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190D 2.5 tachometer; Tachometer troubleshooting
Topic Started: Sat Dec 31, 2011 7:46 pm (757 Views)
mlr
Newbie
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I've spent the day trying to find out why the tachometer on my 190 2.5D doesn't work. I checked all the usual suspects - fuses, OVP relay and engine speed sensor. They all appeared to be OK. There was 12v at the tachometer, but no signal from the speed sensor. Checking the circuit from the speed sensor input at the tachometer back to the speed sensor showed that all was seemingly as it should be, as was the earthing. So why no signal?

The next move was to disconnect the speed sensor from the loom and run a wire from it directly to the tachometer signal input, making sure that the other side of the speed sensor was earthed. (I also rigged a temporary 12v supply.) Hey presto, the tachometer worked, so there had to be a problem with the signal wiring somewhere. Next, I connected the temporary wire from the speed sensor to the tachometer, with the tachometer plugged into the loom. This time, the tachometer did not work, so although as I said earlier, at first sight the wiring looked OK, it cannot be.

With the engine running and the tachometer connected with the temporary wiring only, the tachometer works. If the signal wire from the tachometer loom plug is connected to the tachometer signal input, (with the temporary connection still in place), it ceases to work. Clearly there is leakage to earth somewhere in the loom wiring. Rather than try to find the problem, I plan to run a new signal cable to the speed sensor, leaving the original disconnected.

Does anyone know if I am right to assume that nothing else on the car is dependent on the speed sensor? I can't see that anything would be, but the wiring diagrams are far from clear, (to me, at least!), when it comes to the tachometer circuitry.

Sorry that this is so wordy - Happy New Year to one and all!

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MBZ190D
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Hi there had a quick glance at your post, check the normal fuse box the one that does the electromagnet fan also Dose the rev counter, apoligises if you've checked this but I'm on a night out and haven't read your post fully!
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mlr
Newbie
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Thanks for that. The fuses are all fine and everything else is working as it should, as far as I can see. I did forget to mention that with the engine running, or just the ignition switched on, there is 6v DC present on the speed sensor signal cable when it is unplugged at both ends. That should not be there. According to the wiring diagrams, nothing else connects to the signal cable, except the diagnostic socket. The trouble is, I can't find a diagnostic socket on my car, unless it's the rectangular one with a cap on and only two connections, just in front of the battery.

I'm really clutching at straws here, because the main length of the signal wire is buried under the wipers etc. As I said in my first post, I'm thinking of just running a new wire, then it will all work properly......
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mlr
Newbie
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Just found this:

Mercedes diagnostic plug pinout

which confirms my suspicion that the speed sensor on diesels is not connected to the diagnostic plug. (Only pins 1 and 4 are connected on my vehicle.)
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mlr
Newbie
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Further investigation has revealed that the engine speed signal is fed via the idle speed control unit. Maybe my next move will be to find another to see if substituting it resolves the problem.
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