'85 300D 123.133

Tachometer has stopped functioning. Somewhere along the way I had a breakdown of how to figure out the problem, as this has happened before. The last time I used compressed air and cleaned the sensor mounted above the ring gear, while it was mounted on the bell housing. This time, compressed air does not work.
I looked in the archives, only found information relating to the tach sensor mounted on the drivers side fender next to the glow plug relay. In a way, the car has this, a three wire harness coming off the balancer, and two separate wires leading into the main harness. There is no circuit board.
I have the first version of the 123 CD, and cannot find a thing related to the tachometer. There are several pages missing from the first versions, Mercedes came out with updated CD's later.
With the tachometer inop, the a/c compressor will not engage. Are there any suggestions? Specs, other ways to test the sensor, tach ???????
Alan

The tach in my '85 300D sometimes doesn't work at startup. A light slap to the top of the dash above the cluster wakes it up. Probably a bad solder joint on the tach circuit board. Just haven't had time to pull the cluster (risking more problems disturbing 20 year old everythings) to investigate. You MAY have a similar issue.

Tach amp is most likely the casue of this...........the little black garbage can looking thing on left front fender.

__________________
Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
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Section 609 MVAC Certified
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"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

Note, the '85 300D does not have a little black plastic cylinder on the L fender. The circuit is in an aluminum box, similar to the climate control box, located under the dash on the Rt side, you have to remove the glove box to get to it.
I have never seen the schematic diagram, if anyone has a copy of that I would like to see it.
Are you sure the tach sender at the crank isn't bad, like the wire is torn or something? Using a spare module to swap in place of the questionable one is the first thing I would try. I use a module for that. And then swapping the dash gauge (or entire instrument cluster would be the next thing to try (more difficult than swapping the tach module).

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!

There's a modern style type of fuse located behind the glove box on the '85's. You need to pull the glove box to get to it plus move the air duct to find it.

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DJ


84 300D Turbodiesel 190K with 4 speed manual sold in 03/2012

After work today, I removed the magnetic pick-up (mounted behind the oil filter housing), cleaned the magnetic probe, and re-installed. What I noticed upon startup was this:
When turning the key to "on", the tach needle barely moves.
The tach still does not function with the engine running.
When turning the engine off, the tach "jumps" to 500 RPM then to 0 when engine is off.

I also checked continuity between the two terminals (brown wire and white wire), coming off the pick-up. Nothing. This might be the problem, as the seal where the wires enter the sensor housing is cracked, probably moisture and crud have done their damage. I used a little compressed air to clean out any crap, still nothing.
I finally found the schematic for the cluster, the tach pick-up is nothing more than a coil with a wire at each end. I think there should be continuity at all times. Even with the engine running, I did not pick up any continuity. (Using a digital meter with audio function for continuity). I am leaning towards replacing the tach pick-up (aka engine speed sensor). Any thoughts? Alan

There probably is too much resistance in the pickup coil to trigger the continuity function in your meter. Continuity usually means little to no resistance. To test the coil, start with the ohms x 1 setting and work your way up to see if the value appears. Not sure of the specific value of resistance for that coil. Not sure where to find out (we all know how good the manuals and cd's are), maybe test a good unit on another car. Hold the test probes tight to the wires and don't let them contact your skin, as this will affect the reading.

I didn't think of this until after posting but, since I have the identical car, I could test my pickup coil to get the value of resistance for reference if you need me to. I will have to do it at work tomorrow as my multimeter is at the shop.

I would appreciate this very much. I did spin the dial around various ohm settings, and still did not read anything. But as you say, it may not read anything, but hopefully yours will.
Yes the manuals are limited, message boards like this are worth their weight in gold.
Post your results for all to see!!!! Thank you Alan

Ok, I will post the result tomorrow when I get home (around 6pm).

If the tach senser on an MB is like othe pick up coils, and it probably is. an ohms ready probably won't diagnous it. changing the senser is the easiest test. or after put your ohm meter on ac volts (about10v setting) and connect to the two senser leads. crank the engine and look for any sign of voltage. If the senser is out of the car moving a metal object (screwdriver etc.) across the tip will simulate the engine cranking.

BenQ
2 87 fords, looking for am MB to replace one
1 88 ford
1 72 ford

I disagree with that. The sensor is a coil of wire, an electrodynamic pickup. I checked one once with an Ohmeter when I installed it on a non-turbo car. It was a fairly low reading, maybe 35 Ohms or something. If you read infinity its been damaged for sure.

__________________
'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!

Hey, I checked the sensor today on my identical car and got a reading of 2.15k ohms taken at the disconnected sensor plug at the firewall by the monovalve. I put the meter at the 2k setting and it was too low, went up to the 20k setting and got 2.15 or 2150 ohms. Let me know what you find. I will check back tomorrow, going out tonight.

Thanks for the results, I have a reading of 1.8 ohms on the 2k scale, 1.8 ohms on the 20k scale. Bumping the engine around does not produce a different reading as I thought it would. I have ordered a new sensor (aka Engine RPM Sensor).
Sunday, after church the tach started working!!! Looking at the sensor wiring, it's apparent there is a break inside the sensor itself, affected by outside conditions (heat, cold, wind speed). Alan

Hey guys

I have exact same problem on my 1995. Dead tacho and aircon. I have searched the forum an not found enough info like the stuff in this thread however I need to know if it applies to my W124 or if not what differences .

Can anyone advise ?

thanks
Keith