Replace Tach Pickup/Crank Sensor in 10 Minutes - W123 thru early '84 - Mercedes-Benz Forum
 
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post #1 of 4 (permalink) Old 11-03-2017, 12:40 PM Thread Starter
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Replace Tach Pickup/Crank Sensor in 10 Minutes - W123 thru early '84

Posted this on peach as well. This applies to the years in which the tach pickup(speed sensor) is on the crankshaft up until some time in early '84.

I had read the factory/advertised method of changing the crankshaft pickup/speed sensor for my tachometer which involved removing the upper radiator hose, radiator fan, fan shroud, alternator belt, water pump belt, and the water pump pulley to access an 8mm nut. From the DIY article: "Due to the location of the sensor behind the water pump pulley, a lot of components need to come off the vehicle so you can access the sensor. This is not a quick job so make sure you factor in all the time needed to remove and replace all the other systems before starting." Now I understand why people just go without a tach.

I poked around and tried accessing it from the side. Couldn't connect so I took photos down where I couldn't see. Discovered you certainly can come in from the side with small diameter extensions and an 8mm socket+swivel. The trick is you can't go straight across to the nut because the vacuum pump housing "bells out" and basically holds your extension down and kills the little bit of play you need to get the socket "up" onto the nut. The attack angle is forward of the nut.

Used one of those dinky little mini bit driver ratchets to turn everything and it backed right off. Push on it the whole time you're turning with a little downward pressure against the radiator hose and the socket will stay on the nut. As you can see the old sender's cable came out while I was investigating everything but I don't think that had an effect either way as the install with a new cable in the mix went just as smoothly. Once the nut backed off I switched to the wheel on the ratchet head to turn everything. Instead of risking the nut not making the ride all the way out I opted to pull it straight up and out first with an extension magnet. I sprayed degreaser on the area before and during the part change as the tolerances in there are very tight. The odd piece of grit here will definitely jam you up.

To install the new sensor I came in from the side as well. Unlike the swivel+socket the ideal path for the cable was direct. After taking a few stabs at it and adding a curve in the cable here and there to make it "point" a certain way it went right into place. You can get your hand pretty close to everything. I kept inward pressure on the cable which put curves in it so I used one of the curves to hold everything still with a screwdriver. This is all on the fly with my right arm pushing in from the side and grabbing for a solution with other free hand. As soon as I was freed up I packed the socket with a piece of paper towel to backstop the nut and a dab of grease to hold it and started it by hand. Finished with the ratchet and it all tightened down the same way. It was 30 minutes. Actual tool-time 6 minutes, jockeying the new pickup into place 4 minutes, lighting & photos 20 minutes.
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post #2 of 4 (permalink) Old 11-03-2017, 01:16 PM
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Very informative. This technique will save others lots of time. It’s a lot better than taking apart half the car.

When I replaced mine, I just use some WD-40 to clean a bit and then used a little 8mm wrench to loosen the nut and took it out with 2 of my fingers. Then I replaced the sensor and carefully put the nut back on and hand tightened it. Finally, I used the wrench to tighten it a bit more. I couldn’t see what I was doing, so everything had to be “by feel”. I guess that having small hands and skinny fingers helped.
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post #3 of 4 (permalink) Old 11-03-2017, 01:50 PM Thread Starter
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I'd be very interested to see the little wrench you use to break the nut loose and tighten it. Will you post a pic?
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post #4 of 4 (permalink) Old 11-03-2017, 02:12 PM
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Quote:
Originally Posted by paulma2010 View Post
I'd be very interested to see the little wrench you use to break the nut loose and tighten it. Will you post a pic?
Of course, I will. But I have to look for it because it isn’t in the tool box that I carry in the trunk.
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