Replace Tach Pickup/Crank Sensor in 10 Minutes - W123 thru early '84
 

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.

Pic limit of first post was 5. These are a few others that apply.

I'd like to remove the tach pickup in my 1984 and 85 since just a gunk gunk trap which serves no purpose. The cable routes to an open connector, which presumably dealers could connect a diagnostic instrument to. My cars have a separate pickup on the transmission bell-housing. Problem is that cable is hard-wired into the engine harness so would have to cut wires to remove it.

My 1985 doesn't even have the pin on the crank damper since when I added the support bracket mod for Rollguy's Sanden compressor bracket, the supplied bolt was too long and hit the pickup pin to snap it off. Much better result than cracking the upper oil pan. That Sanden bracket later cracked in several places so went back to an R4 compressor which works fine.

Bill, if you do decide to remove the pick-up, I'd like to have it. I've got a tach install project on a '78 300CD I need to do. I've got the tach binnacle and the amp, just need the pick-up bracket.

This is a good tip. I heard it in a number of other places when I investigated removing the useless crank sensor from my 85 Cali 300d. I may have used a few extensions with those wobble drives. Harbor Freight sells a cheap assortment that works well. I also put a u-joint in there someplace.

My intent was to recover the sensor from my 300d and use it for a test fixture for my tachometer amp project. It didn’t really work out because the cable after being heated by the engine for 35 years was as stiff as a board. I reinstalled it all and picked up the installation tricks too. The grease dab to hold the nut would have helped.

A final tip is if you have a damaged sensor, you can get one off an old gasser at the junkyard. There are a lot of 80s Mercedes cars with these sensors for diagnostic testing. SLs, 300Es, SELs and many others. I ended up buying a new pickup from a 300E. They’re cheaper on eBay because nobody I mean nobody would buy this part and maintain it on a 300E because it is purely for diagnostics. So it’s half the price of a 300D pickup. FYI PN **Beru 0191325006 0021531328. There are other PNs for the others. Mine has a 75 ohm resistance and triggers my tach amp just fine.

Just a tip in case your sensor is damaged on your diesel. You can save beer and pizza money.

**Edited to add - sorry guys I’m wrong about the Beru sensor I mentioned above. The sensor head is the same but the plug is different. It won’t fit in the diesel tachometer amp socket.

I bought one of those tiny ratchets like you used from from Horror Fright the found a nut driver kit on the counter at Ase Hardware. The ratchet in that kit is designed similar to the typical larger ratchet. The ratchet came in a metal box with a dozen or so bits like phillips, slotted and other type bits. Had it included an extension it would have been a lot nicer but for 13 bucks it is what it is.

Those tiny ratches saved my bacon when I was working behind the dash on those 7mm plastic nuts which old the wood trim to the dash.

After hovering on a Sanden kit for years I went with the Klima Design one solely due to the series of revisions/mods of the kit you used. I'm sure every big undertaking involves revisions but normally it happens privately one-on-one. It's got to be tougher when the problems have spectators. If it's ever time to go back to a Sanden, Klima will sell just their bracket set. The component that really dropped vent temps was their parallel flow condenser. It also reduces the amount of refrigerant required. A W123 OE charge weight for R12 is right near 42oz(3 x 14oz cans). After the PF condenser 28oz was the magic number.

Thanks for sharing 300