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.

junqueyardjim · #7 02-08-2006, 01:38 PM

No sheatait! Didn't sound much that way to me. Anyway, the original question was, "Where is the tach pickup on a W123, and since it is in the diesel forum, I would assume he is talking about a diesel.

pawoSD · #8 02-08-2006, 01:40 PM

This goes back to an unanswered question I had posted in another thread, has anyone ever actually had the pickup over the crank fail? Mine is an 82 so would the overvoltage protection even be an issue? What else does the overvoltage protection protect....as everything else on my car works. I see they do sell replacement tach pickups, but I still wonder if they can even fail...or maybe be out of adjustment?

dieseldiehard · #9 02-08-2006, 01:48 PM

Quote:

Originally Posted by pawoSD

This goes back to an unanswered question I had posted in another thread, has anyone ever actually had the pickup over the crank fail? Mine is an 82 so would the overvoltage protection even be an issue? What else does the overvoltage protection protect....as everything else on my car works. I see they do sell replacement tach pickups, but I still wonder if they can even fail...or maybe be out of adjustment?

No, the pickup is a coil of wire with almost no current flowing in it, it generates a weak electrical pulse when the small metal (magnetic?) tip of the projection on the crank pulley passes by.
The cable to the pick up coil is something that can break though and I've seen that happen during an engine removal or swap
Usually the failure is in the tach amp module on the fender or in the case of the '85 diesels the EGR box in the car behind the glove box.

pawoSD · #10 02-08-2006, 02:15 PM

Quote:

Originally Posted by dieseldiehard

No, the pickup is a coil of wire with almost no current flowing in it, it generates a weak electrical pulse when the small metal (magnetic?) tip of the projection on the crank pulley passes by.
The cable to the pick up coil is something that can break though and I've seen that happen during an engine removal or swap
Usually the failure is in the tach amp module on the fender or in the case of the '85 diesels the EGR box in the car behind the glove box.

Thats what I thought, I tried to fix my existing amp (the gauge would occassionally "go crazy") then later on it all but died. The amp messing did nothing, so I bought a brand new one, it also made no change, so I cleaned and made sure the contacts were good in the socket, nothing. So I bought a new tach-gauge pod, still nothing.....so I tested the wires that plug into the tach pod in the dash, it had 13.4 volts at idle. So the problem lies somewhere beyond that....I just have no clue what else to check. The pickup appears to be ok.....how does this system actually work....is the gauge reading a "sine wave" from the electrical supply coming into it? Mine has never so much as even quivered in the years since I messed with it.

pawoSD · #11 02-09-2006, 07:20 PM

I took my multimeter out to my cruise amp today, with the car running I get 14.12 volts in two pins of the socket that the amp plugs into, I get the same at the two pins on the back of the gauge pod in the dash. On the other holes in the socket (the ones from the pickup) I get no reading on any setting of my multimeter, nothing at all. Is there a way of testing the pickup to be sure its actually sending some sort of signal? It was said it emits a weak A/C current pulse....but I picked up nothing, not even in the hundredths of a volt....

The only way I could think of to test it is to have the stealership plug their "diagnostic" tool into the amp socket, as it reads RPM. If it showed no reading, I'd assume the pickup is the culprit, if it did, then something is screwy with my gauge, which is pretty much impossible.

Will this ever be solved? Who knows......any help?

MB-Owner-in-ind · #12 02-09-2006, 07:35 PM

You really need a scope to pickup the weak signal. You could try a frequency counter but a scope will tell you the strength and shape of the signal which should be a square wave.

pawoSD · #13 02-09-2006, 08:28 PM

Quote:

Originally Posted by MB-Owner-in-ind

You really need a scope to pickup the weak signal. You could try a frequency counter but a scope will tell you the strength and shape of the signal which should be a square wave.

Any idea where I could find one of these? Or someplace that might have one? I am right about the MB "diagnostic" tool aren't I? If so, they should be able to plug that in and get an RPM readout right? If they don't then I'd presume there's a pickup problem and that I'd just replace it.

Diesel Giant · #14 02-09-2006, 09:03 PM

If your talking about the 85 model, you may want to check your egr computer. If that or the egr relay is not working properly, then your tach will not work. I have spent countless hours chasing down a non functioning tach when it ended up being the egr computer. The contacts that plug into the harness get a green fluffy corrosion and it flake the tach out. This is only on the 85 w123. The other years do not run the tach through the EGR and the Klima relay.

pawoSD · #15 02-09-2006, 09:25 PM

I am talking about my 1982 SD, how would I check to see that the pickup over the crank works other than going to the stealership?