Where is the RPM sensor on a W123 diesel? - PeachParts Mercedes-Benz Forum

Carrameow · #1 09-16-2004, 12:20 PM

I lent all my CDs and manuals to a friend. I just did a mess of work on my W123 300D and I want afraid to do it without manuals. I have even rebuilt a few of these and I never remember hooking up a wire that Picks up the RPM. It can't be on the fuel pump--no wires except a few pumps have 3 for emissions, or cylinder head--there is only a glo plug harness and temp sensor--and the block has a TDC sensor that doesnt even have to be reconnected--and for the life of me, I cant "THINK OUT" where the RPM sensor goes--

Reason is , my RPMs on my dash guage are suddenly zero/Dead. but the car runs fine...does anyone know what I did? The last thing i did before I noticed this was re time my Pump...

Old300D · #2 09-16-2004, 12:28 PM

It's located on the front of the engine above the crank balancer.

Eskimo · #3 09-16-2004, 01:00 PM

On the earlier years, I think that the TDC sensor is used, in conjunction with the "tach amp" which plugs into the socket on the driver's side of the engine bay.

I think the '85 federal (and the '84 version in California) still have the TDC sensor, but have no tach amp in the socket - the socket's there solely for diagnostic purposes (external tachometer, injection timing by the RIV method, etc.) There's another sensor on these "which is screwed in on intermediate flange to automatic transmission," according to my FSM. In the picture, it looks to me as though it's on the driver's side (I think I see the oil filter housing), but I'm just looking at a picture; no such animal on my car. I think this sensor is "read" by the electronic control unit (located in the passenger side kick panel) for the EGR system. This unit is protected by the overvoltage protection module (behind the glove box, with a 10A fuse), so that's another thing that one can check if the tach is dead. If you've been working under the hood, though, I agree that I'd find the sensor and make sure that it hasn't become unplugged first.

If I've got anything wrong here, someone please step in and correct me. I'm working more from what I've read, here and in the FSM, than from personal experience.

-- eskimo

Godhand · #4 02-08-2006, 07:33 AM

why you all think that the rpm sure got thing todo with the tdc or crank sensor? Why? the true was the rpm signal is from the alternater. the pin which connect to the rpm meter are mark with W. Shock! that was the true! by the way not all alternater had the W socket, only Diesel type got and most of it was 55Amps to 65Amps!

junqueyardjim · #5 02-08-2006, 01:07 PM

Man, I have read some badly written English, but yours gets first prize. Anyway, I am quite sure that the pickup for the factory is mounted tach is in a little holder mounted kind of to the top and driver side behind the harmonic balancer. I took it off my 83 300 Turbo parts car and I am sure that is what and where it is.

pawoSD · #6 02-08-2006, 01:28 PM

He's saying that on diesels the alternator can be used as a source for the tachometer, which is true, via inductive pickup. Autometer and some other brands sell gauges/tachometers where it has a small pickup that straps to the alternator to measure the rpms. I am thinking of getting one of these. The instructions say to hook it all up, then calibrate it at a mph/engine speed you know to be accurate. It has all the instructions for this. I thought it was pretty cool. I'd probably just calibrate mine at idle to around 700-750, then I don't have to factor in torque converter slip....

The gauge is $114 though, so I am looking to mess with my OEM one some more before I abandon it and get the alternative.