Did some searching on this forum, can't seem to find much on this topic, or I'm searching wrong ?
Have a newly aquired 85 300 SD tach was working for a short time, then quite. Can anyone give me some ditrction as to what this system involves? or if theres likely weak areas I should check first?

thanks

It's electronic, not mechanical, and on my '83 SD it's a plug-in/screw-down module on the driver's side fender. I've heard replacing the module can fix the problem; I've also heard using contact cleaner on it might help.

Mine is flaky; sometimes it works fine, sometimes it works for a little while then goes completely dead, and then comes back.

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Lots of diesel W126s in the past
Current: 1991 Toyota 4Runner with an OM603

There have been threads on this forum about putting a wad of paper between the plug-in module and the screw-down cap to force better contact between the pins. I suppose a careful cleaning of the male and female pins would be equally helpful. In most cases, Mercedes specified silver plated pins to their subcontractors; excess scrubbing removes the plating so be gentle. A q-tip and metal polish works well for me.

Jeremy

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

Heres my write up of how I permanently fixed my tachometer recently. The PERMANENT Tachometer Amplifier Fix - Benzworld.org - Mercedes-Benz Discussion Forum

I too have an 85 300 SD and for the longest time tach did not work put a differant OVP (over volt protection) relay in and it took right off.

There is a fuse on top of the OVP relay, check that.

Not sure where it is tho. On a 123, it's near the glove box.

Jeremy, the OP has the new style tach sending unit, no amp.

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83 SD

84 CD

OK, thanks, I'm not as familiar with the S-class cars as perhaps I should be.

J.

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

On the '85 the tach signal doesn't come from the vibration damper but rather from the flywheel. It also doesn't have a tach amp, rather the signal to the gauge comes from the EGR Computer.

Here are a couple of diagrams that show what is going on
The first image shows the wiring diagram of the EGR system and you can see that the tach pickup (82) is a sensor for the EGR computer. The output to the tach is shown at a in a detail from the first image. The third image is from the wiring diagram.

The computer gets its power through the OVP so the fuse referenced above is a definite first place to check if the tach goes out.

The last image is the tach pickup on the flywheel.

Attached Thumbnails

 

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Current Stable

• 380SL (diesel)
• Corvette C5
• Manx
• Baja Bug
• F350 Powerstroke
• Auburn Boattail Speedster replica

Very nice post. This very thing was going thru my mind and there it is on virtual print! Nice screen name too!

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Thanks,
Mark in NC

"Spark plugs?...We don't need no stinking spark plugs!"
1985 300SD "Der Silberne Schlitten" 420,000 mi


Wish these were diesel:
2003 Ford Club Wagon 130,000 mi

No prob, MB changed the set up in 1984, my CD doesn't have an amp.

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83 SD

84 CD

Mach4
Thanks so much for the info. ( and all others) this should narrow it down.
Let me toss this in the ring, I diss connected the EGR vac line and fresh air recirc line ( and plugged the ends). If the tach is pulling a signal from a part of the EGR circuit Could this be the reason I lost the tach?

No.

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

From all I have read, post 84 tach's still have a tach amp, its circuitry, (the round module) has been relocated to the EGR computer.

Interesting that in the 1983 240D I had, which has a EGR computer, but no tach, I was able to find and extract the signal from the EGR computer to drive a tach I took from a 300DT.

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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

No, what you changed was part of the vacuum system, the tach is electrical. Many people have removed the EGR stuff completely and still have tach. My belief is that the engineers just stuck the tach circuit into the EGR computer because "it was handy", rather than create another separate device. Same thing today with the car computers - they do everything.

I would suggest troubleshoot by doing the following

[1] Trace the wires between the tach sensor and the EGR controller and make sure they are good (you can't actually test the sensor unless you've got a scope) - pins 20 & 16
[2] Check the OVP fuse
[3] Check the power to the computer - pin 1
[4] Check the ground wire - pin 14
[5] Check the wire to the tach - pin 25
[6] Check the connector itself - the pins can get corroded
[7] Pull the gauge cluster and check the tach module itself

The last thing would be the computer itself. If you can borrow one from a known good system, that would be the only real way to check it.

Here's a picture of the plug with wire colors indicated that may help a bit.

__________________
Current Stable

• 380SL (diesel)
• Corvette C5
• Manx
• Baja Bug
• F350 Powerstroke
• Auburn Boattail Speedster replica

Mach4
I hate to ask, but, this is how new I am to these cars. Is the OVP located in the fuse box under the hood? I have one relay that has a red spade type fuse built onto the top. It's good. I also found a set of fuses on the fender wall next to the blue vac amp. I'm I getting warmer?
Thanks again. Your discription and photos Are really helpful