1984 300SD - Can someone identify this part? - PeachParts Mercedes-Benz Forum

750KMILZ · #1 09-02-2013, 03:55 PM

Hello,

I just replaced the engine in my 1984 300SD (myself). I have some sort of sensor that I can't figure out what it is. The wire comes from the right side, hooking into a wiring harness below the battery, through a connector in front of the washer fluid tank and across the partition behind the engine. I have pictures of the wire before I removed the original engine, and it looks like it goes straight down behind the engine, but none of my pictures show where it came from. I swear, I don't remember taking it off before I pulled the engine, but evidently I did.

Anywho, can someone tell me where this thing goes and exactly what it is? I can't see an empty hole anywhere that it would plug into. The photo with the wire itself circled in red is how it sits now. It should go straight down behind the engine somehow...

Thanks much!

mach4 · #2 09-02-2013, 03:58 PM

Looks like the VR sensor for the tach. If so, it goes on the intermediate plate and reads the teeth of the flywheel to get an RPM signal.

Here's a picture.

Mölyapina · #3 09-02-2013, 04:35 PM

That's a tach feed if I ever saw one.

750KMILZ · #4 09-02-2013, 04:42 PM

Thanks so much for the help, and especially the photo! I'll take a look and see if I can get this thing in there. Tach isn't working, so that could explain!

Sincerely appreciate the help!

toomany MBZ · #5 09-02-2013, 08:31 PM

The tach sending unit changed in 1984, but my CD doesn't have anything like that.

vstech · #6 09-02-2013, 08:43 PM

what year did the engine you swapped in come from?

you may need to pull it out, and swap the IMplate from the original engine... or convert to the old style tack pickup... (pulling the engine would be less work)

750KMILZ · #7 09-02-2013, 08:49 PM

Well, I saw the hole where the sensor was supposed to go on the engine I took out. Looked with a flashlight this afternoon and didn't see the hole anywhere on the car. I'll take a closer look at it again tomorrow. Looks like a tough place to get to. My SD still had the original engine, and finally, at nearly 729K miles, I figured it was just time. Broken valve guides replaced at 703K, new timing chain and some other things done. Finally she just got to the point where she would barely move and just barely got me home on the last trip. She was just too tired to go on! Rattling that sounded like it was coming from the bottom end pretty much made the decision for replacement. Replacement engine supposedly only has about 130-140K on it (no documentation, no proof of that mileage), so hopefully it will last a while. Trying to get the cooling system working right, but otherwise just getting the speedometer/odometer working (my fault, I failed to remove the sensor before I pulled the engine and stripped the wires out of it), and getting the tach working, transmission adjustment (it shifts a little too smoothly for my taste), and a transmission fluid/filter change, I think we'll be set. Gave her a bath today, and she's still a pretty sight!

@mach4 - I saw your signature line (I need to get mine set up). How did the 380SL diesel conversion work out? I drove one of those cars many years ago (it was actually given to me as a loaner for two days when I had my 300SDL in the shop), and it was a great little car. That's the only one I've ever driven. I also like the Ford diesel pickups. Never had a Power Stroke like you have, but my old beater that I use on our cattle farm is one of the first 7.3L diesels, a 1988 F250, built in late 1987. Had an aftermarket turbo on it when I bought it, and it's a great machine!

@ toomany MBZ, I was wondering about that as well. My car is a 1984 year model (built Sept. 1983), and I wonder if I couldn't spot that hole for the sensor because maybe it was slightly moved in the engine I put in. I'm not 100% certain on the year model of the replacement engine. I'd assume it has to be pretty close. I'll take a closer look tomorrow.

Thanks again for your help.

750KMILZ · #8 09-02-2013, 08:54 PM

@ vstech - Honestly, I'm not completely sure about the year model of the engine I bought. I had a 1982 transmission in the car hooked to the original 1984 engine (late 83), but I don't suppose that had anything to do with it. Are older (pre late 83) sensors mounted somewhere else?

Honestly, if it means pulling the engine to switch something, I'll just do without the tach. I have never done anything this major in my life. Working in a cramped 100-year-old garage with limited lighting and space, that's completely out of the question, lol! It was nice, with all those miles, that everything still worked (including the tach), but no way I'm pulling this thing again!

