I just got a new tach amp from Techguy512, I get the RPM sweep at start up, but I'm still not getting any action from the tach. I cleaned some gunk off the pickup, but still no go. I came across this thread trying to figure out the next step and just found out that when I honk the horn, the tach moves.

Did any of you solve the issue?

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Blue '83 300D "Domobile"
Red '73 Cimatti City Bike (moped = fun)

Not me; gave up. Too frustrating.

I found the problem Guys. The wire coming from crankshaft sender plugging underneath the amp you screw on.Moves down with engine vibrations,and simply srewing tach amp on.The key is to wedge something under the tach socket(my case a key ring spread open).
Tach fixed for good.

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1999 w140, quit voting to old, and to old to fight, a god damned veteran

This is almost guaranteed to be weak signal from the pickup. Most people with this issue have fixed it by adjusting the gap between the face of the pickup and the pin when they are in close proximity. 1-2mm works best.

I did fix this for one guy by programming more sensitive limits into the pulse detector. Custom software just for him. I was worried that noise sensitivity would increase, but he reports perfect functionality.

Check your gap and if it's good, let me know....

Thanks,

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Bob
'82 300D Petrol B-G Metallic

Thanks for the advice. I just discovered that the pin on the balancer apears to be busted - it is just about flush with the surface. I am contemplating drilling it out or just trying to crazy glue a nut (or something) over it instead (after cleaning the area).
Does anyone know what kind of metal that pin is?
Does the crazy glue idea sound stupid?

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Blue '83 300D "Domobile"
Red '73 Cimatti City Bike (moped = fun)

You want something highly permeable to magnetic flux, such as a ferromagnetic material (Iron or Iron/Nickle alloy), but anything that is strongly attracted to a magnet will likely work. Stainless steel or aluminum are out.

I don't think I'd trust adhesives, but if you use one, use a strong epoxy instead of a cyanoacrylate (Crazy glue).

I would drill out the pin and tap the hole, screwing a threaded dowel rod of the proper material into the hole.....

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Bob
'82 300D Petrol B-G Metallic

Are the ground wires in the dash color coded ? I've had a major problem for years. Fuel & temp ga. pegged, tach cuts out when high beams are on. Started when I put new ignition lock in. Must have twisted some wires too far.

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1984 MB 300SD Turbo

On Mercedes are almost always Brown.

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'84 300SD sold
124.128

So I used epoxy and glued a nut that is attracted to a magnet onto the crank pully next to where the pin used to be. This was easier than drilling and seems to be holding on there for now. It is a little wider than the pin and basically there is a hole where you would normally thread a bolt in. The pickup is quite close to the nut. I'm still not getting any tach action except for the sweep at startup, when I honk the horn, and a little blip when I turn the ignition to off.

I've read conflicting info on getting readings from the pickup wires. With the tach amp off the base, I get about 70 ohms across pins 9 to 7 on the amp base and OL 9 to 8 and 7 to 8. If I put the amp back on the base I get the same 70 ohms from 9 to 7 and 9 to 8, but 0 (short) between 7 and 8. I am getting the proper 12V across 2 and 6 with the ignition in glow position.

One of Funola's posts states that I should get 80 ohms between 8 and 9, another post somewhere (might have been techguy512) says I should get between 50 and 80 ohms across 9 and 7.
Which one is it?

Right now I'm thinking the pickup is good, the gap is good, the tach gauge and wiring to it is good. I might glue a cap on top of the nut to cover the hole in the middle.

I plan on adding a ground from the back of the tach gauge and another one across the instrument cluster in hopes of solving the horn thing, but I’m also wondering if I should be tracking down the horn ground(s). I can live with the horn tach thing as I rarely use the horn. Now that I’ve started trying to fix the tach, I can’t seem to give up!

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Blue '83 300D "Domobile"
Red '73 Cimatti City Bike (moped = fun)

My tach just died last week. Came to life for one day again and then hasn't worked since. Found this thread with search.

I've poked around the engine compartment and under the dash and concluded it must be electronic as the only mechanical shaft comes from the back of the tranny so that must be the speedometor. But haven't found where the sensor is yet to check connections. Can anybody bring me up to speed on this?

Thanks

'85 300TD

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1985 300TD 214k. Elsbett VO system + Fattywagon injection line heaters. (SOLD!)
1996 VW Passat TDI wagon 327k. No VO
Two white german turbodiesel wagons. One analog, one digital.

Deezulmark - I've been doing lots of research on the tach lately. The 85 tach set up is different... sensor pickup is on the tranny flywheel (not crank pully) and the OVP is involved. Here are some links that might help:

Tachometer link threads

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Blue '83 300D "Domobile"
Red '73 Cimatti City Bike (moped = fun)

Racebannon, I've emailed some additional information to you in the form of an instruction sheet I made for a tach diagnostic tool I made from one of my tach amps. It has resistance and voltage measurements that can be made both with the amp in and out of the socket. You point out correctly that there was some confusion way back when in the discussions of pins 7, 8, and 9.

This should clear them up...

Thanks,

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Bob
'82 300D Petrol B-G Metallic

Thanks Bob!

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Blue '83 300D "Domobile"
Red '73 Cimatti City Bike (moped = fun)

Did you start off with a known working tach from a 300D turbo? Do you have other instrument cluster glitches? Did you copy exactly the wiring from the pics I posted? My 83 240D tach is still working fine since I installed it..

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Thanks! Lots of good links there. Didn't take too long to figure out that OVP is Overvoltage Protection (or Overload Relay) and where it is hidden. Now I know why my A/C compresor won't engage either!

Fuse was good. Will not click when 12V applied to coil even though about 675 ohms shows on coil. Jumped pin 30 and 87 and ran car for a few moments and the tach and A/C came to life!

New ones costing $50 to $80. Hmm, maybe try to fix first - nothing to lose. Pulled the cover off. Manually flip relay and pins 30-87 are connected OK. Find resitor wired in parallel with coil (with color code for about 680 ohms) which explains why coil doesn't show open.

Remelted the solder joints even though they look OK. Still no good, so coil must be bad. Can't fix that (easily!).

New one on order ($53.23 with shipping). Can do without tach a few more days and not much warm weather yet so will surve no A/C a little longer too.

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1985 300TD 214k. Elsbett VO system + Fattywagon injection line heaters. (SOLD!)
1996 VW Passat TDI wagon 327k. No VO
Two white german turbodiesel wagons. One analog, one digital.