How do I get the tach working agai0? (W124, 300TD, 5 speed)
 

I've spent a lot of time and money resurrecting my 1987 300TD. It's been a great learning experience and I love the car. My stick shift conversion is fully sorted, as is the HVAC, suspension, and brakes. Not bad for a car that I think was left for dead over the course of several years. Unless some unknown horror is lurking in the background, there are only niggling things left to fix.



One such niggling thing is that the tachometer ceased to work after I installed the 5-speed. My understanding is that in an automatic car, the tach pickup gets its signal from the ring gear. Now that there's a .411 in place of the auto trans, how do I get the tach going again?


Many thanks,


Corvo Gold
1987 300TD, converted to stick shift

Few years back I also did a conversation in a 87 300TD and the tachometer kept working so is not because of the swap check all the wires coming out of the rear of engine near the transmission

My 87 300d tach kept working after 5spd swap. Years later it failed due to dried out capacitors in the tach. Replaced those and all was well again.

My understanding is that in an automatic car, the tach pickup gets its signal from the ring gear. Now that there's a .411 in place of the auto trans, how do I get the tach going again?

This is correct the location of the pickup is rear engine oil pan drivers side. That feeds a signal to the ECU which in turn drives the tach.

Re-flow the solder joints on the tach amp. Thats the most common issue. For my year, its a round black piece on the driver's side inner fender. Lots of info on how to do it and its pretty easy. Fixed mine in ~30 minutes.

I didn't add any solder. Just used my iron to melt and re-flow each solder joint on the back side of the unit. It's covered in glue so take your time to clean it off.

https://www.benzworld.org/threads/the-permanent-tachometer-amplifier-fix.1652194/

W124 and late W126 cars do not have a tach amp. The tach pulse is generated from the EDS computer and gets it's signal from the magnetic pickup located at the ring gear as mentioned above multiple times.

Gotcha. Thanks for clarification.

Replace the OVP relay before you make yourself nuts. When the OVP fails, the EDS has no power.

I confirmed the cable to the tach pickup is still hooked up. I did not remove it when I did my stick shift conversion. I wonder if the flywheel I used has something to do with it, like it doesn't have the peg the pickup senses. The tach has never worked since I competed the conversion.

BTW, the OVP relay is new.

Corvo Gold
1987 300TD, converted to stick shift

What flywheel did you use? Did you damage the crank sensor on installation?

You can view the signals at the diagnostic port
The pickup counts each tooth 144 teeth per revolution.

Or check the little red fuse on top of the relay.

OP - the fact your tach worked before the swap means something changed. The fact that others have done the same swap and maintain tach function is confusing.

The tach signal on the diagnostic port is for the sensor at the harmonic balancer up front. Kind of a different animal with a different frequency...I think. Correct me if I’m wrong.

I removed the tach pickup from the bell housing. Although it was undamaged, it had lots of metal shavings stuck to it that I removed. No change.

Where precisely is this diagnostic port and how do I check the signal? This is a new one for me.

Appreciate the responses!

Corvo Gold
1987 300TD, converted to stick shift

The diagnostic port is on the drivers side fender. It’s a little cylindrical connector under a cap. Pins 7,8,9 lead to a coil pickup at the front of the engine that gives one pulse per revolution. On older w123 turbos diesels this is converted to a square wave using a tachometer amplifier and fed to a two pin tach pre 85. After 85 and on your car there is a 3 pin tach that takes the bell housing sensor output which is many pulses per revolution.

This is not how to fix your problem. Conceivably you could put in an older model amplifier and somehow squeeze in an older tach and you’ll have a tach. But this isn’t advisable. It’s way too much extra electronics.

So there’s an incompatibility there. On your car as well as on 80s gassers the diagnostic port was used to hook up a piece of diagnostic gear to check rpm or TDC.

http://www.peachparts.com/shopforum/attachments/diesel-discussion/42916d1175638043-1985-300d-ca-emissions-tachometer-repair-pictach.jpg

I was just pointing out that this diagnostic plug signal is not compatible with your dash gauge even though the same pickup is used to run the tach on a w123 turbo pre 85.

At this point if I were in your shoes I’d start at your sensor like you have. I think it is supposed to read off the ring gear. If you have an oscilloscope you can check to see if you are getting a pulse out of it and it is getting delivered to the electronic brain box on the passenger side of the dash. My guess is it isn’t. You’re probably right, something has gone wrong between your old flywheel assembly and new. But that is only a guess.

Basically you need a wiring diagram and a oscilloscope and you need to trace the signal. Somewhere between the gauge and trans it gets lost. Hopefully it doesn’t require pulling the trans again.

Make sure to check the little red fuse on top of the ovp relay if there is one. On my 300d 1985 it was burned out and that was what I fixed. Easy things first.

I now have a working tachometer in my stick shift 1987 300TD. The other day I got a wild hair and swapped the tach with a spare that came from a diesel w124. I think the inop tach that I had in there was from a gas model.

Either the tach itself was broken or there is a difference between diesel and gas tachs.

FWIW.

Corvo Gold
1987 300TD, converted to stick shift