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  #1  
Old 03-18-2014, 02:38 AM
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Adding a tachometer to a w123

So, My moms car has the big clock. We've decided it needs the tachometer. So I read this thread:

Tachometer retrofit - shortcut option?

And that sounds like the best way. I assume from reading that, the 84/85 with the pick up in the adapter plate has the tach amp built into the gauge. So all you've have to do is wire the tach pickup to the gauge. This sounds like the simpleast solution electonicly speaking. I've never had a problem with the tach in my 84, so this makes me think this is a sound solution.

BUT

Is this the simplest solution? My mom has a new tach amp she bought for her first car. So if I do set up the early style tach I have a tach amp. The engine still has the bracket on the front. And I don't have to drop the transmission and pull the flywheel to install it. So this seems like the easiest option.

Has anyone retrofitted the early style tach?

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  #2  
Old 03-18-2014, 04:14 AM
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Here's how far I've got with the same problem

Help identifying diesel tachometer req'd - want to fit it to my car!

Earlier tachometer for the cluster - adjusted the holder at the front of the engine - got me an amp - probably going to wire from the glow plug relay supply - got a bracket for the amp from a friendly chap on the forum
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1992 W201 190E 1.8 171,000 km - Daily driver
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1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!
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  #3  
Old 03-18-2014, 09:57 AM
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If you have the OE sensor on the flywheel, wire it directly to an 85 tach and that should be it. In my recent work building a timing light, I looked at the output of my 85 300D flywheel sensor with an oscilloscope and it was 4 V P-P sine wave at idle, high enough to drive an 85 tach directly, bypassing the EGR computer. Do not use just any VR sensor, it may not have high enough output to drive the 85 tach. Get the OE Mercedes flywheel VR sensor if you don't have one!
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Old 03-18-2014, 12:05 PM
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The car doesn't have the port in the adapter plate for the 84/85 tach. So if I want to go that route, I have to pull the transmission and flywheel, and and install a different adapter plate. Not really wanting to pull the transmission and flywheel. But I do think this the better of the 2 types of tachs.

I think the earlier style tach with the amp on the fender may be the easiest to install at this point. Now I just need to find a car to pull the parts from.
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  #5  
Old 03-18-2014, 04:16 PM
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Quote:
Originally Posted by SirNik84 View Post
The car doesn't have the port in the adapter plate for the 84/85 tach. So if I want to go that route, I have to pull the transmission and flywheel, and and install a different adapter plate. Not really wanting to pull the transmission and flywheel. But I do think this the better of the 2 types of tachs.

I think the earlier style tach with the amp on the fender may be the easiest to install at this point. Now I just need to find a car to pull the parts from.
Pulling engine tranny to install a tach is nuts. Don't! Make sure you get a tach that is compatible with the signal from the harmonic balancer sensor.
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  #6  
Old 03-19-2014, 01:07 AM
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Tonight I did a little looking around at my tach situation. I found the pickup in place on the engine with the wire clipped. The tach-amp has about 6 inches of wire sticking on its side. Sadly the wires arnt't long enough to reconnect. Any idea on what kind of cable to replace it with? I have some #16-TSP that we use at work for 4-20ma signals, I was thinking about that.



There is a loom of wire coming out of the tach-amp holder and going into a loom headed to the fire wall. the other part of that loom, not going to the tach amp holder is a mess of wire. It looks like this is where the glow plug relay should be wired up. But since the car started life as a 280E I assume these wires were cut as part of the conversion to diesel.



These are the wires under the tach amp. One of the wires is tied to the harness going to the new GP relay. Its located near the battery. It looks like the GP relay for a 116 SD (dang didn't grab a picture of that) Here are the wires.



If you fallow the red wire that is extended under the tach-amp back tward the fire wall it joins some other wires in a loom. A green wire is coming from the firewall and 2 others are coming from anothe harness at the firewall. These 4 wires go into that loom and over to the GP relay.



I haven't looked into any of this wiring yet. But I'm hoping to figure it out. This car has some intresting wiring thanks to its past. Did the 280 engine use the same kind of tachamp base? maybe as a place for a diognostic tool? I know on gas cars the distributor gives the RPM signal, but did it use the same tach amp? Or maybe the person who swapped the engines added it? maybe he swapped the whole harness, and thats why there are wires under the tach amp (for the GP relay?) But that doesn't explain why it has the other style GP relay... This is a mystery!

