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Old 05-09-2005, 08:02 AM
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1991 300sl tach not working

Can anyone tell me where the tach gets its signal from? About a month ago the ignition switch and voltage regulator both went at the same instant and the tach hasn't worked since. I tried using another instrument cluster with the same results. When I start the car, the idle is high then settles out after 30 seconds when cold, and is worse when the engine is hot. The car has the M104 engine. Thanks

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Old 05-09-2005, 08:59 PM
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I'm not sure I understand your question. What do you mean when you say it is worse when the engine is hot? I've never looked into the tach pulse circuit, so I've no first hand experience. I do know that an RPM signal is used by the transmission and also the radio ( Yes, volume control ). Anyone here know if there is a seperate fuse for the tach display?
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Old 05-10-2005, 12:11 AM
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Thanks for the fast reply. By worse I mean that the rpm's go higher and stay higher for around 45 seconds. Tonight I ran the diagnostics in the cluster ( updated to digital cluster ) and it showed '0' rpm's in function '4'. Using the diagnostics that check that the tach itself is actually working, (functions 6-10) the tach goes to the correct position. I have disconnected the radio harness since I installed a JVC mp3 player last summer. The transmission part is really bugging me since fifth gear doesn't kick in. Talk about bad fuel economy!!

The MB service dvd points me to the ignition module, but I can't see where it gets its signal from.

Thanks again.
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Old 05-11-2005, 02:39 AM
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The ignition module generates an RPM signal by processing the input from the crankshaft position sensor. This signal is output to the engine systems module (MAS) which passes it to several systems including the radio, instrument cluster, and automatic transmission controller. There is no dedicated fuse.
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Old 06-13-2005, 08:02 AM
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Tach working again!!

Well, it looks like I narrowed it down to the MAS module. Out of desperation I tapped on the top of the module with the handle of a screwdriver while the car was running and the tach started working. I plan on opening it up and looking for a cracked solder joint when I get a chance.

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