W123 tach
 

G'day all, well I got myself a 2 pin diesel tach from the USA, i also got a tach amp

the engine i got is om617A and it has the pickup on the front of the engine

the pickup wire is installed, the tach amp is in place

I'm just having trouble wiring it.


I tried following this guide. ( see attached picture )


i used a multimeter to see which wire was which, the green + yellow wire is number 3, however i'm struggling to find where it's supposed to go in the wiring loom

I can't find the correstonpding enf to connect it to.

the car has the tach plug behind the instument cluster, but for the life of me i can't find the other end, i tried tracing it, but it goes into a 2 inch thick loom of so many wires under the dashboard and from there i lose it :/

with the connecting wire behind the dash
I see the + is for straight 12v( black + red stripe ) , and the other straight black ) is for signal. But no luck, i tried running a wire straight from the number 3 output from the tach amp to it, and no go :'(

the car

1985 280CE ( manufactured in 1985 ) with om617A engine transplanted into it. I'll grab some pictures


off of the tach amp i have these wires.

+ 1 more for power.

i have

1x brown = ground
1x red = +12V ?
1x black + red = unknown
1x green = unknown
1x green + yellow = signal to tach.

If you are looking for ends that seem to be hidden in looms and you're not sure where they go the only way I know is to get a multimeter and start performing continuity checks

I assume the wiring loom is still from the 280CE - as in original to the car - so if I were looking for the other end of the wiring for a tachometer on a petrol engined car I'd be off searching around where the alternator sits...

...I've got a feeling (wild guess alert!) that that's how "they" did it for petrol engine cars (not sure why they did it differently for the diesel come to think of it - I'm sure some electrickery genius will jump in and explain...

...another alternative to the alternator way might be via the CIS system that was once fitted where there probably a crank position sensor for the tachometer on the front pulley (that's where it is on my M102 / W201). There's another sensor on the flywheel too on my M102 and the wires all join up on the left hand side of the car near to where the ABS pump on a LHD car is fitted.

As you can tell I'm not up to speed om M110 + W123 engine configurations - hope the comments help enough for you to find the wires.

When I back-fit a tachometer to my '85 300TD, I just ran all new wiring from the tach amp to the tachometer and to a fuse for power. Left a lot more hair on my head.

Thank you. I think i'll just go and buy a new line to put in because i don't want to have to undo and cut open sheathes of wire bundles.

what size/ voltage/amp rating fuse did you use ?

subscribed .

Put new lines in today. Last thing i need is a new pickup sensor/cable because the one i have it gone. Reading of 85 Ohms resistance and according to Pierre Hedary anything over about 70 ohms is where failure starts to occure

https://www.youtube.com/watch?v=5glxTyJbgTo

So, I test fitted a blue wire through the firewall along side the standard..

cut what i needed from the plug with as much lead as possible

figured out the wiring.

put a fuse in on the positive line, used a 15Amp fuse, which i think needs to be replaced for a much smaller one..

I think i should put a fuse on the " signal" line as well, not sure.

So yeah, it's time to get a new pickup wire. the grey one that goes to the front of the engine that picks up the signal.

the part number code for it is A 001 153 83 28 for anyone needing it for future reference.

"Too Much Resistance"
 

? measured where ? .

there are 3 pins on the signal pickup wire.

1 is the sheilding sheath, the other 2 are the ones you measure across :)

W123 Tachometer Leads
 

? I thought some were two wire and others were three..... ?

there are 2 kinds of tachs, a 3 wire and a 2 wire. The 2 wire/pin is easier.


Also I was talking about the pickup sensor wire which is 3 pins/wires within it.


Just did a little MS paint to help show what i mean :)

the end of the sensor wire is a plug which has 3 pins in it :)

W123 Tachometer Leads
 

THANK YOU VERY MUCH ! .

One of these days I want to add a tachometer to my '82 240D.....

This document will come in handy when I get a 'roundtuit' ;) .

I’m want the tachometer to work in my 190 I did a conversion from gas to diesel so it’s not working. I wonder if the same procedures will work ? since the engine it’s different my is a 602 turbo. Anyone knows ?

At an absolute minimum you'll have to swap the tach in the cluster. The gas and diesels use a different pulse rate and therefore have incompatible tachs. Not sure about the 201 chassis, but the 124 and 126's with the 603's had the tach signal sent through the EDS/EGR computer before being sent to things like the tach, cruise control, A/C, and radio (in later cars).

Sorry for the very late update.

I got a engine issue which put my car off the road and engine out time ( weird knock noise, we think it's a spun bearing, stopped the car immediately and trailer'd it home. Here's hoping no damage, just a new bearing )

Any ways, I got to the point where the tach would work- sort of. It wouldn't pickup the signal at Idle speed, but would read RPM when revving it, so it was always behind 800-1000 rpm.

I Measured the resistance in the signal pickup sensor wire and it's too much out of spec, so i need a new pickup wire.