If the admins would like to rename this thread, I think I'm just scratching the surface of the mysteries that is the wiring in this car. I found these wires going to the engine by the alternator.



The black/red wire continuse around the head to the temp sensor.


You can also see in this pictuer that the GPs are linked together. They are all linked together and then one wire goes the GP relay through a 50A strip fuse. It was getting power directly from the + post on the battery.

I bought a new strip fuse holder as the one in the car was melted, the fues is good and the GPs work, maybe an old fuse blew out in the holder. I'm going to replace it. I was thinking of fusing the it up pstream of the relay. Any idea why the swapper fused it after the relay?

I'll take some more pics tomorrow.

What is funny is everything in the car appears to work, so the fellow tho did this wireing did it right, well, enough to get the car to work. I'd just like to clean it up and get a tach in the car. Its got the huge clock... but the wiring seems to lead me to believe maybe the car once had a tach. maybe the guy didn't have a tach so he swapped in a clock? Mysteries!!!

Thanks for reading all about my mystery wiring. Feel free to chime in with any clues!
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Last edited by SirNik84; 03-19-2014 at 01:26 AM.
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  #7  
Old 03-19-2014, 01:17 PM
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Since yours is a converted 280E and there are spliced/ crimped wires to what you call the tach amp holder (I call it a diagnostic port- I'll explain later), I assume whoever did the conversion did not swap over the engine harness from the diesel donor.

If it were my car and I want to add a tach, I would not bother trying to use the existing wiring. I would add new wires: mag pickup to tach amp, tach amp to tach in cluster. You just have to supply switched 12V power to the tach amp and the tach.

The 77 280E elec diag. from the W123 service manual 2 CD set that I have indicate there is a TDC sensor and a diagnostic plug. The 78, 79 280E elec diags are multi use, i.e. for diesel + gas engines and more confusing. The 80 280E elec diagram is plain wrong (a 240D diagram) so don't bother looking at it.

I think the TDC sensor and diagnostic plug (in both diesel and gas models) is strictly for factory test equipment, i.e. to read RPM only. My 85 300D has the diagnostic plug which has wires to the TDC sensor and has power and ground and no where else (no tach amp output pin like on pre 85 300D models, which use a tach amp to drive the tach). The tach on 85 is driven off a sensor on the flywheel/EGR computer. On the 85, the TDC sensor and diagnostic plug is again, IMO, for factory test equipment to measure RPM only for the digital RIV injection timing method.

If you decide to use the diagnostic plug, you will need to add a pin for the tach amp output and run that up to the tach, or get a tach amp socket from pre 85 turbo diesel models.

6" long pig tail on the TDC sensor is long enough to splice if you take it off. Use shielded cable to minimize noise pickup. Old RCA video or audio cable should do nicely. Instead of splicing, go to the JY and get both the TDC sensor and tach amp holder from a pre 85 turbo diesel?
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  #8  
Old 03-19-2014, 02:04 PM
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I'm headed to the junk yard this weekend to see what I can find. I'm going to try to pull all the wiring, from TDC sensor to tach... If I can find it. It been my experience that here in Sacramento there is someone who pulls all the instrument clusters. Its rare to find gauges for Mercedes here in the yards. So I might have to travel to the bay area or something to find a tach. But I should be able to get all the wiring. we'll see what I come home with.
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  #9  
Old 09-19-2014, 03:00 PM
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i'm trying to get mine working right now too. it looks like like the amp only needs +, -, and the signal wire to the cluster. you may be able to wire those yourself using similar sized wire, unless your concern is using the same wire color as stock (get the whole connectors if you don't want splices, the pins and sockets can be removed intact).

the harness on the back of the tachometer itself has either + and signal or +, -, signal. i don't see why one shouldn't power the tach amp from somewhere in the engine bay instead of using the original multiconductor loom through the firewall; so any single wire of 18-22 gauge could carry the signal from the amp to the cluster, no junkyard harnesses needed....

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