[posted this yesterday on STD, but maybe there are more om617 owners, and certainly more W123 owners, here.]

butchering a five-cyl tach, i noticed that the other one i had on hand was different. the VDO part numbers differ as do the signal jacks and circuitboards. the numbers visible on are: 230/154/1 14000Hz, on the one i was cutting up; and 230/107/15 6000Imp., on the one from a cluster which is elsewhere stamped 1982. unfortunately i don't know where the butchered one came from. there is no difference on the gauge faces. do these two units work the same? the numbers 6000 and 14000 sure make me wonder.

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'77 240D, 504H, OM617.952, etc.

A past post talked about the wiring changes when the under-hood "tach amp" was replaced by the 1984+ cabin "EGR computer". One guy traced it out and determined the wires simply route thru the later w/ no signal conditioning (formerly by tach amp), so that function probably then takes place in the dash tachometer. Thus, a different PN makes sense. May apply only to CA cars, but that probably means you, Bay guy.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

Not all 1984's have the cabin EGR computer. My '84 does NOT have the EGR computer and OVP fuse in the glove box, but it does not have a tach amp either. It's direct pickup from the flywheel into the tach.

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"The MB W123 is so bulletproof, you can drive them forever. Which is a good thing as it takes that long to get anywhere."
Betsie: 1984 W123 300D (hobby, 280k miles)
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The Turd: 2007 Toyota Camry (wife's car, 118k miles)

Good point, Ceristimo, and I think such was mentioned in the former post I referred to (someone please link if they find it). That gives even more credence to the statement that the EGR computer simply runs the wires thru and the dash tach does the conditioning in 1984+ cars.

To clarify, in earlier 300D's, there is a removable tach amp that plugs into the "diagnostic port" on the driver's inner fender (might be a later term). I assume the tach amp takes the pulses from the crank pickup and outputs a steady DC voltage signal that goes to the dash tach gage. But, there are posts on rebuilding the tach amp that probably detail that. Whatever it does, must be done in the dash in 1984-85 300D's and probably within the gage itself.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

Thanks, Bill and Ceristimo for jumping in. yeah, the presence or absence of the amp may be a key point here, and i was working blind, without the donor car. i can hook them up to a simple low DC voltage and check if the circuit works that way, but i'm leery of releasing the magic smoke.

Ceristimo, any chance you could supply the imprinted type on the back of your tach to compare?

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'77 240D, 504H, OM617.952, etc.

Try post 34 here Help identifying diesel tachometer req'd - want to fit it to my car!


It claims that :

- crankshaft tachometer, on the back it says 6000 Imp
- flywheel tachometer, on the back it says 14000 Hz
- alternator tachometer, aftermarket, no indication, rare
~~~

Working backwards,

6,000 Imp ( = Impulses but no time base listed ) Trying 6,000 pulses per second = 100 pulses per minute / 6,000 max RPM = 0.016667 teeth, this can't be right

6,000 Imp ( = Impulses but no time base listed ) Trying 6,000 pulses per minute = 6,000 / 6,000 max RPM = 1 tooth ( 1 pulse per revolution ) This looks possible.


14,000 HZ = 14,000 cycles per second = 840,000 cycles per minute / 6,000 max RPM = 140 teeth ( or pulses per revolution ) . Do these flywheels have 140 teeth and does the tack top out at 6,000 ? If not 140 teeth, perhaps an amp is altering the frequency.

It is very common for industrial diesels to use a magnetic prox sensor to count flywheel teeth for an RPM signal. Have a look at Murphy by Enovation Controls

With an oscilloscope I measured a 1666 HZ tach signal on my 85. My idle is around 700 rpm.

(1666 * 60)/700 = flywheel around 142 teeth

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

thanks folks, i hadn't realized there were two kinds of pickups. mine is on the crankshaft, so i was working on the wrong kind of tach. cheers!

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'77 240D, 504H, OM617.952, etc.

... gasser 123's would have a different tach also... what's the redline?

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
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1987 190D 2.5Turbo
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