As the title states. We have a '85 W123TD parts car with the tach/clock and a '78 W123 300CD with no tach. I don't understand the reasoning of having a tach in an automatic transmission car but it is not my car and not my place to question why.

What would be required to transfer the clock/tach into the 300CD instrument cluster and make it operable?

I'm sure a bit of wiring would need to be altered/added, the amplifier, gauge and the sending unit used from the 300TD but is there anything which would make this an absolute "no go"?

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

Should bolt in.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

You'll need to ensure your pre E.G.R. Coupe has the TDC sensor attached to the engine near the bottom crank pulley and there's an tachometer amplifier some where in the car .

The gauge proper is a pretty much direct drop in to the dash binnacle but making it work might be a bit of a job, fiddly but not impossible .

Maybe look up the old post about adding a tachometer to an '81 / '82 240D, I've assembled the parts but am dragging my feet on the project as under dash work is difficult and very pain full for me these days .

IMO, adding a factory dynamic gauge is a great idea, slushbox tranny or not .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

Does the '85 with the EGR computer use the crank-mounted tach pickup, or does it count the flywheel teeth like the '86+ cars? It definitely matters because the pulse rate is very different between the two setups!

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Don't know and that's why I'm asking. The '85 OM617 is at a friend's place thirty plus miles away and the '78 is here with me. It's too danged hot for a "pleasure cruise" at the moment so I am trying to avoid a wasted trip.

I'm hoping Nate's right. I'm not interested enough in installing a tach if I have to split the engines/transmissions and swap flywheels. I guarantee he is not interested enough to pay me what I would charge if I had to go that route.

__________________
“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

? You didn't fix the wretched Klima I with the evil servo ?! .

I did on my NA 1979 300CD and it darn near froze me out of the car in the Mojave Desert .

It was always a pleasure to drive it any where hot or cold .

I imagine you'll have to change the damper pulley as it has the little rod sticking up that the RPM sensor reads as it whizzes by....

__________________
-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

Yeah, the ACC's working fine. It's the getting out of the car and walking over to where the car is located part I'm trying to avoid. Mostly because once I get to his place, I know there's going to be 25 or 30 "little things" I'll want to do. You know, since I'm there already. No sense in driving that far, we're talking two lane country roads followed by a couple of miles of a dirt "road" to the field where the beast is stored, just to turn around and come back.

I read on one of the postings (I think it was Funola's thread) about drilling and tapping a bolt into the existing dampener to create the trigger for the sensor. I'd imagine it would serve the same purpose as having the trigger on the flywheel. If so there would be no need to swap flywheels which would make the project MUCH more feasible.

__________________
“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

I feel your pain .

I agree, doing the damper mod will be simpler .

I'm so crazy from being cooped up I'm talking drives and walks in the 100+ degree heat and parking far from the store when I go shopping to force my self to get exercise =8-) .

__________________
-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

That isn't the point of my question asked earlier. If the EGR computer style tachs from the 1985 and California 1984 models are counting teeth on the flywheel like the 1986+ cars that all used that system, there are however many pulses as there are ring gear teeth per revolution of the engine, not 1 from the pin on the crank damper.

The reason it matters is because if the '85 is using the flywheel teeth for it's tach signal, it will not work with the single pin in the crank damper. You'd need to install the magnetic pickup on the flywheel to use the '85 tach, or you'd need to install the tach sensor on the crank damper and use an earlier tach.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Man, you sure know how to rain on a guy's parade. So, it is all going to depend on where the tach pick-up sensor is mounted.

If the sensor is mounted on the front of the engine then it should be a comparatively easy addition. Mounted on the rear side of the block it's going to be a matter of whether there is an access port to install the sensor plus transferring (and wiring) the EGR computer.

Guess there's nothing for it but to go and make a visual inspection.

__________________
“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

No rain spells cast, just asking an important question that could be a "game over" for your conversion. It would suck to pull the tach and go through the trouble of wiring it all up to find out it doesn't work.

Where are the OM617 gurus? I'm really surprised one of them hasn't chimed in here to confirm where the EGR computer OM617's get their tach pickup from for the EGR computer.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

I own a California spec. 1984 300CD and it has the crank pulley mounted tach sensor .

__________________
-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better