity, anyway? - Damn, why did I sell it?!
'59 Jaguar 3.4 'Le Chat Noir' - Damn, why did I sell it?!

It's difficult to make predictions, especially about the future.
- Niels Bohr

Don't know exactly the frequency of sensor going bad. Wild guess is 100 to 1 (tach amp vs sensor) going bad?

You should get around 81 ohms for the front crankshaft sensor. The gap should be about a matchbook thickness.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

I've got the researchers dream here, a functional amp and tach in one of my w123s and a non functional one in the other (changing the amp makes no difference).

I'll bust out the Oscilloscope and function generator over the weekend and see what I can turn up for the function of this thing.

I've toyed with making my own digital "amp" using an attiny or something of the likes.
I suppose I may as well give it a whirl and see what happens.

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1982 300D (w123, "Grey Car")
1982 300D (w123, "Blue Car")
2001 Ford F150 "Clifford" (The Big Red Truck)
1997 Dodge Ram 2500 12V Cummins
1996 Dodge Ram 2500 12V Cummins
Previous Vehicles:
1995 E300D, 1980 300SD, 1992 Buick Century, 2005 Saturn Ion

It's a magnetic pickup which is a coil of wire wound around an iron core. If you're measuring 70 ohms that sensor is good. The resistance scale measures resistance not voltage, revving the motor will not change the resistamce reading. Put it on the most sensitive AC scale and you should be able to measure a few millivolts.

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Jarod, would be interested to see your findings on that.

funola - Yes, a simple induction coil. Makes sense. I didn't test in the millivolt range, however (also makes sense!); I'll have to do that.

But if the output is simply a millivolt burst on an interval of 't' - effectively reporting the engine rpms - I don't see the electronic purpose of the '8' connector.

__________________
'83 300 SD
'68 Triumph TR 250 - The only car I ever loved more than the Mercedes; who needs electricity, anyway? - Damn, why did I sell it?!
'59 Jaguar 3.4 'Le Chat Noir' - Damn, why did I sell it?!

It's difficult to make predictions, especially about the future.
- Niels Bohr

Same connector is used as a diagnostic port on other models, which would use some of the unused pins. On yours, only pins 1,4,5,8 are unused.

7&9 sensor coil
2 gnd
3 tach amp output
6 +12V

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

my tach was non-working when i picked up my 82 w123 this year. I tried a type of cig trick (rolled up some paper towel scraps) and it worked... for a few months. topped working so I refreshed the paper towel thing because it had obviously compressed.

now it seems to be working, but at idle it's resting at zero - and i can't tell if the whole tach is now miss-calibrated (lower reading than it should be) or if it's just not registering the RPMs until there is a bit more ooomf.

is it possible the amp is now fine (for the meantime) and the sensor is malfunctioning? maybe I'll try the re-solder route first?

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Andrew
Montclair, NJ
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1982 300D Turbo
120k
Petrol Blue Green

does anyone think using a silicon remover like this

http://www.amazon.com/Mckanica-Caulk-3-Oz-Silicone/dp/B000PVT1P4

would be harmful to the tach amp circuit board?

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Andrew
Montclair, NJ
--------------
1982 300D Turbo
120k
Petrol Blue Green