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Euro-spec 280 SE - Erratic tachometer issue

Started by piotr0r, 30 October 2023, 01:07 PM


Hello everyone,

first, I'd like to thank the forum/community as a whole for the invaluable information I've found since March, when I bought my 1975 280 SE with K-Jet at an auction. I got it cheap (3100 EUR) and had only 126 000 km on the clock, but it was a non runner. But I like challenges.

I knew absolutely nothing about this particular generation, and knew nothing about K-Jet (and boy did I learn). Long story short, I got absolutely everything sorted out, despite the unbelievable amount of crud in the fuel system. I had the FD/WUR professionally rebuilt in Germany to save time.

Now, the car drives beautifully, it's a joy to drive, and it has plenty of power. I took it to 170 km/h on the motorway and it had power to spare, so I take it the fuel tuning is alright.

Now for my question: when I reach 4500 rpm, the tacho needle suddenly bounces, then goes back to zero. The engine obviously still revs normally and keeps picking up RPM without issue. With the four speed auto gearbox, it's above 150-ish km/h.

All the other instruments seem to work fine (temp, oil, fuel, and speed). The first time it did this was when I used the kickdown on a slip road.

As soon as the engine speed goes below 4500 rpm, it works just fine.

I'm not too familiar with the inner workings of the tachometer, and I'm baffled by this issue, so any guidance would be appreciated.

Thank you!


Ignition switch box of on its way out, replace it with a known good one


Thanks for your reply. Just so we're sure, it's the module on the left on the engine bay with the green wires going to the ignition distributor ?

Bosch ref. nr. on mine is 0 227 100 001B - I hope I'll be able to find one here in Europe.

I don't know if these could be repaired. I fix pinball machines and other arcade games for a living, I repair boards at component level, so I know how to read schematics and use a soldering iron, but these would be new to me. But I'd definitely wait to have another working module to get started.

And I do hope it won't leave me stranded without notice.


Also, while I'm at it, while looking at a couple of listings of eBay, there is this module that popped up on the suggestions. It's a 190 4 cylinder ignition module, and it still available as new from some vendors, and it's cheap.. The pinouts looks the same, but since I've got no schematics for the internals, I can't be sure.

It's not original, true, but I'd rather have a car that would not leave me stranded. I wonder if these could work.


Yes that's it. Get a good serving have one, but there are people who say you can re solder the connections to fix them


'68 250S
'77 6.9 Euro
'91 300SE,
'98 SL500
'14 CLS550,
'21 E450 Cabrio


You can repair, and rebuild them. I've reverse engineered that specific module years ago, and built two working prototypes.

Schematic is here:

I also have an unshared version that uses a FET instead of a BJT for the power stage, but haven't prototyped it yet.

Also, these are super rare to actually fail.
I'd check grounds first, make sure if they're all clean.

The tacho is fed from the TD signal in the box. You can attach a scope there and see what the output looks like in terms of noise. Should be a nice, square wave. The frequency depends on number of cylinders.
1993 "Pearl Blue" W124 280TE
1988 "Arctic White" W124 200T
1979 "Icon Gold" W116 450SE


Well, gentlemen, thank you very much for your plethora of details and leads. I'll get to work as soon as my day job schedule permits, as well as the weather.

ptashek: the schematic you provided is super interesting. I figured these boxes should be quite reliable by design, due to the nature of the components. It's not that different from the hardware inside the first electronic Gottlieb pinball machines I fix (down to the 1N4148 small signal diodes on the CPU). If I find a replacement box (original or otherwise) I'll look into it. I'll also check the aspect of the signal.

I'll share my findings here. I hate when a problem is solved but no solution is posted.