Thanks much for the input!

Junkman · #9 09-02-2013, 08:55 PM

That wire is on the 84SD. I didn't pay attention because it didn't need to come out. You may be able to start the bolt form underneath then tighten with swivel & extension from above.

750KMILZ · #10 09-02-2013, 08:57 PM

Quote:

Originally Posted by Junkman

That wire is on the 84SD. I didn't pay attention because it didn't need to come out. You may be able to start the bolt form underneath then tighten with swivel & extension from above.

Thanks; I'll get under it tomorrow and just see if I can get to it (or at least find the hole!)

Mölyapina · #11 09-02-2013, 09:24 PM

My '84 300D has the style that mach4 pictured. My '82 'SD doesn't have the wire that yours does.

750KMILZ · #12 09-02-2013, 09:29 PM

That's odd. I have a 1982 300SD parts car as well (actually had two of them). The one I have now, I don't see the wire either. No telling what kind of can of worms I've opened. Like you mentioned, the picture from mach4 looks like the one that came out of my SD originally too. I just need get out there and look at it closer tomorrow and see what I can find out.

Thanks,

mach4 · #13 09-03-2013, 12:19 AM

Quote:

Originally Posted by 750KMILZ

That's odd. I have a 1982 300SD parts car as well (actually had two of them). The one I have now, I don't see the wire either. No telling what kind of can of worms I've opened. Like you mentioned, the picture from mach4 looks like the one that came out of my SD originally too.

I believe that the tach pickup on the transmission was an 84-85 only item. No reason it wouldn't work other engines, but you need the proper intermediate plate that has the hole for the VR pickup.

If you don't have that setup, it's a pretty easy task to pull the tach signal off the harmonic balancer and use the tach amp on the fender well. Unfortunately you'll need the correct tach module in the cluster too as the harmonic balancer sensor gets one signal per revolution while the flywheel sensor gets about 50 signals per revolution.

The 84-85 tach get's its signal through the EGR controller - at least that's the way it's wired. An important piece of new information is that you can bypass the EGR controller and hook the VR sensor wires directly to the tach module in the cluster! This is one less piece of crap that is needed to adapt the later tach system to an earlier vehicle.

Quote:

Originally Posted by 750KMILZ

@mach4 - I saw your signature line (I need to get mine set up). How did the 380SL diesel conversion work out? I drove one of those cars many years ago (it was actually given to me as a loaner for two days when I had my 300SDL in the shop), and it was a great little car. That's the only one I've ever driven. I also like the Ford diesel pickups. Never had a Power Stroke like you have, but my old beater that I use on our cattle farm is one of the first 7.3L diesels, a 1988 F250, built in late 1987. Had an aftermarket turbo on it when I bought it, and it's a great machine!

The conversion is really working out well. I've already got over 26000 miles on it and it's my daily driver. In the spring we took it from San Diego to Arkansas and are planning on taking it to North Carolina the end of the month. Mileage is great (well over 30 highway) and it's a dream to drive.

If you're interested in the conversion, it's completely documented here - 380SL Diesel Conversion Project and the web site for the car is here - 380SL Diesel Swap Project Web Site

750KMILZ · #14 09-03-2013, 02:36 PM

Cool. It's not the biggest of my problems right now, so I'll figure out the tach later. Right now I need to get the speedometer sending unit or whatever it is, in the back of the transmission replaced, and even more importantly, try to figure out why I can't get the cooling right. Runs great with the overflow cap loose, but starts getting hot when the cap is tightened downed (system pressurized I suppose). This has been a long frustrating process.

mach4, Love the info on your 380SL. I'll look it over in more detail after while. I've read about a third of the post so far, and I'm really impressed. Where in NC are planning to head to? I live about in the middle of the state, outside of Greensboro.

Thanks much,

mach4 · #15 09-03-2013, 02:53 PM

Quote:

Originally Posted by 750KMILZ

mach4, Love the info on your 380SL. I'll look it over in more detail after while. I've read about a third of the post so far, and I'm really impressed. Where in NC are planning to head to? I live about in the middle of the state, outside of Greensboro.

Kernersville